Description:TECHNICAL FIELD
The present disclosure relates to a multilayer coating on a steel substrate comprising a zinc
(Zn) layer on the steel substrates, a copper (Cu) layer on the Zn layer, a nickel (Ni) layer on
the Cu layer, and a chromium (Cr) layer on the Ni layer. In particular, the disclosure provides
the multilayer coating, a method of depositing the multilayer coating, 5 and substrates
comprising the multilayer coating.
BACKGROUND OF THE DISCLOSURE
Coated steel sheets are being considered as a replacement for stainless steel due to the higher
10 cost of stainless-steel sheets (Rs.150-350/kg). Numerous metallic or non-metallic coating
systems have been studied to coat steel sheets for different applications. Examples of such
coatings include Ni/Cr or Cu/Ni/Cr electroplated coating which are available commercially but
these coatings are also very costly (Rs. 60-170/kg steel sheet with thickness from 3.5 mm to
1.2 mm) due to the higher cost of thick Ni layers (15-40 µm). Therefore, there are tremendous
15 opportunities to explore new metallic coating systems on steel sheet which could serve as a
potential replacement for stainless steel as well as commercially available Ni/Cr or Cu/Ni/Cr
multilayer coatings in terms of cost and durability.
Commercially available Ni/Cr or Cu/Ni/Cr multilayer coatings have been used for
20 indoor/outdoor corrosive and decorative applications for many years. To understand the
corrosion behavior of these coatings, it is important to understand the architecture of the Ni/Cr
or Cu/Ni/Cr multilayer coating. The coating begins with a thin layer of cyanide (non-acid) Cuflash
to protect the steel/zinc die cast against the acidity of subsequent baths. Next is a thicker
layer of acid copper plate, which serves to make the surface more uniform and assures good
25 electrical conductivity. This is followed with one or more layers of nickel, which provides a
continuous corrosion resistant barrier. Finally, a chromium layer is applied to give the desired
shiny, silvery appearance and to protect the nickel against mechanical forces such as wear and
erosion. Both, a two-nickel-layer (bi-nickel) or three-nickel-layer (tri-nickel) system, can be
applied. The first (the bi-nickel layer system) is commonly referred to as an automotive grade,
30 and the second (the tri-nickel layer) is sometimes called marine grade, as it is used for more
aggressive chloride environment applications. In case of three layers of nickel in a
Cu/Ni/Ni/Ni/Cr tri-nickel plating system, the composition of the intermediate Ni layer is
adjusted to be anodic to both the upper and lower Ni layers so that it can sacrificially protect
the layers from corrosion (Knapp, Trans. Inst. Met. Finishing, 1958, 35, 139-165). In most
3
cases, however, the metal corrodes too rapidly causing blistering or scaling or staining or
coloring of the decorative surface. The corrosion field test of the bi-layer Ni/Cr and tri-layer
Cu/Ni/Cr was extensively studied by Biestek et al. (T. Biestek, Surf. Technol. 21 (1984), pages
283 and 295). The field results showed that coating systems containing external layers of
microporous or microcracked chromium have markedly better protective 5 properties than
conventional Ni40-Cr0.5 (Ni 40 µm and Cr 0.5 µm) and Cu20-Ni25-Cr0.5 coating systems (Cu
20 µm, Ni 25 µm and Cr 0.5 µm). Moreover, two-layer coatings of bright-nickel and regular
chromium and three-layer coatings of bright copper, bright-nickel and regular chromium, do
not provide the protection usually required under exceptionally severe service conditions, even
10 for 1 year. From the two systems of coatings examined, a better corrosion resistance was
demonstrated by the three-layer coatings of bright copper, bright nickel and regular chromium
(Cu20-Ni25-Cr0.5). They have also demonstrated the possibility of reducing the thickness of
the nickel layer in three-layer coatings with micro-discontinuous chromium from 30-25 µm or
eventually even to 20 µm for exceptionally severe service conditions. Similarly, the corrosion
15 study through CASS (Copper Accelerated Salt Spray Test), Corrodkote accelerated tests and
exposure tests in tropical marine atmosphere by Chandran et al. (K. Chandran, S.
Sriveeraraghavan, R. M. Krishnan, S. R. Natarajan, S. Guruviah, Bulletin of Electrochemistry
2 (5) (1986) 457) for various types of decorative Cu/Ni/Cr multilayer electrodeposits on mild
steel substrates show that duplex nickel with microporous chromium and a copper undercoat
20 could be the best system for corrosion protection.
It is evident from the literatures and ASTM B456-17 standard that a Ni/Cr or Cu/Ni/Cr
multilayer coating on steel requires very high coating thickness of Cu (10-20 µm) and Ni layers
(25-35 µm) to achieve desired corrosion life in different service conditions (Service Condition
25 Number: 4 and 5). A high thickness of Cu and Ni layer could be attributed from their inherent
material property. Cu is electrochemically cathodic/inert (-ve oxidation potential Eox
0 = -0.34
V), therefore, provides only barrier corrosion protection to steel (Eox
0 = +0.44 V). Although Ni
layer provides a unique combination of corrosion, wear resistance, brightness, lustre and
appeal, Ni acts as electrochemically cathodic (Eox
0 = +0.25 V) compared to steel and provides
30 barrier protection. The high coating thickness of Cu and Ni layers in Ni/Cr or Cu/Ni/Cr
multilayer coating is required to attain desired corrosion life through their barrier nature of
corrosion protection on steel. Although intermediate high-sulfur Ni layer in tri-nickel plating
system was introduced as sacrificial anodic layer to both the upper and lower Ni layers from
corrosion, the layer is still cathodic to steel substrate. Therefore, any corrosion cell reached to
4
steel substate will continue to corrode steel leaving Cu or Ni layers. Moreover, providing Cu
and Ni layers with a high coating thickness of about 10-20 µm (Cu layer) and 25-35 µm (Ni
layers) makes the Ni/Cr and Cu/Ni/Cr coating systems expensive.
GB1408285A discloses a multilayer coating on a steel fastener which comprise 5 of: at least
one from Cu, Co, Zn, Ni, Cd, Sn with a thickness of 0.25-2.5 µm as the first layer; at least one
from Cd, Zn, Sn, Zn-Cu ally, Cd-Cu alloy as a second layer different from the first layer with
a thickness of 1.25-2.25 µm; Cu as a third layer with a thickness of 0.25-12.5 µm; at least one
from Ni, Co, Co-Sn alloy, Ni-Sn with a thickness of 2.5-17.8 µm as the forth layer and Cr as
10 the top layer with a thickness of 0.125-0.5 µm. However, this coating system only fulfilled the
16-22 h of standard CASS corrosion test (ASTM B368-65) and 96 h of SST (ASTM B117)
both of which is not sufficient in today’s standard. With some modification in the coating
layers, Jacob Hyner (U.S. Patent No. 5,275,892) studied 6 layers coating on steel fastener in
the sequence of non-strike Ni alloy, Zn alloy, Zn, Cu, Ni and Cr with different ranges of
15 individual layer thickness. However, this multilayer system exhibited CASS life of only up to
24 hours. Increasing layer numbers will impart extra cost and time for plating as well as
additional rinsing steps.
Thus, there is a need in the art to provide a multilayer coating which exhibits a high corrosion
20 resistance but comprises lesser number of layers, and at the same time, the thickness of the
layers is reduced to save costs and time associated with depositing these coatings. The present
disclosure attempts to address said need.
STATEMENT OF THE DISCLOSURE
25 The present disclosure relates to a multilayer coating on a steel substrate, the coating
comprising: a Zn layer on the steel substrate; a Cu layer on the Zn layer; a Ni layer on the Cu
layer; and a Cr layer on the Ni layer.
.
The present disclosure also relates to a method for depositing the multilayer coating described
30 above on a steel substrate, comprising: depositing a Zn layer on the steel substrate; contacting
the Zn layer on the steel substrate with an activating composition comprising ammonium
bifluoride for about 5-15 seconds; depositing a Cu layer on the Zn layer; depositing a Ni layer
on the Cu layer; and depositing a Cr layer on the Ni layer as a top layer.
.
5
The present disclosure also provides a steel substrate comprising the multilayer coating
described above.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the cross-sectional EDS elemental map analysis of 5 Acid Zn/Cu/Ni/Cr
multilayer coating on steel according to the present disclosure.
Figure 2 shows photographs of different Zn/Cu/Ni/Cr multi-layered electroplated steel sheet
samples, according to the present disclosure.
10
Figure 3 shows the SEM topography and corresponding EDS area analysis of different Znbased
multilayer coatings of the present disclosure.
.
Figure 4 shows the cross-sectional SEM micrographs and corresponding EDS point analysis
15 (wt%) of different Zn-based multilayer coatings of the present disclosure on steel.
Figure 5 shows the cross-sectional EDS elemental map analysis of Acid Zn/Cu/Ni/Cr
multilayer coating of the present disclosure on steel.
20 Figure 6 depicts the Tafel curves of different Zn/Cu/Ni/Cr multi-layered coated steels of the
present disclosure, a commercial Ni/Cr coated sample and stainless steel sample.
Figure 7 shows the Salt spray test (SST) results of the Acid Zn/Cu/Ni/Cr (Zn-layer thickness:
6 µm) multilayer electroplated steel according to the present disclosure and a 200 series
25 stainless steel sample.
Figure 8 shows the CASS test results of the different Zn/Cu/Ni/Cr multilayer electroplated
steel substrates according to the present disclosure.
30 Figure 9 shows the Coating Adherence test results of the Zn/Cu/Ni/Cr multilayer coated steel
substrates prepared with and without activation using ammonium bifluoride.
6
DETAILED DESCRIPTION OF THE DISCLOSURE
As used herein, the term ‘comprising’ when placed before the recitation of steps in a method
means that the method encompasses one or more steps that are additional to those expressly
recited, and that the additional one or more steps may be performed before, between, and/or
after the recited steps. For example, a method comprising steps a, b, and 5 c encompasses a
method of steps a, b, x, and c, a method of steps a, b, c, and x, as well as a method of steps x,
a, b, and c.
With respect to the use of substantially any plural and/or singular terms herein, those having
10 skill in the art can translate from the plural to the singular and/or from the singular to the plural
as is appropriate to the context and/or application. The various singular/plural permutations
may be expressly set forth herein for sake of clarity. As used in this specification and the
appended claims, the singular forms ‘a’, ‘an’ and ‘the’ includes both singular and plural
references unless the content clearly dictates otherwise.
15
The use of the expression “at least” or “at least one” suggests the use of one or more elements
or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve
one or more of the desired objects or results. Throughout this specification, the word
“comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or
20 “having”, or “including but not limited to” wherever used, will be understood to imply the
inclusion of a stated element, integer or step, or group of elements, integers or steps, but not
the exclusion of any other element, integer or step, or group of elements, integers or steps.
Numerical ranges stated in the form ‘from x to y’ include the values mentioned and those values
25 that lie within the range of the respective measurement as known to the skilled person. If several
preferred numerical ranges are stated in this form, of course, all the ranges formed by a
combination of the different end points are also included.
Reference throughout this specification to “some embodiments”, “one embodiment”, “an
30 embodiment”, “a preferred embodiment”, “a non-limiting embodiment” or “an exemplary
embodiment” means that a particular feature, structure or characteristic described in connection
with the embodiment may be included in at least one embodiment of the present disclosure.
Thus, the appearances of the phrases “in some embodiments”, “in one embodiment” or “in an
embodiment” in various places throughout this specification may not necessarily all refer to
7
the same embodiment. It is appreciated that certain features of the disclosure, which are, for
clarity, described in the context of separate embodiments, may also be provided in combination
in a single embodiment. Conversely, various features of the disclosure, which are, for brevity,
described in the context of a single embodiment, may also be provided separately or in any
suitable 5 sub-combination.
The term “about” as used herein encompasses variations of +/-5% and more preferably +/-
2.5%, as such variations are appropriate for practicing the present invention.
10 The inventors tested whether providing a sacrificial anodic layer adjacent to a steel substrate
could provide both barrier and sacrificial protection of steel. The inventors explored whether
Zn having an oxidation potential of Eox
O = +0.76 V, Mg having an oxidation potential of Eox
O
= +2.36 V, Al having an oxidation potential of Eox
O = +1.66 V, or Cd having an oxidation
potential of Eox
O = +0.4 V would provide better performance than having Ni layers or a Cu
15 layer adjacent to the steel to provide sacrificial corrosion protection to steel. Since Al and Mg
are not possible to electrodeposit from an aqueous electrolyte, and Cd electro-deposition is
environmentally hazardous, the inventors tested whether providing a Zn layer adjacent to the
steel substrate could eliminate or reduce the thickness of Ni layers and provide better corrosion
resistance than having Ni layers or a Cu layer adjacent to the steel substrate. Moreover,
20 elimination or reduction of Ni layer thickness with a Zn layer will reduce the cost of the
multilayer coating. The inventors found that by providing a multilayer coating comprising a
Zn layer on (i.e., adjacent to) the steel substrate, a Cu layer on the Zn layer, a Ni layer on the
Cu layer, and a Cr layer on the Ni layer, the inventors were able to reduce the number and
thickness of layers in the multilayer coating and at the same time, the coating exhibited a very
25 high corrosion resistance compared to commercially available Ni/Cr or Cu/Ni/Cr multilayer
coatings or stainless steel series 200. Moreover, the Zn layer was found to be compatible with
Cu and Ni layers as well to provide adherent and corrosion resistant decorative multilayer
coatings.
30 Accordingly, the present disclosure provides a multilayer coating on a steel substrate, the
coating comprising:
a) a Zn layer on the steel substrate,
b) a Cu layer on the Zn layer,
c) a Ni layer on the Cu layer, and
8
d) a Cr layer on the Ni layer.
Accordingly, the multilayer coating of the present disclosure is interchangeably referred to
herein as a Zn/Cu/Ni/Cr coating, a Zn/Cu/Ni/Cr multilayer coating or a Zn-based multilayer
coating throughout the description.
The first layer of the present Zn/Cu/Ni/Cr multilayer coating adjacent to the 5 steel substrate is
the Zn layer. The second layer from the steel substrate is the Cu layer which is deposited on
top of the Zn layer. The third layer from the steel substrate is the Ni layer which is deposited
on top of the Cu layer. The fourth layer from the steel substrate is the Cr layer which is
deposited on top of the Cu layer. The Cr layer is the topmost layer that provides a bright or
10 semi-bright Cr finish to the steel substrate.
In some embodiments of the present disclosure, the Zn layer has a thickness ranging from about
6-9 µm, including values and ranges therebetween.
For example, in some embodiments, the multilayer coating comprises a Zn layer on the steel
substrate, having a thickness of about 6 µm, about 6.5 µm, about 7 µm, about 7.5 µm, about 8
15 µm, about 8.5 µm, or about 9 µm, including values and ranges therebetween.
In some embodiments of the present disclosure, the Cu layer has a thickness ranging from about
0.5-2 µm, including values and ranges therebetween.
For example, in some embodiments, the multilayer coating comprises a Cu layer on the Zn
layer, having a thickness of about 0.5 µm, about 0.6 µm, about 0.7 µm, about 0.8 µm, about
20 0.9 µm, about 1 µm, about 1.1 µm, about 1.2 µm, about 1.3 µm, about 1.4 µm, about 1.5 µm,
about 1.6 µm, about 1.7 µm, about 1.8 µm, about 1.9 µm or about 2 µm, including values and
ranges therebetween.
In some embodiments of the present disclosure, the Ni layer has a thickness ranging from about
1-2 µm, including values and ranges therebetween.
25 For example, in some embodiments, the multilayer coating comprises a Ni layer on the Cu
layer, having a thickness of about 1 µm, about 1.1 µm, about 1.2 µm, about 1.3 µm, about 1.4
µm, about 1.5 µm, about 1.6 µm, about 1.7 µm, about 1.8 µm, about 1.9 µm or about 2 µm,
including values and ranges therebetween.
In some embodiments of the present disclosure, the Cr layer has a thickness ranging from about
30 0.25-0.5 µm, including values and ranges therebetween.
9
For example, in some embodiments, the multilayer coating comprises a Cr layer on the Ni
layer, having a thickness of about 0.25 µm, about 0.28 µm, about 0.30 µm, about 0.32 µm,
about 0.34 µm, about 0.36 µm, about 0.38 µm, about 0.40 µm, about 0.42 µm, about 0.44 µm,
about 0.46 µm, about 0.48 µm or about 0.5 µm, including values and ranges therebetween.
Thus, in some embodiments, the present disclosure provides a multilayer 5 coating on a steel
substrate, the coating comprising:
a) a Zn layer on the steel substrate having a thickness ranging from about 6-9 µm,
including values and ranges therebetween as described above,
b) a Cu layer on the Zn layer having a thickness ranging from about 0.5-2 µm, including
10 values and ranges therebetween as described above,
c) a Ni layer on the Cu layer having a thickness ranging from about 1-2 µm, including
values and ranges therebetween as described above, and
d) a Cr layer on the Ni layer having a thickness ranging from about 0.25-0.5 µm, including
values and ranges therebetween as described above.
15 In some embodiments of the present disclosure, the total thickness of the multilayer coating
described above ranges from about 7.5-12 µm, including values and ranges therebetween, such
as about 7.5-11.5 µm, about 8-12 µm, about 8-11.5 µm, about 8-11 µm, about 8-10 µm, about
8-9 µm, about 8.5-12 µm, about 8.5-11 µm, about 8.5-10.5 µm,, about 8.5-10 µm, about 8.5-
9.5 µm, about 9-12 µm, about 9-11.5 µm, about 9-11 µm, about 9-10 µm, about 9.5-12 µm,
20 about 9.5-11.5 µm, about 9.5-11 µm, about 10-12 µm, about 10-11 µm, including values and
ranges therebetween.
For example, in some embodiments, the total thickness of the multilayer coating is about 7.5
µm, about 7.75 µm, about 8 µm, about 8.25 µm, about 8.5 µm, about 8.75 µm, about 9 µm,
about 9.25 µm, about 9.5 µm, about 9.75 µm, about 10 µm, about 10.25 µm, about 10.5 µm,
25 about 10.75 µm, about 11 µm, about 11.25 µm, about 11.5 µm, about 11.75 µm, or about 12
µm, including values and ranges therebetween.
In some embodiments of the present disclosure, the Zn layer is deposited on the steel substrate
from an acidic Zn electroplating bath, a sulphate Zn electroplating bath, or an alkaline Zn
electroplating bath.
30 Acidic Zn electroplating baths
In some embodiments, the present disclosure provides an acidic Zn/Cu/Ni/Cr multilayer
coating, comprising:
10
a) a Zn layer on the steel substrate having a thickness ranging from about 6-9 µm,
including values and ranges therebetween as described above; wherein the zinc layer is
deposited on the steel substrate from an acidic Zn electroplating bath,
b) a Cu layer on the Zn layer having a thickness ranging from about 0.5-2 µm, including
values and ranges therebetween as 5 described above,
c) a Ni layer on the Cu layer having a thickness ranging from about 1-2 µm, including
values and ranges therebetween as described above, and
d) a Cr layer on the Ni layer having a thickness ranging from about 0.25-0.5 µm, including
values and ranges therebetween as described above.
10 In some embodiments, the acid Zn/Cu/Ni/Cr multilayer coating comprises:
a) a Zn layer on the steel substrate having a thickness ranging from about 6-8 µm,
including values and ranges therebetween; wherein the zinc layer is deposited on the
steel substrate from an acidic Zn electroplating bath,
b) a Cu layer on the Zn layer having a thickness ranging from about 0.5-1.5 µm, including
15 values and ranges therebetween,
c) a Ni layer on the Cu layer having a thickness of about 1.5 µm, and
d) a Cr layer on the Ni layer having a thickness of about 0.25 µm.
In some embodiments, the Zn layer has a thickness of about 6 µm, about 6.5 µm, about 7 µm,
about 7.5 µm, or about 8 µm, including values and ranges therebetween; the Cu layer has a
20 thickness of about 0.5 µm, about 0.6 µm, about 0.7 µm, about 0.8 µm, about 0.9 µm, about 1
µm, about 1.1 µm, about 1.2 µm, about 1.3 µm, about 1.4 µm, or about 1.5 µm, including
values and ranges therebetween; the Ni layer has a thickness of about 1.5 µm; and the Cr layer
has a thickness of about 0.25 µm.
In some embodiments, the total thickness of the acid Zn/Cu/Ni/Cr multilayer coating described
25 above ranges from about 8-11 µm, including values and ranges therebetween.
For example, the total thickness of the acid Zn/Cu/Ni/Cr multilayer coating is about 8-11 µm,
about 8-11 µm, about 10-11 µm, about 8.25-8.75 µm, about 10.25-10.75 µm, about 8 µm, about
8.25 µm, about 8.5 µm, about 8.75 µm, about 9 µm, about 9.25 µm, about 9.5 µm, about 9.75
µm, about 10 µm, about 10.25 µm, about 10.5 µm, about 10.75 µm, or about 11 µm, including
30 values and ranges therebetween.
In some embodiments, the acidic Zn electroplating bath comprises about 42-84 g/L of zinc
chloride that provides about 20-40 g/L of Zn metal, about 205-223 g/L of potassium chloride,
11
and about 24-32 g/L of boric acid. The total amount of zinc chloride and potassium chloride in
the acidic Zn electroplating bath is such that it provides about 120-150 g/L of total chloride.
In some embodiments, the acidic Zn electroplating bath comprises about 1-3 mL/L of
brightener, including values and ranges therebetween. In some exemplary, non-limiting
embodiments of the present disclosure, the acidic Zn electroplating bath 5 comprises about 2
mL/L of the brightener.
Accordingly, in some embodiments, the acidic Zn electroplating bath comprises about 42-84
g/L of zinc chloride that provides about 20-40 g/L of Zn metal, about 205-223 g/L of potassium
chloride, about 24-32 g/L of boric acid, and about 1-3 mL/L of brightener.
10 For example, in some embodiments, the acidic Zn electroplating bath comprises zinc chloride
in an amount of about 42-80 g/L, about 42-70 g/L, about 42-65 g/L, about 42-60 g/L, about 42-
55 g/L, about 50-80 g/L about 50-75 g/L, about 50-70 g/L, about 50-60 g/L, about 60-82 g/L
about 60-75 g/L, about 60-70 g/L, including values and ranges therebetween; potassium
chloride in an amount of about 205-220 g/L, about 205-215 g/L, about 210-223 g/L, or about
15 215-223 g/L, including values and ranges therebetween; boric acid in an amount of about 24-
32 g/L, about 24-30 g/L, about 25-28 g/L, about 26-31 g/L, about 27-29 g/L, about 24-26 g/L,
about 24 g/L, about 25 g/L, about 26 g/L, about 27 g/L, about 28 g/L, about 29 g/L, about 30
g/L, about 31 g/L or about 32 g/L, including values and ranges therebetween; and brightener
in an amount of about 1-3 mL/L, about 2-3 ml/L, about 1-2 ml/L, about 1 ml/L, about 2 ml/L
20 or about 3 ml/L of brightener, including values and ranges therebetween.
In some embodiments, the total chloride content of an acidic Zn electroplating bath is about
120-150 g/L, about 120-140 g/L, about 130-140 g/L, about 130-150 g/L, about 140-150 g/L,
about 120-130 g/L, about 120 g/L, about 125 g/L, about 130 g/L, about 135 g/L, about 140 g/L,
about 145 g/L or about 150 g/L, including values and ranges therebetween.
25 In some embodiments, the brightener is selected from a group comprising methacrylic acid,
acrylic acid, acrylonitrile, methacrylonitrile, vinyl C1 -C5 alkyl esters, vinyl halide,
epihalohydrin, vinylidine halide, alkylene oxide, or a combination thereof. In an exemplary
embodiment, the brightener comprises a mixture of methacrylic acid, acrylic acid, acrylonitrile,
methacrylonitrile, vinyl C1 -C5 alkyl esters, vinyl halide, epihalohydrin, vinylidine halide, and
30 alkylene oxide.
12
Alkaline Zn electroplating baths
In some embodiments, the present disclosure provides an alkaline Zn/Cu/Ni/Cr multilayer
coating, comprising:
a) a Zn layer on the steel substrate having a thickness ranging from about 6-9 µm,
including values and ranges therebetween as described above; wherein 5 the zinc layer is
deposited on the steel substrate from an alkaline Zn electroplating bath,
b) a Cu layer on the Zn layer having a thickness ranging from about 0.5-2 µm, including
values and ranges therebetween as described above,
c) a Ni layer on the Cu layer having a thickness ranging from about 1-2 µm, including
10 values and ranges therebetween as described above, and
d) a Cr layer on the Ni layer having a thickness ranging from about 0.25-0.5 µm, including
values and ranges therebetween as described above.
In some exemplary non-limiting embodiments, the alkaline Zn/Cu/Ni/Cr multilayer coating
comprises:
15 a) a Zn layer on the steel substrate having a thickness ranging from about 6-8 µm,
including values and ranges therebetween; wherein the zinc layer is deposited on the
steel substrate from an alkaline Zn electroplating bath,
b) a Cu layer on the Zn layer having a thickness ranging from about 1-2 µm, including
values and ranges therebetween,
20 c) a Ni layer on the Cu layer having a thickness ranging from about 1.5-2 µm, and
d) a Cr layer on the Ni layer having a thickness of about 0.25 µm.
In some non-limiting embodiments, the Zn layer has a thickness of about 6 µm, about 6.5 µm,
about 7 µm, about 7.5 µm, or about 8 µm, including values and ranges therebetween; the Cu
layer has a thickness of about 1 µm, about 1.1 µm, about 1.2 µm, about 1.3 µm, about 1.4 µm,
25 about 1.5 µm, about 1.6 µm, about 1.7 µm, about 1.8 µm, about 1.9 µm or about 2 µm including
values and ranges therebetween; the Ni layer has a thickness of about 1.5 µm, about 1.6 µm,
about 1.7 µm, about 1.8 µm, about 1.9 µm or about 2 µm, including values and ranges
therebetween; and the Cr layer has a thickness of about 0.25 µm.
In some exemplary non-limiting embodiments, the total thickness of the alkaline Zn/Cu/Ni/Cr
30 multilayer coating described above ranges from about 9-11 µm, including values and ranges
therebetween.
13
For example, the total thickness of the alkaline Zn/Cu/Ni/Cr multilayer coating is about 9 µm,
about 9.5 µm, about 9.75 µm, about 10 µm, about 10.25 µm, about 10.5 µm, about 10.75 µm,
or about 11 µm, including values and ranges therebetween.
In some embodiments, the alkaline Zn electroplating bath comprises Zn oxide that provides
about 11-13 g/L of Zn metal, about 110-130 g/L of sodium hydroxide, and 5 about 27-30 ml/L
of additives.
In some embodiments, the alkaline Zn electroplating bath further comprises about 1-3 ml/L of
brightener, including values and ranges therebetween.
Accordingly, in some embodiments of the present disclosure, the alkaline Zn electroplating
10 bath comprises Zn oxide that provides about 11-13 g/L of Zn metal, about 110-130 g/L of
sodium hydroxide, about 27-30 ml/L of additives, and about 1-3 ml/L of brightener.
For example, in some embodiments, the alkaline Zn electroplating bath comprises Zn oxide
that provides about 11-12 g/L, about 11.5-13 g/L, about 11.5-12.5 g/L, about 12-13 g/L, about
11 g/L, about 11.5 g/L, about 12 g/L, about 12.5 g/L, or about 13 g/L of Zn metal, including
15 values and ranges therebetween; about 110-130 g/L, about 110-120 g/L, about 115-130 g/L,
about 115-125 g/L, about 120-130 g/L, about 110 g/L, about 115 g/L, about 120 g/L, about 125
g/L, or about 130 g/L of sodium hydroxide, including values and ranges therebetween; about
27-30 ml/L, about 28-30 ml/L, about 29-30 ml/L, about 27-29 ml/L, about 27-28 ml/L, about
28-29 ml/L, about 27 ml/L, about 28 ml/L, about 28.5 ml/L, about 29 ml/L or about 30 ml/L
20 of additives, including values and ranges therebetween; and about 1-3 ml/L, about 2-3 ml/L,
about 1-2 ml/L, about 1 ml/L, about 2 ml/L or about 3 ml/L of brightener.
In some exemplary non-limiting embodiments of the present disclosure, the alkaline Zn
electroplating bath comprises Zn oxide that provides about 12 g/L of Zn metal, about 110 g/L
of sodium hydroxide, about 28.5 ml/L of additives, and about 2 ml/L of brightener.
25 In some embodiments of the present disclosure, the additives are selected from a group
comprising N-benzyl-3-carboxylpyridinium chloride sodium salt (NCP), polyethylene amine
and thiourea, or a combination thereof.
In some embodiments of the present disclosure, the brightener is selected from a group
comprising N-benzyl-3-carboxylpyridinium chloride (NCP), a sodium salt of NCP,
30 polyethylene amine, thiourea, or a combination thereof.
14
Zn Sulphate electroplating baths
In some embodiments, the present disclosure provides a sulphate Zn/Cu/Ni/Cr multilayer
coating, comprising:
a) a Zn layer on the steel substrate having a thickness ranging from about 6-9 µm,
including values and ranges therebetween as described above; wherein 5 the zinc layer is
deposited on the steel substrate from a sulphate Zn electroplating bath,
b) a Cu layer on the Zn layer having a thickness ranging from about 0.5-2 µm, including
values and ranges therebetween as described above,
c) a Ni layer on the Cu layer having a thickness ranging from about 1-2 µm, including
10 values and ranges therebetween as described above, and
d) a Cr layer on the Ni layer having a thickness ranging from about 0.25-0.5 µm, including
values and ranges therebetween as described above.
In some exemplary non-limiting embodiments, the sulphate Zn/Cu/Ni/Cr multilayer coating
comprises:
15 a) a Zn layer on the steel substrate having a thickness ranging from about 6-8 µm,
including values and ranges therebetween; wherein the zinc layer is deposited on the
steel substrate from a sulphate Zn electroplating bath,
b) a Cu layer on the Zn layer having a thickness ranging from about 1-2 µm, including
values and ranges therebetween,
20 c) a Ni layer on the Cu layer having a thickness ranging from about 1-2 µm, and
d) a Cr layer on the Ni layer having a thickness of about 0.25 µm.
In some non-limiting embodiments, the Zn layer has a thickness of about 6 µm, about 6.5 µm,
about 7 µm, about 7.5 µm, or about 8 µm, including values and ranges therebetween; the Cu
layer has a thickness of about 1 µm, about 1.1 µm, about 1.2 µm, about 1.3 µm, about 1.4 µm,
25 about 1.5 µm, about 1.6 µm, about 1.7 µm, about 1.8 µm, about 1.9 µm or about 2 µm,
including values and ranges therebetween; the Ni layer has a thickness of about 1 µm, about
1.1 µm, about 1.2 µm, about 1.3 µm, about 1.4 µm, about 1.5 µm, about 1.6 µm, about 1.7 µm,
about 1.8 µm, about 1.9 µm or about 2 µm, including values and ranges therebetween; and the
Cr layer has a thickness of about 0.25 µm.
30 In some exemplary non-limiting embodiments, the total thickness of the sulphate Zn/Cu/Ni/Cr
multilayer coating described above ranges from about 9-11.5 µm, including values and ranges
therebetween.
15
For example, the total thickness of the sulphate Zn/Cu/Ni/Cr multilayer coating is about 9 µm,
about 9.25 µm, about 9.5 µm, about 9.75 µm, about 10 µm, about 10.25 µm, about 10.5 µm,
about 10.75 µm, about 11 µm, about 11.25 µm or about 11.5 µm, including values and ranges
therebetween.
In some embodiments of the present disclosure, the sulphate Zn electroplating 5 bath comprises
about 172-444 g/L of zinc sulphate that provides about 70-180 g/L of Zn metal, about 12-16
g/L of potassium chloride, about 18-24 g/L of boric acid, and about 8-10 ml/L of additives.
For example, in some embodiments, the sulphate Zn electroplating bath comprises zinc
sulphate in an amount of about 172-430 g/L, about 172-425 g/L, about 172-415 g/L, about 172-
10 400 g/L, about 172-380 g/L, about 172-350 g/L, about 172-320 g/L, about 172-280 g/L, about
172-250 g/L, about 200-444 g/L, about 200-430 g/L, about 200-425 g/L, about 200-415 g/L,
about 200-400 g/L, about 200-380 g/L, about 200-350 g/L, about 200-320 g/L, about 200-280
g/L, about 200-250 g/L, about 250-430 g/L, about 250-425 g/L, about 250-415 g/L, about 250-
400 g/L, about 250-380 g/L, about 250-350 g/L, about 250-320 g/L, about 250-280 g/L, about
15 300-430 g/L, about 300-425 g/L, about 300-415 g/L, about 300-400 g/L, about 300-380 g/L,
about 300-350 g/L, about 350-430 g/L, about 350-425 g/L, about 350-415 g/L, about 350-400
g/L, or about 400-444 g/L, including values and ranges therebetween; potassium chloride in an
amount of about 12-16 g/L, about 13-15 g/L, about 14-16 g/L, about 12-15 g/L, about 13-16
g/L, about 14-15 g/L, about 12 g/L, about 13 g/L, about 14 g/L, about 15 g/L, or about 16 g/L,
20 including values and ranges therebetween; boric acid in an amount of about 18-24 g/L, about
19-23 g/L, about 20-22 g/L, about 21-23 g/L, about 22-24 g/L, about 18-23 g/L, about 19-24
g/L, about 20-24 g/L, about 21-24 g/L, about 18 g/L, about 19 g/L, about 20 g/L, about 21 g/L,
about 22 g/L, about 23 g/L or about 24 g/L, including values and ranges therebetween; and
about 8-10 ml/L, about 9-10 ml/L, about 8-9 ml/L, about 8 ml/L, about 9 ml/L or about 10 ml/L
25 of additives, including values and ranges therebetween.
In some embodiments, the sulphate Zn electroplating bath comprises zinc sulphate in an
amount that provides about 70-180 g/L, about 80-180 g/L, about 90-170 g/L, about 100-160
g/L, about 110-150 g/L, about 120-140 g/L, about 130-170 g/L, about 140-160 g/L, about 150-
180 g/L, about 160-180 g/L, about 170-180 g/L, about 70 g/L, about 80 g/L, about 90 g/L,
30 about 100 g/L, about 110 g/L, about 120 g/L, about 130 g/L, about 140 g/L, about 150 g/L,
about 160 g/L, about 170 g/L or about 180 g/L of Zn metal, including values and ranges
therebetween.
16
In some embodiments of the present disclosure, the additives are selected from a group
comprising cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS),
dextrin, glycine and thiourea, or any combination thereof.
In some embodiments, the multilayer coating of the present disclosure exhibits a corrosion
potential of about -0.90 to -0.95 V or about -0.91 to -0.94 V, including 5 values and ranges
therebetween, such as about -0.90 V, about -0.91 V, -0.92 V, about -0.93 V, about -0.94 V or
about -0.95 V, including values and ranges therebetween.
In some exemplary, non-limiting embodiments of the present disclosure, the multilayer coating
exhibits a corrosion potential of about -0.91 or -0.94 V.
10 In some embodiments, the multilayer coating of the present disclosure exhibits a corrosion
current of about 5.4 × 10-7 A/cm2 to 8.7 × 10-7 A/cm2, including values and ranges
therebetween.
For example, in some embodiments of the present disclosure, the multilayer coating exhibits a
corrosion current of about 5.4 × 10-7 A/cm2 , about 5.5 × 10-7 A/cm2, about 5.6 × 10-7 A/cm2 ,
15 about 5.7 × 10-7 A/cm2 , about 5.8 × 10-7 A/cm2 , about 5.9 × 10-7 A/cm2 , about 6 × 10-7 A/cm2,
about 6.1 × 10-7 A/cm2 , about 6.2 × 10-7 A/cm2 , about 6.3 × 10-7 A/cm2 , about 6.4 × 10-7
A/cm2 , about 6.5 × 10-7 A/cm2 , about 6.6 × 10-7 A/cm2 , about 6.7 × 10-7 A/cm2 , about 6.8 ×
10-7 A/cm2 , about 6.9 × 10-7 A/cm2 , about 7 × 10-7 A/cm2 , about 7.1 × 10-7 A/cm2 , about 7.2
× 10-7 A/cm2 , about 7.3 × 10-7 A/cm2 , about 7.4 × 10-7 A/cm2 , about 7.5 × 10-7 A/cm2 , about
20 7.6 × 10-7 A/cm2 , about 7.7 × 10-7 A/cm2 , about 7.8 × 10-7 A/cm2 , about 7.9 × 10-7 A/cm2,
about 8 × 10-7 A/cm2 , about 8.1 × 10-7 A/cm2 , about 8.2 × 10-7 A/cm2 , about 8.3 × 10-7 A/cm2,
about 8.4 × 10-7 A/cm2 , about 8.5 × 10-7 A/cm2 , about 8.6 × 10-7 A/cm2 , or about 8.7 × 10-7
A/cm2.
In some exemplary, non-limiting embodiments of the present disclosure, the multilayer coating
25 of the present disclosure exhibits a corrosion current of about 5.42 × 10-7 A/cm2.
In some exemplary, non-limiting embodiments of the present disclosure, the multilayer coating
of the present disclosure exhibits a corrosion current of about 8.64 × 10-7 A/cm2.
In some embodiments, the multilayer coating of the present disclosure exhibits a corrosion rate
of about 0.3 to 0.55 mils per year (mpy), including values and ranges therebetween.
30 For example, in some embodiments of the present disclosure, the multilayer coating exhibits a
corrosion rate of about 0.3 mpy, about 0.32 mpy, about 0.35 mpy, about 0.37 mpy, about 0.39
17
mpy, about 0.4 mpy, about 0.42 mpy, about 0.44 mpy, about 0.46 mpy, about 0.48 mpy, about
0.5 mpy, about 0.51 mpy, about 0.53 mpy or about 0.55 mpy.
In some exemplary, non-limiting embodiments of the present disclosure, the multilayer coating
exhibits a corrosion rate of about 0.32 mpy.
In some exemplary, non-limiting embodiments of the present disclosure, the 5 multilayer coating
exhibits a corrosion rate of about 0.51 mpy.
In some embodiments, the multilayer coating of the present disclosure exhibits a Salt Spray
Test 5% Red Rust life of about 800 hours or more.
In some embodiments, the multilayer coating of the present disclosure exhibits a Salt Spray
10 Test 5% Red Rust life of about 800 hours to about 1600 hours, including values and ranges
therebetween, such as about 800 hours to 1500 hours, about 800 hours to about 1400 hours,
about 800 hours to about 1200 hours, about 800 hours to about 1000 hours, about 1000 hours
to about 1600 hours, about 1000 hours to about 1500 hours, about 1000 hours to about 1400
hours, about 1000 hours to about 1200 hours, about 1200 hours to about 1600 hours, about
15 1200 hours to about 1400 hours, or about 1400 hours to about 1600 hours, including values
and ranges therebetween.
For example, the multilayer coating of the present disclosure exhibits a Salt Spray Test 5% Red
Rust life of about 800 hours, about 900 hours, about 1000 bours, about 1080 hours, about 1100
hours, about 1200 hours, about 1280 hours, about 1300 hours, about 1400 hours, about 1500
20 hours, or about 1600 hours.
In some embodiments, the multilayer coating of the present disclosure exhibits a Copper
Accelerated Acetic Acid Salt Spray (CASS) test life of more than 40 hours.
In some embodiments of the present disclosure, the multilayer coating exhibits a Copper
Accelerated Acetic Acid Salt Spray (CASS) test life from about 40 hours to about 80 hours,
25 including values and ranges therebetween, such as about 40 hours to 70 hours, about 40 hours
to about 65 hours, about 40 hours to about 60 hours, about 40 hours to about 50 hours, 48 hours
to about 80 hours, about 50 hours to about 80 hours, about 50 hours to 70 hours, or about 60
hours to about 80 hours, including values and ranges therebetween.
For example, in some embodiments, the multilayer coating exhibits a Copper Accelerated
30 Acetic Acid Salt Spray (CASS) test life of about 40 hours, about 45 hours, about 48 hours,
18
about 50 hours, about 55 hours, about 60 hours, about 65 hours, about 70 hours, about 75 hours
or about 80 hours.
In some embodiments, the multilayer coating of the present disclosure exhibits a corrosion
potential of about -0.90 to -0.95 V, including values and ranges therebetween as described
above; a corrosion current of about 5.4 × 10-7 A/cm2 to 8.7 × 10-7 A/cm2, including 5 values and
ranges therebetween as described above; a corrosion rate of about 0.3 to 0.55 mils per year
(mpy), including values and ranges therebetween as described above; a Salt Spray Test 5% Red
Rust life of about 800 hours or more; and a Copper Accelerated Acetic Acid Salt Spray (CASS)
test life of more than 40 hours.
10 In some non-limiting embodiments, the multilayer coating exhibits a corrosion potential of
about -0.91 to -0.94 V, including values and ranges therebetween as described above; a
corrosion current of about 5.42 × 10-7 A/cm2 or about 8.64 × 10-7 A/cm2; a corrosion rate of
about 0.32 mpy or about 0.51 mpy; a Salt Spray Test 5% Red Rust life of about 800 hours to
about 1600 hours, including values and ranges therebetween as described above; and a Copper
15 Accelerated Acetic Acid Salt Spray (CASS) test life of about 48 hours to about 80 hours,
including values and ranges therebetween as described above.
In some embodiments, the above described coatings of the present disclosure do not exhibit
peeling or flaking from the steel substrate.
The present disclosure also relates to a method for depositing the multilayer coating described
20 above on a steel substrate, comprising:
a) depositing a Zn layer on the steel substrate;
b) contacting the Zn layer on the steel substrate with an activating composition
comprising ammonium bifluoride for about 5-15 seconds;
c) depositing a Cu layer on the Zn layer;
25 d) depositing a Ni layer on the Cu layer; and
e) depositing a Cr layer on the Ni layer as a top layer.
In some embodiments, the Zn layer is deposited on the steel substrate using an acidic Zn
electroplating composition, a sulphate Zn electroplating composition, or an alkaline Zn
electroplating composition. These electroplating compositions are described above.
30 Deposition of a Zn layer from acidic Zn electroplating compositions
For acidic Zn electroplating compositions, the Zn layer is deposited on the steel substrate at a
current density of about 250-1000 A/m2, including values and ranges therebetween; a pH of
19
about 5-5.5, including values and ranges therebetween; and a temperature of about 20-40°C,
including values and ranges therebetween.
For example, the Zn layer is deposited on the steel substrate at a current density of about 250-
1000 A/m2, about 300-950 A/m2, about 400-900 A/m2, about 500-800 A/m2, about 600-700
A/m2, about 250-900 A/m2, about 700-1000 A/m2, about 800-1000 A/5 m2, about 900-1000
A/m2, about 300-800 A/m2, about 400-1000 A/m2, about 500-950 A/m2, about 250 A/m2, about
300 A/m2, about 400 A/m2, about 500 A/m2, about 600 A/m2, about 700 A/m2, about 800 A/m2,
about 900 A/m2, or about 1000 A/m2, including values and ranges therebetween; a pH of about
5, about 5.1, about 5.2, about 5.3, about 5.4 or about 5.5, including values and ranges
10 therebetween; and a temperature of about 20-40°C, about 20-30°C, about 30-40°C, about 20°C,
about 22°C, about 24°C, about 26°C, about 28°C, about 30°C, about 32°C, about 34°C, about
36°C, about 38°C, or about 40°C, including values and ranges therebetween.
In some embodiments, the Zn layer is deposited on the steel substrate from acidic Zn
electroplating compositions at a voltage of about 1-4 V, including values and ranges
15 therebetween and for a time ranging from about 10-20 minutes, including values and ranges
therebetween.
For example, the Zn layer is deposited on the steel substrate at a voltage of about 1 V, about 2
V, about 3 V or about 4 V, including values and ranges therebetween; and for a time of about
10-20 minutes, about 11-19 minutes, about 13-18 minutes, about 12-16 minutes, about 15-17
20 minutes, about 18-20 minutes, about 10 minutes, about 11 minutes, about 12 minutes, about 13
minutes, about 14 minutes, about 15 minutes, about 16 minutes, about 17 minutes, about 18
minutes, about 19 minutes, or about 20 minutes, including values and ranges therebetween.
In some exemplary, non-limiting embodiments of the present disclosure the Zn layer is
deposited on the steel substrate for a time ranging from about 12-16 minutes, including values
25 and ranges therebetween; for example, for about 12 minutes, about 13 minutes, about 14
minutes, about 15 minutes, or about 16 minutes, including values and ranges therebetween.
In some non-limiting embodiments of the present disclosure, the Zn layer is deposited on the
steel substrate at a current density of about 250-1000 A/m2, including values and ranges
therebetween as described above; a pH of about 5-5.5, including values and ranges
30 therebetween as described above; a temperature of about 20-40°C, including values and ranges
therebetween as described above; a voltage of about 1-4 V, including values and ranges
20
therebetween as described above; and for a time ranging from about 10-20 minutes, including
values and ranges therebetween as described above.
In some exemplary non-limiting embodiments of the present disclosure, the Zn layer is
deposited on the steel substrate at a current density of about 250-1000 A/m2, including values
and ranges therebetween as described above; a pH of about 5-5.5, including 5 values and ranges
therebetween as described above; a temperature of about 30°C; a voltage of about 1-4 V,
including values and ranges therebetween as described above; and for a time ranging from
about 12-16 minutes, including values and ranges therebetween as described above.
Deposition of a Zn layer from Zn sulphate electroplating compositions
10 For Zn sulphate electroplating compositions, the Zn layer is deposited on the steel substrate at
a current density of about 250-1000 A/m2, including values and ranges therebetween; a pH of
about 3-4, including values and ranges therebetween; and a temperature of about 40-60°C,
including values and ranges therebetween.
For example, the Zn layer is deposited on the steel substrate at a current density of about 250-
15 1000 A/m2, about 300-950 A/m2, about 400-900 A/m2, about 500-800 A/m2, about 600-700
A/m2, about 250-900 A/m2, about 700-1000 A/m2, about 800-1000 A/m2, about 900-1000
A/m2, about 300-800 A/m2, about 400-1000 A/m2, about 500-950 A/m2, about 250 A/m2, about
300 A/m2, about 400 A/m2, about 500 A/m2, about 600 A/m2, about 700 A/m2, about 800 A/m2,
about 900 A/m2, or about 1000 A/m2, including values and ranges therebetween; a pH of about
20 3, about 3.1, about 3.2, about 3.3, about 3.4 or about 3.5, about 3.6, about 3.7, about 3.8, about
3.9, or about 4, including values and ranges therebetween; and a temperature of about 40-60°C,
about 40-50°C, about 50-60°C, about 40°C, about 42°C, about 44°C, about 46°C, about 48°C,
about 50°C, about 52°C, about 54°C, about 56°C, about 58°C, or about 60°C, including values
and ranges therebetween.
25 In some embodiments of the present disclosure, the Zn layer is deposited on the steel substrate
at a voltage of about 5-20 V, including values and ranges therebetween and for a time ranging
from about 5-20 minutes, including values and ranges therebetween.
For example, the Zn layer is deposited on the steel substrate at a voltage of about 5-20 V, about
5-18 V, about 6-20 V, about 7-17 V, about 8-18 V, about 9-15 V, about 10-20 V, about 12-18
30 V, about 14-19 V, about 15-20 V, about 17-20 V, about 5 V, about 6 V, about 7 V, about 8 V,
about 9 V, about 10 V, about 11 V, about 12 V, about 13 V, about 14 V, about 15 V, about 16
V, about 17 V, about 18 V, about 19 V, or about 20 V, including values and ranges
21
therebetween; and for a time of about 5-20 minutes, about 5-18 minutes, about 6-20 minutes,
about 7-17 minutes, about 8-16 minutes, about 9-15 minutes, about 10-20 minutes, about 12-
18 minutes, about 14-19 minutes, about 15-20 minutes, about 17-20 minutes, about 5 minutes,
about 6 minutes, about 7 minutes, about 8 minutes, about 9 minutes, about 10 minutes, about
11 minutes, about 12 minutes, about 13 minutes, about 14 minutes, about 15 5 minutes, about 16
minutes, about 17 minutes, about 18 minutes, about 19 minutes, or about 20 minutes, including
values and ranges therebetween.
In some exemplary, non-limiting embodiments of the present disclosure, the Zn layer is
deposited on the steel substrate at a voltage of about 8-18 V including values and ranges
10 therebetween; and for a time ranging from about 8-16 minutes, including values and ranges
therebetween.
In some non-limiting embodiments of the present disclosure, the Zn layer is deposited on the
steel substrate at a current density of about 250-1000 A/m2, including values and ranges
therebetween as described above; a pH of about 3-4, including values and ranges therebetween
15 as described above; a temperature of about 40-60°C, including values and ranges therebetween,
as described above; a voltage of about 5-20 V, including values and ranges therebetween as
described above; and for a time ranging from about 5-20 minutes, including values and ranges
therebetween as described above.
In some exemplary non-limiting embodiments of the present disclosure, the Zn layer is
20 deposited on the steel substrate at a current density of about 250-1000 A/m2, including values
and ranges therebetween as described above; a pH of about 3-3.6, including values and ranges
therebetween as described above; a temperature of about 50°C; a voltage of about 8-18 V,
including values and ranges therebetween as described above; and for a time ranging from
about 8-16 minutes, including values and ranges therebetween as described above.
25 Deposition of a Zn layer from alkaline Zn electroplating compositions
For alkaline Zn electroplating compositions, the Zn layer is deposited on the steel substrate at
a current density of about 250-1000 A/m2, including values and ranges therebetween; a pH of
about 3-4, including values and ranges therebetween; and a temperature of about 20-40°C,
including values and ranges therebetween.
30 For example, the Zn layer is deposited on the steel substrate at a current density of about 250-
1000 A/m2, about 300-950 A/m2, about 400-900 A/m2, about 500-800 A/m2, about 600-700
A/m2, about 250-900 A/m2, about 700-1000 A/m2, about 800-1000 A/m2, about 900-1000
22
A/m2, about 300-800 A/m2, about 400-1000 A/m2, about 500-950 A/m2, about 250 A/m2, about
300 A/m2, about 400 A/m2, about 500 A/m2, about 600 A/m2, about 700 A/m2, about 800 A/m2,
about 900 A/m2, or about 1000 A/m2, including values and ranges therebetween; a pH of about
3, about 3.1, about 3.2, about 3.3, about 3.4, about 3.5, about 3.6, about 3.7, about 3.8, about
3.9, or about 4, including values and ranges therebetween; and a temperature 5 of about 20-40°C,
about 20-30°C, about 30-40°C, about 20°C, about 22°C, about 24°C, about 26°C, about 28°C,
about 30°C, about 32°C, about 34°C, about 36°C, about 38°C, or about 40°C, including values
and ranges therebetween.
In some embodiments of the present disclosure, the Zn layer is deposited on the steel substrate
10 at a voltage of about 5-20 V, including values and ranges therebetween and for a time ranging
from about 5-20 minutes, including values and ranges therebetween.
For example, the Zn layer is deposited on the steel substrate at a voltage of about 5-20 V, about
5-18 V, about 6-20 V, about 7-17 V, about 8-18 V, about 9-15 V, about 10-20 V, about 12-18
V, about 14-19 V, about 15-20 V, about 17-20 V, about 5 V, about 6 V, about 7 V, about 8 V,
15 about 9 V, about 10 V, about 11 V, about 12 V, about 13 V, about 14 V, about 15 V, about 16
V, about 17 V, about 18 V, about 19 V, or about 20 V, including values and ranges
therebetween; and for a time of about 5-20 minutes, about 5-18 minutes, about 6-20 minutes,
about 7-17 minutes, about 8-16 minutes, about 9-15 minutes, about 10-20 minutes, about 12-
18 minutes, about 14-19 minutes, about 15-20 minutes, about 17-20 minutes, about 5 minutes,
20 about 6 minutes, about 7 minutes, about 8 minutes, about 9 minutes, about 10 minutes, about
11 minutes, about 12 minutes, about 13 minutes, about 14 minutes, about 15 minutes, about 16
minutes, about 17 minutes, about 18 minutes, about 19 minutes, or about 20 minutes, including
values and ranges therebetween.
In some exemplary non-limiting embodiments of the present disclosure, the Zn layer is
25 deposited on the steel substrate at a voltage of about 8-18 V including values and ranges
therebetween; and for a time ranging from about 8-16 minutes, including values and ranges
therebetween.
In some non-limiting embodiments of the present disclosure, the Zn layer is deposited on the
steel substrate at a current density of about 250-1000 A/m2, including values and ranges
30 therebetween as described above; a pH of about 3-4, including values and ranges therebetween
as described above; a temperature of about 20-40°C, including values and ranges therebetween,
as described above; a voltage of about 5-20 V, including values and ranges therebetween as
23
described above; and for a time ranging from about 5-20 minutes, including values and ranges
therebetween as described above.
In some exemplary non-limiting embodiments of the present disclosure, the Zn layer is
deposited on the steel substrate at a current density of about 250-1000 A/m2, including values
and ranges therebetween as described above; a pH of about 3-3.6, including 5 values and ranges
therebetween as described above; a temperature of about 30°C; a voltage of about 8-18 V,
including values and ranges therebetween as described above; and for a time ranging from
about 8-16 minutes, including values and ranges therebetween as described above.
After deposition of the Zn layer on the steel substrate in any of the manner described above,
10 the Zn layer is contacted with an activating composition comprising ammonium bifluoride for
about 5-15 seconds, including values and ranges therebetween, for example for about 5
seconds, about 6 seconds, about 7 seconds, about 8 seconds, about 9 seconds, about 10 seconds,
about 11 seconds, about 12 seconds, about 13 seconds, about 14 seconds or for about 15
seconds.
15 In some embodiments of the present disclosure, ammonium bifluoride is present in the
activating composition in an amount of about 2-3 wt%, including values and ranges
therebetween, for example, about 2 wt%, about 2.1 wt%, about 2.2 wt%, about 2.3 wt%, about
2.4 wt%, about 2.5 wt%, about 2.6 wt%, about 2.7 wt%, about 2.8 wt%, about 2.9 wt% or about
3 wt%.
20 In some exemplary, non-limiting embodiments of the present disclosure, the Zn layer on the
steel substrate is contacted with an activating composition comprising about 2.5 wt% of
ammonium bifluoride for about 10 seconds.
The step of contacting the Zn layer on the steel substrate with an activating composition
comprising ammonium bifluoride enhances adhesion of the coating and also results in a
25 smooth, bright and adherent coating, with no peel off or delamination of the coating and no
blister formation on the coating.
Deposition of Cu layer
In some embodiments, the Cu layer is deposited on the Zn layer using an electroplating
composition comprising about 28-42 g/L of copper cyanide that provides a copper content of
30 about 20-30 g/L, including values and ranges therebetween; and a free cyanide content of about
4-6 g/L, including values and ranges therebetween.
24
In some embodiments, the Cu layer is deposited on the Zn layer using an electroplating
composition comprising about 28-40 g/L, about 28-38 g/L, about 28-35 g/L, about 28-32 g/L,
about 28 g/L, about 30 g/L, about 32 g/L, about 35 g/L, about 40 g/L, or about 42 g/L of copper
cyanide, including values and ranges therebetween; and a free cyanide content of about 4 g/L,
about 5 g/L, or about 6 g/L, including values and ranges 5 therebetween.
In some exemplary, non-limiting embodiments of the present disclosure, the Cu layer is
deposited on the Zn layer using an electroplating composition comprising copper cyanide that
provides a copper content of about 24 g/L; and a free cyanide content of about 4-6 g/L,
including values and ranges therebetween.
10 In some embodiments, the Cu layer is deposited on the Zn layer at a current of about 2.5-3
A/dm2, including values and ranges therebetween; and a temperature of about 50-60°C,
including values and ranges therebetween.
In some embodiments of the present disclosure, the Cu layer is deposited on the Zn layer at a
current of about 2.5 A, about 2.6 A, about 2.7 A, about 2.8 A, about 2.9 A, or about 3 A,
15 including values and ranges therebetween; and a temperature of about 50°C, about 51°C , about
52°C, about 53°C, about 54°C, about 55°C, about 56°C, about 57°C, about 58°C, about 59°C,
or about 60°C, including values and ranges therebetween.
In some embodiments of the present disclosure, the Cu layer is deposited on the Zn layer at a
voltage of about 3-3.2 V, including values and ranges therebetween; and for a time of about 1-
20 5 minutes, including values and ranges therebetween.
In some embodiments of the present disclosure, the Cu layer is deposited on the Zn layer at a
voltage of about 3 V, about 3.1 V, or about 3.2 V, including values and ranges therebetween;
and for a time of about 1 minute, about 2 minutes, about 3 minutes, about 4 minutes, or about
5 minutes, including values and ranges therebetween.
25 In some embodiments of the present disclosure, the Cu layer is deposited on the Zn layer at a
current of about 2.5-3 A, including values and ranges therebetween; a temperature of about 50-
60°C, including values and ranges therebetween; a voltage of about 3-3.2 V, including values
and ranges therebetween; and for a time of about 1-5 minutes, including values and ranges
therebetween.
30 In some exemplary, non-limiting embodiments of the present disclosure, the Cu layer is
deposited on the Zn layer at a current of about 2.7 A; a temperature of about 55°C; a voltage
25
of about 3-3.1 V; and for a time of about 1-4 minutes, including values and ranges
therebetween.
Deposition of Ni layer
In some embodiments of the present disclosure, the Ni layer is deposited on the Cu layer using
an electroplating composition comprising about 280-320 g/L of nickel sulphate, 5 including
values and ranges therebetween; about 50-70 g/L of nickel chloride, including values and
ranges therebetween; about 35-45 g/L of boric acid, including values and ranges therebetween;
and about 5-7 ml/L of additives, including values and ranges therebetween.
In some embodiments of the present disclosure, the Ni layer is deposited on the Cu layer using
10 an electroplating composition comprising about 280-300 g/L, about 290-320 g/L, about 290-
310 g/L, about 300-320 g/L, about 280 g/L, about 290 g/L, about 300 g/L, about 310 g/L or
about 320 g/L of nickel sulphate, including values and ranges therebetween; about 50 g/L, 52
g/L, about 54 g/L, about 56 g/L, about 58 g/L, about 60 g/L, about 62 g/L, about 64 g/L, about
66 g/L, about 68 g/L or about 70 g/L of nickel chloride, including values and ranges
15 therebetween; about 35 g/L, about 36 g/L, about 37 g/L, about 38 g/L, about 39 g/L, about 40
g/L, about 41 g/L, about 42 g/L, about 43 g/L, about 44 g/L, or about 45 g/L of boric acid,
including values and ranges therebetween; and about 5 ml/L, about 6 ml/L or about 7 ml/L of
additives, including values and ranges therebetween.
In some exemplary, non-limiting embodiments of the present disclosure, the Ni layer is
20 deposited on the Cu layer using an electroplating composition comprising about 300 g/L of
nickel sulphate; about 60 g/L of nickel chloride; about 40 g/L of boric acid; and about 6 ml/L
of additives.
In some embodiments, the total amount of nickel salts in the electroplating composition
employed for depositing the Ni layer is such that it provides about 70-90 g/L of nickel metal,
25 including values and ranges therebetween, such as, about 75-85 g/L or about 80 g/L of nickel.
In some embodiments, the additives are selected from a group comprising Formaldehyde
chloral hydrate, o-sulpho benzaldehyde, allyl sulphonic acid, 2-butyne-1, 4-diol, thiourea,
coumarin, or a combination thereof. In an exemplary embodiment, the additive is a mixture of
Formaldehyde chloral hydrate, o-sulpho benzaldehyde, allyl sulphonic acid, 2-butyne-1, 4-diol,
30 thiourea, and coumarin.
26
In some embodiments, the Ni layer is deposited at a current density of about 70-100 A/m2,
including values and ranges therebetween; a pH of about 3.5-4.5, including values and ranges
therebetween; and a temperature of about 50-60°C, including values and ranges therebetween.
In some embodiments of the present disclosure, the Ni layer is deposited at a current density
of about 70-90 A/m2, about 70-80 A/m2, about 80-100 A/m2, about 80-90 5 A/m2, about 75-95
A/m2, about 70 A/m2, about 75 A/m2, about 80 A/m2, about 85 A/m2, about 90 A/m2, about 95
A/m2, or about 100 A/m2, including values and ranges therebetween; a pH of about 3.5-4.0,
about 4-4.5, about 3.5, about 3.6, about 3.7, about 3.8, about 3.9, about 4.0, about 4.1, about
4.2, about 4.3, about 4.4, or about 4.5, including values and ranges therebetween; and a
10 temperature of about 50°C, about 51°C, about 52°C, about 53°C, about 54°C, about 55°C,
about 56°C, about 57°C, about 58°C, about 59°C, or about 60°C, including values and ranges
therebetween.
In some embodiments, the Ni layer is deposited at a voltage of about 1-2.5 V, including values
and ranges therebetween; and for a time ranging from about 2-8 minutes, including values and
15 ranges therebetween.
In some embodiments, the Ni layer is deposited at a voltage of about 1-2.5 V, about 1-2 V,
about 1.5-2 V, about 1.5-2.5 V, about 1 V, about 1.1 V, about 1.2 V, about 1.3 V, about 1.4 V,
about 1.5 V, about 1.6 V, about 1.7 V, about 1.8 V, about 1.9 V, about 2 V, about 2.1 V, about
2.2 V, about 2.3 V, about 2.4 V or about 2.5 V, including values and ranges therebetween; and
20 for a time ranging from about 2 minutes, about 3 minutes, about 4 minutes, about 5 minutes,
about 6 minutes, about 7 minutes, or about 8 minutes, including values and ranges
therebetween.
In some embodiments of the present disclosure, the Ni layer is deposited at a current density
of about 70-100 A/m2, including values and ranges therebetween; a pH of about 3.5-4.5,
25 including values and ranges therebetween; a temperature of about 50-60°C, including values
and ranges therebetween; a voltage of about 1-2.5 V, including values and ranges therebetween;
and for a time ranging from about 2-8 minutes, including values and ranges therebetween.
In some exemplary, non-limiting embodiments of the present disclosure, the Ni layer is
deposited at a current density of about 70-100 A/m2, including values and ranges therebetween;
30 a pH of about 4; a temperature of about 55°C; a voltage of about 1.5-2.0 V, including values
and ranges therebetween; and for a time ranging from about 2-6 minutes, including values and
ranges therebetween.
27
Deposition of a Cr layer
In some embodiments, the Cr layer is deposited on the Ni layer using an electroplating
composition comprising about 220-230 g/L of chromic acid, including values and ranges
therebetween; and about 0.8-1.2 g/L of a sulphate, including values and ranges therebetween.
In some embodiments, the Cr layer is deposited on the Ni layer using 5 an electroplating
composition comprising about 220 g/L, about 221 g/L, about 222 g/L, about 223 g/L, about
224 g/L, about 225 g/L, about 226 g/L, about 227 g/L, about 228 g/L, about 229 g/L, or about
230 g/L of chromic acid, including values and ranges therebetween; and about 0.8 g/L, about
0.9 g/L, about 1 g/L, about 1.1 g/L or about 1.2 g/L of a sulphate, including values and ranges
10 therebetween.
In some embodiments of the present disclosure, the Cr layer is deposited on the Ni layer using
an electroplating composition comprising about 225 g/L of chromic acid; and about 0.8-1.2
g/L of a sulphate, including values and ranges therebetween.
In some non-limiting embodiments of the present disclosure, the source of sulphate is sulphuric
15 acid.
In some embodiments of the present disclosure, the Cr layer is deposited at a current density
of about 1200-1300 A/m2, including values and ranges therebetween; and a temperature of
about 50-60°C, including values and ranges therebetween.
In some embodiments of the present disclosure, the Cr layer is deposited at a current density
20 of about 1200 A/m2, about 1210 A/m2, about 1220 A/m2, about 1230 A/m2, about 1240 A/m2,
about 1250 A/m2, about 1260 A/m2, about 1270 A/m2, about 1280 A/m2, about 1290 A/m2, or
about 1300 A/m2, including values and ranges therebetween; and a temperature of about 50°C,
about 51°C, about 52°C, about 53°C, about 54°C, about 55°C, about 56°C, about 57°C, about
58°C, about 59°C, or about 60°C, including values and ranges therebetween.
25 In some embodiments of the present disclosure, the Cr layer is deposited at a voltage of about
9-10 V, including values and ranges therebetween; and for a time period up to about 1 minute.
In some embodiments of the present disclosure, the Cr layer is deposited at a voltage of about
9 V, about 9.1 V, about 9.2 V, about 9.3 V, about 9.4 V, about 9.5 V, about 9.6 V, about 9.7
V, about 9.8 V, about 9.9 V or about 10 V, including values and ranges therebetween; and for
30 a time period of about 0.1 minute, about 0.2 minute, about 0.3 minute, about 0.4 minute, about
28
0.5 minute, about 0.6 minute, about 0.7 minute, about 0.8 minute, about 0.9 minute or about 1
minute, including values and ranges therebetween.
In some embodiments of the present disclosure, the Cr layer is deposited at a current density
of about 1200-1300 A/m2, including values and ranges therebetween; a temperature of about
55°C; a voltage of about 9.8 V; and for a time period of 5 about 0.5 minute.
While the subsequent embodiments focus on a steel substrate comprising the multilayer
coatings of the present disclosure,
The present disclosure also relates to a steel substrate comprising the multilayer coating as
described above. The features and characteristics of the multilayer coatings described above,
10 such as the corrosion rate, corrosion current, corrosion potential and the like, are also applicable
to the steel substrates comprising the multilayer coatings. For the sake of brevity, and avoiding
repetition, each of those embodiments are not being reiterated here again with respect to the
specific steel substrate. However, each of the said embodiments completely fall within the
purview of the steel substrates described herein.
15 The present disclosure also relates to a steel substrate comprising the acid Zn/Cu/Ni/Cr
multilayer coating as described above.
The present disclosure also relates to a steel substrate comprising the sulphate Zn/Cu/Ni/Cr
multilayer coating as described above.
The present disclosure also relates to a steel substrate comprising the alkaline Zn/Cu/Ni/Cr
20 multilayer coating as described above.
In some embodiments of the present disclosure, the steel substrate is selected from hot rolled
sheets, cold rolled sheets, tubes, wires and other steel substrates having complicated shapes or
components.
It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and
25 not a limitation. While considerable emphasis has been placed herein on the particular features
of this disclosure, it will be appreciated that various modifications can be made, and that many
changes can be made in the preferred embodiments without departing from the principles of
the disclosure. Those skilled in the art will recognize that the embodiments herein can be
practiced with modification within the spirit and scope of the embodiments as described herein.
30 Similarly, additional embodiments and features of the present disclosure will be apparent to
one of ordinary skill in art based upon description provided herein.
29
Descriptions of well-known/conventional methods/steps and techniques are omitted so as to
not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for
examples illustrating the above-described embodiments, and in order to illustrate the
embodiments of the present disclosure certain aspects have been employed. The examples used
herein for such illustration are intended merely to facilitate an understanding 5 of ways in which
the embodiments herein may be practiced and to further enable those of skill in the art to
practice the embodiments herein. Accordingly, the following examples should not be construed
as limiting the scope of the embodiments herein.
10 EXAMPLES
Example 1: Preparation of Zn/Cu/Ni/Cr multilayer coated steel substrate
Different Zn/Cu/Ni/Cr multilayer coated steel sheets were prepared by a multiple bath
electroplating process using commercial electroplating solutions. The bath compositions and
15 operating parameters for all the coatings are listed in Table 1, below. Initially, Zn layer plating
was carried out on a steel substrate using one of the three different electroplating bath
compositions, i.e., acidic, sulphate or alkaline as shown in Table 1. The plated Zn layer was
then dipped in an activating bath comprising about 2.5 wt% Ammonium bifluoride [NH4][HF2]
for about 10 seconds at room temperature. Post this, Cu layer was deposited on the activated
20 Zn layer using the Cu plating bath composition shown in Table 1. Over this, a Ni layer was
deposited using the Ni plating bath composition shown in Table 1, finally followed by
depositing a Cr layer using the Cr plating bath composition shown in Table 1. An activation
step was carried out after deposition of the Zn layer.
Table 1: Electroplating bath compositions and operating parameters
Coated Layer Bath Composition Operating Parameters
Acidic Zn Metal (from ZnCl2): 20-
40 g/l
Total Chloride (from ZnCl2
+ KCl): 120-150 g/l,
Boric Acid: 24-32 g/l,
Brightener: 2 ml/l
Voltage: 1-4 V
Temperature: 30°C
Cathode Current Density: 250-1000 A/m2
pH: 5-5.5
Time: 12-16 minutes
Agitation: air agitation
30
Zn
Sulphate Zn Metal (from ZnSO4): 70-
180g/l
KCl: 12-16 g/l,
Boric Acid: 18-24 g/l,
Additives: 8-10 ml/l
Voltage: 8-18 V
Temperature: 50°C
Cathode Current Density: 250-1000 A/m2
pH: 3-3.6
Time: 8-16 minutes
Agitation: air agitation
Alkaline Zn Metal (from Zn Oxide):
11-13 g/l
NaOH: 110-130 g/l,
Additives: 28.5 ml/l
Brightener: 2 ml/l
Voltage: 8-18 V
Temperature: 30°C
Cathode Current Density: 250-1000 A/m2
pH: 3-3.6
Time: 8-16 minutes
Agitation: air agitation
Cu
Copper Content (from
Copper Cyanide): 24 g/l
Free Cyanide Content (as
NaCN): 4-6 g/l
Voltage: 3.0-3.1 V
Temperature: 55°C
Current: 2.7 A/dm2
Time: 1-4 minutes
Bright Ni
NiSO4, 6 H2O: 300 g/l
NiCl2, 6 H2O: 60 g/l
Boric Acid: 40 g/l
Additives: 6 ml/l
Total Ni Metal: 80 g/l
Voltage: 1.5-2 V
Temperature: 55°C
Cathode Current Density: 70-100 A/m2
pH: 4.0
Time: 2-6 minutes
Agitation: air agitation
Bright Cr
Chromic Acid: 225 g/l
Sulphate (from H2SO4): 0.8-
1.2 g/l
Voltage: 9.8 V
Temperature: 55°C
Cathode Current: 1200-1300 A/m2
Time: 0.5 min
Agitation: No
The thickness of different layers for all the samples prepared were determined from Energy
Dispersive spectroscopy (EDS) line analysis and scanning electron microscopy (SEM) crosssections
analysis. The cross-sectional SEM micrographs and corresponding EDS line analysis
31
(wt%) of Acid Zn/Cu/Ni/Cr multilayer coating on steel is shown in Figure 1. From the EDS
line analysis, the individual thickness of acid Zn layer was found to be 7 µm, Cu: 1.5 µm, Ni:
1.5 µm, Cr: 0.25 µm. Total thickness: 10.25 µm was determined which is consistent with the
thickness determined from Faraday’s law.
5
Similarly, the thickness of the individual layers as well as the total thickness for all the samples
prepared was determined and the values are shown in Table 2 below:
Table 2: Individual layer and total thickness of different Zn/Cu/Ni/Cr multilayer coated
steel samples.
10
Example 2: Coating Characterization of the Zn/Cu/Ni/Cr multilayer coated steel
A series of characterization tests were performed on Zn/Cu/Ni/Cr multilayer coated steel sheets
to evaluate coating morphology.
The top surface and cross-section of samples were characterized using scanning electron
15 microscopy (SEM) and composition of each sample was analyzed using energy dispersive
spectroscopy (EDS).
Figure 2 shows the photographs of top surface of Zn/Cu/Ni/Cr multi-layered electroplated
samples using three different Zn plating baths represented as Acid Zn, Sulphate Zn and
Alkaline Zn. The visual observation indicates that Acid Zn-based (Acid Zn/Cu/Ni/Cr) and
20 Alkaline Zn-based (Alkaline Zn/Cu/Ni/Cr) multilayer coatings appeared as bright finish.
Zn
Plating
Bath
Layer Thickness (µm) Total
Thickness
(µm)
Zn Cu Bright Ni Bright Cr
Acid 6 0.5-1 1.5 0.25 8.25-8.75
Acid 8 0.5-1 1.5 0.25 10.25-10.75
Acid 7 1.5 1.5 0.25 10.25
Sulphate 8 2 1 0.25 11.25
Sulphate 7 1 2 0.25 10.25
Sulphate 6 1 2 0.25 9.25
Alkaline 8 1 1.5 0.25 10.75
Alkaline 7 1 1.5 0.25 9.75
Alkaline 6 2 2 0.25 10.25
32
Sulphate Zn-based (Sulphate Zn/Cu/Ni/Cr) multilayer coating showed semi-bright finish of Cr
layer outside. All coatings were highly adherent to the steel substrate.
Figure 3 shows the SEM topography of different Zn-based multilayer coatings on steel sheet
surface and their corresponding EDS analysis. The top surface shows smooth 5 appearance with
no defects for Acid and Alkaline Zn based multilayer coating. Sulphate Zn based multilayer
coating shows higher surface roughness/pit formation compared to other which is consistent
with the semi-bright visual appearance of the sample in Figure 2. The EDS analysis of the top
surface shows that the top surface contains Zn, Cu, Cr and Ni. A substantial amount of Ni is
10 present due to the Ni layers (1-2µm) underneath the thin Cr top layer (~0.25µm). The presence
of Zn and Cu in the top surface layer are very less due to the presence of the Ni layer.
Figure 4 shows the cross-sectional SEM microstructures of different Zn-based multilayer
coatings on steel sheet surface and their corresponding EDS analysis. SEM micrograph clearly
15 shows that the coating consisted of multi-layered structure of Zn, Cu, Ni and Cr layers. The
layers were continuous with smooth interface. EDS point analysis indicated the presence of
Zn, Cu, Ni and Cr in the layers. The presence of Fe as seen in the EDS point analysis is due to
the steel substrate. Further EDS mapping confirmed the presence of distinctive layer structure
of Zn, Cu, Ni and Cr as shown in Figure 5. The interface surface was very smooth and
20 continuous.
Example 3: Performance Characterization of the Zn/Cu/Ni/Cr Multilayer Coated Steel
A series of tests were performed on the Zn/Cu/Ni/Cr multilayer coated steel sheets to evaluate
their performance.
Potentiodynamic Polarization Tests (Tafel Test)
25 The corrosion rate was measured by potentiodynamic polarization test (Tafel Test) using
VersaSTAT MC®, Princeton Applied Research instrument. The test was carried out in three
electrodes system. The working electrode was Al-alloy coated sample, counter electrode was
platinum and reference electrode was calomel electrode. The test was conducted in 3.5%
sodium chloride (NaCl) solution with a scan rate of 0.5mV/s. The corrosion rate was measured
30 by Tafel extrapolation technique using VersaStudio® software module. For comparison of
corrosion performances, similar tests were conducted with commercially available 200 series
stainless steel sheet.
33
Figure 6 depicts the comparison in Tafel curves of acid and alkaline Zn/Cu/Ni/Cr multi-layered
coated samples of the present disclosure with commercially available Ni/Cr plated steel and
stainless steel. The Ecorr, icorr and corrosion rate (mpy) determined from the potentiodynamic
polarization curves are listed in Table 3 below:
Table 3: Average values of Ecorr, icorr and corrosion rate (mpy) 5 determined from
potentiodynamic polarization curves in 3.5 wt% NaCl solution.
As can be seen, the corrosion rate of Zn/Cu/Ni/Cr multi-layered coated samples of the present
disclosure varied closely around ~0.40 mpy. In comparison to commercial Ni/Cr coated
10 sample, the Zn/Cu/Ni/Cr multi-layered coated samples of the present disclosure showed several
times lower corrosion current and corrosion rate, demonstrating the much superior corrosion
resistance shown by the Zn/Cu/Ni/Cr multi-layered coated samples of the present disclosure.
Corrosion potential (Ecorr) is a reliable guide for predicting the current flow in galvanic coupling
15 and thereby assessing the sacrificial property of any coating. More anodic or negative corrosion
potential of the coatings of the present disclosure compared to steel substrate implies that the
coating will anodically corrode to protect the steel substrate.
As can be seen from the above table, the average corrosion potential of the Zn/Cu/Ni/Cr multi20
layered coated samples of the present disclosure (~ -0.93 V vs. SCE) was more negative
compared to commercial Ni/Cr (-0.48 V vs. SCE) and 200 series stainless steel (-0.13 V vs.
SCE) due to anodic Zn layer in the coating. Therefore, the Zn/Cu/Ni/Cr multi-layered coatings
of the present disclosure possess superior sacrificial corrosion behaviour compared to
commercial Ni/Cr coatings and stainless steel. Further considering the Zn/Cu/Ni/Cr coating25
steel substrate galvanic couple, the coating had much lower corrosion potential compared to
steel substrate (-0.54 V vs. SCE). Therefore, the present Zn/Cu/Ni/Cr coating can provide a
sacrificial cathodic protection to the steel substrate when the steel substrate is exposed to
corrosion environment due to scratch or abrasion.
Sample Ecorr (vs.SCE) (V) icorr (A/cm2) Corrosion Rate (mpy*)
Ni/Cr Coating -0.48 29.7 × 10-7 1.26
200x Stainless Steel -0.13 4.39 × 10-7 0.187
Acid Zn/Cu/Ni/Cr -0.91 5.42 × 10-7 0.32
Alkaline Zn/Cu/Ni/Cr -0.94 8.64 × 10-7 0.51
34
Salt Spray Test (SST)
The corrosion performance of the Zn/Cu/Ni/Cr multilayer coating of the present disclosure on
steel was measured in a Weiss Tenik SC 450® salt spray test (SST) amber as per ASTM B117
standard. The results of the salt spray test (SST), as per ASTM B117 of the Zn/Cu/Ni/Cr coated
steel and 200 series stainless steel are shown 5 in Table 4 below:
Table 4: SST life (5% red rust) of different Zn-based multilayer coated (Zn/Cu/Ni/Cr)
steel samples
It is evident from Table 4, that the corrosion resistance of all types of Zn/Cu/Ni/Cr coated steels
10 is much more superior compared to 200 series stainless steel. SST tests showed that there was
no visible sign of red rust formation and coating delamination of all types of Zn/Cu/Ni/Cr
multilayer coated steels even after 500 hours. For example, as can be seen in Figure 7, the
Acid Zn/Cu/Ni/Cr multilayer electroplated steel (Zn-layer thickness: 6 µm) samples shows no
red rust formation after 500 hours, although white rust formed on the surface due to the
15 presence of Zn base layer. At 1000 hours of SST, the samples show red rust initiation, and 5%
red rust was observed after 1080h of SST. On the contrary, the stainless steel substrate showed
significant red rust formation at 500 hours and 1000 hours of SST.
Upon increasing the coating thickness of the Acid Zn-layer from 6 µm to 8 µm, the SST life of
20 the multilayer coating increased significantly from 1080 hours to 1600 hours as can be from
Table 4 above. Alkaline Zn-based multilayer coating of the present disclosure also showed
Zn
Plating
Bath
Layer Thickness (µm) Total
Thickness
(µm)
SST
Life (h)
(5% Red Rust)
Zn Cu Bright Ni Bright Cr
Acid 6 0.5-1 1.5 0.25 8.25-8.75 1080
Acid 8 0.5-1 1.5 0.25 10.25-10.75 1600
Acid 7 1.5 1.5 0.25 10.25 1280
Sulphate 8 2 1 0.25 11.25 900
Sulphate 7 1 2 0.25 10.25 800
Sulphate 6 1 2 0.25 9.25 800
Alkaline 8 1 1.5 0.25 10.75 900
Alkaline 7 1 1.5 0.25 9.75 900
Alkaline 6 2 2 0.25 10.25 900
200 Series Stainless Steel 650
35
comparable SST life with respect to the acid Zn-based multilayer coating. Sulphate Zn-based
multilayer coating showed SST life of 800-900 hours compared to the other two varieties of
acid and alkaline Zn-based coatings with similar or even higher coating thickness.
In the case of 200 series stainless steel, red rust formation started after 60 5 hours. After 600
hours of SST, a significant formation (>10%) of red rust was observed on the surface as well
as at the edges. These tests demonstrate the ability of the Zn/Cu/Ni/Cr multi-layered
electroplated steel of the present disclosure to resist corrosion in aggressive chloride
environment better than 200 series stainless steel.
10
Higher corrosion resistance of the Zn/Cu/Ni/Cr multilayer coating was achieved due to the
presence of highly anodic sacrificial Zn layer underneath the bright Ni layer. The corrosion
potential of Zn layer is more negative than the bright Ni layer and therefore any corrosion pits
which go through the barrier Ni layer will be arrested under the sacrificial Zn layer. Zn layer
15 dissolves preferentially and protects the steel substrate. The outer chromium layer provides the
desired shiny, silvery appearance and to protect the Zn, Cu and bright Ni layers against
mechanical forces such as wear and erosion. It is worthwhile to mention that conventional 2-
layer (Ni/Cr) or 3-layer (Cu/Ni/Cr) plating cannot provide any sacrificial property in the
coating to protect the underlying steel. Even in the case of Cu/Ni/Ni/Ni/Cr tri-nickel plating
20 system, the intermediate Ni layer which acts as an anodic layer is far less anodic compared to
Zn.
Copper Accelerated Acetic Acid Salt Spray Test (CASS)
The Copper Accelerated Acetic Acid Salt Spray (CASS) test was carried out as per ASTM
25 standard method B368-09. A salt solution containing 5% NaCl + 1.3-1.5 ml/L acetic acid (for
pH adjustment) +0.25g/L cupric chloride (CuCl2.H2O) was injected into the test chamber with
compressed air for atomized spray. A pH of 3.2±0.1 and temperature of 49°C±1 was
maintained inside the chamber. After testing, the samples were examined to determine the
extent of corrosion.
30 The results of the Copper Accelerated Acetic Acid Salt Spray (CASS) test carried out on the
Zn/Cu/Ni/Cr coated sheets of the present disclosure are shown in Figure 8 and Table 5 below.
The panels were rated as per the following scale:
36
10 = no visible corrosion,
9 = trace of corrosion (one or two small red rust spots),
8 = slight corrosion (some small red rust spots)
7 = light corrosion (many small red rust spots, approx. 10% of area)
6 = moderate corrosion (medium size rust spots, 5 10-40% of area).
Table 5: CASS life of different Zn-based multilayer coated (Zn/Cu/Ni/Cr) steel samples
of the present disclosure and Ni/Cr or Cu/Ni/Cr coating on steel as per ASTM B456-17.
10
15
20
The service condition number indicates the severity of exposure for which the grade of coating
25 is intended:
SC 5 extended severe service
SC 4 very severe service,
SC 3 severe service,
SC 2 moderate service, and
Sample
CASS Test
(Hour)/Rating
Ni/Cr Cu/Ni/Cr
CASS Test (Hour) Requirement
Acid
Zn/Cu/Ni/Cr
(Zn-layer
thickness: 6
µm)
48
Rating : 8
22
(For Service
Condition Number: 4
Ni: 30µm
Cr: 0.25-0.5 µm)
66
(For Service
Condition Number: 4
Cu: 15 µm
Ni: 25 µm
Cr: 0.25-0.5 µm)
Acid
Zn/Cu/Ni/Cr
(Zn-layer
thickness: 8
µm)
80
Rating : 8
22
(For Service
Condition Number: 5
Ni: 35 µm
Cr: 0.25-0.5 µm)
66
(For Service
Condition Number: 5
Cu: 15 µm
Ni: 30 µm
Cr: 0.25-0.5 µm)
Sulphate
Zn/Cu/Ni/Cr
(Zn-layer
thickness: 8
µm)
48
Rating: 8
37
SC 1 mild service.
For all service conditions (SC), Rating 8 and above is acceptable.
It is evident from Table 5 and Figure 8 that the corrosion resistance of all Zn/Cu/Ni/Cr coated
steels of the present disclosure exceeded the CASS test requitement compared to conventional
Ni/Cr or Cu/Ni/Cr coatings on steel as per ASTM B456-17 for Service condition 5 number: 4.
With an increase in the Zn layer thickness to 8 µm in the Zn/Cu/Ni/Cr coated steel of the present
disclosure, the CASS test result (80 hours) well exceeded beyond 66 hours for Service
condition number: 5 as per ASTM B456-17. The results indicate the Zn/Cu/Ni/Cr coating of
the present disclosure is suitable for SC 5 extended severe service and SC 4 very severe service.
10 All Zn/Cu/Ni/Cr coatings of the present disclosure have a total thickness of <12 µm, which is
much lower than the thickness of conventional Ni/Cr or Cu/Ni/Cr coatings, thereby providing
better CASS test hour. Therefore, it can be concluded that the Zn/Cu/Ni/Cr coated steel
possesses better corrosion resistance compared to conventional Ni/Cr or Cu/Ni/Cr coating on
steel.
15 As can be seen from the above table, Acid Zn/Cu/Ni/Cr (Zn-layer thickness: 6 µm) coating
showed CASS test life of 48 hours due to lower thickness compared to Zn-layer thickness of 8
µm. Additionally, the Sulphate Zn/Cu/Ni/Cr coating showed least CASS test life of 48 hours,
even at a high Zn-layer thickness of 8 µm.
20 Example 4: Coating Adherence of the Zn/Cu/Ni/Cr Multilayer Coated Steel
Coating adherence with the substrate was measured through peel test using Permacel 99 tape
as per ASTM B571-97. After testing, the samples were visually inspected and photographed
in high-resolution camera to observe any peeling or flaking of the coating from the substrate.
The ammonium bifluoride “activation” step after deposition of the Zn layer is an important
25 step to enhance coating adhesion and a comparison with and without activation step is shown
in Figure 9. It is evident from Figure 9 that the multilayer coating process without ammonium
bifluoride “activation” step resulted in blister formation after coating and also resulted in
coating peel off and flaking after the peel test. This sample hence failed the adherence test. On
the contrary, samples prepared with the activation step mentioned above resulted in smooth,
30 bright and adherent deposits, no blister formation was observed post coating and no peel off,
flaking or delamination was observed after the peel test. Therefore, it can be concluded that the
38
activation step is important during the preparation of the multilayer coatings of the present
disclosure for better adherence and quality.
Additional embodiments and features of the present disclosure will be apparent to one of
ordinary skill in art based on the description provided herein. The embodiments 5 herein provide
various features and advantageous details thereof in the description. Descriptions of wellknown/
conventional methods and techniques are omitted so as to not unnecessarily obscure the
embodiments herein.
10 The foregoing description of the specific embodiments reveal the general nature of the
embodiments herein that others can, by applying current knowledge, readily modify and/or
adapt for various applications such specific embodiments without departing from the generic
concept, and, therefore, such adaptations and modifications should and are intended to be
comprehended within the meaning and range of equivalents of the disclosed embodiments. It
15 is to be understood that the phraseology or terminology employed herein is for the purpose of
description and not of limitation. Therefore, while the embodiments in this disclosure have
been described in terms of preferred embodiments, those skilled in the art will recognize that
the embodiments herein can be practiced with modification within the spirit and scope of the
embodiments as described herein.
20
Throughout this specification, the term ‘combinations thereof’ or ‘any combination thereof’ or
‘any combinations thereof’ are used interchangeably and are intended to have the same
meaning, as regularly known in the field of patents disclosures.
25 As regards the embodiments characterized in this specification, it is intended that each
embodiment be read independently as well as in combination with another embodiment. For
example, in case of an embodiment 1 reciting 3 alternatives A, B and C, an embodiment 2
reciting 3 alternatives D, E and F and an embodiment 3 reciting 3 alternatives G, H and I, it is
to be understood that the specification unambiguously discloses embodiments corresponding
30 to combinations A, D, G; A, D, H; A, D, I; A, E, G; A, E, H; A, E, I; A, F, G; A, F, H; A, F, I;
B, D, G; B, D, H; B, D, I; B, E, G; B, E, H; B, E, I; B, F, G; B, F, H; B, F, I; C, D, G; C, D, H;
C, D, I; C, E, G; C, E, H; C, E, I; C, F, G; C, F, H; C, F, I, unless specifically mentioned
otherwise.
39
While considerable emphasis has been placed herein on the particular features of this
disclosure, it will be appreciated that various modifications can be made, and that many
changes can be made in the preferred embodiments without departing from the principles of
the disclosure. These and other modifications in the nature of the disclosure or the preferred
embodiments will be apparent to those skilled in the art from the disclosure 5 herein, whereby it
is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely
as illustrative of the disclosure and not as a limitation. , Claims:We claim:
1. A multilayer coating on a steel substrate, the coating comprising:
a) a Zn layer on the steel substrate,
b) a Cu layer on the Zn layer,
c) a Ni layer on 5 the Cu layer, and
d) a Cr layer on the Ni layer.
2. The multilayer coating as claimed in claim 1, wherein the Zn layer has a thickness ranging
from about 6-9 µm.
3. The multilayer coating as claimed in claim 1 or 2, wherein the Cu layer has a thickness
10 ranging from about 0.5-2 µm.
4. The multilayer coating as claimed in any one of claims 1-3, wherein the Ni layer has a
thickness ranging from about 1-2 µm.
5. The multilayer coating as claimed in any one of claims 1-4, wherein the Cr layer has a
thickness ranging from about 0.25-0.5 µm.
15 6. The multilayer coating as claimed in any one of claims 1-5, wherein the Zn layer has a
thickness ranging from about 6-9 µm, the Cu layer has a thickness ranging from about 0.5-
2 µm, the Ni layer has a thickness ranging from about 1-2 µm, and the Cr layer has a
thickness ranging from about 0.25-0.5 µm.
7. The multilayer coating as claimed in any one of claims 1-6, wherein a total thickness of the
20 multilayer coating ranges from about 7.5-12 µm.
8. The multilayer coating as claimed in any one of claims 1-7, wherein the Zn layer is
deposited on the steel substrate from an acidic Zn electroplating bath, a sulphate Zn
electroplating bath, or an alkaline Zn electroplating bath.
9. The multilayer coating as claimed in any one of claims 1-8, wherein the coating exhibits a
25 corrosion potential of about -0.90 to -0.95 V.
10. The multilayer coating as claimed in any one of claims 1-9, wherein the coating exhibits a
corrosion current of about 5.4 × 10-7 A/cm2 to 8.7 × 10-7 A/cm2.
11. The multilayer coating as claimed in any one of claims 1-10, wherein the coating exhibits
a corrosion rate of about 0.3 to 0.55 mils per year (mpy).
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12. The multilayer coating as claimed in any one of claims 1-11, wherein the coating exhibits
a Salt Spray Test 5% Red Rust life of about 800 hours or more.
13. The multilayer coating as claimed in any one of claims 1-12, wherein the coating exhibits
a Copper Accelerated Acetic Acid Salt Spray (CASS) test life of more than 40 hours.
14. The multilayer coating as claimed in any one of claims 1-13, wherein the 5 coating does not
exhibit peeling or flaking from the steel substrate.
15. A method for depositing the multilayer coating as claimed in any one of claims 1-14 on a
steel substrate, comprising:
a) depositing a Zn layer on the steel substrate;
10 b) contacting the Zn layer on the steel substrate with an activating composition
comprising ammonium bifluoride for about 5-15 seconds;
c) depositing a Cu layer on the Zn layer;
d) depositing a Ni layer on the Cu layer; and
e) depositing a Cr layer on the Ni layer as a top layer.
15 16. The method as claimed in claim 15, wherein the Zn layer is deposited on the steel substrate
using an acidic Zn electroplating composition comprising about 42-84 g/L of zinc chloride,
about 205-223 g/L of potassium chloride, and about 24-32 g/L of boric acid.
17. The method as claimed in claim 16, wherein the Zn layer is deposited on the steel substrate
at a current density of about 250-1000 A/m2, pH of about 5-5.5, and a temperature of about
20 20-40°C.
18. The method as claimed in claim 15, wherein the Zn layer is deposited on the steel substrate
using a sulphate Zn electroplating composition comprising about 172-444 g/L of zinc
sulphate, about 12-16 g/L of potassium chloride, about 18-24 g/L of boric acid, and about
8-10 ml/L of additives.
25 19. The method as claimed in claim 18, wherein the Zn layer is deposited on the steel substrate
at a current density of about 250-1000 A/m2, pH of about 3-4, and a temperature of about
40-60°C.
20. The method as claimed in claim 15, wherein the Zn layer is deposited on the steel substrate
using an alkaline Zn electroplating composition comprising about 11-13 g/L of Zn oxide,
30 about 110-130 g/L of sodium hydroxide, and about 27-30 ml/L of additives.
42
21. The method as claimed in claim 20, wherein the Zn layer is deposited on the steel substrate
at a current density of about 250-1000 A/m2, pH of about 3-4, and a temperature of about
20-40°C.
22. The method as claimed in any one of claims 15-21, wherein ammonium bifluoride is
present in the activating composition in an amount 5 of about 2-3 wt%.
23. The method as claimed in any one of claims 15-22, wherein the Cu layer is deposited on
the Zn layer using an electroplating composition comprising about 28-42 g/L of copper
cyanide and a free cyanide content of about 4-6 g/L.
24. The method as claimed in claim 23, wherein the Cu layer is deposited on the Zn layer at a
10 current of about 2.5-3 A/dm2 and a temperature of about 50-60°C.
25. The method as claimed in any one of claims 15-24, wherein the Ni layer is deposited on
the Cu layer using an electroplating composition comprising about 280-320 g/L of nickel
sulphate, about 50-70 g/L of nickel chloride, about 35-45 g/L of boric acid, and about 5-7
ml/L of additives.
15 26. The method as claimed in claim 25, wherein the Ni layer is deposited at a current density
of about 70-100 A/m2, pH of about 3.5-4.5, and a temperature of about 50-60°C.
27. The method as claimed in any one of claims 15-26, wherein the Cr layer is deposited on
the Ni layer using an electroplating composition comprising about 220-230 g/L of chromic
acid and about 0.8-1.2 g/L of a sulphate.
20 28. The method as claimed in claim 27, wherein the Cr layer is deposited at a current density
of about 1200-1300 A/m2, and a temperature of about 50-60°C.
29. A steel substrate comprising the multilayer coating as claimed in any one of claims 1-14.
30. The steel substrate as