The present invention relates to a product of Cu/Ni/Cr multilayer coated steel
substrate by electro-plating method. The coating demonstrates excellent
corrosion behaviour compared to stainless steel kitchen used in commercial
applications. More specifically, the invention related to a multilayered Cu/Ni/Cr
electro-plating on a steel substrate product for kitchen sink and household
appliances.
BACKGROUND OF THE INVENTION:
Main driver to replace stainless steel sink with coated steel sheet is the higher
cost of stainless steel sheet (Rs.100-150/kg vs. Rs.40-50/kg for steel sheet).
Although numerous metallic or non-metallic coating systems are available to coat
steel sheet for different applications, so far no commercial kitchen sink material is
available which is manufactured by electroplating with multilayer Cu/Ni/Cr
coatings. So, there are tremendous opportunities to explore new metallic coating
systems on steel sheet which could be a potential replacement for stainless steel
sink in terms of cost and durability. There are numerous literatures available on
Cu/Ni/Cr coating system, few of them are referred below which stated the
corrosion behavior, coating thickness and morphology of the coatings.
Biestek et al [1,2] studied the corrosion properties of by-layer Ni/Cr and tri-layer
Cu/Ni/Cr by field test. The field results showed that coating systems containing
external layers of microporous or microcracked chromiµm have markedly better
protective properties than conventional Ni40-Cr0.5 (Ni 40 µm and Cr 0.5 µm) and
Cu20-Ni25-Cr0.5 coating systems (with regular chromiµm layers). Moreover, two-
layer coatings of bright nickel and regular chromiµm and three-layer coatings of
bright copper, bright nickel and regular chromiµm, do not provide the protection
usually required under exceptionally severe service conditions, even 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 chromiµm (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 to 25 µm or eventually even to 20 µm for
exceptionally severe service conditions.
Medeliene et al [3] have studied the morphology and corrosion Properties of
electroplated Ni–Cr alloy coatings on Cu substrate in salt solutions. Co-
deposition of chromium with nickel changes the morphology of the coating,
through the micro-cracks of which the corrosion of copper starts and accelerates
by forming corrosion cells (between the substrate and coating) in an acidic 5%
NaCl solution (pH 3.1). In neutral media, the substrate is protected from
corrosion not only by the passive Ni–Cr coating, but also by insoluble copper
compounds that block the coating microcracks, which manifests itself in inhibiting
the corrosion. Upon a long exposure to acidic NaCl solutions, uniform corrosion
of the Ni coating gives way to pitting corrosion.
Chandran et al [4] have investigated various types of decorative copperr-nickel-
chromium electrodeposits were produced on mild steel substrates and their
corrosion resistance evaluated by means of CASS and Corrodkote accelerated
tests and also by exposure tests at a locality with a tropical marine atmosphere.
They have concluded that duplex nickel with microporous chromium and a
copper undercoat can be considered as a better system for corrosion protection.
Most of the patents could be found on multilayered Cu/Ni/Cr coating for
automobile applications. Where, steel panels were plated with about 0.8 mil
copper from a typical cyanide copper plating solution such as disclosed in Martin
(U.S. Patent No. 2,861,929) and then with about 0.7 mil nickel from a typical
nickel plating solution such as disclosed in Towle (U.S. Patent No. 2,972,571)
and finally with about 0.01 mil chromium from a chromic acid plating bath, for
example, as disclosed in Brown (U.S. Patent No. 2,750,334). This represents a
plating finish of the kind used commercially today. Similar steel panels were next
plated as in the pre ceding paragraph with copper, nickel and chromium with the
exception that a 0.1 mil layer of cadmium was plated between the copper and
nickel plates. The cadmium was plated from a typical cadmium plating bath such

as disclosed in Hendrick’s (U.S. Patent No. 2,085,747).
When using three layers of nickel, the composition of the intermediate layer is
adjusted so as to be anodic to both the upper and lower layers and preferably the
composition of the upper layer is adjusted so as to be anodic to the lower layer.
By the use of some triple-layer Ni systems, marked corrosion protection was
effected [5]. In most cases, however, the metal corroded too rapidly causing
blistering or scaling or staining or colored the decorative surface.
1. T. Biestek, Surf. Technol. 21 (1984) 283.
2. T. Biestek, Surf. Technol. 21 (1984) 295.
3. V. Medeliene, E. Matulionis, Protection of Metals 38 (2002) 238.
4. K. Chandran, S. Sriveeraraghavan, R. M. Krishnan, S. R. Natarajan,
S. Guruviah, Bulletin of Electrochemistry 2 (5) (1986) 457.
5. Knapp, Trans. Inst. Met. Finishing, 1958, 35, 139-165
OBJECTS OF THE INVENTION:
An object of the invention is to propose a multilayer coating that has excellent
corrosion behaviour compared to stainless steel.
An object of the present invention is to propose a Cu/Ni/Cr multilayered coating
that can be applied on a steel substrate.
Another object of the invention is to produce a Cu/Ni/Cr multilayered coated steel
substrate.
Still another object of the invention is to propose a Cu/Ni/Cr multilayered coating
that can be applied on a steel substrate using an electro-plating process.
Another object of the invention is to manufacture Cu/Ni/Cr multilayered coated
steel substrate for kitchen sink and household appliances.
SUMMARY OF THE INVENTION:
Cu/Ni/Cr multilayer coated steel substrate was produced electroplating a steel

substrate into a sequence of electroplating baths. It begins with a thin layer of
cyanide Cu-flash (2-3µm), followed by semi-bright Ni layer (3-7 µm); high-sulfur
Tri-Ni layer (1-2 µm); bright Ni layer (3-7 µm) and finally bright Cr layer (0.5-1
µm) on top. The coating was bright and adherent. Salt spray test demonstrated
that the multilayered Cu/Ni/Cr coating can resist red rust formation even after
1700 h of exposure. Furthermore, the coating can withstand over 0.99 Kg.m
impact force without coating break-up. This type of coated steels can be used for
kitchen sink material and manufacturing of household appliances.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - Photographs of Cu/Ni/Cr multilayered electroplated steel sheet samples.
Fig. 2 - Top surface SEM micrographs and corresponding EDS area analysis of
Cu/Ni/Cr multilayered electroplated steel sheet (Sample A).
Fig. 3 - Cross-sectional SEM micrographs and corresponding EDS point analysis
(wt%) of Cu/Ni/Cr multilayered electroplated steel sheet (Sample A).
Fig. 4 - Cross-sectional SEM micrographs and corresponding EDS line analysis
(wt%) of Cu/Ni/Cr multilayered electroplated steel sheet (Sample A).
Fig. 5 - GDOES sputter depth profile of Cu/Ni/Cr multilayered electroplated steel
sheet (Sample A).
Fig. 6 - Salt spray test (SST) results of Cu/Ni/Cr multilayered electroplated steel
samples and actual stainless steel kitchen sink sample.
Fig. 7 - Tafel curves of Cu/Ni/Cr coated (Sample A) and stainless steel kitchen
sink sample.
Fig. 8 - Photographs of samples before and after conical mandrel test.
Fig. 9 - Photographs of Sample A and S. S. Kitchen Sink sample after 40 days
caustic immersion test and heating up to 100°C.
DETAILED DESCRIPTION OF THE INVENTION:
The invention proposes a Cu/Ni/Cr multilayered electroplated coating on a steel
substrate. Table 1 provides details of the coating layer sequence along with
individual coating layer thickness. The steel substrate coated with the Cu/Ni/Cr

multilayer was evaluated the corrosion properties using different tests and was
compared with commercially available stainless steel kitchen sink material. It was
observed that Cu/Ni/Cr multilayer possess superior corrosion resistance in salts
spray test.
Table 1: Multilayered Cu/Ni/Cr electro-plating sequence and individual layer
thickness.

Preparation of Cu/Ni/Cr multilayer coated steel:
Cu/Ni/Cr multilayer coated steel sheets were prepared by multiple bath
electroplating process. The bath composition and operating parameters are listed
in Table 2 and three types of samples (A, B and C) with individual electroplated
layers calculated from Faraday’s Law are tabulated in Table 3.
The top surface and cross-section of samples were characterized using scanning
electron microscopy (SEM) and energy dispersive X-ray (EDX). Elemental depth-
profiling of coating was recorded by Glow Discharge Optical Emission
Spectroscopy (GDOES) to confirm the coating thicknesses of different phases
obtained by SEM on cross-sections.
The corrosion performance of Cu/Ni/Cr multilayer coating on steel was measured
in a Weiss Tenik SC 450® salt spray test (SST) amber as per ASTM B117
standard. Furthermore, corrosion rate was measured by DC 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 platinµm and reference electrode
was calomel electrode. The test was conducted in 3.5% sodiµm chloride (NaCl)
solution with scan rate of 0.5mV/s. The corrosion rate was measured by Tafel

extrapolation technique using VersaStudio® software module. For comparison of
corrosion performances, similar tests were conducted with commercially
available stainless steel kitchen sink material.
Coating adherence was measured The Impact testing was carried out using 1m
Sheen Tubular Impact Tester® according to ISO 6272, BS 3900: E13, DIN
55669 with an impacting load of 1 kg. Indenter diameter is 20mm and the die
diameter is 27mm. After testing, those samples were photographed in high-
resolution camera to observe any macroscopic crack formation as well as visual
observation.


Table 3: Individual layer thickness of different Cu/Ni/Cr multilayer coated
samples.

Characterization and Performance of Cu/Ni/Cr multilayer coated steel:
A series of characterization tests were performed on Cu/Ni/Cr multilayer coated
steel sheets to evaluate coating morphology and performance.
Coating Characterization:
Fig. 1 shows the photographs of top surface of Cu/Ni/Cr multilayered
electroplated samples (A, B and C). The visual observation shows the bright Cr
layer outside and the coating is very adherent with steel substrate.
Fig. 2 shows the SEM topography of Cu/Ni/Cr multilayered electroplated steel
sheet surface (Sample A) and their corresponding EDS analysis. Top surface
shows smooth appearance with no defects. EDS analysis top surface contains
Cr but substantial Ni is present due the thick Ni layers (~13µm) underneath the
thin Cr top layer (~0.5µm). The presence of other elements such as Fe and C
could be justified due to the interference from the steel substrate.
Fig. 3 shows the cross-sectional SEM microstructure of Cu/Ni/Cr multilayered
electroplated steel sheet and their corresponding EDS analysis. SEM micrograph
clearly shows the coating consists of multilayered structure of Cu, Ni and Cr. The
layers are continuous with smooth interface. EDS point analysis indicates the
presence of Cu, Ni and Cr layer as identified in Fig. 3. Fig. 4 shows Cross-
sectional SEM micrographs and corresponding EDS line elemental intensity of

Cu, Ni, Cr and Fe of multilayered electroplated steel sheet. The thickness of
different layers determined from the intensity curve of Cu: 3-4µm, total Ni: 11-
13µm and Cr: ~1 µm which is consistent with the thickness determined from
Faraday’s law.
Fig. 4 Cross-sectional SEM micrographs and corresponding EDS line analysis
(wt%) of Cu/Ni/Cr multilayered electroplated steel sheet (Sample A).
Fig. 5 illustrates the GDOES elemental depth profiles of Fe, Cu, Ni and Cr. The
results show consistency with the total coating thickness and structure compared
to cross-sectional EDS line analysis shown in Fig. 3. It must be noted that it is
quite difficult to determine the exact individual structures of very thin layers from
GDOES analysis.
Salt Spray Test (SST)
The results of the salt spray test (SST), as per ASTM B117, are shown in Fig. 6.
It is evident from the photographs that there was no visible sign of red rust
formation and coating delamination of all Cu/Ni/Cr multilayered electroplated
steel sheet samples up to 650 h, whereas in the case of stainless steel the red
rust formation starts after 60 h of test. After 300 h and 650 h of test, actual
stainless steel sink material shows significant formation of red rust on the surface
as well as at the edges. These tests demonstrate the ability of the Cu/Ni/Cr
multilayered electroplating to resist corrosion in aggressive chloride environment.
After salt exposure over 71 days (1700 h), for Sample A shows small spot of red
rust formation and the probable reason could be the presence of pore during Cr
plating. On the other hand, stainless steel sink material depicts significant
amount of red rust formation. The results emphatically highlighted the superior
corrosion resistance of Cu/Ni/Cr multilayer coating compared to stainless steel
sink material.
Higher corrosion resistance of the coating was further enhanced by the

application of high-sulfur Nickel plating between semi-bright and bright Ni layers.
The corrosion potential of high-sulfur Tri-Nickel is most negative of Ni deposits,
so this high-sulfur Nickel layer dissolves preferentially even when the pitting
corrosion reaches to the surface of a semi-bright Ni deposit. However, since the
high-sulfur Nickel layer reacts together with a bright Ni layer, pitting corrosion
doesn’t pass through the high-sulfur Nickel layer in the tunneling form.
Tafel Test
The corrosion behavior was also evaluated by anodic potentiodynamic
polarization experiments, which were conducted in 3.5 wt.% sodiµm chloride
solution with a scan rate of 0.5 mV/s. The corrosion rate (mpy) was measured by
Tafel extrapolation technique using VersaStudio® software module. Fig. 7
depicts the comparison in Tafel curves of Cu/Ni/Cr multilayered coated sample
(Sample A) with stainless steel. The Ecorr, icorr and corrosion rate (mpy)
determined from potentiodynamic polarization curves for all samples are listed in
Table 3. The Ecorr of -0.484 V of Sample A actually correspond to the standard
potential to Cr which is the outermost of the coating and consistent with literature
[6]. But the corrosion rate our sample is much less (1.26 mpy) compared to pure
chrome plating (5.18 mpy) reported in literature [6] due to the presence of Ni
layer underneath which has a very low corrosion rate of ~ 1 mpy [7]. Similar
observation was found for Sample B and C.


But unfortunately, polarization experiment reveals that corrosion rate of Cu/Ni/Cr
coated is almost 6 times compared to stainless steel kitchen sink sample as
shown in Table 4. Although we have seen far superior corrosion performance of
Cu/Ni/Cr multilayered electroplated steel sheet in salt sprat test compared to
stainless steel, the Tafel test does not translate the similar behavior. The
probable reason for higher corrosion rate could be the presence of porous Cr
outer layer. The Ecorr of -0.132V for stainless steel consistent with Jang et. al [8].
In summary, it could be concluded from Tafel test that Cu/Ni/Cr multilayered
electroplated steel sheet can’t be represented as a true corrosion resistance of
the material due to lower depth of reaction during Tafel Test.
6. H. A. Ameen, K. S. Hassan, B. R. Mohameed, Am. J. Sci. Ind. Res., 1(3)
(2010) 565-572.
7. D. Pradhan, A. K. Guin, M. Manna, P. Raju, Tata Search, Vol II (2013)
209-214.
8. S. K. Jang, M. S. Han, S. J. Kim, Trans. Nonferrous Met. Soc. China 19 (2009)
930-934.
Impact Test
Table 5 shows the photographs of impact test samples at maximµm impact
height of 39ʺ with impacting load of 1 kg as per ISO 6272, BS 3900: E13, DIN
55669. All samples show no coating breakup or peel-off on both sides of sample
even after maximµm impact force of 0.99 kg.m. The impact test results conclude
that the coating has enough adherences for kitchen sink and related applications.


Conical Mandrel Test
The conical mandrel test was carried out using Sheen Conical Mandrel Tester®
according to ASTM D522-(A). The test was conducted on all Cu/Ni/Cr coated
steel samples. Fig. 8 show the high resolution digital images of Sample A and
Sample B before and after the conical mandrel test. No crack or delamination
was observed throughout length of cone (5mm diameter end to 35 mm diameter
end). This test concludes that all samples of Cu/Ni/Cr multilayer coating have
sufficient resistance to cracking and/or detachment from its steel substrate.

Alkali Immersion Test
Although chemical immersion tests was not followed in accordance with ASTM
C868-02 which is applicable for polymer coating, rather we have followed the
similar concentration alkali solution (NaOH) for immersion of all types of samples
to simulate the similar behavior observed in household/industrial use of kitchen
sink as no standard was available for metallic coating.
Fig. 9 shows Cu/Ni/Cr coated steel sheet sample (Sample A) after 40 days of
exposure in alkali solution of NaOH (100g/l, pH=12) after periodical heating
(everyday 4h at up to 100°C). It clearly indicates that there was no red rust
formation on all types of samples. Furthermore, no coating delamination was
observed. Similar results were obtained for Sample B and C. Hence, Cu/Ni/Cr
multilayer coated steel sheets have sufficient caustic resistance even at higher
temperature.

WE CLAIM
1. A multi-layered steel substrate with improved corrosion resistance,
characterized by coating consisting of copper (Cu), nickel (Ni) and
cromium (cr), wherein the coating sequence comprising one bottom layer
of cu; three intermediate layers of Ni; and one top layer of cr, wherein the
coating appearances and the thicknesses of the coated layers exhibits
cyanide Bright copper between 2-3nm for the bottom layer; bright chrome
between 0.5 to 1.00^m for the top layer; and sulfer-free semi bright nickel
between 3-7µm, high sulfer tri-nickel between 1-2km, and bright nickel
between 3-7µm for the three intermediate layers respectively when viewed
from top, and wherein the coating is applied on the substrate by
electroplating process.
2. The multi-layered steel substrate as claimed in claim 1, wherein the
coated substrate can be used as a kitchen sink material.