TECHNICAL FIELD
The present disclosure relates to the field of electroplating. Particularly, the present disclosure
relates to electroplating compositions to provide zinc-manganese (Zn-Mn) coatings, methods
of preparing them, methods of depositing electroplating compositions on steel substrates and
steel substrates obtained therefrom.
BACKGROUND OF THE DISCLOSURE
Zinc coatings have extensively been used for improving the corrosion resistance of steel. Zinc
is well known for imparting sacrificial corrosion resistance to steel, along with the benefit of a
low cost. Hot-dip galvanising of zinc is the most popular process for applying zinc coatings on
steel. However, hot-dip galvanising has certain disadvantages like poor coating thickness
control, and impossibility of single sided coatings. Another problem with the hot-dipping of
zinc for high strength alloy steels is that they suffer from selective oxidation of alloying
elements in the steel during annealing, resulting in poor wettability of molten zinc [1]. Also,
zinc coatings have poor weldability due to the low melting point of zinc (420ᵒC). The
weldability is poorer for thicker coatings [2,3].
One of the ways by which these problems can be addressed is by electrodeposition of zinc
(electrogalvanising) instead of hot-dipping. Electrogalvanising offers many advantages in
comparison to hot-dipping, like coating at room temperature, production of more compact
coatings [4], better control over coating thickness, and the possibility of one-sided coating.
However, zinc being an active metal has a high corrosion rate itself. Generally, a secondary
passivation layer is applied to zinc coated steels to retard the corrosion of active zinc. Using
electrodeposition, this passivation property may be achieved by depositing zinc alloy coatings.
Alloying elements like nickel (Ni) are known to slow down the corrosion process and stabilize
the corrosion product that provides passivation.
Electrodeposited Zn-Ni alloy coatings with about 12% Ni are extremely popular and provide
higher corrosion resistance to steel than pure zinc coatings even at very low coating thickness
[5]. The commercial electrodeposited Zn-Ni alloy coating on steel provides excellent corrosion
resistance to steel due to the retardation of corrosion by nickel. However, since nickel is a more
noble metal than zinc, Zn-Ni alloys exhibit lower sacrificial corrosion properties than pure zinc.

Nickel is also a costly material, and there have been multiple attempts to replace nickel with
cheaper metals like manganese [6].
Manganese is an extremely promising alloying element for zinc as it provides higher sacrificial
corrosion resistance to steel than zinc itself. Moreover, Zn-Mn alloy coatings are known to
have exceptional corrosion resistance due to the formation of a dense protective corrosion
product that inhibits further corrosion [7-9]. Zn-Mn alloys are also very promising as Mn is
much cheaper than Ni and Zn.
The major problem of Zn-Mn co-deposition is that the standard reduction potentials Zn and
Mn are significantly different, (-0.76 V/SHE for Zn and -1.18 V/SHE for Mn). As a result, Zn
gets deposited more readily and hinders the deposition of Mn, leading to little or no manganese
in the final deposit [4]. Hence, zinc deposition needs to be inhibited to allow an appreciable
amount of manganese in the coating. Present studies on electroplating baths for Zn-Mn alloy
coatings include mostly acidic baths containing complexing agents like citrates, gluconates,
tartarates, pyrophosphates and fluoroborates that are used to push Zn deposition potentials to
more negative values, and hence obtain appreciable amounts of Mn in the final deposit [7,10-
15]. The acidic sulphate-citrate baths are most popular as they are environment friendly and
are known to produce smooth, fine-grained deposits with Mn content up to 95% providing very
high corrosion resistance to steel [13,15]. However, Zn-Mn deposition comes at the price of
low current efficiency, due to the parasitic hydrogen evolution reaction along with Mn
deposition [16]. This also results in lower deposition rates and formation of porous coatings,
especially at high current densities [6,7,17-20]. In this regard, organic additives have been
reported to suppress hydrogen evolution and provide better current efficiency [15]. However,
these additives may contain thiocyanates which are not very environment friendly. Another
major issue with sulphate baths with citrate complexants is their poor bath stability, due to the
oxidation of Mn(II) to Mn(III) in the bath and subsequent precipitation of an insoluble
manganese citrate complex Mn(III)Cit [7,17,21,22]. There have been attempts to combat these
problems by using additives, as well as changing the bath pH and temperature, but with little
success [7,23].
Thus, there is a need in the art to develop Zn-Mn electroplating compositions that are stable,
environment-friendly, show better current efficiency and deposition rates, and provide coatings

with uniform microstructure, desired Mn content and corrosion resistance. The present
disclosure attempts to address said need.
STATEMENT OF THE DISCLOSURE
The present disclosure relates to an electroplating composition comprising zinc sulphate,
manganese sulphate, sodium citrate, sodium thiosulphate, and ascorbic acid. In some
embodiments, the electroplating composition comprises zinc sulphate in an amount of about
30-50 g/L, manganese sulphate in an amount of about 40-70 g/L, sodium citrate in an amount
of about 175-200 g/L, sodium thiosulphate in an amount of about 0.1-0.3 g/L, and ascorbic
acid in an amount of about 0.1-6 g/L.
The present disclosure also relates to a method for preparing the electroplating composition
described herein, said method comprising: a) heating water to about 60-70℃ to obtain a heated
water; b) adding ascorbic acid to the heated water to obtain a first solution; c) adding sodium
thiosulphate to the first solution to obtain a second solution; d) adding sodium citrate to the
second solution to obtain a third solution; e) adding manganese sulphate to the third solution
to obtain a fourth solution; and f) adding zinc sulphate to the fourth solution to provide the
electroplating composition.
The present disclosure relates to a method for depositing the electroplating composition on a
steel substrate, comprising a) providing the steel substrate; and b) depositing the
electroplating composition on the steel substrate at a current density of about 20-90 mA/cm2,
at a stirring rate of about 100-300 rpm, at a temperature of about 20-30ᵒC, and at a pH of about
4.5-5.5 to provide a steel substrate comprising a zinc-manganese (Zn-Mn) coating.
The present disclosure further relates to a steel substrate comprising a Zn-Mn coating, wherein
the Zn-Mn coating comprises about 10-35% by weight of manganese.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the effect of adding ascorbic acid on the stability of the electroplating
composition.
Figure 2 shows the schematic of an exemplary method of preparing the electroplating
composition.

Figure 3 shows the surface morphology of the Zn-Mn coatings deposited at different current
densities.
Figure 4 shows the surface morphology of the Zn-Mn coatings deposited at different current
densities and at different stirring speeds.
Figure 5 shows the surface morphology (panel “a”) and the cross-sectional microstructure
(panel “b”) of the Zn-Mn coating.
DETAILED DESCRIPTION OF THE DISCLOSURE
With respect to the use of substantially any plural and/or singular terms herein, those having
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. 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 “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.
Reference throughout this specification to “some embodiments”, “one embodiment” or “an
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
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 sub-combination.

As used herein, the term “electroplating composition” refers to an electroplating bath
comprising electrolytes (Zn and Mn salts) and additives such as a complexing agent, a
stabilizing agent, and a current efficiency enhancer.
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.
The present disclosure provides an electroplating composition comprising zinc sulphate,
manganese sulphate, sodium citrate as a complexing agent, ascorbic acid as a stabiliser
(reducing agent) and sodium thiosulphate as a current efficiency enhancer. In some
embodiments, the electroplating composition comprises zinc sulphate in an amount of about
30-50 g/L, manganese sulphate in an amount of about 40-70 g/L, sodium citrate in an amount
of about 175-200 g/L, sodium thiosulphate in an amount of about 0.1-0.3 g/L, and ascorbic
acid in an amount of about 0.1-6 g/L. An exemplary electroplating composition is shown in
Table 1 below.
Table 1: Electroplating Composition
Function I Constituent I Concentration (g/L)
"Zn salt Zinc sulphate heptahydrate "30-50
"Mn Salt Manganese sulphate "40-70
monohydrate
Complexing Agent Tri-sodium citrate dihydrate 175-200
Current Efficiency Enhancer Sodium thiosulphate "0.1-0.3
Stabilizing/Reducing Agent Ascorbic acid "0.1-6
In some embodiments, zinc sulphate is present in the electroplating composition in an amount
of about 30-50 g/L, including values and ranges thereof. For example, in some embodiments,
zinc sulphate is present in the electroplating composition in an amount of about 30, 31, 32, 33,
34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 g/L, including values and
ranges thereof. In some embodiments, zinc sulphate is present in the electroplating composition
in an amount of about 35-50, 40-50, or 45-50 g/L, including values and ranges thereof. In some
embodiments, zinc sulphate is present in the electroplating composition in an amount of about
50 g/L. In some embodiments, zinc sulphate is ZnSO4.xH2O where x is 0 to 7. In an exemplary

embodiment, the electroplating composition comprises the heptahydrate form of zinc sulphate
(ZnSO4.7H2O).
In some embodiments, manganese sulphate is present in the electroplating composition in an
amount of about 40-70 g/L, including values and ranges thereof. For example, in some
embodiments, manganese sulphate is present in the electroplating composition in an amount of
about 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,
63, 64, 65, 66, 67, 68, 69, or 70 g/L, including values and ranges thereof. In some embodiments,
manganese sulphate is present in the electroplating composition in an amount of about 40-65,
40-60, 40-50, 40-45, 45-70, 45-60, 45-50, 50-70, 55-70, or 60-70 g/L, including values and
ranges thereof. In some embodiments, manganese sulphate is present in the electroplating
composition in an amount of about 40 g/L. In some embodiments, manganese sulphate is
MnSO4.xH2O where x is 0 to 7. In an exemplary embodiment, the electroplating composition
comprises the monohydrate form of manganese sulphate (MnSO4.H2O).
The concentration of zinc sulphate and manganese sulphate in the electroplating composition
affects the amount of Mn deposited on steel substrates. Lower manganese sulphate
concentration in the composition leads to lower manganese in the deposit. The inventors have
found that a manganese content of about 20-30% by weight of the deposit exhibits lowest
corrosion rate.
In some embodiments, sodium citrate is present in the electroplating composition in an amount
of about 175-200 g/L, including values and ranges thereof. For example, in some embodiments,
sodium citrate is present in the electroplating composition in an amount of about 175, 176, 177,
178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196,
197, 198, 199, or 200 g/L, including values and ranges thereof. In some embodiments, sodium
citrate is present in the electroplating composition in an amount of about 175-195, 175-190,
175-185, 175-180, 180-200, 180-195, 180-190, 180-185, 185-200, 185-195, 185-190, 190-200,
190-195, or 195-200 g/L, including values and ranges thereof. In some embodiments, the
electroplating composition comprises the dihydrate form of sodium citrate. Sodium citrate acts
as a complexing agent for zinc to push Zn deposition potentials to more negative values.
In some embodiments, sodium thiosulphate is present in the electroplating composition in an
amount of about 0.1-0.3 g/L, including values and ranges thereof. For example, in some

embodiments, sodium thiosulphate is present in the electroplating composition in an amount
of about 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.25,
0.26, 0.27, 0.28, 0.29 or 0.3 g/L, including values and ranges thereof. In some embodiments,
sodium thiosulphate is present in the electroplating composition in an amount of about 0.1-
0.25, 0.1-0.2, 0.1-0.15, 0.15-0.3, 0.15-0.25, 0.15-0.2, 0.2-0.3, 0.2-0.25, or 0.25-0.3 g/L,
including values and ranges thereof. Sodium thiosulphate is added to the electroplating
composition as a current efficiency enhancer. In some embodiments, the addition of sodium
thiosulphate provides the current efficiency of about 45-60%, 45-55%, 50-60%, or 55-60% in
the electroplating process (e.g., see Table 4).
In some embodiments, the addition of sodium thiosulphate increases the cathodic current
efficiency of the electroplating process by about 50-90%, 50-80%, 50-75%, 50-70%, 55-90%,
55-85%, 55-80%, 55-75%, 60-90%, 60-85%, 60-80%, 70-90%, 70-85% or 70-80%, including
values and ranges therebetween, compared to the electroplating process where the
electroplating composition does not comprise sodium thiosulphate. In an exemplary
embodiment, if the current efficiency of the electroplating process in the absence of sodium
thiosulphate is about 31%, the addition of sodium thiosulphate increases the current efficiency
to about 47% (when 0.15 g/L of sodium thiosulphate is added) or to about 57% (when 0.3 g/L
of sodium thiosulphate is added). See Table 4. That is an increase of about 57% to 87%.
As sodium thiosulphate increases current efficiency, it also increases the deposition rate. In
some embodiments, sodium thiosulphate increases the deposition rate from about 0.6 µm/min
to about 0.9-1.1 or 0.9-1.05 µm/min (e.g., see Table 4). In some embodiments, the addition of
sodium thiosulphate increases the deposition rate by about 50-75%, 50-70%, 50-60%, 60-75%,
or 60-70%, including values and ranges therebetween, compared to the electroplating process
where the electroplating composition does not comprise sodium thiosulphate. In an exemplary
embodiment, if the deposition rate in the absence of sodium thiosulphate is about 0.6
µm/minute, the addition of sodium thiosulphate increases the deposition rate to about 0.9
µm/minute (when 0.15 g/L of sodium thiosulphate is added) or to about 1.03 µm/minute (when
0.3 g/L of sodium thiosulphate is added). See Table 4. That is an increase of about 50% to 72%.
In some embodiments, ascorbic acid is present in the electroplating composition in an amount
of about 0.1-6 g/L, including values and ranges thereof. For example, in some embodiments,
ascorbic acid is present in the electroplating composition in an amount of about 0.5, 1, 1.5, 2,

2.5, 3, 3.5, 4, 4.5, 5, 5.5, or 6 g/L, including values and ranges thereof. In some embodiments,
ascorbic acid is present in the electroplating composition in an amount of about 0.5-6, 0.5-5,
0.5-4.5, 0.5-4, 0.5-3.5, 0.5-3, 0.5-2.5, 0.5-2, 1-6, 1-5.5, 1-5, 1-4.5, 1-4, 1-3.5, 1-3, 1-2.5, 1-2,
1.5-6, 1.5-5.5, 1.5-5, 1.5-4.5, 1.5-4, 1.5-3.5, 1.5-3, 2-6, 2-5.5, 2-5, 2-4.5, 2-4, 2-3.5, 2-3, 2.5-6,
2.5-5, 2.5-4.5, 2.5-4, 2.5-3.5, 2.5-3, 3-6, 3-5.5, 3-5, 3-4.5, 3-4, 3.5-6, 3.5-5.5, 3.5-5, 3.5-4.5,
3.5-4, 4-6, 4-5.5, 4-5, 4.5-6, 4.5-5.5, 4.5-5, 5-6, or 5.5-6 g/L, including values and ranges
thereof. In some embodiments, ascorbic acid is present in the electroplating composition in an
amount of about 2-3 or 2.5-3 g/L. Ascorbic acid acts as a stabilizing/reducing agent to improve
stability of the electroplating composition by preventing oxidation of Mn(II) to Mn(III) and the
subsequent formation of the Mn(III) precipitate. The stability of the electroplating composition
is inspected visually. For example, as shown in Table 5 and FIG. 1, in the absence of ascorbic
acid, the colour/appearance of the electroplating composition changes in 1 day from clear to
orange; whereas in the presence of ascorbic acid, the colour/appearance of the electroplating
composition remains clear up to 3-7 days. In some embodiments, the addition of ascorbic acid
increases the stability of the electroplating composition by 1-8 days, 1-7 days, 1-6 days, 1-5
days, 1-4 days, 1-3 days, 2-8 days, 2-7 days, 2-6 days, 2-5 days, 2-4 days, 2-3 days, 3-8 days,
3-7 days, 3-6 days, 3-5 days, 4-8 days, 4-7 days, 4-6 days, 5-8 days, or 5-7 days. In some
embodiments, the addition of ascorbic acid increases the stability of the electroplating
composition by 1, 2, 3, 4, 5, 6, 7, or 8 days.
One of ordinary skill in the art would understand that the electroplating composition of the
present disclosure comprises zinc sulphate, manganese sulphate, sodium citrate, sodium
thiosulphate, and ascorbic acid in any of the amounts described herein. That is, any
combination of the individual amounts of zinc sulphate, manganese sulphate, sodium citrate,
sodium thiosulphate, and ascorbic acid disclosed herein is contemplated by the present
disclosure.
In some embodiments, the electroplating composition comprises zinc sulphate in an amount of
about 30-50 g/L, manganese sulphate in an amount of about 40-70 g/L, sodium citrate in an
amount of about 175-200 g/L, sodium thiosulphate in an amount of about 0.15-0.3 g/L, and
ascorbic acid in an amount of about 2-6 g/L.
In some embodiments, the electroplating composition comprises zinc sulphate in an amount of
about 30-50 g/L, manganese sulphate in an amount of about 40-70 g/L, sodium citrate in an

amount of about 175-200 g/L, sodium thiosulphate in an amount of about 0.15-0.3 g/L, and
ascorbic acid is present in an amount of about 2-3 g/L.
In some embodiments, the electroplating composition comprises zinc sulphate in an amount of
about 50 g/L, manganese sulphate in an amount of about 40 g/L, sodium citrate in an amount
of about 175-200 g/L, sodium thiosulphate in an amount of about 0.15-0.3 g/L, and ascorbic
acid in an amount of about 2-3 g/L.
The electroplating compositions of the present disclosure have a pH of about 4-6 such as the
pH of about 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8,
5.9, or 6. In some embodiments, the pH of the electroplating composition is about 4.5-5.5, 4.5-
5, or 5-5.5, including values and ranges thereof.
The present disclosure also provides methods for preparing the electroplating compositions
described herein. The inventors have found that the order of addition of the components while
preparing the electroplating composition affects the stability of the composition. For example,
adding the Zn and Mn salts and the complexing agent to water before addition of the reducing
agent leads to oxidization of Mn(II) ions and causes precipitation. The inventors have observed
that the corrosion rate of the coating deposited from such a composition increases. The
inventors found that when the components are added in a specific order, it provides the
electroplating composition with the highest stability. Accordingly, the method for preparing
the electroplating composition comprises: a) heating water to about 60-70ᵒC to obtain heated
water; b) adding ascorbic acid to the heated water to obtain a first solution; c) adding sodium
thiosulphate to the first solution to obtain a second solution; d) adding sodium citrate to the
second solution to obtain a third solution; e) adding manganese sulphate to the third solution
to obtain a fourth solution; and f) adding zinc sulphate to the fourth solution to provide the
electroplating composition. An exemplary schematic of the process is shown in FIG. 2. The
electroplating composition prepared in this manner is stable over time and provides a coating
that that exhibits a lower corrosion rate.
In some embodiments, the method for preparing the electroplating composition comprises
adjusting the pH of the electroplating composition to about 4-6, 4.5-5.5, 4.5-5, or 5-5.5. In an
exemplary embodiment, the pH of the fourth solution is adjusted to a desired value after

addition of zinc sulphate. In an exemplary embodiment, the pH of the electroplating
composition is adjusted using sulphuric acid and sodium hydroxide.
The present disclosure also provides methods for depositing the electroplating compositions
described herein on a steel substrate to provide steel substrates comprising Zn-Mn coatings. In
some embodiments, the method for depositing the electroplating composition on a steel
substrate comprises a) providing the steel substrate; and b) depositing the electroplating
composition on the steel substrate at a current density of about 20-90 mA/cm2, at a stirring rate
of about 100-300 rpm, at a temperature of about 20-30ᵒC, and at a pH of about 4.5-5.5 to
provide a steel substrate comprising a zinc-manganese (Zn-Mn) coating. Exemplary
parameters for depositing the electroplating composition are shown in Table 2.
Table 2: Exemplary parameters for depositing a Zn-Mn coating
Electroplating parameter pValue
Current density of deposition 20-90 mA/cm2
Stirring rate of deposition 100-300 rpm
Temperature 20-30ᵒC 20
pH of deposition 4.5-5.5
Current density employed during the process of deposition affects the Mn content and the
morphology of the deposit which in turn affects the current efficiency of the deposition process
and the corrosion rate exhibited by the coating. The inventors have observed that Zn-Mn
coatings with a fine and compact globular morphology show better corrosion resistance than
those with plate-like morphology. As the current density employed in the deposition process
influences the morphology of the coating, the current density that provides a fine and compact
globular morphology is employed in the methods of the disclosure.
In some embodiments, the current density employed in the method of deposition ranges from
about 20-90 mA/cm2, 20-85 mA/cm2, 20-80 mA/cm2, 20-75 mA/cm2, 20-70 mA/cm2, 20-65
mA/cm2, 20-60 mA/cm2, 20-50 mA/cm2, 40-90 mA/cm2, 40-85 mA/cm2, 40-80 mA/cm2, 40-
75 mA/cm2, 40-70 mA/cm2, 40-60 mA/cm2, 50-90 mA/cm2, 50-85 mA/cm2, 50-80 mA/cm2,
50-75 mA/cm2, 60-90 mA/cm2, 60-85 mA/cm2, 60-80 mA/cm2, 65-90 mA/cm2, 65-85 mA/cm2,
65-80 mA/cm2, 70-90 mA/cm2, 70-85 mA/cm2, or 70-80 mA/cm2, including values and ranges
thereof. In some embodiments, the current density employed in the method of deposition is

about 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90 mA/cm2. In an exemplary
embodiment, the current density ranges from about 60-80 mA/cm2.
Unlike prior studies on Zn-Mn electrodeposition, the present disclosure has investigated the
effect of stirring the electroplating composition during the process of deposition. The inventors
observed that the stirring rate affects the Mn content, the morphology, and the corrosion rate
of the deposit. In some embodiments, the stirring rate of the electroplating composition in the
method of depositing ranges from about 100-300 rpm, such as about 150-300 rpm, 150-250
rpm, or 200-300 rpm, including values and ranges thereof. In some embodiments, the stirring
rate of the electroplating composition is about 100, 150, 200, 250, or 300 rpm.
In the method for depositing the electroplating composition, the temperature of the
electroplating composition is maintained at about 20-30℃, such as about 20, 21, 22, 23, 24,
25, 26, 27, 28, 29, or 30℃.
In the method for depositing the electroplating composition, the pH of the electroplating
composition is as described herein. For example, the pH of the electroplating composition is
about 4-6, 4.5-5.5, 4.5-5, or 5-5.5.
In some embodiments, the method for depositing the electroplating composition is carried out
for about 5-8 minutes such as for about 5, 6, 7, or 8 minutes. In an exemplary embodiment, the
method is carried out for about 5 minutes.
In some embodiments, the method for depositing the electroplating composition described
herein provides a current efficiency of deposition of about 40-80%, 40-70%, 40-60%, 40-50%,
50-80%, 50-70%, 50-60%, 60-80%, or 60-70%.
In some embodiments, the method for depositing the electroplating composition described
herein provides a deposition rate of about 0.7-1.5, 0.7-1.4, 0.7-1.3, 0.7-1.2, 0.7-1.1, 0.7-1, 0.7-
0.9, 0.8-1.5, 0.8-1.4, 0.8-1.3, 0.8-1.2, 0.8-1.1, 0.8-1, 0.8-0.9, 0.9-1.5, 0.9-1.4, 0.9-1.3, 0.9-1.2,
0.9-1.1, 0.9-1, 1-1.5, 1-1.4, 1-1.3, 1-1.2, 1-1.5, 1-1.4, 1-1.3, 1-1.2, 1.1-1.5, 1.1-1.4, 1.1-1.3, 1.2-
1.5, or 1.2-1.4 µm/minute. In an exemplary embodiment, the deposition rate is about 0.8-1.1
µm/minute.

In some embodiments, the method for depositing the electroplating composition described
herein provides a Zn-Mn coating comprising about 10-35%, 10-30%, 10-25%, 15-30%, 15-
25%, 20-30%, or 25-30% by weight of manganese. In an exemplary embodiment, the method
provides Zn-Mn coatings comprising about 20-30% by weight of manganese.
In some embodiments, the method for depositing the electroplating composition described
herein provides a Zn-Mn coating that exhibits a corrosion rate of about 0.008-0.05 mm/year.
In some embodiments, the Zn-Mn coatings provided by the method exhibits a corrosion rate of
about 0.008-0.04, 0.008-0.03, 0.008-0.02, 0.008-0.015, 0.008-0.01, 0.009-0.04, 0.009-0.03,
0.009-0.02, 0.009-0.015, 0.01-0.02, or 0.01-0.015 mm/year.
In an exemplary embodiment, a set up for depositing the electroplating composition on a steel
substrate comprises placing the steel substrate as a cathode and a pure zinc plate as an anode
in the electroplating composition, stirring the electroplating composition, and passing an
electric current between these two electrodes.
The present disclosure also provides steel substrates comprising Zn-Mn coatings, wherein the
Zn-Mn coatings exhibit desired Mn content; a globular microstructure; and better corrosion
resistance. In some embodiments, provided herein are steel substrates comprising Zn-Mn
coatings, wherein the coating has the Mn content of about 10-35%, 10-30%, 10-25%, 15-30%,
15-25%, 20-30%, or 25-30% by weight. In an exemplary embodiment, the steel substrate
provided by the present disclosure comprises a Zn-Mn coating that has the Mn content of about
20-30% by weight.
In some embodiments, provided herein are steel substrates comprising Zn-Mn coatings,
wherein the coating exhibits a coarse or fine globular morphology. In some embodiments, the
Zn coatings exhibit uniform, fine, and compact globular morphology.
In some embodiments, provided herein are steel substrates comprising Zn-Mn coatings,
wherein the coating exhibits a corrosion rate of about 0.008-0.05 mm/year. In some
embodiments, the Zn-Mn coatings on the steel substrate exhibits a corrosion rate of about
0.008-0.04, 0.008-0.03, 0.008-0.02, 0.008-0.015, 0.008-0.01, 0.009-0.04, 0.009-0.03, 0.009-
0.02, 0.009-0.015, 0.01-0.02, or 0.01-0.015 mm/year.

In some embodiments, provided herein are steel substrates comprising Zn-Mn coatings,
wherein the coating exhibits a corrosion potential of about -1.0 to -1.15 V, -1.0 to -1.14 V, -1.0
to -1.13 V, -1.0 to -1.12 V, -1.0 to -1.11 V, or -1.0 to -1.10 V.
The electroplating compositions, methods of producing them, methods of depositing them on
a steel substrate and steel substrates comprising Zn-Mn coatings disclosed herein provide many
advantages. First, while there have been many attempts to develop Zn-Mn coatings in the art,
the present disclosure provides electroplating compositions, which when produced in the
manner described herein and deposited on a steel substrate in the manner described herein,
provide coatings that show uniform, fine and compact morphology and a thick, continuous
layer on the steel substrate. This fine and compact morphology and a continuous, thick layer
of coating in turn provides excellent corrosion properties. Zn-Mn coatings developed in
previous studies may have higher Mn content such as up to 90%; however, these coatings do
not show desired corrosion resistance. The corrosion resistance is dependent on two things, the
right Mn content and a fine and compact microstructure. The present disclosure shows that the
coating with a Mn content of about 20-30 wt% and fine and compact microstructure provides
a high corrosion resistance that has not been achieved before. The Zn-Mn coating provided by
the compositions and methods of the present disclosure shows better corrosion resistance than
the commercially used Zn-Ni coating (Table 8). As nickel is much more costly than Mn, the
Zn-Mn coating of the present disclosure are cost-effective and at the same time provide
superior corrosion properties than the commercially used Zn-Ni coatings.
It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and
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.
Similarly, additional embodiments and features of the present disclosure will be apparent to
one of ordinary skill in art based upon description provided herein.
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 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.
References
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[3] G. Barceló, M. Sarret, C. Müller, J. Pregonas, Corrosion resistance and mechanical
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[16] D Thippeswamy, Y Arthoba Nayaka, Electrochemical Studies of Zn-Mn Alloy Plating
from Acid Sulphate Bath using Condensation Product 4-Chloro-2-Nitro-N-Phenyl
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acid electrolyte for deposition.
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EXAMPLES
Example 1: Effects of varying concentrations of zinc sulphate and manganese sulphate
Electroplating compositions comprising sodium citrate (180 g/L), sodium thiosulphate (0.3
g/L), ascorbic acid (2 g/L) and varying amounts of zinc sulphate and manganese sulphate were
prepared. The amount of zinc sulphate and manganese sulphate in the electroplating
composition was varied as shown in Table 3 to test the effect of their concentration on the
deposited coating. The compositions comprising zinc sulphate and manganese sulphate were
deposited on steel substrates and the Mn weight % and corrosion rate of the coating deposited
on the steel substrates were determined. The results are summarized in Table 3. Lower
manganese sulphate concentration in the composition led to lower manganese in the deposit.

Manganese content up to 77% was obtained by adjusting the zinc and manganese salt content
in the composition. A manganese content of about 25 wt. % in the deposited coating provided
the lowest corrosion rate of the coating.
Table 3: Deposit properties at different ZnSO4 and MnSO4 concentrations in the
composition
Zinc I Manganese I Manganese I Corrosion
sulphate sulphate wt. % in rate
(g/L) (g/L) coating (mm/yr)
30 70 7 0.028
40 60 36 0.026
50 40 26 0.015
Example 2: Effect of sodium thiosulphate on the current efficiency of the deposition
The effect of sodium thiosulphate on the current efficiency of the deposition was studied. For
this, electroplating compositions comprising zinc sulphate (50 g/L), manganese sulphate (40
g/L), sodium citrate (180 g/L), ascorbic acid (2 g/L) and varying amounts of sodium
thiosulphate were prepared. The results are summarized in Table 4. As seen in Table 4, the
addition of sodium thiosulphate increased the current efficiency of the deposit from about 30%
to about 47% (with 0.15 g/L sodium thiosulphate) or to about 56% (with 0.3 g/L sodium
thiosulphate). This also led to an increase in the deposition rate due to the suppression of side
reactions such as hydrogen evolution.
Table 4: Current efficiency of deposition with addition of sodium thiosulphate
Sodium thiosulphate I Current I Deposition rate
concentration (g/L) efficiency of (µm/min)
deposition (%)
0 31.6 06
015 47.3 09
03 56.4 1. 03
Example 3: Effect of addition of ascorbic acid on the stability of the electroplating
composition
Electroplating compositions comprising zinc sulphate (50 g/L), manganese sulphate (40 g/L),
sodium citrate (180 g/L), sodium thiosulphate (0.3 g/L) and no ascorbic acid or 2, 4, or 6 g/L
of ascorbic acid were prepared. The stability of these compositions was inspected visually. The

results are shown in Table 5 and FIG. 1. The compositions as prepared were clear (Day 0).
With no ascorbic acid in the composition, the colour of the composition changed to orange in
one day indicating oxidation of Mn(II). This colour became deeper with time and on day 7
white precipitates were observed in the composition. The addition of ascorbic acid delayed this
colour change and improved the bath stability. With addition of 2 g/L ascorbic acid, the first
colour change was observed on day 7; i.e., the stability of the composition increased by 6 days.
Table 5: Stability of the electroplating composition with ascorbic acid addition
Composition I Ascorbic acid I Colour of the composition
number concentration
(g/L) Day 0 I Day 1 I Day 3 |Day 7
S1 0 Clear Orange Deep Deep red with white
red precipitate
S2 2 Clear Clear Clear Light yellow
S3 4 Clear Clear Light Deep yellow with precipitate
yellow
S4 6 Clear Light Deep Deep yellow with precipitate
yellow | yellow
Example 4: Effects of current density on the properties of Zn-Mn coatings
An electroplating composition comprising zinc sulphate (50 g/L), manganese sulphate (40
g/L), sodium citrate (180 g/L), sodium thiosulphate (0.3 g/L), and ascorbic acid (2 g/L) was
prepared. The effects of varying the current density were tested by depositing the electroplating
composition using a steel substrate as a cathode and a pure zinc plate as an anode at a current
density of 20 mA/cm2 to 80 mA/cm2. The Zn-Mn coating deposited at each current density was
analysed for the Mn content, top surface morphology, current efficiency, deposition rate, and
corrosion rate. The results are summarized in Table 6 and FIG. 3.
A higher current density produced Zn-Mn coatings with higher Mn content. However, these
high Mn coatings showed lower current efficiency and lower deposition rate. The
microstructures of the coatings shown in FIG. 3 indicate that at low current densities, fine plate
like morphology was obtained whereas at higher current densities, a globular morphology was
obtained. The corrosion rate was also affected by the current density due to the change in the
Mn content of the coating as well as the change in morphology. Coatings with a fine globular
morphology showed better corrosion resistance. Plate or needle like morphologies obtained at
low current densities were not very effective in corrosion resistance. The corrosion rate of the

coating was the least when the Mn content is about 25% and current density of deposition is
60 mA/cm2.
Table 6: Coating properties at different current densities of deposition (without stirring
of the electroplating composition)
Current I Mn wt. I Top surface I Current I Deposition I Corrosion
Density % in morphology of the efficiency of rate rate
(mA/cm2) coating coating deposition (µm/min) (mm/yr)
(%)
20 0 Fine non-uniform plate 79.5 1.36 0.038
like grains
40 7 Fine uniform plate like 57.3 1. 01 0.042
grains
60 26 Globular grains 473 09 0.015
80 31.3 Coarse globular grains 42 08 0.045
Example 5: Effect of stirring on the properties of Zn-Mn coatings
An electroplating composition comprising zinc sulphate (50 g/L), manganese sulphate (40
g/L), sodium citrate (180 g/L), sodium thiosulphate (0.3 g/L), and ascorbic acid (2 g/L) was
prepared. The effect of stirring the electroplating composition during the deposition process
was studied. For this, the electroplating composition was deposited at a current density of 60
mA/cm2 or 80 mA/cm2 in the absence of stirring or at a stirring rate of 200, 250, or 300 rpm.
The Zn-Mn coatings deposited at each current density and different stirring rates were analysed
for the Mn content, top surface morphology, and corrosion rate. The results are summarized in
Table 7 and FIG. 4.
Table 7: Deposit properties at different stirring rates during deposition
Current I Stirring rate I Mn wt. % in I Top surface I Corrosion
density (rpm) coatings morphology rate
(mA/cm2) (mm/yr)
60 0 26 Coarse 0.015
globular
200 23 Fine plates 0.022
250 11 Fine plates 0.017
300 6 Fine plates 0.107
80 0 31 Coarse 0.045
globular
200 28 Coarse 0.011
globular

250 26 Fine globular 0.009
300 "25 Fine globular 0.041
At a current density of 60 mA/cm2, higher stirring produced deposits with lower Mn content,
and the corrosion rate of these deposits was much higher than with no stirring. At a current
density of 80 mA/cm2, the Mn content is not reduced much with stirring, and remains in the
range of 20-30 wt.% which provides a low corrosion rate. The stirring rate also affected the
morphology of the deposit. As seen in FIG. 4, at low stirring rates, the morphology is a coarse
globular morphology whereas at higher stirring rate, the grains became finer. For 60 mA/cm2
current density, the grains lost their globular nature with increasing stirring rates whereas at
higher current density (80 mA/cm2), the grains became finer along with retaining their globular
morphology. This fine globular morphology was responsible for very low corrosion rates. The
lowest corrosion rate was obtained at 250 rpm stirring rate and a current density of 80 mA/cm2.
Example 6: Surface morphology and microstructure of Zn-Mn coatings
An electroplating composition comprising zinc sulphate (50 g/L), manganese sulphate (40
g/L), sodium citrate (180 g/L), sodium thiosulphate (0.3 g/L) and ascorbic acid 2 (g/L) was
deposited on a steel substrate at a current density of 80mA/cm2 and stirring rate of 250 rpm.
The surface morphology and the cross-sectional microstructure of the Zn-Mn coating deposited
on the steel substrate were studied. FIG. 5 shows the top surface and cross-sectional
microstructures of the developed Zn-Mn coating. The top surface microstructure shows that
the Zn-Mn coating is very compact and contains fine globular grains which provide excellent
corrosion resistance. The cross-sectional microstructure shows a continuous thick layer of
coating on the steel substrate.
Example 7: Comparison with commercially available coating
The current commercially available products for corrosion resistance are electrodeposited Zn-
Ni alloys on steel with Ni contents in the range of 12-15%. The Zn-Mn coating obtained in

Example 6 was compared with the commercial Zn-Ni coating. The results are summarized in
Table 8.
Table 8: Comparison of final properties of developed Zn-Mn coating with commercially
available Zn-Ni coating
Property I Developed coating I Commercially available coating
(Zn-Mn) (Zn-Ni)
Corrosion potential vs SCE (V) -1.06 -082
Corrosion rate (mm/yr) 0.009 0.03
Deposition rate (µm/min) 1.08 1
The corrosion potential for the Zn-Mn coating of the present disclosure is more negative than
that of commercially available Zn-Ni coating, implying a higher sacrificial corrosion
resistance. The corrosion rate is also more than three times lower than that of the commercially
available coating. Also, the deposition rate of the present coating is similar to, or albeit higher
than, that of the commercial coating. Thus, the developed Zn-Mn coating shows a higher
sacrificial protection to steel along with a much lower corrosion rate than the commercially
available coating and shows a similar deposition rate as the commercial coating.

We Claim:
1. An electroplating composition comprising zinc sulphate, manganese sulphate, sodium
citrate, sodium thiosulphate, and ascorbic acid.
2. The electroplating composition as claimed in claim 1, wherein the zinc sulphate is present
in an amount of about 30-50 g/L, the manganese sulphate is present in an amount of about
40-70 g/L, the sodium citrate is present in an amount of about 175-200 g/L, the sodium
thiosulphate is present in an amount of about 0.1-0.3 g/L, and the ascorbic acid is present
in an amount of about 0.1-6 g/L.
3. The electroplating composition as claimed in claim 1, wherein the zinc sulphate is present
in an amount of about 30-50 g/L, the manganese sulphate is present in an amount of about
40-70 g/L, the sodium citrate is present in an amount of about 175-200 g/L, the sodium
thiosulphate is present in an amount of about 0.15-0.3 g/L, and the ascorbic acid is present
in an amount of about 2-6 g/L.
4. The electroplating composition as claimed in claim 1, wherein the zinc sulphate is present
in an amount of about 30-50 g/L, the manganese sulphate is present in an amount of about
40-70 g/L, the sodium citrate is present in an amount of about 175-200 g/L, the sodium
thiosulphate is present in an amount of about 0.15-0.3 g/L, and the ascorbic acid is present
in an amount of about 2-3 g/L.
5. The electroplating composition as claimed in claim 1, wherein the zinc sulphate is present
in an amount of about 50 g/L, the manganese sulphate is present in an amount of about 40
g/L, the sodium citrate is present in an amount of about 175-200 g/L, the sodium
thiosulphate is present in an amount of about 0.15-0.3 g/L, and the ascorbic acid is present
in an amount of about 2-3 g/L.
6. The electroplating composition as claimed in any one of claims 1-5, wherein the pH of the
composition is about 4-6.
7. A method for preparing the electroplating composition as claimed in any one of claims 1-
5, comprising:
a. heating water to about 60-70℃ to obtain a heated water;
b. adding ascorbic acid to the heated water to obtain a first solution;
c. adding sodium thiosulphate to the first solution to obtain a second solution;
d. adding sodium citrate to the second solution to obtain a third solution;
e. adding manganese sulphate to the third solution to obtain a fourth solution; and

f. adding zinc sulphate to the fourth solution to provide the electroplating
composition.
8. A method for depositing the electroplating composition as claimed in any one of claims 1-
6 on a steel substrate, comprising:
a. providing the steel substrate;
b. depositing the electroplating composition on the steel substrate at a current
density of about 20-90 mA/cm2, at a stirring rate of about 100-300 rpm, at a
temperature of about 20-30℃, and at a pH of about 4.5-5.5to provide a steel
substrate comprising a zinc-manganese (Zn-Mn) coating.
9. The method as claimed in claim 8, wherein the current density is about 60-80 mA/cm2 and
the stirring rate is about 200-300 rpm.
10. The method as claimed in claim 8 or 9, wherein the current density is 60 mA/cm2 or 80
mA/cm2 and the stirring rate is about 200, 250, or 300 rpm.
11. The method as claimed in any one of claims 8-10, wherein the method provides a current
efficiency of deposition of about 40-80%.
12. The method as claimed in claim 11, wherein the method provides a current efficiency of
deposition of about 40-60%.
13. The method as claimed in any one of claims 8-12, wherein the method provides a deposition
rate of about 0.7-1.5 µm/minute.
14. The method as claimed in claim 13, wherein the method provides a deposition rate of about
0.8-1.1 µm/minute.
15. The method as claimed in any one of claims 8-14, wherein the Zn-Mn coating provided by
the method comprises about 10-35% by weight of manganese.
16. The method as claimed in claim 15, wherein the Zn-Mn coating provided by the method
comprises about 20-30% by weight of manganese.
17. The method as claimed in any one of claims 8-16, wherein the Zn-Mn coating provided by
the method exhibits a corrosion rate of about 0.008-0.05 mm/year.
18. The method as claimed in claim 17, wherein the Zn-Mn coating provided by the method
exhibits a corrosion rate of about 0.008-0.02 mm/year.

19. The method as claimed in any one of claims 8-18, wherein the electroplating composition
is deposited at a constant current with the steel substrate as a cathode.
20. A steel substrate comprising a zinc-manganese (Zn-Mn) coating, wherein the Zn-Mn
coating comprises about 10-35% by weight of manganese.
21. The steel substrate as claimed in claim 20, wherein the Zn-Mn coating comprises about 20-
30% by weight of manganese.
22. The steel substrate as claimed in claim 20 or 21, wherein the Zn-Mn coating has a coarse
or fine globular morphology.
23. The steel substrate as claimed in any one of claims 20-22, wherein the Zn-Mn coating
exhibits a corrosion rate of about 0.008-0.05 mm/year.
24. The steel substrate as claimed in any one of claims 20-23, wherein the Zn-Mn coating
exhibits a corrosion rate of about 0.008-0.02 mm/year.
25. The steel substrate as claimed in any one of claims 20-24, wherein the Zn-Mn coating
exhibits a corrosion potential of about -1.0 to -1.15 V.