TECHNICAL FIELD
The present disclosure relates to the field of electroplating. Particularly, the present disclosure relates to methods of depositing zinc-manganese (Zn-Mn) coatings on steel substrates and steel substrates obtained therefrom.

BACKGROUND OF THE DISCLOSURE
Zinc (Zn) coatings on steel are well known for their excellent sacrificial corrosion properties. Galvanised or zinc coated steels are extremely popular in the market even today. The most commonly used process for zinc coatings on steel is hot-dip galvanising, in which steel is dipped into a molten zinc bath and then cooled to form a zinc coating. However, zinc electroplating commonly known as electrogalvanising is an emerging competitor to the conventional hot dipping due to its numerous advantages such as better coating thickness control and ability to produce compact, defect free coatings, which enables better corrosion properties at lower coating thickness. Zinc alloy coatings, especially with nobler metals such as nickel can also be used to further enhance the corrosion properties of the coating. Electrodeposited zinc-nickel alloy coatings are very popular as nickel being a noble metal is known to slow down the corrosion process and also stabilise the zinc corrosion products that provide passivation to the otherwise highly active zinc [1]. Commercial Zn-Ni coatings with about 12 wt. % Ni are known to provide higher corrosion resistance to steel than pure zinc coatings at much lower coating thicknesses [2]. However, since nickel is more noble than zinc, Zn-Ni alloy coatings come at the cost of lower sacrificial corrosion properties than pure zinc. Moreover, nickel is also a costly material, and therefore commercial Zn-Ni coatings have an added disadvantage of a high cost. There have been several attempts to replace nickel with cheaper metals, however Zn-Ni coatings continue to remain the most popular in the market.

The commercial electrogalvanising process is currently limited to the use of direct current (DC) electrodeposition. This DC deposition process comes with its own limitations, which include low limiting current densities and non-uniform deposits, especially in the case of alloy deposition [3]. There have been major attempts to modify the conventional DC electrodeposition process to achieve better coating properties at lower coating thickness. Pulsed current electrodeposition has attracted a lot of attention over the years as it is known to produce highly compact, fine-grained uniform coatings that offer improved corrosion and mechanical properties [4]. During pulsed current deposition, due to the repeated switch on and switch off of current, metal ions are replenished near the cathode surface during the off time. This makes it possible to go to higher deposition current densities without depletion of metal ions near the cathode, and hence avoiding burnt deposits. It is also possible to obtain a greater range of alloy compositions by careful optimisation of the pulse parameters, which cannot be imagined using conventional DC deposition techniques [4].

There has been significant work done on Zn and Zn-Ni electrodeposition with the use of pulsed current deposition [5-7]. Pulsed current deposition has proved to be extremely beneficial for Zn and Zn-Ni electrodeposition, due to the possibility of operation at very high peak current densities without forming burnt or powdery deposits. Moreover, high peak current densities provide much smoother, finer and homogeneous deposits which is ideally achieved by adding organic additives such as grain refiners, levellers and brighteners [4, 8]. Hence, pulsed deposition can be used to replace these organic additives and complexants, many of which are toxic in nature. However, the high cost of Ni along with the added complexity of pulsed current deposition makes it impractical to be commercialised on a large scale. The present inventors hypothesized that a cheaper alloying element along with the use of pulsed current deposition needs to be explored that could provide high corrosion resistance at lower coating thicknesses and at lower costs.

Manganese (Mn) can be a great alternative to nickel as an alloying element for zinc, as it is much cheaper and also much less noble than nickel, or even less noble than zinc. This means that it has the potential to provide a higher sacrificial corrosion resistance to steel than Zn-Ni or even pure Zn coatings. Moreover, electrodeposited Zn-Mn alloy coatings are known to have exceptional corrosion resistance as the mechanism of Zn-Mn corrosion is such that it enables the formation a dense protective corrosion product consisting of zinc hydroxy-chloride (ZHC) and ?-Mn2O3 that inhibits further corrosion from taking place [9].

A direct current (DC) deposition of Zn-Mn has been widely explored and Zn-Mn coatings with varied Mn content, phase compositions and microstructures have been reported [10-16]. However, the standard reduction potentials Zn and Mn are significantly different, (-0.76 V/SHE for Zn and -1.18 V/SHE for Mn) which makes it difficult for Mn to get co-deposited along with Zn. Most of the reported electrolytic baths for Zn-Mn deposition are acidic in nature which contain complexing agents like citrates, gluconates, tartarates, pyrophosphates and fluoroborates to push Zn deposition potentials to more negative values, and obtain high Mn deposits [10]. The acidic sulphate-citrate baths have proved to be the most popular due to their environment friendliness and ability to produce smooth, fine-grained and high Mn containing deposits [14, 16]. However, Zn-Mn deposition from acidic sulphate-citrate baths has its own drawbacks such as low current efficiency due to extensive hydrogen evolution catalysed by the Mn deposition [17]. This results in low deposition rates and formation of rough and porous coatings at high current densities [18-21]. Organic additives such as thiosulphates have been reported to suppress hydrogen evolution and increase current efficiency [16]. However, even with the use of organic additives, the current efficiency and deposition rate are limited for sulphate citrate baths using DC deposition.

Electrodeposited Zn-Mn alloy coatings from sulphate citrate baths have the potential to provide excellent corrosion resistance of steel. However, the DC deposition method is associated with the problems such as low current efficiency and deposition rates. A few studies have studied pulsed deposition of Zn-Mn using acidic sulphate-citrate baths. Muller et al. [22] reported high Mn contents and fine-grained microstructures using pulsed current deposition but the current efficiency obtained was not greater than 50 %. Danilov et al. [23] reported high Mn contents with DC superimposed pulsed current deposition. However, enhancement of corrosion properties along with high Mn contents and high current efficiencies is not yet reported.

Thus, there is a need in the art to develop Zn-Mn alloy coatings on steel substrates from sulphate-citrate based electrolytic baths using pulsed current electrodeposition that show a high current efficiency, deposition rate, fine grained microstructure with excellent corrosion resistance. The present disclosure attempts to address said need.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to a method for depositing a zinc-manganese (Zn-Mn) coating on a steel substrate, comprising: a) providing the steel substrate as a cathode; and b) depositing an electroplating composition comprising about 60-80 g/L zinc sulphate, about 25-35 g/L manganese sulphate, about 170-190 g/L sodium citrate, about 0.1-0.2 g/L sodium thiosulphate, and about 1.5-2.5 g/L ascorbic acid on the steel substrate by employing a pulsed current with an average current density of about 75-85 mA/cm2, a pulse duty cycle of about 25%-75% and a pulse frequency of about 25-250 Hz to provide a steel substrate comprising the Zn-Mn coating. The pulsed deposition method shows an improved current efficiency of deposition, an improved deposition rates and provides coatings with a refined microstructure and lower or similar corrosion rates compared to the DC deposition method.

The present disclosure also relates to a steel substrate comprising a Zn-Mn coating obtained by the pulsed current method described herein. In some embodiments, the Zn-Mn coating comprises about 15-40% by weight of manganese.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the microstructural refinement due to pulsed current deposition (sample P1) compared to a DC deposition of a Zn-Mn coating.

Figure 2 shows an increase in the Mn content in the pulsed current deposited coating (sample P10) compared to a DC deposition of a Zn-Mn coating.

Figure 3 shows the microstructural refinement in pulsed current deposition (sample P3) compared to a DC deposition of a Zn-Mn coating.

Figure 4 shows an increase in the deposition current efficiency and deposition rate (due to an increase in average coating thickness) in the pulsed current deposition (sample P3) compared to a DC deposition of a Zn-Mn coating.

Figure 5 shows the microstructural refinement and a decrease in the corrosion rate in the pulsed current deposited coating (sample P9) compared to a DC deposition of a Zn-Mn coating.

Figure 6 shows an increase in the deposition current efficiency and deposition rate (due to increase in average coating thickness) in pulsed current deposition (sample P9) compared to a DC deposition of a 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 bath stabiliser 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 a pulsed current method of depositing a Zn-Mn coating on a steel substrate where the coating exhibits a very fine-grained microstructure, a high Mn content in the coating, and excellent corrosion resistance and the method provides high deposition rates and current efficiencies. The deposition rates and current efficiency of deposition provided by the present method are superior compared to the pulsed deposition methods reported in the art. For example, in some embodiments, a current efficiency as high as about 60-80% or about 75-80% is provided by the present method and at the same time, the Mn content of the deposit is high or similar to that obtained by the DC-deposition method, the deposit exhibits a fine and uniform microstructure and excellent corrosion resistance.

In some embodiments, the method for depositing a Zn-Mn coating on a steel substrate comprises: a) providing the steel substrate as a cathode; b) depositing an electroplating composition comprising about 60-80 g/L zinc sulphate, about 25-35 g/L manganese sulphate, about 170-190 g/L sodium citrate, about 0.1-0.2 g/L sodium thiosulphate, and about 1.5-2.5 g/L ascorbic acid on the steel substrate by employing a pulsed current with an average current density of about 75-85 mA/cm2, a pulse duty cycle of about 25%-75% and a pulse frequency of about 25-250 Hz to provide a steel substrate comprising the Zn-Mn coating.
Exemplary parameters for depositing the electroplating composition are shown in Table 1.
Table 1: Exemplary parameters for depositing a Zn-Mn coating by pulsed current
Pulsed current parameter Value
Average current density of deposition 75-85 mA/cm2
Pulse duty cycle 25%-75%
Pulse frequency 25-250 Hz

In some embodiments, the average current density of the pulsed current employed in the method of deposition ranges from about 75-85 mA/cm2, including values and ranges thereof, such as about 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, or 85 mA/cm2. In some embodiments, the average current density of the pulsed current is about 80 mA/cm2.

The duty cycle of the pulsed current varies from about 25-75%, such as about 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or 75%. In an exemplary embodiment, the duty cycle is about 25%, 50%, or 75%.

In some embodiments, the frequency of the pulsed current is about 25-250 Hz, such as about 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, or 250 Hz. In some embodiments, the frequency is about 25, 75, 150, 200, or 250 Hz.

In some embodiments, the pulsed current has a duty cycle of about 25% and the frequency of about 25, 75, or 150 Hz. In some embodiments, the pulsed current has a duty cycle of about 50% and the frequency of about 25, 75, or 150 Hz. In some embodiments, the pulsed current has a duty cycle of about 75% and the frequency of about 25, 75, 150, 200, or 250 Hz.

In some embodiments, the pulsed current has a duty cycle of about 50% and the frequency of about 25Hz. In some embodiments, the pulsed current has a duty cycle of about 75% and the frequency of about 200 Hz. In some embodiments, the pulsed current has a duty cycle of about 25% and the frequency of about 150 Hz. In some embodiments, the pulsed current has a duty cycle of about 75% and the frequency of about 150 Hz.

In some embodiments, the electroplating composition is stirred during the process of deposition. In some embodiments, the stirring rate ranges from about 200-300 rpm or about 225-275 rpm, including values and ranges thereof. In some embodiments, the stirring rate of the electroplating composition is about 200, 225, 250, 275 or 300 rpm.

In some embodiments, the pulsed deposition is carried out at a temperature of about 285-310?, about 290-305?, or about 293-303?, including values and ranges thereof, such as at about 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, or 310?.

In some embodiments, the pulsed 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 pulsed method is carried out for about 5 minutes.

In some embodiments, the pulsed method for depositing the Zn-Mn coating provides a current efficiency of deposition of about 35-80%, 35-75%, 35-70%, 35-65%, 35-60%, 40-80%, 40-75%, 40-70%, 40-65%, 40-60%, 50-80%, 50-75%, 50-70%, 50-65%, 50-60%, 60-80%, 60-75%, 60-70%, or 70-80%, including values and ranges thereof. In some embodiments, the method provides a current efficiency of deposition of about 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or about 80%.

In some embodiments, the pulsed method for depositing the Zn-Mn coating provides a deposition rate of about 0.8-1.7, 0.8-1.6, 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, 1-1.7, 1-1.6, 1-1.5, 1-1.4, 1-1.3, 1-1.2, 1-1.1, 1.1-1.7, 1.1-1.6, 1.1-1.5, 1.1-1.4, 1.1-1.3, 1.1-1.2, 1.2-1.7, 1.2-1.6, 1.2-1.5, 1.2-1.4, 1.3-1.7, 1.3-1.6, 1.3-1.5, 1.4-1.7, 1.4-1.6, or 1.5-1.7 µm/minute, including values and ranges thereof. In some embodiments, the deposition rate is about 1.3-1.7 µm/minute. In some embodiments, the deposition rate is about 1.37 µm/minute. In some embodiments, the deposition rate is about 1.67 µm/minute.

In some embodiments, the present method for depositing the Zn-Mn coating provides a coating comprising about 15-40%, 15-35%, 15-30%, 20-40%, 20-35%, 20-30%, 25-40%, 25-35%, or 30-40% by weight of manganese, including values and ranges thereof. In an exemplary embodiment, the method provides Zn-Mn coatings comprising about 25-40% by weight of manganese.

In some embodiments, the method for depositing the Zn-Mn coating provides a coating that exhibits a corrosion rate of about 0.008-0.1 mm/year. In some embodiments, the Zn-Mn coatings provided by the method exhibits a corrosion rate of about 0.008-0.08, 0.008-0.06, 0.008-0.05, 0.008-0.02, 0.008-0.01, 0.01-0.1, 0.01-0.08, 0.01-0.05, 0.02-0.1, 0.02-0.08, 0.02-0.05, 0.04-0.1, 0.04-0.08, 0.05-0.1, or 0.05-0.08 mm/year, including values and ranges thereof. In some embodiments, the Zn-Mn coatings provided by the method exhibits a corrosion rate of about 0.008, 0.009, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1 mm/year.

In some embodiments, the method for depositing the Zn-Mn coating provides a coating that exhibits a corrosion potential of about -1.02 to -1.10 V, -1.02 to -1.09 V, -1.02 to -1.08 V, -1.02 to -1.07 V, -1.02 to -1.05 V, -1.04 to -1.10 V, -1.04 to -1.08 V, -1.05 to -1.10 V, -1.05 to -1.09 V, -1.05 to -1.08 V, -1.06 to -1.10 V, -1.06 to -1.09 V, or -1.06 to -1.08 V, including values and ranges thereof. In some embodiments, the method for depositing the Zn-Mn coating provides a coating that exhibits a corrosion potential of about -1.05 to -1.10 V, including values and ranges thereof.

The electroplating composition employed in the pulsed deposition method of the present disclosure comprises 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 60-80 g/L, manganese sulphate in an amount of about 25-35 g/L, sodium citrate in an amount of about 170-190 g/L, sodium thiosulphate in an amount of about 0.25-0.35 g/L, and ascorbic acid in an amount of about 1.5-2.5 g/L.
In some embodiments, zinc sulphate is present in the electroplating composition in an amount of about 60-80 g/L or 65-75 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 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or 80 g/L, including values and ranges thereof. In some embodiments, zinc sulphate is present in the electroplating composition in an amount of about 70 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 25-35 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 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, or 35 g/L, including values and ranges thereof. In some embodiments, manganese sulphate is present in the electroplating composition in an amount of about 30 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).

In some embodiments, sodium citrate is present in the electroplating composition in an amount of about 170-190 g/L or about 175-185 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 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, or 190 g/L, including values and ranges thereof. In some embodiments, sodium citrate is present in the electroplating composition in an amount of about 180 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 0.1-0.2 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 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or 0.2 g/L, including values and ranges thereof. In some embodiments, sodium thiosulphate is present in the electroplating composition in an amount of about 0.15 g/L, including values and ranges thereof. Sodium thiosulphate is added to the electroplating composition as a current efficiency enhancer.

In some embodiments, ascorbic acid is present in the electroplating composition in an amount of about 1.5-2.5 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 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, or 2.5, g/L, including values and ranges thereof. In some embodiments, ascorbic acid is present in the electroplating composition in an amount of about 2 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.

In one embodiment, the electroplating composition comprises about 70 g/L zinc sulphate, about 30 g/L manganese sulphate, about 180 g/L sodium citrate, about 0.15 g/L sodium thiosulphate, and about 2 g/L ascorbic acid.

One of ordinary skill in the art would understand that the electroplating compositions employed in the pulsed deposition method 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 pH of the electroplating composition is about 4.5-5.5, 4.5-5, or 5-5.5.

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 a pulsed electric current between these two electrodes.

The present disclosure also provides steel substrates comprising Zn-Mn coatings, wherein the Zn-Mn coatings exhibit a desired Mn content, fine-grained microstructure, a high current efficiency of deposition and a corrosion rate that is similar to Zn-Mn coatings deposited by a direct current method.

In some embodiments, provided herein are steel substrates comprising Zn-Mn coatings, wherein the coating has a Mn content of about 15-40%, 15-35%, 15-30%, 20-40%, 20-35%, 20-30%, 25-40%, 25-35%, or 30-40% by weight, including values and ranges thereof. In an exemplary embodiment, the steel substrate provided by the present disclosure comprises a Zn-Mn coating that has a Mn content of about 25-40% by weight.

In some embodiments, the Zn-Mn coatings on the steel substrate exhibit a corrosion rate of about 0.008-0.1 mm/year, including values and ranges thereof. In some embodiments, the Zn-Mn coatings on the steel substrate exhibit a corrosion rate of about 0.008-0.08, 0.008-0.06, 0.008-0.05, 0.008-0.02, 0.008-0.01, 0.01-0.1, 0.01-0.08, 0.01-0.05, 0.02-0.1, 0.02-0.08, 0.02-0.05, 0.04-0.1, 0.04-0.08, 0.05-0.1, or 0.05-0.08 mm/year, including values and ranges thereof. In some embodiments, the Zn-Mn coatings on the steel substrate exhibits a corrosion rate of about 0.008, 0.009, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1 mm/year.

In some embodiments, the Zn-Mn coatings on the steel substrate exhibit a corrosion potential of about -1.02 to -1.10 V, -1.02 to -1.09 V, -1.02 to -1.08 V, -1.02 to -1.07 V, -1.02 to -1.05 V, -1.04 to -1.10 V, -1.04 to -1.08 V, -1.05 to -1.10 V, -1.05 to -1.09 V, -1.05 to -1.08 V, -1.06 to -1.10 V, -1.06 to -1.09 V, or -1.06 to -1.08 V, including values and ranges thereof. In some embodiments, the Zn-Mn coatings on the steel substrate exhibit a corrosion potential of about -1.05 to -1.10 V, including values and ranges thereof.

In some embodiments, the Zn-Mn coatings on the steel substrate exhibit a fine-grained microstructure. In some embodiments, the Zn-Mn coatings exhibit a compact, ultrafine, pyramidal morphology.

The pulsed current deposition method of the present disclosure offers several advantages compared to conventional direct current (DC) deposition. Depending upon the pulse parameters (e.g., duty cycle and frequency), new coating microstructures, deposition current efficiencies, deposition rates as well as corrosion rates are achieved.

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.

EXAMPLES

Example 1: Pulsed deposition of a Zn-Mn coating on a steel substrate
An electroplating composition comprising about 70 g/L zinc sulphate, about 30 g/L manganese sulphate, about 180 g/L sodium citrate, about 0.15 g/L sodium thiosulphate, and about 2 g/L ascorbic acid was deposited on a steel substrate by employing a pulsed current with an average current density of about 75-85 mA/cm2, a pulse duty cycle of about 25%-75% and a pulse frequency of about 25-250 Hz to provide a steel substrate comprising the Zn-Mn coating. The pulse parameters employed for deposition are shown in Table 2 below:
Table 2: Pulse Parameters for Zn-Mn electrodeposition
Sample Name pH of deposition Temp. of deposition
(K) Time of deposition
(min) Stirring rate
(rpm) Avg
Current Density (mA cm-2) Pulse Duty Cycle (%) Peak Current Density (mA cm-2) Pulse Frequency (Hz) Pulse on time (ms) Pulse off time (ms)
DC 4.5-5.5 293-303 5 250 80 NA
P1
25 320 25 (P1) 10 30
P2 75 (P2) 3.33 10
P3 150 (P3) 1.67 5
P4
50 160 25 (P4) 20 20
P5 75 (P5) 6.67 6.67
P6 150 (P6) 3.33 3.33
P7
75
106.67 25 (P7) 30 10
P8 75 (P8) 10 3.33
P9 150 (P9) 5 1.67
P10 200 (P10) 3.75 1.25
P11 250 (P11) 3 1

Samples P1 to P11 represent pulsed current deposited samples at different duty cycle and frequency combinations. The coating properties are compared with DC deposited coating and shown in Table 3:
Table 3
Sample Name Microstructure Manganese in coating
(wt. %) Current efficiency (%) Deposition rate (µm/min) Corrosion rate
(mm/yr)
DC Coarse, Globular, Compact 28 49.4 1.08 0.009
P1 Fine, Pyramidal,
Compact 16.4 64 1.4 0.036
P2 Fine,
Pyramidal,
Compact with some voids 15.2 59.5 1.3 0.106
P3 Ultrafine,
Pyramidal,
Compact 24.3 76.4 1.67 0.03
P4 Fine,
Pyramidal, Compact 14.6 36.6 0.8 0.018
P5 Fine,
Pyramidal,
Compact 15.6 48.9 1.07 0.043
P6 Fine,
Pyramidal, Compact 14.6 71.3 1.56 0.02
P7 Fine,
Pyramidal,
Compact with some voids 18.8 81 1.77 0.04
P8 Ultrafine,
Globular,
Cracked 29.5 66.3 1.45 0.066
P9 Ultrafine,
Pyramidal,
Compact 25.3 62.7 1.37 0.0087
P10 Coarse,
Globular,
Compact 39.6 38.4 0.84 0.025
P11 Coarse,
Globular,
Cracked 32.5 37.5 0.82 0.025

The pulse parameters (duty cycle and frequency) affect the coating microstructures, coating compositions, deposition current efficiencies, deposition rates as well as corrosion rates as demonstrated below through various pulse duty cycle and pulse frequency combinations.
Sample: P4 (Medium duty cycle and low frequency)
Figure 1 shows the top surface SEM microstructures of the conventional DC deposited coating and the pulse deposited coating at 50 % pulse duty cycle and 25 Hz pulse frequency (P4). The microstructural refinement is quite evident. The large globular grains of the DC coating are changed to very fine pyramidal grains using pulsed current deposition. This level of grain refinement is not observed in DC Zn-Mn coatings and hence can only be obtained using pulsed current deposition.
Sample: P10 (High duty cycle and very high frequency)
The top surface SEM microstructures along with the Mn content of the conventional DC deposited coating and pulsed current deposited coating at 75 % duty cycle and 200 Hz frequency (P10) are shown in Figure 2. An EDS analysis shows an increase in the Mn content of the coating for P10 as compared to the conventional DC deposited coating, indicating that by employing pulsed current deposition at high duty cycle and very high frequencies, the Mn content of the coating can be increased by almost 1.5 times.
Sample: P3 (Low duty cycle and high frequency)
Figure 3 shows the top surface SEM microstructures of the conventional DC deposited coating and the pulse deposited coating at 25 % pulse duty cycle and 150 Hz pulse frequency (P3). These pulse parameters also provide a much finer grained coating microstructure which is also very uniform, unlike the large globular morphology of the conventional DC deposited coating.
Figure 4 shows the cross-sectional SEM images of the DC and pulse deposited coating (P3). It can be seen that the coating thickness is higher for P3 than for the conventional DC deposited coating. Since the deposition time and average current density of both deposition processes were identical, the increased coating thickness in the case of P3 implies a higher cathodic current efficiency and a higher deposition rate. This shows that the pulsed current deposition is advantageous over conventional DC deposition.
Sample: P9 (High duty cycle and high frequency)
The top surface SEM microstructures along with the Mn content of the conventional DC deposited coating and pulsed current deposited coating at 75 % duty cycle and 150 Hz frequency (P9) are shown in Figure 5. The corrosion rates obtained from potentiodynamic polarisation are also shown. The refinement of grains with the use of pulsed current deposition is very prominent for P9, like P3 and P4. In the case of P9, a decrease in the corrosion rate can also be seen compared to the conventional DC coating, indicating an enhanced corrosion resistance property.
Figure 6 shows the cross-sectional SEM images of the DC and pulse deposited coating P9. A higher average coating thickness can be observed for P9, indicating a higher current efficiency and a higher coating deposition rate, similar to P3 (Figure 4).
Thus, the pulse current deposited Zn-Mn coating P9 shows a corrosion rate that is similar or lower than the conventional DC-deposited coating and at the same time shows refined microstructure, increased current efficiency and increased deposition rate compared to conventional DC deposited Zn-Mn coating.

Example 2: Comparison with a commercially used coating
The current commercially available products for superior corrosion resistance are electrodeposited Zn-Ni alloys on steel with Ni contents in the range of 12-15 %. Table 4 below shows the comparison of the properties between the commercial Zn-Ni, Zn-Mn DC-deposited coating and the pulsed electrodeposited Zn-Mn coating of the present disclosure. The corrosion potential for the developed Zn-Mn coating is like the DC-deposited coating, but more negative than that of commercially available Zn-Ni coating, implying a higher sacrificial corrosion resistance. The corrosion rate for the developed pulse deposited Zn-Mn coating is slightly lower than the DC deposited coating but more than three times lower than that of the commercially available coating. The deposition rate of the pulse deposited coating is also significantly higher than the commercially available coating as well as the DC deposited coating. Hence the developed coating shows a high sacrificial protection to steel along with a very low corrosion rate and a high rate of deposition.
Table 4: Comparison of final properties of developed Zn-Mn coating with DC deposited Zn-Mn coating and commercially available Zn-Ni coating
Property Pulse deposited Zn-Mn coating (P9) DC deposited Zn-Mn coating Commercially available coating (Zn-Ni)
Corrosion potential vs SCE (V) -1.06 -1.06 -0.82
Corrosion rate (mm/yr) 0.0087 0.009 0.03
Deposition rate (µm/min) 1.37 1.08 1

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Claims:We Claim:

1. A method for depositing a zinc-manganese (Zn-Mn) coating on a steel substrate, comprising:
a. providing the steel substrate as a cathode;
b. depositing an electroplating composition comprising about 60-80 g/L zinc sulphate, about 25-35 g/L manganese sulphate, about 170-190 g/L sodium citrate, about 0.1-0.2 g/L sodium thiosulphate, and about 1.5-2.5 g/L ascorbic acid on the steel substrate by employing a pulsed current with an average current density of about 75-85 mA/cm2, a pulse duty cycle of about 25%-75% and a pulse frequency of about 25-250 Hz to provide a steel substrate comprising the Zn-Mn coating.
2. The method as claimed in claim 1, wherein the duty cycle is about 50% and the frequency is about 25Hz.
3. The method as claimed in claim 1, wherein the duty cycle is about 75% and the frequency is about 200 Hz.
4. The method as claimed in claim 1, wherein the duty cycle is about 25% and the frequency is about 150 Hz.
5. The method as claimed in claim 1, wherein the duty cycle is about 75% and the frequency is about 150 Hz.
6. The method as claimed in any one of claims 1-5, wherein the electroplating composition is stirred at a rate of about 200-300 rpm.
7. The method as claimed in any one of claims 1-6, wherein the method provides a current efficiency of deposition of about 35-80%.
8. The method as claimed in any one of claims 1-7, wherein the method provides a current efficiency of deposition of about 60-80%.
9. The method as claimed in any one of claims 1-8, wherein the current efficiency of deposition is about 76%.
10. The method as claimed in any one of claims 1-9, wherein the method provides a deposition rate of about 0.8-1.7 µm/min.
11. The method as claimed in any one of claims 1-10, wherein the method provides a deposition rate of about 1.3-1.7 µm/min.
12. The method as claimed in any one of claims 1-11, wherein the Zn-Mn coating provided by the method comprises about 15-40% by weight of Mn.
13. The method as claimed in any one of claims 1-12, wherein the Zn-Mn coating provided by the method exhibits a corrosion rate of about 0.008-0.1 mm/yr.
14. The method as claimed in any one of claims 1-13, wherein the Zn-Mn coating provided by the method exhibits a corrosion potential of about -1.02 to -1.10 V.
15. A steel substrate comprising a zinc-manganese (Zn-Mn) coating obtained by the method as claimed in any one of claims 1-14.
16. The steel substrate as claimed in claim 15, wherein the Zn-Mn coating comprises about 15-40% by weight of Mn.
17. The steel substrate as claimed in claim 15 or 16, wherein the Zn-Mn coating exhibits a corrosion rate of about 0.008-0.1 mm/yr.
18. The steel substrate as claimed in any one of claims 15-17, wherein the Zn-Mn coating exhibits a corrosion potential of about -1.02 to -1.10 V.
19. The steel substrate as claimed in any one of claims 15-18, wherein the coating exhibits a compact, ultrafine, pyramidal morphology.