Description:TECHNICAL FIELD
The present disclosure provides chemically stable electroplating compositions and methods thereof for electrodeposition of zinc-iron (Zn-Fe) alloy coatings.

BACKGROUND OF THE DISCLOSURE
Zinc and zinc alloy electroplating baths of various types have been used or proposed for use for depositing a metal / alloy coating of a decorative or functional type on a variety of conductive substrates. Zinc alloy coatings provide for improved corrosion resistance than plain zinc due to decreased sacrificial nature by shifting the corrosion potential more towards steel. While electroplated zinc-nickel coatings have become popular over time, there is a need to find more benign alternatives as nickel is a known allergen to about 10 -15% of population and its salts are carcinogenic.

Iron is a benign metal and is abundant in nature, hence it is a suitable alloying element with zinc. Moreover, such alloy coating is expected to be easily weldable. However, aqueous solutions of iron salts are unstable due to easy oxidation and precipitation. Due to this reason, there are no commercially available zinc-iron plating baths.

It has been observed in other cases of metal / alloy plating that electrodeposition from acidic baths can be done at a faster rate than from alkaline baths. There are several studies that describe electrodeposition from acidic or alkaline baths, but none of them are stable beyond a few days. For example, U.S. Patent No. 4,543,300 to Hara et al.; Praveen et al. (International Journal of Electrochemistry doi:10.4061/2011/132138); U.S. Patent No. 4,541,903 to Kyono et al.; Rashmi et al. (International Journal of Applied Engineering and Management 3 (2019) 38 – 44); Bhat et al. (Journal of Metals, Materials and Minerals, 20(2010).43-51); and Nayana et al. (Surface & Coatings Technology 235 (2013) 461–468) disclose describe electrodeposition from acidic or alkaline baths. These baths lack strong complexing agents and/or reducing agents. For example, the bath compositions disclosed by Praveen et al. didn’t contain any chelating agents; while the bath composition in Rashmi et al. employ only small amounts of glycine and citrate and operated up to a current density of 8 A/sq.dm. Jayakumar et al. employed ferrous ammonium sulphate salt and glycine and citric acid as complexing agents but operated at current densities lower than 2 A/sq. dm. [N. D. Jayakumar, G. Devaraj, G. N. K. Ramesh Bapu, J. Ayyapparaju Bulletin of Electrochemistry 4 (1988) 711 – 715]. Both Hara and Kyono employed a high velocity electrolyte flow to operate at a high current density and increase the rate of deposition. These bath compositions lack sufficient complexing and/or reducing agents; therefore, they are not stable enough for continuous long-term operations. In these baths, ferrous ions oxidize to ferric state, undergo hydrolysis and form a precipitate.

Therefore, there is a need to develop chemically stable baths for electrodeposition of zinc-iron alloy coatings. The present disclosure attempts to address this need.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to an electroplating composition comprising zinc sulphate, ferrous sulphate, citric acid, sodium citrate, potassium chloride, a chelating agent, and one or more additives selected from sodium gluconate, sodium sulphate, ascorbic acid, and nitrilotriacetic acid, wherein the electroplating composition has a pH of about 2.5-3.5.

The present disclosure provides a direct current method for depositing the electroplating composition on a steel substrate, comprising: a) providing the steel substrate as a cathode; b) contacting the steel cathode with the electroplating composition and passing current at a current density of about 5-21 A/dm2 and at a temperature of about 25-35? to provide a steel substrate comprising a zinc-iron (Zn-Fe) alloy coating.

The present disclosure further relates to a steel substrate comprising a zinc-iron (Zn-Fe) coating, wherein the Zn-Fe coating comprises about 0.5-40% by weight of Fe.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the potentiodynamic polarization curve comparison of uncoated mild steel sheet and the Zn-Fe coated product.

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 Fe salts); buffering agents such as citric acid and sodium citrate; potassium chloride; chelating agents; and additives.

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 electroplating compositions for depositing zinc-iron (Zn-Fe) alloy coatings on steel substrates. The present disclosure also provides methods for depositing/electroplating said compositions on steel substrates by a direct current (DC) method. The electroplating compositions and the electroplating methods of the present disclosure show improved deposition kinetics and provide Zn-Fe alloy coatings with a higher iron content, and/or improved corrosion resistance.

In some embodiments, the present disclosure provides an electroplating composition comprising zinc sulphate, ferrous sulphate, citric acid, sodium citrate, potassium chloride, a chelating agent, and one or more additives selected from sodium gluconate, sodium sulphate, ascorbic acid, and nitrilotriacetic acid, wherein the electroplating composition has a pH of about 2.5-3.5.

Zinc sulphate is present in the electroplating compositions in an amount of about 100-250 g/L, including values and ranges thereof, such as about 100-200 g/L, 100-150 g/L, 100-120 g/L, 110-140 g/L, 150-250 g/L, 150-200 g/L, 200-250 g/L, 100 g/L, 110 g/L, or about 220 g/L. In an exemplary embodiment, zinc sulphate is present in the amount of about 110 g/L.

Ferrous sulphate is present in the electroplating compositions in an amount of about 180-220 g/L, including values and ranges thereof, such as about 190-210 g/L, 190 g/L, or about 200 g/L. In an exemplary embodiment, ferrous sulphate is present in the amount of about 200 g/L.

Potassium chloride is present in the electroplating compositions in an amount of about 8-12 g/L, including values and ranges thereof, such as about 10 g/L. Citric acid is present in the electroplating compositions in an amount of about 16-20 g/L, including values and ranges thereof, such as about 18 g/L. Sodium citrate is present in the electroplating compositions in an amount of about 2-4 g/L, including values and ranges thereof, such as about 3 g/L.

In some embodiments, the chelating agent present in the electroplating compositions is selected from Ethylenediamine-N, N, N’, N’-tetra -2-propanol (EDTP), 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP), or a combination thereof. In some embodiments, the chelating agent is present in the electroplating compositions in an amount of about 10-30 g/L, including values and ranges thereof, such as about 10-20 g/L, 10 g/L, 15 g/L, 20 g/L, 25 g/L, or 30 g/L.

The electroplating compositions of the present disclosure comprise one or more additives selected from sodium gluconate, sodium sulphate, ascorbic acid, and nitrilotriacetic acid. In some embodiments, the electroplating composition comprises sodium gluconate as an additive in an amount of about 20 g/L. In some embodiments, the electroplating composition comprises sodium sulphate as an additive in an amount of about 10 g/L. In some embodiments, the electroplating composition comprises sodium gluconate and sodium sulphate as additives in an amount of about 20 g/L and about 10 g/L, respectively. In some embodiments, the electroplating composition comprises sodium sulphate and ascorbic acid as an additive in an amount of about 10 g/L and about 5 g/L, respectively. In some embodiments, the electroplating composition comprises sodium sulphate and nitrilotriacetic acid as an additive in an amount of about 10 g/L and about 20 g/L, respectively.

In some embodiments, the electroplating composition comprises about 110 g/L zinc sulphate, about 200 g/L ferrous sulphate, about 18 g/L citric acid, about 3 g/L sodium citrate, about 10 g/L potassium chloride, about 10 g/L sodium sulphate, about 20 g/L sodium gluconate, and about 10-20 g/L EDTP as a chelating agent.

In some embodiments, the electroplating composition comprises about 110 g/L zinc sulphate, about 200 g/L ferrous sulphate, about 18 g/L citric acid, about 3 g/L sodium citrate, about 10 g/L potassium chloride, about 10 g/L sodium sulphate, and EDTP and HEDP as chelating agents in an amount of about 10 g/L and 20 g/L, respectively.

In some embodiments, the electroplating composition comprises about 110 g/L zinc sulphate, about 200 g/L ferrous sulphate, about 18 g/L citric acid, about 3 g/L sodium citrate, about 10 g/L potassium chloride, about 10 g/L sodium sulphate, about 5 g/L ascorbic acid, and about 20 g/L HEDP.

The electroplating compositions of the present disclosure have a pH of about 2.5 to about 3.5.

The inventors found that the addition of a chelating agent such as EDTP and/or HEDP and secondary complexing agents such as sodium gluconate and sodium citrate stabilize the iron salt-containing electroplating compositions. In some embodiments, the electroplating compositions of the present disclosure are stable for at least 60 days, at least 100 days, or about 60-180 days, about 80-180 days, about 100-180 days, about 100-150 days, about 120-180 days, about 120-150 days, or about 150 to 180 days. In some embodiments, stability is determined by inspecting the electroplating compositions for the formation of a precipitate.

The electroplating compositions of the present disclosure allow high current density operations in a moderately agitated bath. Operating at high current densities allows faster rate of deposition and increased incorporation of iron in the zinc-iron coatings.

The present disclosure further provides a direct current (DC) method for depositing the electroplating compositions on steel substrates to provide substrates with Zn-Fe alloy coatings.

In some embodiments, a method for depositing the electroplating composition on a steel substrate comprises: a) providing the steel substrate as a cathode; b) contacting the steel cathode with the electroplating composition and passing current at a current density of about 5-21 A/dm2 and at a temperature of about 25-35? to provide a steel substrate comprising a Zn-Fe alloy coating.

In some embodiments, the current density employed in the DC method of deposition is about 10-21 A/dm2, including values and ranges thereof.

In some embodiments, the electroplating composition is stirred during electrodeposition by air agitation. In some embodiments, the electroplating composition is stirred during electrodeposition by air agitation at a rate of about 2-4 litres per minute (LPM). In an exemplary embodiment, the electroplating composition is stirred during electrodeposition by air agitation at a rate of about 3 LPM.

In some embodiments, the temperature of the electroplating composition is maintained at about 25?-35?, including values and ranges thereof, such as about 30?.

In some embodiments, the rate of deposition provided by the compositions and methods of the present disclosure is about 2-7 µm/min or about 2-5 µm/min, including values and ranges thereof, such as about 2-4 µm/min or 3-5 µm/min.

In some embodiments, the compositions and methods of the present disclosure provide a Zn-Fe alloy coating comprising about 0.5-40% by weight of iron, including values and ranges thereof, such as about 1-10%, 5-20%, 5-25%, 15-25%, or 10-25% by weight of iron. In some embodiments, the compositions and methods of the present disclosure provide a Zn-Fe alloy coating comprising about 15-25% by weight of iron. In some embodiments, the compositions and methods of the present disclosure provide a Zn-Fe alloy coating comprising about 1-10% by weight of iron.

In some embodiments, the Zn-Fe alloy coating provided by the compositions and methods of the present disclosure exhibit a corrosion current density of about 1-2.5 µA/cm2, including values and ranges thereof.
The present disclosure also provides a steel substrate comprising a Zn-Fe alloy coating.

In some embodiments, the steel substrate comprises a Zn-Fe coating comprising about 0.5-40% by weight of Fe, including values and ranges therebetween. For example, in some embodiments, the steel substrate comprises a Zn-Fe coating comprising 1-10%, 5-20%, 5-25%, 15-25%, or 10-25% by weight of iron.

In some embodiments, the steel substrate comprising a Zn-Fe coating exhibits a corrosion current density of about 1-2.5 µA/cm2, including values and ranges thereof.

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:
Table 1:
Chemical name Quantity per liter of solution
Zinc chloride solution (46%) 100 grams (equivalent to ~ 22 g Zn metal)
Ammonium ferrous sulphate 200 grams (equivalent to ~ 28 g Fe metal)
Glycine 10 grams
Citric acid 18 grams
Sodium citrate 3 grams

Electroplating parameters:
Cathode Current density = 10 – 21 A/dm2
Temperature = 30?
pH = 3
Anode = zinc blocks
Cathode = copper or mild steel sheets
When plated under conditions of mild air agitation in the bath, uniform dark grey coatings were obtained that had about 20 – 30% Fe content (the rest being Zn). A plating time of one minute provided 2-5 micron thick coatings. The iron in solution is stabilized mainly by the ammonium chelate, and secondary complexing agents - glycine and citrate. The citric acid – citrate mix acts as buffer as well as mild reducing agent.

Example 2:
Table 2:
Chemical name Quantity per liter of solution
Zinc sulphate Heptahydrate 220 grams (equivalent to ~ 49 g Zn metal)
Ferrous sulphate Heptahydrate 200 grams (equivalent to ~ 40 g Fe metal)
Citric acid Monohydrate 18 grams
Sodium citrate Dihydrate 3 grams
Potassium Chloride 10 grams
Sodium gluconate 20 grams
EDTP
(Ethylenediamine – N, N, N’, N’-tetra -2-propanol) 20 grams

Electroplating parameters:
Cathode Current density = 10 – 21 A/dm2
Temperature = 30?
pH = 3
Anode = zinc blocks
Cathode = copper or mild steel sheets
When plated under conditions of mild air agitation in the bath, uniformly bright coatings were obtained that had about 0.5-5% Fe content (the rest being Zn). A plating time of one minute provided 2-3 micron thick coatings at the current densities mentioned above. In this example, the iron in solution is stabilized mainly by the EDTP chelate, and secondary complexing agents - gluconate and citrate. The citric acid-citrate mix acts as a buffer as well as a mild reducing agent.

Example 3:
Table 3:
Chemical name Quantity per liter of solution
Zinc sulphate Heptahydrate 110 grams (equivalent to ~ 25 g Zn metal)
Ferrous sulphate Heptahydrate 200 grams (equivalent to ~ 40 g Fe metal)
Citric acid Monohydrate 18 grams
Sodium citrate Dihydrate 3 grams
Potassium Chloride 10 grams
Sodium sulphate 10 grams
Sodium gluconate 20 grams
EDTP
(Ethylenediamine – N, N, N’, N’-tetra -2-propanol) 10 grams

Electroplating parameters:
Cathode Current density = 10-21 A/dm2
Temperature = 30?
pH = 3
Anode = zinc blocks
Cathode = copper or mild steel sheets
When plated under conditions of mild air agitation in the bath, fairly uniform bright coatings were obtained that had about 15-25% Fe content (the rest being Zn). The electrodeposition was carried out for one minute to provide 3-5-micron thick coatings at the current densities mentioned above. In this example, the iron in solution is stabilized mainly by the EDTP chelate, and secondary complexing agents - gluconate and citrate. The citric acid-citrate mix acts as a buffer as well as a mild reducing agent.

Example 4:
Table 4:
Chemical name Quantity per liter of solution
Zinc sulphate Heptahydrate 110 grams (equivalent to ~ 25 g Zn metal)
Ferrous sulphate Heptahydrate 200 grams (equivalent to ~ 40 g Fe metal)
Citric acid Monohydrate 18 grams
Sodium citrate Dihydrate 3 grams
Potassium Chloride 10 grams
Sodium sulphate 10 grams
Sodium gluconate 20 grams
EDTP
(Ethylenediamine – N, N, N’, N’-tetra -2-propanol) 20 grams

Electroplating parameters:
Cathode Current density = 10-21 A/dm2
Temperature = 30?
pH = 3
Anode = zinc blocks
Cathode = copper or mild steel sheets
When plated under conditions of mild air agitation in the bath, fairly uniform bright coatings were obtained that had about 1-10% Fe content (the rest being Zn). A plating time of one minute provided 3-4 micron thick coatings at the current densities mentioned above. This example demonstrates the effect of increased EDTP compared to Example 3. Higher amount of EDTP shifted the chemical equilibrium towards greater stability of the EDTP - Fe chelate, and hence lowered deposition of Fe in the coating.

Example 5:
Table 5:
Chemical name Quantity per liter of solution
Zinc sulphate Heptahydrate 110 grams (equivalent to ~ 25 g Zn metal)
Ferrous sulphate Heptahydrate 200 grams (equivalent to ~ 40 g Fe metal)
Citric acid Monohydrate 18 grams
Sodium citrate Dihydrate 3 grams
Potassium Chloride 10 grams
Sodium sulphate 10 grams
HEDP
(1-Hydroxyethylidene-1,1-diphosphonic acid) 20 grams
EDTP
(Ethylenediamine – N, N, N’, N’-tetra -2-propanol) 10 grams

Electroplating parameters:
Cathode Current density = 5-21 A/dm2
Temperature = 30?
pH = 3
Anode = zinc blocks
Cathode = copper or mild steel sheets
When plated under conditions of mild air agitation in the bath, uniform semi-bright coatings were obtained that had about 5-20% Fe content (the rest being Zn). A plating time of one minute provided 3-5 micron thick coatings at the current densities mentioned above. In this example, the iron in solution is stabilized mainly by the EDTP chelate and HEDP chelate. The citric acid-citrate mix acts as a buffer as well as a mild reducing agent.

Example 6:
Table 6:
Chemical name Quantity per liter of solution
Zinc sulphate Heptahydrate 110 grams (equivalent to ~ 25 g Zn metal)
Ferrous sulphate Heptahydrate 200 grams (equivalent to ~ 40 g Fe metal)
Citric acid Monohydrate 18 grams
Sodium citrate Dihydrate 3 grams
Potassium Chloride 10 grams
Sodium sulphate 10 grams
HEDP
(1-Hydroxyethylidene-1,1-diphosphonic acid) 20 grams
Ascorbic acid 5 grams

Electroplating parameters:
Cathode Current density = 5-21 A/dm2
Temperature = 30?
pH = 3
Anode = zinc blocks
Cathode = copper or mild steel sheets
When plated under conditions of mild air agitation in the bath, uniform whitish grey coatings were obtained that had about 2-26% Fe content (the rest being Zn). A plating time of one minute provided 4-7 micron thick coatings at the current densities mentioned above. In this example, the iron in solution is stabilized mainly by the HEDP chelate. The citric acid-citrate mix acts as a buffer as well as a mild reducing agent. Ascorbic acid is a stronger anti-oxidant.

Example 7:
Table 7:
Chemical name Quantity per liter of solution
Zinc sulphate Heptahydrate 110 grams (equivalent to ~ 25 g Zn metal)
Ferrous sulphate Heptahydrate 200 grams (equivalent to ~ 40 g Fe metal)
Citric acid Monohydrate 18 grams
Sodium citrate Dihydrate 3 grams
Potassium Chloride 10 grams
Sodium sulphate 10 grams
HEDP
(1-Hydroxyethylidene-1,1-diphosphonic acid) 10 grams
Nitrilotriacetic acid 20 grams

Electroplating parameters:
Cathode Current density = 5-21 A/dm2
Temperature = 30?
pH = 3
Anode = zinc blocks
Cathode = copper or mild steel sheets
When plated under conditions of mild air agitation in the bath, uniform whitish grey coatings were obtained that had about 2-40% Fe content (the rest being Zn). A plating time of one minute provided 4-7 micron thick coatings at the current densities mentioned above. In this example, the iron in solution is stabilized mainly by the HEDP and Nitrilotriacetic acid. The citric acid-citrate mix acts as a buffer as well as a mild reducing agent.

Example 8: Coating properties and stability of different baths
Table 8: Coating properties and stability of different baths
Example Deposition Kinetics (µm/min) Fe wt.% Stability
1 2-5 20-30 30 days
2 2-3 0.5-5 60 days
3 3-5 15-25 Above 100 days
4 2-4 1-10 Above 100 days
5 3-5 5-20 Above 60 days
6 4-7 2-26 Above 60 days
7 4-7 2-40 Above 60 days

Table 9: Bath stability with time
Example Bath Appearance after 1 month Bath Appearance after 2 months Bath Appearance after 3 months
1 No precipitation No precipitation till 58 days
2 No precipitation No precipitation till 59 days
3 No precipitation No precipitation No precipitation till 103 days
4 No precipitation No precipitation No precipitation till 100 days
5 No precipitation No precipitation No precipitation till 61 days
6 No precipitation No precipitation No precipitation till 61 days
7 No precipitation No precipitation No precipitation till 61 days

Example 9: Electrochemical Corrosion measurements
For corrosion studies, a solution of 3.5% NaCl was used. Electrochemical measurements (open circuit potential and polarization curves) were carried out using Gamry software (Make: Gamry). Saturated Calomel Electrode was used as the reference electrode.

Before the polarization measurements, the open circuit potential (OCP) was recorded during 1 hour, until it was stabilized.
For evaluation of corrosion resistance, the scan rate was 0.5 mV/sec, and the sweep direction was from cathodic to anodic region {input from and to voltages}. Corrosion tests were conducted at room temperature.

The Icorr values for coatings of Example 3 on mild steel sheets were between 1.0 – 2.5 µA/cm2; whereas the Icorr values for the bare or uncoated mild steel was 43-48 µA/cm2. This decrease in the Icorr value by about 20 times demonstrates the corrosion protective property of the electrodeposited alloy coating.

, C , Claims:We Claim:
1. An electroplating composition comprising zinc sulphate, ferrous sulphate, citric acid, sodium citrate, potassium chloride, a chelating agent, and one or more additives selected from sodium gluconate, sodium sulphate, ascorbic acid, and nitrilotriacetic acid, wherein the electroplating composition has a pH of about 2.5-3.5.
2. The electroplating composition as claimed in claim 1, wherein zinc sulphate is present in an amount of about 100-250 g/L, ferrous sulphate is present in an amount of about 200 g/L, citric acid is present in an amount of about 18 g/L, sodium citrate is present in an amount of about 3 g/L, potassium chloride is present in an amount of about 10 g/L, and the chelating agent is present in an amount of about 10-30 g/L.
3. The electroplating composition as claimed in claim 1 or 2, wherein the chelating agent is selected from Ethylenediamine – N, N, N’, N’-tetra -2-propanol (EDTP), 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP), or a combination thereof.
4. The electroplating composition as claimed in any one of claims 1-3, wherein the chelating agent is EDTP present in an amount of 20 g/L and the additive is sodium gluconate present in an amount of about 20 g/L.
5. The electroplating composition as claimed in any one of claims 1-3, wherein the chelating agent is EDTP present in an amount of about 10 g/L or about 20 g/L, and the additive is sodium gluconate present in an amount of about 20 g/L and sodium sulphate present in an amount of about 10 g/L.
6. The electroplating composition as claimed in any one of claims 1-3, wherein the chelating agent is EDTP present in an amount of about 10 g/L and HEDP present in an amount of about 20 g/L, and the additive is sodium sulphate present in an amount of about 10 g/L.
7. The electroplating composition as claimed in any one of claims 1-3, wherein the chelating agent is HEDP present in an amount of about 20 g/L, and the additive is sodium sulphate present in an amount of about 10 g/L and ascorbic acid present in an amount of about 5 g/L.
8. The electroplating composition as claimed in any one of claims 1-3, wherein the chelating agent is HEDP present in an amount of about 10 g/L, and the additive is sodium sulphate present in an amount of about 10 g/L and nitrilotriacetic acid present in an amount of about 20 g/L.
9. The electroplating composition as claimed in any one of claims 1-8, wherein the composition is stable for at least 60 days or at least 100 days.
10. The electroplating composition as claimed in any one of claims 1-8, wherein the composition provides a zinc-iron (Zn-Fe) alloy coating that exhibits a corrosion current of about 1-2.5 µA/cm2.
11. A method for depositing the electroplating composition as claimed in any one of claims 1-10 on a steel substrate, comprising:
a. providing the steel substrate as a cathode;
b. contacting the steel cathode with the electroplating composition and passing current at a current density of about 5-21 A/dm2 and at a temperature of about 25-35? to provide a steel substrate comprising a zinc-iron (Zn-Fe) alloy coating.
12. The method as claimed in claim 11, wherein the method provides a deposition rate of about 2-7 µm/min.
13. The method as claimed in claim 11 or 12, wherein the Zn-Fe alloy coating provided by the method comprises about 0.5-40% by weight of Fe.
14. The method as claimed in any one of claims 11-13, wherein the Zn-Fe alloy coating provided by the method comprises about 15-25% by weight of Fe.
15. The method as claimed in any one of claims 11-14, wherein the Zn-Fe alloy coating provided by the method exhibits a corrosion current of about 1-2.5 µA/cm2.
16. A steel substrate comprising a zinc-iron (Zn-Fe) alloy coating, wherein the coating comprises about 0.5-40% by weight of Fe.
17. The steel substrate as claimed in claim 16, wherein the coating comprises about 15-25% by weight of Fe.
18. The steel substrate as claimed in claim 16 or 17, wherein the coating exhibits a corrosion current density of about 1-2.5 µA/cm2.