TECHNICAL FIELD
The present disclosure relates to the field of electroplating. Particularly, the present disclosure relates to electroplating compositions to provide zinc-iron (Zn-Fe) alloy coatings on steel substrates, methods for preparing them, direct and pulsed current methods for depositing these 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. Also, zinc coatings have poor weldability due to the low melting point of zinc (420°C). The weldability is poorer for thicker coatings.
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, better control over coating thickness, and the possibility of one-sided coating. However, zinc being an active metal has a high corrosion rate itself. Zinc-coated surfaces are prone to form white rust (a complex compound of zinc hydroxide). Iron group metals like Ni, Fe and Co improve the protectiveness of Zn coatings. Zn-Fe coatings have been studied and are found to provide excellent corrosion resistance, weldability, paintability and formability. Since iron is very cheap and readily available metal, cost effective coatings can be formed with superior corrosion and functional properties. It has also been found that Zn-Fe alloy coating has excellent paintability due to phosphyllite phase (Zn2Fe(PO4)2·4H2O). The mechanism by which Zn-Fe alloy coatings provide corrosion resistance is shown in FIG. 1.
The present disclosure aims to provide Zn-Fe alloy coatings comprising lower amounts of iron and at the same time, showing refined morphology and providing improved corrosion

resistance. The present disclosure further aims to obtain these Zn-Fe coatings by employing bath compositions comprising leaner amounts of zinc and iron salts.
STATEMENT OF THE DISCLOSURE
The present disclosure relates to an electroplating composition comprising zinc sulphate, ferrous sulphate, zinc chloride, and boric acid. In some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 50-250 g/L, ferrous sulphate in an amount of about 5-50 g/L, zinc chloride in an amount of about 1-10 g/L, and boric acid in an amount of about 10-50 g/L, and has a pH of about 3-5.
The present disclosure also relates to a method for preparing the electroplating composition described herein, comprising: a) heating water to about 60-70°C to obtain a heated water; b) adding boric acid to the heated water to obtain a first solution; c) adding zinc sulphate to the first solution to obtain a second solution; d) adding ferrous sulphate to the second solution to obtain a third solution; and e) adding zinc chloride to the third solution to obtain the electroplating composition.
The present disclosure provides direct current and pulsed current methods for depositing the electroplating compositions. The direct current method for depositing the electroplating composition on a steel substrate, comprises: a) providing the steel substrate as a cathode; b) depositing the electroplating composition on the steel substrate at a constant current with a current density of about 160-200 mA/cm2, at a stirring rate of about 300 rpm, and at a pH of about 3.5 to provide a steel substrate comprising a zinc-iron (Zn-Fe) coating.
The pulsed current method for depositing the electroplating composition on a steel substrate, comprises: a) providing the steel substrate as a cathode; b) depositing the electroplating composition on the steel substrate by employing a pulsed current with an average current density of about 190 mA/cm2, a duty cycle of 15-80% and a frequency of 25-400Hz to provide a steel substrate comprising a zinc-iron (Zn-Fe) coating.
The present disclosure further relates to steel substrate comprising a zinc-iron (Zn-Fe) coating, wherein the Zn-Fe coating comprises about 0.6-1.4% by weight of iron.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES Figure 1 shows an exemplary corrosion mechanism of zinc-iron alloy coating.
Figure 2 shows the schematic of an exemplary method of preparing the electroplating composition.
Figure 3 shows the corrosion current and corrosion potential of Zn-Fe coatings obtained at varying deposition current densities.
Figure 4 shows morphologies of the Zn-Fe coatings deposited using the direct current method at varying current densities.
Figure 5 shows the Scanning Electron Microscope (SEM) cross-section of a Zn-Fe coating deposited using the direct current method from which the thickness of the coating is measured.
Figure 6 shows the corrosion current and corrosion potential of Zn-Fe coatings deposited using the pulsed current method at different duty cycles and frequencies.
Figure 7 shows morphologies of the Zn-Fe coatings deposited using the pulsed current method at different duty cycles.
Figure 8 shows the SEM cross-section of a Zn-Fe coating deposited using the pulsed current method from which the thickness of the coating is measured.
Figure 9 shows the corrosion current, corrosion potential, and the deposition rate of the Zn-Fe coatings of the present disclosure and those of the commercial 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 Fe salts) and a buffer.
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, ferrous sulphate, zinc chloride and boric acid. In some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 50-250 g/L, ferrous sulphate in an amount of about 5-50 g/L, zinc chloride in an amount of about 1-10 g/L, and boric acid in an amount of about 10-50 g/L, and the composition has a pH of about 3-5.
In some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 175-225 g/L, ferrous sulphate in an amount of about 35-45 g/L, zinc chloride in an amount of about 5-8 g/L, and boric acid in an amount of about 25-35 g/L, and the electroplating composition has a pH of about 3-4. In some embodiments, the electroplating composition

comprises zinc sulphate in an amount of about 195-205 g/L, ferrous sulphate in an amount of about 35-45 g/L, zinc chloride in an amount of about 5-8 g/L, and boric acid in an amount of about 25-35 g/L, and the electroplating composition has a pH of about 3-4. In an exemplary embodiment, the electroplating composition comprises zinc sulphate in an amount of about 200 g/L, ferrous sulphate in an amount of about 40 g/L, zinc chloride in an amount of about 6 g/L, and boric acid in an amount of about 30 g/L, and the composition has a pH of about 3.5.
In some embodiments, zinc sulphate is present in the electroplating composition in an amount of about 50-250, 50-225, 50-200, 75-250, 75-225, 75-200, 100-250, 100-225, 100-200, 125-250, 125-225, 125-200, 150-250, 150-225, 150-200, 175-250, 175-225, or 185-215 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 50, 75, 100, 125, 150, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, or 225 g/L, including values and ranges thereof. In some embodiments, zinc sulphate is present in the electroplating composition in an amount of about 180-220, 185-215, or 190-210 g/L, including values and ranges thereof. In some embodiments, zinc sulphate is present in the electroplating composition in an amount of about 200 g/L.
In some embodiments, ferrous sulphate is present in the electroplating composition in an amount of about 5-50, 5-40, 5-30, 10-50, 10-45, 10-40, 20-50, 20-45, 20-40, 25-50, 25-45, 25-40, 30-50, 30-45, 35-50, or 35-45 g/L, including values and ranges thereof. For example, in some embodiments, ferrous sulphate is present in the electroplating composition in an amount of about 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50 g/L, including values and ranges thereof. In some embodiments, ferrous sulphate is present in the electroplating composition in an amount of about 37-44, 38-43, or 39-42 g/L, including values and ranges thereof. In some embodiments, ferrous sulphate is present in the electroplating composition in an amount of about 40 g/L.
In some embodiments, zinc chloride is present in the electroplating composition in an amount of about 1-10, 2-8, 3-7, 4-8, or 5-8 g/L, including values and ranges thereof. For example, in some embodiments, zinc chloride is present in the electroplating composition in an amount of about 1, 2, 3, 4, 5, 6, 7, or 8 g/L, including values and ranges thereof. In some embodiments, zinc chloride is present in the electroplating composition in an amount of about 5-7 or 5.5-6.5

g/L. In some embodiments, zinc chloride is present in the electroplating composition in an amount of about 6 g/L.
In some embodiments, boric acid is present in the electroplating composition in an amount of about 10-50, 10-45, 10-40, 10-35, 10-30, 20-50, 20-45, 20-40, 20-35, 25-50, 25-45, 25-40, 25-35, 30-50, 30-45, or 30-40 g/L, including values and ranges thereof. In some embodiments, boric acid is present in the electroplating composition in an amount of about 25-34, 25-32, 25-30, 26-35, 26-32, 26-30, 27-35, 27-33, 27-31, 27-30, 28-33, 28-32, or 29-31 g/L, including values and ranges thereof. In some embodiments, boric acid is present in the electroplating composition in an amount of about 10, 15, 20, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 40, 45, or 50 g/L. In some embodiments, zinc chloride is present in the electroplating composition in an amount of about 30 g/L.
One of ordinary skill in the art would understand that any combination of the individual amounts of zinc sulphate, ferrous sulphate, zinc chloride, and boric acid disclosed herein is contemplated by the present disclosure.
The electroplating compositions of the present disclosure have a pH of about 3-5 such as the pH of about 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, or 5. In some embodiments, the pH of the electroplating composition is about 3.2-5, 3.2-4.5, 3.2-4.3, 3.2-4.2, 3.2-4.1, 3.2-3.8, 3.5-5, 3.5-4.5, 3.5-4.4, 3.5-4.3, 3.5-4.2, 3.5-4.1, 3.7-5, 3.7-4.5, 3.7-4.3, 3.7-4.2, 3.8-5, 3.8-4.5, 3.8-4.4, 3.8-4.3, 3.8-4.2, 3.9-5, 3.9-4.5, 3.9-4.4, 3.9-4.3, 3.9-4.2, 4-4.5, 4-4.4, or 4-4.3, including values and ranges thereof. In some embodiments, the pH of the electroplating composition is about 3-4 or 3.2-3.8, including values and ranges thereof. In an exemplary embodiment, the pH of the electroplating composition is about 3.5.
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 and homogeneity of the composition and affects the morphology and corrosion properties. In some embodiments, the method for preparing the electroplating composition comprises: a) heating water to about 60-70℃ to obtain a heated water; b) adding boric acid to the heated water to obtain a first solution; c) adding zinc sulphate to the first solution to obtain a second solution; d) adding ferrous sulphate to the second solution to obtain a third solution; and e) adding zinc chloride to the

third solution to obtain the electroplating composition. After addition of zinc chloride, the solution is stirred for about 2 hours at a rate of about 350-450 rpm. An exemplary schematic of the process is shown in FIG. 2.
The pH of the electroplating composition is adjusted to about 3-5. In some embodiments, the pH of the electroplating composition is adjusted to about 3-4. In an exemplary embodiment, the pH of the electroplating composition is adjusted to about 3.5. In some embodiments, the pH of the electroplating composition is adjusted using sulphuric acid and/or 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-Fe coatings. In some embodiments, the electroplating compositions are deposited using a direct current (DC) method. In some embodiments, the electroplating compositions are deposited using a pulsed current method.
Direct Current (DC) Deposition
In some embodiments, the method for depositing the electroplating composition on a steel substrate comprises: a) providing the steel substrate as a cathode; and b) depositing the electroplating composition on the steel substrate at a constant current with a current density of about 160-200 mA/cm2, at a stirring rate of about 300 rpm, and at a pH of about 3.5 to provide a steel substrate comprising a Zn-Fe coating.
The inventors have found that electroplating compositions comprising zinc sulphate at a concentration of about 200 g/L and ferrous sulphate at a concentration of 40 g/L when deposited by employing a constant current having a current density of about 170-190 mA/cm2 provide Zn-Fe coatings with a uniform morphology, fine grain structure, desired Fe content, and better corrosion properties. FIG. 3 shows corrosion current (Icorr) and corrosion potential (Ecorr) values for Zn-Fe coatings obtained at 200 g/L zinc sulphate and 40 g/L ferrous sulphate when deposited by employing a constant current having current densities of 160, 170, 180, 190, 200, and 220 mA/cm2.
In some embodiments, the current density employed in the DC method of deposition is about 170, 175, 180, 185, or 190 mA/cm2, including values and ranges thereof. In some embodiments,

the current density employed in the DC method of deposition is about 185-195 mA/cm2, including values and ranges thereof. In an exemplary embodiment, the current density for the DC deposition is about 190 mA/cm2.
Current density affects the morphology of the deposit which in turn affects the current efficiency and corrosion rate of the coating. The inventors found that at a current density about 160 mA/cm2, a needle like morphology is obtained whereas at a current density of about 190 mA/cm2, uniform and compact morphology with finer grain structure is observed. At a higher current density of about 220 mA/cm2, a globular morphology is obtained. See FIG. 4.
Further, the current density also affects the amount of Fe deposited in the coating. Table 1 shows exemplary Fe wt.% in the final deposits obtained at different current densities.

Although at higher (220 mA/cm2) and lower current density (160 mA/cm2), Fe wt.% in the deposits is higher (above 2%); the coatings deposited at these current densities either did not show uniform and compact morphology (see the morphology obtained at 160 mA/cm2 in FIG. 4) or showed a high corrosion current (FIG. 3). The coatings deposited between 170-200 mA/cm2 had lower Fe wt.% (0.5-1.2) but showed uniform morphology, fine grain structure, and low corrosion current.
Unlike prior studies on Zn-Fe electrodeposition, the present disclosure has explored the effect of stirring the electroplating composition during the process of deposition. The inventors observed that the stirring rate affects the Zn and Fe 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 250-350 rpm, 275-325 rpm, 280-320 rpm, or about 290-310 rpm, including values and ranges thereof. In some embodiments, the stirring rate of the electroplating composition is about 300 rpm.
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 3-5, 3-4, or 3.5.
In some embodiments, the DC 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 DC method is carried out for about 5 minutes.
In some embodiments, the DC method for depositing the electroplating composition provides a deposition rate of about 2-3.5 or 2.5-3.2 µm/min, including values and ranges thereof. In some embodiments, the DC method provides a deposition rate of about 2, 2.1, 2.2, 2.3, 2.4, 2.5,
2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, or 3.5 µm/min. In exemplary embodiments, the DC
method provides a deposition rate of about 2.8 µm/min.
In some embodiments, the Zn-Fe coatings provided by the DC method of depositing the electroplating composition exhibit a corrosion current density (Icorr) of about 0.8-2.3 μA/cm2, including values and ranges thereof. In some embodiments, the Zn coatings obtained by the DC method exhibit a corrosion current density of about 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6,
1.7, 1.8, 1.9, 2, 2.1, 2.2, or 2.3 μA/cm2.
In some embodiments, the Zn-Fe coatings provided by the DC method of depositing the electroplating composition exhibit a corrosion potential (Ecorr) of about -1.08 to -1.09 V, including values and ranges thereof.
In some embodiments, the DC method for depositing the electroplating composition described herein provides a Zn-Fe coating comprising about 0.6-1.4%, including values and ranges thereof, by weight of iron. In some embodiments, the Zn-Fe coatings provided by the DC method of deposition comprise about 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, or 1.4% by weight of iron. The Zn-Fe coatings provided by the present disclosure with about 0.6-1.4% Fe content

show lower corrosion potential, lower corrosion current, and higher deposition rate. See FIG. 9.
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 direct current between these two electrodes.
Pulsed Current Deposition
In some embodiments, the method for depositing the electroplating composition on a steel substrate comprises: a) providing the steel substrate as a cathode; and b) depositing the electroplating composition on the steel substrate by employing a pulsed current with an average current density of about 190 mA/cm2, a duty cycle of 15-80% and a frequency of 25-400 Hz to provide a steel substrate comprising a zinc-iron (Zn-Fe) coating.
The duty cycle and the frequency of the pulsed current affect the corrosion properties of the deposited coatings. FIG. 6 shows corrosion current (Icorr) and corrosion potential (Ecorr) values for Zn-Fe coatings obtained at different duty cycles and frequencies. Zn-Fe coatings showing lower corrosion current were obtained with moderate duty cycle (25-50%) with high frequency (200 Hz) and high duty cycle (75%) with high frequency (300Hz). Accordingly, in some embodiments, the electroplating compositions are deposited by employing a pulsed current having a duty cycle of about 25-75% and a frequency of about 200-300 Hz. In some embodiments, the pulsed current has a duty cycle of about 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or 75% and a frequency of about 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, or 300 Hz. In exemplary embodiments, the pulsed current has a duty cycle of 75% and the frequency of 200 Hz or a duty cycle of 50% and the frequency of 300 Hz.
Duty cycles also affect the morphology of the coating which in turn affects the corrosion rate of the coating. FIG. 7 shows a comparison of the morphologies of the Zn-Fe coatings obtained at different duty cycles. At low duty cycle (15%), fine and compact globule like morphology is obtained. At medium duty cycles (25-75%), even more compact and finer grain structure and uniform morphology is obtained. Coatings with this type of morphology showed better

corrosion resistance. At higher duty cycle, fine plate-like morphology is obtained and the coating with this morphology showed lower corrosion resistance.
In some embodiments, pulsed current with moderate duty cycle (25-50%) with high frequency (200 Hz) or high duty cycle (75%) with high frequency (300Hz) provides Zn-Fe coatings with uniform morphology, fine grain structure, and lower corrosion current.
The electroplating composition is stirred during the pulsed current deposition. Stirring rate and different pulse parameters (frequency and duty cycle) affect the Zn and Fe content, the morphology, and the corrosion rate of the deposit. In some embodiments, the stirring rate of the electroplating composition in the pulsed method of depositing ranges from about 250-350 rpm, 275-325 rpm, 280-320 rpm, or about 290-310 rpm, including values and ranges thereof. In some embodiments, the stirring rate during deposition is about 300 rpm.
In an exemplary embodiment, the electroplating compositions described herein are deposited on a steel substrate by employing a pulsed current having a current density of about 170-200 mA/cm2, a duty cycle of about 25-50%, and a frequency of about 200 Hz. In another exemplary embodiment, the electroplating compositions described herein are deposited on a steel substrate by employing a pulsed current having a current density of about 170-200 mA/cm2, a duty cycle of about 75%, and a frequency of about 300 Hz.
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 electroplating composition provides a deposition rate of about 1.8-2.8 µm/min, including values and ranges thereof. In some embodiments, the pulsed method provides a deposition rate of about 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, or 2.8 µm/min. In an exemplary embodiment, the pulsed method provides a deposition rate of about 2.3 µm/min.
In some embodiments, the Zn-Fe coatings provided by the pulsed method of depositing the electroplating composition exhibit a corrosion current density (Icorr) of about 0.8-2.2 μA/cm2, including values and ranges thereof. In some embodiments, the Zn-Fe coatings obtained by the

pulsed method exhibit a corrosion current density of about 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, or 2.2 μA/cm2.
In some embodiments, the Zn-Fe coatings provided by the pulsed method of depositing the electroplating composition exhibit a corrosion potential (Ecorr) of about -1.04 to -1.09 V, including values and ranges thereof. In some embodiments, the Zn coatings obtained by the pulsed method exhibit a corrosion potential of about -1.04, -1.05, -1.06, -1.07, -1.08, or -1.09.
In some embodiments, the pulsed method for depositing the electroplating composition described herein provides a Zn-Fe coating comprising about 0.6-1.4%, including values and ranges thereof, by weight of iron. In some embodiments, the Zn-Fe coatings provided by the pulsed method of deposition comprise about 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, or 1.4% by weight of iron.
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 the pulsed current between these two electrodes.
The present disclosure also provides steel substrates comprising Zn-Fe coatings, wherein the Zn-Fe coatings exhibit desired Fe content; a uniform, fine and compact morphology; and better corrosion resistance. In some embodiments, provided herein are steel substrates comprising Zn-Fe coatings, wherein the coating has a Fe content of about 0.6-1.4% by weight.
In some embodiments, steel substrates comprise Zn-Fe coatings that show uniform morphology. In some embodiments, steel substrates comprise Zn-Fe coatings that exhibit uniform morphology and fine and compact grain structure.
In some embodiments, provided herein are steel substrates comprising Zn-Fe coatings, wherein the coatings exhibit a corrosion current density (Icorr) of about 0.8-2.3 μA/cm2, including values and ranges thereof. In some embodiments, the steel substrates comprise Zn-Fe coatings that exhibit a corrosion current density of about 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, or 2.3 μA/cm2.

In some embodiments, provided herein are steel substrates comprising Zn-Fe coatings, wherein the coatings exhibit a corrosion potential (Ecorr) of about -1.04 to -1.09 V, including values and ranges thereof. In some embodiments, the steel substrates comprise Zn-Fe coatings that exhibit a corrosion potential of about -1.04, -1.05, -1.06, -1.07, -1.08, or -1.09 V.
The electroplating compositions, methods of producing them, methods of depositing them on a steel substrate and steel substrates comprising Zn-Fe coatings disclosed herein provide many advantages. First, while there have been many attempts to develop Zn-Fe coatings in the art, the present disclosure provides electroplating compositions comprising leaner amounts of zinc and iron salts, which when prepared 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. This morphology in turn provides excellent corrosion properties. The Zn-Fe coatings provided by the compositions and methods of the present disclosure show better corrosion resistance than the commercially used Zn-Ni coatings (FIG. 9). As nickel is much more costly, the Zn-Fe coatings 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.

EXAMPLES
Example 1: Effect of current density on the corrosion properties of Zn-Fe coatings An electroplating composition comprising 200 g/L zinc sulphate, 40 g/L ferrous sulphate, 6 g/L zinc chloride, and 30 g/L boric acid was deposited on a steel substrate at varying current densities from 160 mA/cm2 to 220 mA/cm2 to test the effect of current density on the corrosion properties of deposited coating. The corrosion current density (Icorr) and corrosion potential (Ecorr) exhibited by the deposited coatings were measured. The results are shown in FIG. 3. Lower corrosion current was obtained when the composition was deposited at current densities of 170 mA/cm2 to 200 mA/cm2. At 220 mA/cm2 current density, very high corrosion current was observed, and a burnt deposit was obtained. Corrosion potential remained constant at various current densities.
Example 2: Effect of current density on the morphology of Zn-Fe coatings An electroplating composition comprising 200 g/L zinc sulphate, 40 g/L ferrous sulphate, 6 g/L zinc chloride, and 30 g/L boric acid was deposited on a steel substrate at varying current densities from 160 mA/cm2 to 220 mA/cm2 to test the effect of current density on the morphology of the Zn-Fe coating. FIG. 4 shows a comparison of the morphologies (grain structure) of the Zn-Fe coatings obtained at current densities of 160, 190, and 220 mA/cm2. At low current densities such as 160 mA/cm2, a needle-like morphology was obtained whereas at medium current density (170-200 mA/cm2), a uniform morphology with very compact and finer grain structure was observed. At higher current densities, a globular morphology was obtained.
Example 3: Deposition kinetics for DC deposition
An electroplating composition comprising 200 g/L zinc sulphate, 40 g/L ferrous sulphate, 6 g/L zinc chloride, and 30 g/L boric acid was deposited on a steel substrate at a current density of about 190 mA/cm2. The thickness of the coating was measured by a scanning electron microscope (SEM) cross-sectional study. See FIG. 5. Deposition kinetics was measured by dividing the thickness of the coating with time of deposition. An average coating thickness was

14 µm. The time of deposition was 5 minutes. Accordingly, the deposition rate was 14/5 = 2.8 µm/minute.
Example 4: Effect of duty cycle and frequency of the pulsed current on the corrosion properties of the coating
An electroplating composition comprising 200 g/L zinc sulphate, 40 g/L ferrous sulphate, 6 g/L zinc chloride, and 30 g/L boric acid was deposited on a steel substrate using a pulsed current with duty cycles ranging from 15-80% and frequency ranging from 25-300Hz. The corrosion current density (Icorr) and corrosion potential (Ecorr) exhibited by the deposited coatings were measured. The results are shown in FIG. 6.
Corrosion current was very high for 15% duty cycle compared to other duty cycles. For 25% duty cycle, the lowest corrosion current was observed at 200 Hz frequency. The lowest corrosion current for medium duty cycle (50%) was observed at a higher frequency (200 Hz). For higher duty cycle (75%), lowest corrosion current was observed at 300Hz frequency. At very high duty cycle (80%), the corrosion current was higher than that for the 75% duty cycle. Corrosion potential remained similar for all the coatings generated at different duty cycles and frequencies.
Example 5: Effect of duty cycle of the pulsed current on the morphology of Zn-Fe coatings An electroplating composition comprising 200 g/L zinc sulphate, 40 g/L ferrous sulphate, 6 g/L zinc chloride, and 30 g/L boric acid was deposited on a steel substrate using a pulsed current with duty cycles ranging from 15-80% and at a frequency of 200Hz. FIG. 7 indicates that at 15% duty cycle, fine and compact globule like structure was obtained. At medium duty cycles, 25-75%, a more uniform and very compact and finer grain structure was observed. At 80% duty cycle, fine plate like morphology was obtained.
Example 6: Deposition kinetics for pulsed deposition
An electroplating composition comprising 200 g/L zinc sulphate, 40 g/L ferrous sulphate, 6 g/L zinc chloride, and 30 g/L boric acid was deposited on a steel substrate by employing a pulsed current having a current density of 190 mA/cm2, a duty cycle of 75%, and a frequency of 200Hz. The thickness of the Zn-Fe coating was measured by a scanning electron microscope (SEM) cross-sectional study. See FIG. 8. Deposition kinetics was measured by dividing the thickness of the coating with time of deposition. An average coating thickness was 11.6 µm.

The time of deposition was 5 minutes. Accordingly, the deposition rate was 11.6/5 = 2.3 µm/minute.
Example 7: Comparison with commercially available coating
The current commercially available products to provide corrosion resistance to steel substrates
are EG (electrogalvanized) Zn-Ni alloy coatings with Ni contents in the range of 12-16 wt.%.
The Zn-Fe coatings of the present disclosure were compared with the commercial Zn-Ni
coating.
For this, an electroplating composition comprising 200 g/L zinc sulphate, 40 g/L ferrous sulphate, 6 g/L zinc chloride, and 30 g/L boric acid was deposited on a steel substrate using the DC and the pulsed methods described herein. Deposition parameters employed in the DC and pulsed method are shown in Table 2.

The corrosion potential, corrosion current, and deposition kinetics were measured. The results are shown in FIG. 9.
The corrosion potential for the Zn-Fe coatings of the present disclosure was more negative than that of commercial Zn-Ni coating, implying a higher sacrificial corrosion resistance. The corrosion rate of the present Zn-Fe coatings was more than three times lower and the deposition rate was 3 times higher than that of the commercial Zn-Ni coating. Thus, the developed Zn-Fe coatings show a higher sacrificial protection to steel along with a much lower corrosion rate and a higher deposition rate than the commercial coating.

We Claim:
1. An electroplating composition comprising zinc sulphate, ferrous sulphate, zinc chloride, and boric acid.
2. The electroplating composition as claimed in claim 1, wherein the zinc sulphate is present in an amount of about 50-250 g/L, the ferrous sulphate is present in an amount of about 5-50 g/L, the zinc chloride is present in an amount of about 1-10 g/L, and the boric acid is present in an amount of about 10-50 g/L, and wherein the electroplating composition has a pH of about 3-5.
3. The electroplating composition as claimed in claim 1, wherein the zinc sulphate is present in an amount of about 175-225 g/L, the ferrous sulphate is present in an amount of about 35-45g/L, the zinc chloride is present in an amount of about 5-8 g/L, and the boric acid is present in an amount of about 25-35 g/L, and wherein the electroplating composition has a pH of about 3-4.
4. The electroplating composition as claimed in claim 1, wherein the zinc sulphate is present in an amount of about 200 g/L, the ferrous sulphate is present in an amount of about 40g/L, the zinc chloride is present in an amount of about 6 g/L, and the boric acid is present in an amount of about 30 g/L, and wherein the electroplating composition has a pH of about 3.5.
5. A method for preparing the electroplating composition as claimed in any one of claims 1-4, comprising:
a. heating water to about 60-70°C to obtain a heated water;
b. adding boric acid to the heated water to obtain a first solution;
c. adding zinc sulphate to the first solution to obtain a second solution;
d. adding ferrous sulphate to the second solution to obtain a third solution;
e. adding zinc chloride to the third solution to obtain the electroplating composition.
6. The method as claimed in claim 5, comprising adjusting the pH of composition to about 3-5.
7. A method for depositing the electroplating composition as claimed in any one of claims 1-4 on a steel substrate, comprising:
a. providing the steel substrate as a cathode;
b. depositing the electroplating composition on the steel substrate at a constant
current with a current density of about 160-200 mA/cm2, at a stirring rate of

about 300 rpm, and at a pH of about 3.5 to provide a steel substrate comprising a zinc-iron (Zn-Fe) coating.
8. The method as claimed in claim 7, wherein the current density is about 190 mA/cm2.
9. The method as claimed in claim 7 or 8, wherein the current density is 190 mA/cm2, the stirring rate is about 300 rpm, and the pH is 3.5.
10. The method as claimed in any one of claims 7-9, wherein the method provides a deposition rate of about 2.5-3.2 µm/minute.
11. The method as claimed in claim 10, wherein the method provides a deposition rate of about 2.8 µm/minute.
12. The method as claimed in any one of claims 7-11, wherein the Zn-Fe coating provided by the method comprises about 0.6-1.4% by weight of iron.
13. The method as claimed in any one of claims 7-12, wherein the Zn-Fe coating provided by the method exhibits a corrosion current density of about 0.8-2.3 µA/cm2.
14. The method as claimed in claim 13, wherein the Zn-Fe coating provided by the method exhibits a corrosion potential of about -1.08 to about -1.09V.
15. A method for depositing the electroplating composition as claimed in any one of claims 1-4 on a steel substrate, comprising:
a. providing the steel substrate as a cathode;
b. depositing the electroplating composition on the steel substrate by employing a
pulsed current with an average current density of about 190 mA/cm2, a duty
cycle of 15-80% and a frequency of 25-400Hz to provide a steel substrate
comprising a zinc-iron (Zn-Fe) coating.
16. The method as claimed in claim 15, wherein the duty cycle of the pulsed current is 25-75% and the frequency of the pulsed current is 200-300 Hz.
17. The method as claimed in claim 15, wherein the pulsed current has (i) a duty cycle of 75% and the frequency of 200 Hz or (ii) a duty cycle of 50% and the frequency of 300 Hz.
18. The method as claimed in any one of claims 15-17, wherein the method provides a deposition rate of about 1.8-2.8 µm/minute.
19. The method as claimed in claim 18, wherein the method provides a deposition rate of about 2.3 µm/minute.

20. The method as claimed in any one of claims 15-19, wherein the Zn-Fe coating provided by the method comprises about 0.6-1.4% by weight of iron.
21. The method as claimed in any one of claims 15-20, wherein the Zn-Fe coating provided by the method exhibits a corrosion current density of about 0.8-2.2 µA/cm2.
22. The method as claimed in any one of claims 15-21, wherein the Zn-Fe coating provided by the method exhibits a corrosion potential of about -1.04 to about -1.09V.
23. A steel substrate comprising a zinc-iron (Zn-Fe) coating, wherein the Zn-Fe coating comprises about 0.6-1.4% by weight of iron.
24. The steel substrate as claimed in claim 23, wherein the Zn-Fe coating has a compact and fine morphology.
25. The steel substrate as claimed in claim 23, wherein the Zn-Fe coating exhibits a corrosion current density of about 0.8-2.3 µA/cm2.
26. The steel substrate as claimed in claim 23, wherein the Zn-Fe coating exhibits a corrosion potential of about -1.04 to about -1.09 V.