TECHNICAL FIELD

The present disclosure relates to the field of electroplating. Particularly, the present disclosure relates to electroplating compositions to provide zinc-iron-aluminium or zinc-iron-silicon coatings, methods of preparing them, direct and pulsed current methods of depositing these electroplating compositions on steel substrates and steel substrates obtained therefrom.

BACKGROUND OF THE DISCLOSURE
Three types of coatings are deposited using electrodeposition, namely, pure metals, metal alloys and metal based composite coatings [1]. In case of pure metal deposition, one anode and one cathode are placed in an electrolyte solution and electricity is passed between these two electrodes [2,3]. Anode is generally a pure metal which is to be deposited on the cathode. The electrolyte is composed of the metal salt which is deposited on the cathode surface. When current is passed between anode and cathode, pure metal is deposited on the cathode surface. In the case of alloy deposition, the electrolyte solution is composed of two-different metal salts at definite proportions which are intended for the alloy plating. Some additives are also present to obtain uniform co-deposition of the two metals. In the case of composite coatings, the electrolyte is composed of the salts of the metals along with reinforcement particles and surfactants. These surfactants are used to disperse the particles in the electrolyte solution [1].

A Zn or Zn-alloy composite coating with metallic reinforcement particles is a very promising coating material for providing higher corrosion resistance to steel compared to pure zinc or zinc-iron alloy. Moreover, composite coatings are known to have exceptional hardness and wear resistance properties compared to pure metal or alloy coatings [4-8]. Also, the passivation property is significantly improved by adding metallic particles in metal or alloy matrix [5].

A major challenge in co-depositing metallic particles with different metallic ions from the electrolyte salt is the dispersion of nanoparticles in the electrolyte media. Nanoparticles agglomerate easily in the electrolyte because of their high surface energy. If these inert particles are not dispersed properly, co-deposition becomes very difficult [9-15]. The undispersed particles result in entrapment of large agglomerates in the coated matrix resulting in poor coating performance [12,13]. Two mechanisms are generally employed to disperse the nanoparticles in the electrolyte. One is by surface modification of the particles through surfactant treatments, and another is dispersing the particles with the help of mechanical shear forces through ultrasonication, magnetic stirrer or mechanical stirrer. The first method is termed as chemical dispersion [16] and the second one as physical dispersion [17].

The main challenge in incorporating metallic reinforcement particles in the coating is the dispersion and stability of these metallic particles in the electroplating bath. Nanoparticles have a high specific surface area; hence they tend to agglomerate and settle down in the bath. Various studies have been carried out to study the dispersion of Al and Si particles with and without surfactants [18-24]. Some studies of co-deposition of Al and Si in different metal matrices using electrodeposition have also been carried out [18-24]. Studies on the dispersion of Al and Si particles are briefly discussed below.
There is no literature available on metallic Al co-deposition with a zinc or zinc alloy matrix. However, several authors have studied Al co-deposition in a Ni or Ni-Cu matrix to obtain Ni-Al composite coatings for wear resistance as well as corrosion resistance applications [25-32].
Zhou et al. [25] observed that adding Al particles to a Ni plating bath pushes the cathodic deposition overpotential to more negative values at a constant current density. The change in deposition potential is more for nano-sized particles than for micron-sized particles. The authors further reported that due to increase in the overpotential, finer deposits are observed. Adabi et al. [26] also had similar observations of an increase in cathodic overpotential with Al particles. The authors also reported the use of a surfactant SDS (Sodium dodecyl sulphate) to disperse the Al particles. SDS was reported to increase the zeta potential of Al particles from -4 to -47mV indicating better dispersion.
There are several reports showing that Al particles can be incorporated in Ni or Ni-Cu matrix, and the incorporation of Al particles can be increased by increasing the Al concentration in the bath or by increasing current density of deposition [27-30]. However, a few authors observed that very high amounts of Al incorporation in the matrix may be detrimental to the corrosion resistance of the coating due to galvanic coupling between Ni and Al.
Ni-Al co-deposition using pulsed current has also been studied to some extent. Chhangani et al. [31] obtained smooth and uniform Ni-Al composite coatings by co-depositing nano sized Al particles in a Ni matrix from a sulphamate bath. The authors observed that very high concentrations of Al in the bath (10 g/L) gave rise to cracked deposits with high residual stress. Liu et al. [32] developed a new horizontal setup for deposition of Ni-Cu coatings reinforced with Al particles to increase the incorporation of Al with the help of gravitational forces. The authors carried out pulsed deposition at very low duty cycles and observed that upto 30 % Al can be incorporated using the horizontal setup. However, increasing current density and increasing particle concentration beyond a certain point did not help in further Al particle incorporation.
Particulate Si co-deposition in a metal matrix has been attempted in some reports with matrices like Ni and Fe [33-36]. While Ni-Si composite coatings were attempted for wear and corrosion resistance applications, Fe-Si composite electrodeposition was attempted to obtain a material with properties similar to Fe-6% Si electrical steel.
Ni-Si composite coatings were obtained by Alizadeh et al. [33] by co-deposition of 100nm Si particles with Ni from a simple Watts bath using an anionic surfactant (SDS). The particles were stirred in the bath for 18hrs to allow proper adsorption of the surfactant on the particle surface. Incorporation of Si particles seemed to change the preferential growth direction. As Si concentration in the bath increased, growth along the (200) plane was suppressed and growth along the (111) and (220) planes was preferred. Here as well, it was observed that the corrosion resistance of the coating improved with addition of Si particles, however, above the concentration of 10g/L Si in the bath, corrosion resistance deteriorated. Fellner et al. [34] also studied Si co-deposition in a Ni matrix using Watts bath but without any surfactant. Here Si particles were in the 1-10 micrometer size range and were also coated with Ni using electroless deposition technique. The authors achieved Si content ranging from 6 to 15% in the final deposit. The major observation was that an increase in both Si concentration in the bath and current density increased the Si content in the deposit. A Ni-12wt.%Si deposit on Fe had a hardness of 551HV, much higher than the steel substrate (180HV) as well as pure Ni (297HV).
Si deposition with Fe matrix has been studied by few authors in DC as well as pulsed current deposition conditions. Both CTAB (cetyl trimethyl ammonium bromide) and SDS have been used as cationic and anionic surfactants respectively for the dispersion of the particles in the bath. Long et al. [35] studied DC electrodeposition in the vertical and horizontal setup with an electrolyte containing FeSO4, FeCl2 and NH4Cl along with 10 g/L of 2.5micrometer sized Si particles and CTAB as cationic surfactant. The authors reported a striking difference in the Si content of the deposited coatings from horizontal and vertical setup. The horizontal setup had at least 5-10 times more Si content than the vertical setup which was attributed to the presence of gravitational forces for deposition of Si in the horizontal setup. It was also observed that increasing stirring rate improves particle incorporation to a certain extent but a very high stirring rate reduced the particle incorporation. Increasing current density was observed to reduce particle incorporation.
Zhong et al. studied Fe-Si [36] co-deposition using pulse reverse current and studied the effect of pulse frequency on the Si content of the final deposit. An iron sulphate-based bath was used as electrolyte and 11 g/L of Si particles with average size 150nm were added. SDS was used as an anionic surfactant to disperse the particles. It was reported that pulse reverse current reduced Si content in the deposit due to detachment of Si particles from the coating during the current reversal. An increase in pulse frequency also seemed to have a detrimental effect on the incorporation of Si particles in the coating; a frequency increase from 50 to 500 Hz reduced the Si content in the coating from 5 to 1 wt. %.
Zinc-iron coatings provide superior corrosion resistance, weldability and paintability over pure Zn coatings. Zn-Fe-metallic particles composite coating can be a promising coating that can further enhance the corrosion resistance to steel. However, a suitable electroplating bath, electrolyte preparation process and suitable particle dispersion process are yet to be developed to address the drawbacks of particle agglomeration in coating, stability of electrolyte and uniform dispersion of the metallic particles in the plating bath. The present disclosure attempts to address this need.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to an electroplating composition comprising 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, boric acid in an amount of about 30 g/L, and aluminium or silicon particles at a concentration of about 0.5-10 g/L, wherein the electroplating composition has a pH of about 3.5.

The present disclosure also relates to a method for preparing the electroplating composition described herein, comprising: a) adding boric acid to distilled water; b) after dissolution of boric acid, adding zinc sulphate to the distilled water; c) after dissolution of zinc sulphate, adding ferrous sulphate to the distilled water; d) after dissolution of ferrous sulphate, adding zinc chloride to the distilled water to obtain an electrolyte solution; e) after dissolution of zinc chloride, adding aluminium or silicon particles to a portion of the electrolyte solution to obtain an aluminium or silicon-containing solution; f) stirring the aluminium or silicon-containing solution for about 30 minutes at a speed of about 300-400 rpm; g) ultrasonicating the aluminium or silicon-containing solution after stirring to obtain an ultrasonicated solution; h) adding the ultrasonicated solution to remaining portion of the electrolyte solution; i) adding distilled water to obtain the electroplating composition; j) adjusting pH of the electroplating composition to about 3.5; k) stirring the electroplating composition for about 24 hours at a speed of about 300-400 rpm; and l) ultrasonicating 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 180-200 mA/cm2 and at a stirring rate of about 350 rpm to provide a steel substrate comprising a zinc-iron-aluminium (Zn-Fe-Al) or a zinc-iron-silicon (Zn-Fe-Si) 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 about 50-75% and a frequency of about 75-200Hz to provide a steel substrate comprising a Zn-Fe-Al or a Zn-Fe-Si coating.

The present disclosure further relates to a steel substrate comprising a Zn-Fe-Al coating, wherein the coating comprises about 0.3-0.4% by weight of aluminium. In some embodiments, the Zn-Fe-Al coatings comprising about 0.3-0.4% by weight of aluminium exhibit a corrosion current density of about 0.6-1 µA/cm2.

The present disclosure also relates to a steel substrate comprising a Zn-Fe-Si coating, wherein the coating comprises about 0.95-1% by weight of silicon. In some embodiments, the Zn-Fe-Si coatings comprising about 0.95-1% by weight of silicon exhibit a corrosion current density of about 2.3-2.5 µA/cm2.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the schematic of an exemplary method of preparing the electroplating composition.

Figure 2 shows the zeta potential values of aluminium and silicon particles.

Figure 3 shows corrosion currents of Zn-Fe-Al composite coatings obtained by the direct current method at varying aluminium nanoparticles concentration.

Figure 4 shows morphologies of the Zn-Fe-Al composite coatings obtained by the direct current method at varying aluminium nanoparticles concentration.

Figure 5 shows wt.% of aluminium particles incorporated in the Zn-Fe-Al composite coatings obtained by the direct current method at varying aluminium nanoparticles concentration.

Figure 6 shows the corrosion current of Zn-Fe-Al composite coatings obtained by the pulsed current method at varying aluminium nanoparticles concentration.

Figure 7 shows the top surface morphologies of the Zn-Fe-Al composite coatings obtained by the pulsed current method at varying aluminium nanoparticles concentration.

Figure 8 shows wt.% of aluminium particles incorporated in the Zn-Fe-Al composite coatings obtained by the pulsed current method at varying aluminium nanoparticles concentration.

Figure 9 shows the corrosion current of Zn-Fe-Si composite coatings obtained by the direct current method at varying silicon nanoparticles concentration.

Figure 10 shows morphologies of the Zn-Fe-Si composite coatings obtained by the direct current method at varying silicon nanoparticles concentration.

Figure 11 shows wt.% of silicon particles incorporated in the Zn-Fe-Si composite coatings obtained by the direct current method at varying silicon nanoparticles concentration.

Figure 12 shows the corrosion current of Zn-Fe-Si composite coatings deposited by the pulsed current method at varying silicon nanoparticles concentration.

Figure 13 shows top surface morphologies of the Zn-Fe-Si composite coatings deposited by the pulsed current method at varying silicon nanoparticles concentration.

Figure 14 shows wt.% of silicon particles incorporated in the Zn-Fe-Si composite coatings deposited by the pulsed current method at varying silicon nanoparticles concentration.

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 aluminium or silicon nanoparticles. In some embodiments, the size of the nanoparticles ranges from about 20 nm to about 30 nm, including values and ranges thereof.

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 Zn-Fe-aluminium (Zn-Fe-Al) or Zn-Fe-silicon (Zn-Fe-Si) coatings on steel substrates. Further, the present disclosure provides an improved method for preparing electroplating compositions comprising Zn-Fe salts and aluminium or silicon nanoparticles. The present disclosure also provides methods for depositing/electroplating said compositions on steel substrates by a direct current (DC) method and a pulsed current method. The coatings of the present disclosure show improved deposition kinetics, improved surface microstructure, higher aluminium or silicon content, and/or improved corrosion resistance.

In some embodiments, the present disclosure provides an electroplating composition comprising 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, boric acid in an amount of about 30 g/L, and aluminium or silicon nanoparticles at a concentration of about 0.5-10 g/L, wherein the electroplating composition has a pH of about 3.5.

Aluminium or silicon nanoparticles are present in the electroplating composition at a concentration of about 0.5-10 g/L, including values and ranges therebetween. For example, in some embodiments, aluminium or silicon nanoparticles are present in the electroplating composition at a concentration of about 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or 10 g/L, including values therebetween. In some embodiments, aluminium or silicon nanoparticles are present in the electroplating composition at a concentration of about 0.5-10, 0.5-7, 0.5-5, 0.5-4, 0.5-3.5, 0.5-3, 0.5-2.5, 0.5-2, 1-10, 1-7, 1-5, 1-4.5, 1-4, 1-3.5, 1-3, 1-2.5, 2-10, 2-7, 2-5, 2-4.5, 2-4, 2.5-10, 2.5-7, 2.5-5, 2.5-4.5, 2.5-4, 2.5-3.5, 3-10, 3-7, 3-5, 3-4.5, 3-4, 3.5-10, 3.5-7, 3.5-5, 3.5-4.5, 4-10, 4-7, 4-5, 4.5-10, 4.5-7, 4.5-5.5, 5-10, 5-7, or 7.5-10 g/L, including values and ranges therebetween.

In some embodiments, provided herein are electroplating compositions comprising 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, boric acid in an amount of about 30 g/L, and aluminium nanoparticles at a concentration of about 2.5-5 g/L, wherein the electroplating composition has a pH of about 3.5. In some embodiments, the aluminium nanoparticles are present in the electroplating composition at a concentration of about 2.5 g/L or 5 g/L. In some embodiments, the electroplating composition comprising about 2.5 g/L or 5 g/L aluminium nanoparticles provides a Zn-Fe-Al coating comprising about 0.3-0.35% by weight of aluminium and exhibiting a corrosion current of about 0.6-1 µA/cm2, and a mixed morphology comprising basal and pyramidal microstructure.

In some embodiments, provided herein are electroplating compositions comprising 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, boric acid in an amount of about 30 g/L, and silicon nanoparticles at a concentration of about 4.5-5.5 g/L, wherein the electroplating composition has a pH of about 3.5. In some embodiments, 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, boric acid in an amount of about 30 g/L, and silicon nanoparticles at a concentration of about 5 g/L, wherein the electroplating composition has a pH of about 3.5. In some embodiments, the electroplating composition comprising about 5 g/L silicon nanoparticles provides a Zn-Fe-Si coating comprising about 1% by weight of silicon and exhibiting a corrosion current of about 2.3-2.5 µA/cm2, and a very fine and compact pyramidal morphology.

Also provided herein is a method of preparing the electroplating compositions of the present disclosure. In some embodiments, the method broadly comprises dissolving boric acid, zinc sulphate, ferrous sulphate, and zinc chloride in this order in distilled water to obtain a main electrolyte solution; adding aluminium or silicon particles to a small portion of the main electrolyte; stirring the electrolyte solution containing aluminium or silicon; ultrasonicating the electrolyte solution containing aluminium or silicon particles; adding the ultrasonicated electrolyte containing aluminium or silicon particles to the main electrolyte solution; adjusting the pH to 3.5; stirring the main electrolyte solution for 24 hours; and again ultrasonicating the electrolyte solution briefly. In this method, aluminium or silicon particles are ultrasonicated directly in a portion of the main electrolyte solution comprising boric acid, zinc sulphate, ferrous sulphate, and zinc chloride to obtain aluminium or silicon nanoparticles. The electrolyte solution containing aluminium or silicon nanoparticles is mixed with the remaining portion of the main electrolyte solution and the aluminium or silicon nanoparticles are dispersed in the main electrolyte solution by stirring and ultrasonication, i.e., by physical dispersion. Chemical dispersion such as surface modification by surfactants or additives is not employed in the present method.

In some embodiments, the method for preparing an electroplating composition of the present disclosure comprises: (i) adding boric acid to distilled water; (ii) after dissolution of boric acid, adding zinc sulphate to the distilled water; (iii) after dissolution of zinc sulphate, adding ferrous sulphate to the distilled water; (iv) after dissolution of ferrous sulphate, adding zinc chloride to the distilled water to obtain an electrolyte solution; (v) after dissolution of zinc chloride, adding aluminium or silicon particles to a portion of the electrolyte solution to obtain an aluminium or silicon-containing solution; (vi) stirring the aluminium or silicon-containing solution for about 30 minutes at a speed of about 300-400 rpm; (vii) ultrasonicating the aluminium or silicon-containing solution after stirring to obtain an ultrasonicated solution; (viii) adding the ultrasonicated solution to the remaining portion of the electrolyte solution; (ix) adding distilled water to the electrolyte solution to make up to a desired volume to obtain the electroplating composition; (x) adjusting pH of the electroplating composition to about 3.5; (xi) stirring the electroplating composition for about 24 hours at a speed of about 300-400 rpm; and (xii) ultrasonicating the electroplating composition to obtain the electroplating composition with a uniformly dispersed aluminium or silicon nanoparticles.

In the preparation method, aluminium or silicon particles are added to a portion of the electrolyte solution containing boric acid, zinc sulphate, ferrous sulphate, and zinc chloride. The aluminium or silicon-containing electrolyte solution is stirred for about 30 minutes at a speed of about 300-400 rpm such as about 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, or 400 rpm. In some embodiments, the aluminium or silicon-containing electrolyte solution is stirred for about 30 minutes at a speed of about 340-360 rpm. In an exemplary embodiment, the aluminium or silicon-containing electrolyte solution is stirred for about 30 minutes at a speed of about 350 rpm.

After stirring, the aluminium or silicon-containing electrolyte solution is ultra-sonicated to obtain aluminium or silicon nanoparticles. In some embodiments, the ultra-sonication is carried out at a frequency of about 25-35 kHz for about 30 minutes. In some embodiments, the ultra-sonication is carried out at a frequency of about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, or 35 kHz. In some embodiments, the ultra-sonication is carried out at a frequency of about 28-32 kHz for about 30 minutes. In an exemplary embodiment, the ultra-sonication is carried out at a frequency of about 30 kHz for about 30 minutes.

The ultra-sonicated aluminium/silicon nanoparticles solution is added to the main electrolyte solution. The volume is adjusted with distilled water to a desired level. The pH of the electroplating composition is adjusted to about 3.5. After the pH adjustment, the electroplating composition is stirred for about 24 hours at a speed of about 300-400 rpm. In an exemplary embodiment, the composition is stirred for about 24 hours at a speed of about 350 rpm. The stirring facilitates uniform dispersion of aluminium or silicon nanoparticles in the electroplating composition.

After 24 hours stirring, the electroplating composition is ultra-sonicated to obtain the final electroplating composition/bath. In some embodiments, the ultra-sonication is carried out at a frequency of about 25-35 kHz for about 30 minutes. In some embodiments, the ultra-sonication is carried out at a frequency of about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, or 35 kHz. In some embodiments, the ultra-sonication is carried out at a frequency of about 28-32 kHz for about 30 minutes. In an exemplary embodiment, the ultra-sonication is carried out at a frequency of about 30 kHz for about 30 minutes. The ultrasonication further facilitates uniform dispersion of aluminium or silicon nanoparticles in the electroplating composition.

The present disclosure also provides methods for depositing the electroplating compositions described herein on steel substrates to provide substrates with zinc-iron-aluminium (Zn-Fe-Al) or zin-iron-silicon (Zn-Fe-Si) 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, a 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 180-200 mA/cm2 and at a stirring rate of about 300 rpm to provide a steel substrate comprising a Zn-Fe-aluminium (Zn-Fe-Al) or a Zn-Fe-silicon (Zn-Fe-Si) coating.

The inventors have found that the electroplating compositions of the present disclosure, when deposited by employing a constant current having a current density of about 180-200 mA/cm2, provide a higher rate of deposition compared to alloy coatings or commercially used Zn-Ni coatings. In some embodiments, the rate of deposition provided by the DC deposition method is about 2.4-3 µm/min, including values and ranges thereof. The rate of deposition of commercial coating is 1µm/min and Zn-Fe alloy coating is 2.91 µm/min.

In some embodiments, the current density employed in the DC method of deposition is about 180, 185, 190, 195, or 200 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.

In some embodiments, an electroplating composition comprising about 200 g/L zinc sulphate, about 40 g/L ferrous sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 2.5 g/L aluminium nanoparticles and having a pH of about 3.5 is deposited on a steel substrate at a current density of about 190 mA/cm2 and a stirring rate of about 350 rpm to provide a steel substrate comprising a Zn-Fe-Al coating.

In some embodiments, the DC method provides a Zn-Fe-Al coating comprising about 0.30-0.35% by weight of aluminium and exhibiting a corrosion current of about 0.6-1 µA/cm2, and a mixed morphology comprising basal and pyramidal microstructure. In some embodiments, the DC method provides a Zn-Fe-Al coating comprising about 0.35% by weight of aluminium and exhibiting a corrosion current of about 0.6 µA/cm2, and a mixed morphology comprising basal and pyramidal microstructure. In some embodiments, the DC method provides a Zn-Fe-Al coating comprising about 0.33% by weight of aluminium and exhibiting a corrosion current of about 1 µA/cm2, and a mixed morphology comprising basal and pyramidal microstructure. The corrosion current of the benchmark/commercial Zn-Ni passivated coating is about 0.95 µA/cm2. The corrosion current of the Zn-Fe-Al coatings of the present disclosure is lower (about 0.6 µA/cm2) or similar (about 1 µA/cm2) to the corrosion current of the commercial Zn-Ni passivated coating; at the same time, the deposition kinetics of the present coatings (2.4-3 µm/min) is higher than that of the commercial coating (1 µm/min).

In some embodiments, an electroplating composition comprising about 200 g/L zinc sulphate, about 40 g/L ferrous sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 5 g/L silicon nanoparticles and having a pH of about 3.5 is deposited on a steel substrate at a current density of about 190 mA/cm2 and a stirring rate of about 350 rpm to provide a steel substrate comprising a Zn-Fe-Si coating. In some embodiments, the DC method provides a Zn-Fe-Si coating comprising about 0.9-1% or 0.95-1% by weight of silicon and exhibiting a corrosion current of about 2.5 µA/cm2, and a very fine and compact pyramidal morphology.

Pulsed Current Deposition
In some embodiments, a 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 about 50-75% and a frequency of about 75-200Hz to provide a steel substrate comprising a zinc-iron-aluminium (Zn-Fe-Al) or a zin-iron-silicon (Zn-Fe-Si) coating. The electroplating composition is stirred at a stirring rate of about 350 rpm during the deposition process.

The inventors have found that the electroplating compositions of the present disclosure, when deposited by employing a pulsed current having an average current density of about 190 mA/cm2, a duty cycle of about 50-75% and a frequency of about 75-200Hz, provide a higher rate of deposition compared to alloy coatings or commercially used Zn-Ni coatings. In some embodiments, the rate of deposition provided by the pulsed deposition method is about 2.8-3.3 µm/min, including values and ranges thereof. In some embodiments, the rate of deposition provided by the pulsed deposition method is about 2.8-3 µm/min, including values and ranges thereof. In some embodiments, the rate of deposition provided by the pulsed deposition method is about 3-3.5 µm/min, including values and ranges thereof. The rate of deposition of commercial coating is 1 µm/min.

In some embodiments, the pulsed current has a duty cycle of about 50, 55, 60, 65, 70, or 75% and a frequency of about 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 192 or 200 Hz. In exemplary embodiments, the pulsed current has a duty cycle of about 75% and a frequency of about 200 Hz or a duty cycle of about 50% and a frequency of about 75 Hz.

In some embodiments, an electroplating composition comprising about 200 g/L zinc sulphate, about 40 g/L ferrous sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 2.5 g/L aluminium nanoparticles and having a pH of about 3.5 is deposited on a steel substrate by employing a pulsed current with an average current density of about 190 mA/cm2, a duty cycle of about 75% and a frequency of about 200Hz to provide a steel substrate comprising a Zn-Fe-Al coating.

In some embodiments, the pulsed method provides a Zn-Fe-Al coating comprising about 0.4% by weight of aluminium and exhibiting a corrosion current of about 0.8 µA/cm2, and a fine, compact morphology comprising a mixture of hexagonal and basal microstructure.

In some embodiments, the pulsed method provides a deposition rate of about 2.8 - 3 µm/min for the aluminium-containing electroplating compositions. In some embodiments, the pulsed method provides a deposition rate of about 3.3 µm/min or about 3.7 µm/min for depositing the electroplating composition comprising 10 g/L aluminium nanoparticles.

In some embodiments, an electroplating composition comprising about 200 g/L zinc sulphate, about 40 g/L ferrous sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 5 g/L silicon nanoparticles and having a pH of about 3.5 is deposited on a steel substrate by employing a pulsed current with an average current density of about 190 mA/cm2, a duty cycle of about 50% and a frequency of about 75 Hz to provide a steel substrate comprising a Zn-Fe-Si coating.

In some embodiments, the pulsed method provides a Zn-Fe-Si coating comprising about 1% by weight of silicon and exhibiting a corrosion current of about 2.3 µA/cm2, and a fine, compact pyramidal morphology. In some embodiments, the pulsed method provides a deposition rate of about 3-3.5 µm/min for the silicon-containing electroplating compositions.

The present disclosure provides a steel substrate comprising a zinc-iron-aluminium (Zn-Fe-Al) or zinc-iron-silicon (Zn-Fe-Si) coating.

In some embodiments, the steel substrate comprises a zinc-iron-aluminium (Zn-Fe-Al) coating, wherein the coating comprises about 0.3-0.4% by weight of aluminium, including values and ranges therebetween. For example, in some embodiments, the steel substrate comprises a Zn-Fe-Al coating comprising about 0.30%, 0.31%, 0.32%, 0.33%, 0.34%, 0.35%, 0.36%, 0.37%, 0.38%, 0.39%, or 0.40% by weight of aluminium. In some embodiments, the steel substrates comprising about 0.3-0.4% by weight of aluminium exhibit a corrosion current density of about 0.6-1 µA/cm2, such as about 0.6, 0.7, 0.8, 0.9, or 1 µA/cm2. The Zn-Fe-Al coatings exhibit a basal morphology, a pyramidal morphology, or a mixed morphology having basal and pyramidal microstructure. In some embodiments, the steel substrate comprising a Zn-Fe-Al coating is obtained by the direct current method, wherein the coating comprises about 0.30-0.35% by weight of aluminium, has a corrosion current density of about 0.6-1 µA/cm2, and exhibits a mixed morphology having basal and pyramidal microstructure. In some embodiments, the steel substrate comprising a Zn-Fe-Al coating is obtained by the pulsed current method, wherein the coating comprises about 0.4% by weight of aluminium, has a corrosion current density of about 0.8 µA/cm2, and exhibits a fine, compact, pyramidal morphology.

In some embodiments, the steel substrate comprises a zinc-iron-silicon (Zn-Fe-Si) coating, wherein the coating comprises about 0.95-1% by weight of silicon, including values and ranges therebetween. For example, in some embodiments, the steel substrate comprises a Zn-Fe-Si coating comprising about 0.95%, 0.96%, 0.97%, 0.98%, 0.99%, or 1% by weight of silicon. In some embodiments, the steel substrates comprising about 0.95-1% by weight of silicon exhibit a corrosion current density of about 2.3-2.5 µA/cm2, including values and range thereof. In some embodiments, the Zn-Fe-Si coatings exhibit a fine, compact pyramidal morphology. In some embodiments, the steel substrate comprising a Zn-Fe-Si coating is obtained by the direct current method, wherein the coating comprises about 1% by weight of silicon, has a corrosion current density of about 2.5 µA/cm2, and exhibits a fine, compact pyramidal morphology. In some embodiments, the steel substrate comprising a Zn-Fe-Si coating is obtained by the pulsed current method, wherein the coating comprises about 1-1.2% by weight of silicon, has a corrosion current density of about 2.3 µA/cm2, and exhibits a fine, compact, pyramidal morphology.

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: Preparation of the electroplating composition
The electroplating bath/composition containing 200 g/L zinc sulphate, 40 g/L ferrous sulphate, 30 g/L boric acid as a buffering agent, 6 g/L zinc chloride as a pickling agent to increase the dissolution of Zn anode and 0.5-10 g/L aluminium or silicon nanoparticles was prepared as shown in Figure 1. Specifically, 30 g boric acid was dissolved in 650 ml of freshly prepared distilled water to obtain a solution. After dissolution of boric acid, 200 g of zinc sulphate was dissolved in the solution. After dissolution of zinc sulphate, 40 g of ferrous sulphate was dissolved in the solution. After dissolution of ferrous sulphate, 6 g zinc chloride was dissolved in the solution to obtain the main electrolyte. 250 ml of the main electrolyte was taken in a separate beaker and 0.5-10 g of aluminium or silicon particles were added to the 250 ml electrolyte solution. The aluminium/silicon-containing electrolyte solution was stirred for 30 mins at a stirring rate of 350 rpm followed by ultrasonication at 30kHz frequency for 30 mins. The ultrasonicated solution was added to the main electrolyte. The volume was made up to 1L and the pH was adjusted to 3.5 followed by 24 hrs stirring at 350 rpm. The electroplating composition was again ultrasonicated at 30kHz frequency for 30mins. The detailed process is shown in Figure 1. No additional chemicals like surfactants or additives were added in the electrolyte for particle dispersion. Particle dispersion was achieved through ultrasonication and magnetic stirring.

Example 2: Zeta Potential Measurement
Zeta potential was measured using zeta-sizer instrument (Malvern Nano Sizer). 100ml of freshly prepared distilled water at pH 7 was taken. 1 g/l Al or Si were added in the solution. Agitation speed was maintained between 200-300 rpm. The pH of the solution was adjusted using HCl and NaOH solution. After 30 mins of stirring, the solution was ultrasonicated at 10KHz frequency. After 10 mins, the solution was again stirred for 10 mins. Then, 10µl solution was taken and injected into zeta-sizer cell. The zeta potentials were measured at 3, 6, 9 and 12 pH. The zeta potential values were noted for both the particles (Al and Si) at different pH (Figure 2).

Zeta potential of Al particles was above +30mV at the operating pH (3.5) in the water media. Therefore, Al particles were well dispersed and can be deposited on the cathode surface. At the operating pH, zeta potential of Si particles was in between +30mV to -30mV. Therefore, unlike Al particles, silicon particles were not well dispersed. To deposit silicon particles on cathode surface, stirring was employed.

Zeta potential (Table 1) and hydrodynamic particle size (Table 2) of Al and Si nano-particles were measured in electrolyte and DM water.
Table 1
pH Zeta potential of Al in DM water Zeta of Al in Zn-Fe electrolyte Zeta of Si in DM water Zeta of Si in Zn-Fe electrolyte
3.5 +42mV +15.39 mV +3.2mV +2.51mV

Table 2
pH Hydrodynamic diameter of Al in DM water Hydrodynamic diameter of Al in Zn-Fe electrolyte Hydrodynamic diameter of Si in DM water Hydrodynamic diameter of Si in Zn-Fe electrolyte
3.5 100nm 2.33µm 2µm 10.9µm

Hydrodynamic diameter of Al was increased from 100nm to 2.33µm in Zn-Fe electrolyte. So, the particles were agglomerated in the electrolyte media. Hydrodynamic diameter of Si was also increased in the electrolyte. So, the particles dispersion was hampered in the electrolyte media.

Zeta potential or particle surface charge was reduced in the electrolyte media indicating that the particles are not well dispersed. The zeta potential is within ±30mV for both the particles in the electrolyte. So, the particles were not dispersed properly in the electrolyte.

Example 3: Direct current (DC) deposition of Zn-Fe-Al coating
DC deposition was performed galvanostatically using a Potentiostat (Make: AMETEK) in a two-electrode setup. IF (interstitial free) steel sheet was used as a cathode and pure Zn (99.5% pure) as an anode. Prior to deposition, the IF steel samples were degreased to remove surface oil and then dipped in a dilute HCl solution to remove any oxide film which might be present. The samples were rinsed in distilled water and the deposition was carried out. The electroplating composition was deposited on the IF steel sheet substrate at a constant current with a current density of 190 mA/cm2 and at a stirring rate of about 350 rpm to provide the steel substrate comprising a zinc-iron-aluminium (Zn-Fe-Al) coating.

During deposition, the electrolyte was stirred at a constant rate using magnetic stirrer. The stirrer size was 5cm. The stirring rate was maintained at 350rpm. The current was supplied through a potentiostat. After plating/deposition, the coated samples were rinsed with distilled water and dried.

Various characterizations were performed on the coated steel substrate. Scanning Electron Microscope (SEM) (Make: Nova-Nano) study was conducted to observe the coating morphology, SEM-EDS to obtain wt.% of aluminium/silicon in the coating at different current densities and potentiodynamic polarization test (Make: Gamry) was done to obtain the corrosion potential (Ecorr) and corrosion current density (ICorr) values using Tafel extrapolation.

The potentiodynamic polarization test results showing corrosion current densities of Zn-Fe-Al composite coatings at different particle concentrations are shown in Figure 3. Figure 3 also shows the corrosion current of the benchmark/commercially used coating (Zn-Ni passivated coating) and that of a Zn-Fe alloy coating deposited by a direct current at a current density of 190 mA/cm2 and a stirring rate of 300 rpm. The corrosion current was calculated using Gramry software (Version: 4.35) by Tafel extrapolation method. The corrosion current of the benchmark/commercial Zn-Ni passivated coating was 0.95 µA/cm2 and that of the DC-deposited Zn-Fe alloy coating was 1.1 µA/cm2. For the Zn-Fe-Al coatings, at the highest concentration of aluminium particles (10 g/L), the highest corrosion current (11.25 µA/cm2) was noticed. The lowest corrosion current of 0.6 µA/cm2 and 1 µA/cm2 was observed at a moderate concentration of Al particles, 2.5 g/L and 5 g/L, respectively. As the Al concentration decreased below 2.5g/L, the corrosion current started to increase again implying that corrosion resistance was decreasing. The corrosion potentials (-1.06V) were similar for all these Zn-Fe-Al composite coatings.

The particle concentration affects the morphology and the texture of the deposit which in turn affects the corrosion resistance properties of the coating. At the lowest concentration of particles (0.5g/L), finer structure was observed. For higher particle concentrations, relatively coarser structure was found. Also, some textural difference (based on alignment of basal plane morphology) was observed at different particle concentration. A mixture of pyramidal and basal morphology was seen for 1, 2.5, and 5 g/L of particle concentration whereas only basal morphology was observed at 10 g/L of Al concentration. Pyramidal morphology is more corrosion resistive compared to the basal morphology. For this reason, superior corrosion resistance was observed for moderate concentration of Al particles. Very compact coating was noticed at all concentrations of the particles.

Al wt.% in the final coatings was measured from cross-sectional area analysis using SEM-EDS. Al wt.% in the final coating increased with the increasing concentration of particles in the electrolyte. The highest Al wt.% in the coating (0.35%) was observed at 2.5 g/L concentration leading to the lowest corrosion rate (0.6 µA/cm2) of the coating. A further increase in Al concentration in the electroplating bath beyond 2.5 g/L led to a reduction in Al wt.% in the coating. The lowest particle incorporation (0.01 wt.%) was observed at 1 g/L particle concentration in the electrolyte. Accordingly, a higher corrosion current (3.25 µA/cm2) was observed for 1 g/L of Al concentration. Coating deposition kinetics measured through the rate of deposition (2.4-2.8 µm/min) were similar at all the concentration of particles. The rate of deposition of commercial coating is 1 µm/min.

Example 4: Pulsed current deposition of Zn-Fe-Al coating
The pulsed current deposition was performed galvanostatically using a Potentiostat (Make: AMETEK) in a two-electrode setup. IF (interstitial free) steel sheet was used as a cathode and pure Zn (99.5% pure) as an anode. Prior to deposition, the IF steel samples were degreased to remove surface oil and then dipped in a dilute HCl solution to remove any oxide film which might be present. The samples were rinsed in distilled water and the deposition was carried out. The electroplating composition was deposited on the IF steel sheet substrate by employing a pulsed current having an average current density of about 190 mA/cm2 and two different duty cycles/frequency parameters. P1 represents pulse 1 with 75% duty cycle and 200Hz frequency and P2 represents pulse 2 with 50% duty cycle and 75Hz frequency.

During deposition, the electrolyte was stirred at a constant rate using magnetic stirrer. The stirrer size was 5cm. The stirring rate was maintained at 350 rpm. The current was supplied through a potentiostat. After plating/deposition, the coated samples were rinsed with distilled water and dried.

Various characterizations were performed on the coated steel substrate. Scanning Electron Microscope (SEM) (Make: Nova-Nano) study was conducted to observe the coating morphology, SEM-EDS to obtain wt.% of Al/Si in the coating at different current densities and potentiodynamic polarization test (Make: Gamry) was done to obtain the corrosion potential (Ecorr) and corrosion current density (ICorr) values using Tafel extrapolation.

The potentiodynamic polarization results showing corrosion current densities of all pulsed deposited Zn-Fe-Al composite coatings are shown in Figure 6. At P1 which corresponds to a higher duty cycle and a higher frequency, the lowest corrosion current (0.8 µA/cm2) was observed at a moderate concentration of Al particles (2.5 g/L). As the Al concentration was further increased above 2.5 g/L, the corrosion current started to increase. Corrosion potential was similar (-1.06V) for all the cases. The corrosion current of the benchmark coating was as noted in Example 3 (0.95 µA/cm2). The corrosion current of the Zn-Fe alloy coating deposited by the pulse 1 parameters was 1 µA/cm2 and that deposited by the pulse 2 parameters was 1.94 µA/cm2.

P2 represents lower duty cycles and lower frequency. Here, the lowest corrosion current (1.43 µA/cm2) was observed at the highest Al concentration (10 g/L) and as the Al concentration decreased below 10 g/L, the corrosion current started to increase. The corrosion current was the highest at 1 g/L of Al concentration. The corrosion current of the Zn-Fe alloy coating deposited by the pulse 2 parameters was 1.94 µA/cm2. Similarly, like pulse 1, corrosion potentials were comparable to each other (-1.07V).

The Zn-Fe-Al composite coating deposited at the higher duty cycle and higher frequency showed superior corrosion resistance compared to the lower duty cycle and frequency at 2.5 g/L Al particle concentration.

Figure 7 shows the top surface microstructures of pulsed current deposited Zn-Fe-Al composite coatings at different particle concentrations. For P1, very compact coatings were observed for low and moderate particle concentrations. Porous structures were observed at the highest concentration of particles. A mixture of hexagonal and basal morphology was noticed for 1, 2.5 and 5 g/l of particle concentrations. For 10 g/l of particle concentration, only basal microstructure was observed. Also, the microstructures were coarser at the highest concentration of particles. Thus, better corrosion resistance was observed for moderate concentration of particles.

For P2, very compact coatings were observed for all the sets of particle concentrations. Finer structures were observed for a low concentration of particles (1 g/L) and coarser structures started to form as the concentration of the particles increased. No textural difference was noticed for all sets of particle concentration.

Figure 8 shows the Al wt.% in the pulse current deposited composite coatings. The Al wt.% in the final coating was measured from the cross-sectional area analysis using SEM-EDS. For pulse 1, the highest particle (0.4 wt%) incorporation was observed at 2.5 g/L of particle concentration corresponding to superior corrosion resistance. At lower concentration, less particles (0.03 wt% at 1 g/L) were incorporated in the coating corresponding to a very high corrosion current observed for 1g/L of particle concentration. Highest deposition kinetics was observed for 10 g/L of particle concentration (3.3 µm/min). For other concentrations, deposition kinetics were similar (2.8 - 3 µm/min).

For pulse 2, the highest particle (0.37 wt%) incorporation was observed for 10 g/L of particle concentration in electrolyte corresponding to a better corrosion resistance of the coating deposited at 10 g/L of Al particles. The highest deposition kinetics was observed for 10 g/L of particle concentration (3.7µm/min). For other concentrations, deposition kinetics were similar (3 µm/min).

Example 5: Direct current (DC) deposition of Zn-Fe-Si coating
Zn-Fe-Si containing electroplating compositions were deposited by a direct current as described in Example 3 and various characterizations were performed.

The potentiodynamic polarization results showing corrosion current densities of Zn-Fe-Si composite coating deposited by the direct current method are shown in Figure 9. The corrosion current of the benchmark/commercial Zn-Ni passivated coating was 0.95 µA/cm2 and that of the DC-deposited Zn-Fe alloy coating was 1.1 µA/cm2. With regard to silicon-containing compositions, at a moderate concentration (5 g/L) of silicon particles, the lowest corrosion current was noticed (2.5 µA/cm2). The corrosion current decreased continuously from 1 g/L to 5 g/L silicon concentration. The highest corrosion current (3.8 µA/cm2) was observed at 10 g/L of particles concentration. The corrosion potentials were similar (-1.07V) for all these Zn-Fe-Si composite coatings.

Figure 10 shows the top surface microstructures of Zn-Fe-Si composite coatings deposited at different silicon nanoparticle concentrations. Compact coatings were observed for all concentrations of silicon particles. Apart from 10 g/L of particle concentration, no textural difference was observed. Hexagonal morphology was seen at the top surface for the Zn-Fe-Si composite coating with 10 g/L of particle concentration whereas only pyramidal morphology was observed for the rest of the coatings. Accordingly, a higher corrosion current was observed when the particle concentration increased from 5 g/L to 10 g/L. Finer structures were noticed for composite coatings with 1g/L and 5g/L silicon nanoparticle concentration.

Figure 11 shows the wt.% of Si in the final DC-deposited Zn-Fe-Si composite coatings. The lowest silicon was incorporated (0.15 wt.%) at 1g/L of particle concentration. This explains a higher corrosion current (3.8 µA/cm2) noticed for Zn-Fe-Si composite coating with 1g/L particle concentration. As the particle concentration increased in the electroplating composition, more particles were incorporated on the cathode surface. The highest particles (0.99 wt.%) were incorporated at 5 g/L of silicon particle concentration. Accordingly, the lowest corrosion current (2.5 µA/cm2) was observed for this concentration. The highest deposition kinetics was observed for 1 g/L of particle concentration (3.3 µm/min). For other concentrations, the deposition kinetics were similar (2.9 µm/min).

Example 6: Pulsed current deposition of Zn-Fe-Si coating
Zn-Fe-Si containing electroplating compositions were deposited by a pulsed current as described in Example 4 and various characterizations were performed.

Figure 12 shows potentiodynamic polarization results depicting corrosion current densities of pulse deposited Zn-Fe-Si composite coatings. In the case of pulse 1, the corrosion current for the Zn-Fe alloy coating was 1 µA/cm2. At the moderate concentration of Si particles (5 g/L), the corrosion current was the lowest (3 µA/cm2). The maximum corrosion current was observed at a concentration of 2.5 g/L (4.51 µA/cm2). The corrosion current dropped at a concentration of 5 g/L and increased again at a concentration of 10 g/L. The corrosion potential was similar at all the concentrations (-1.07V).

In terms of corrosion current, promising results were observed for pulse 2 deposits. In the case of pulse 2, the lowest corrosion current (2.31 µA/cm2) was observed at a particle concentration of 5 g/L. Like the P1 deposited coatings, the corrosion potential was similar for all the sets of pulsed current deposited composite coatings (-1.08V).

Figure 13 shows the top surface microstructures of Zn-Fe-Si composite coatings deposited at different particle concentrations. In the case of P1, a finer and compact structure was observed at 1g/L of Si particle concentration. Coarser structures were observed at moderate concentrations of particles. A pyramidal morphology was noticed for Zn-Fe-Si composite coatings with particle concentrations of 1-5 g/L whereas a hexagonal morphology was observed for 10 g/L of particle concentration. Accordingly, an increase in the corrosion current was seen for 10 g/L of particle concentrations.

For pulse 2 as well, a finer and compact structure was observed at 1 g/L of particle concentration and coarser structures were observed at moderate concentrations. A pyramidal morphology was noticed for Zn-Fe-Si composite coatings with particle concentrations of 1-5 g/L whereas a hexagonal morphology was observed for 10 g/L of particle concentration. Accordingly, a higher corrosion current was seen for 10 g/L of particle concentrations.

Figure 14 shows the silicon wt.% in the pulsed deposited composite coatings. For Pulse 1, maximum nanoparticles (1.21 wt.%) were incorporated for 5 g/L of silicon concentration. Therefore, the lowest corrosion current (3 µA/cm2) was observed for Zn-Fe-Si composite coating with 5 g/L of silicon concentration. As the concentration of particles in the electroplating composition increased or decreased from 5 g/L, lower particles were incorporated in the coating leading to an increase in the corrosion current. Deposition kinetics were similar for all the concentrations (3-3.5µm/min).

In the case of pulse 2, Si wt.% in the final deposit steadily increased with increasing concentration of particles in the electroplating composition till 5 g/L and beyond that the Si wt% in the coating reduced. The highest amount of particles (1.01 wt%) were incorporated at 5 g/L of particle concentration resulting in better corrosion resistance than others. Above 5 g/L of particle concentration, the particle incorporation was reduced. Therefore, the corrosion current again increased for 10 g/L of particle concentration. Deposition kinetics were comparable for all the concentrations (3.1-3.5 µm/min).

References
1. M.W. Losey, P.S.R.S., Electrodeposition. Reference Module in Materials Science and Materials Engineering, 2017.
2. Jayakrishnan, D.S., Electrodeposition: the versatile technique for nanomaterials. Corrosion Protection and Control Using Nanomaterials, 2012.
3. Zangari, G., Fundamentals of Electrodeposition. Encyclopedia of Interfacial Chemistry, 2018.
4. M. Sajjadnejad, M. Ghorbani, A. Afshar; Preparation and corrosion resistance of direct current and pulse current electrodeposited Zn-TiO2 nanocomposite coatings, 2014
5. Shirin Dehgahi, Rasool Amini, Morteza Alizadeh; Corrosion, passivation and wear behaviors of electrodeposited Ni–Al2O3–SiC nano-composite coatings
6. H. Gu¨l, F. KilIc, S. Aslan, A. Alp, H. Akbulut, Characteristics of electro-co-deposited Ni– Al2O3 nano-particle reinforced metal matrix composite (MMC) coatings, Wear 267 [5–8] (2009) 976–990.
7. L. Chen, L. Wang, Z. Zeng, J. Zhang, Effect of surfactant on the electrodeposition and wear resistance of Ni–Al2O3 composite coatings, Mater. Sci. Eng. A 434 (2006) 319–325.
8. Q. Feng, T. Li, H. Yue, K. Qi, F. Bai, J. Jin, Preparation and characterization of nickel nanoAl2O3 composite coatings by sediment co-deposition, Appl. Surf. Sci. 254 (2008) 2262–2268.
9. J. Steinbach, H. Ferkel, Scripta Mater. 44 (2001) 1813.
10. A. Mo¨ller, H. Hahn, Nanostruct. Mater. 12 (1999) 259.
11. B. Mu¨ller, H. Ferkel, Nanostruct. Mater. 10 (1998) 1285.
12. S.C. Wang, W.C.J. Wei, Mater. Chem. Phys. 78 (2003) 574.
13. X.H. Chen, F.Q. Cheng, S.L. Li, Surf. Coat. Technol. 155 (2002) 274.
14. I. Garcia, J. Fransaer, J.P. Celis, Surf. Coat. Technol. 148 (2001) 171.
15. A.F. Zimmerman, G. Palumbo, K.T. Aust
16. Arthi Jayaraman; Polymer grafted nanoparticles: Effect of chemical and physical heterogeneity in polymer grafts on particle assembly and dispersion
17. Jamal M. A. Alsharefa , Mohd Raihan Tahaa, Tanveer Ahmed Khan; PHYSICAL DISPERSION OF NANOCARBONS IN COMPOSITES–A REVIEW
18. S.Das, S.Datta, A.K.Mukhopadhyay, K.S.Pal, D.Basu, Al–Al2O3 core–shell composite by microwave induced oxidation of aluminium powder, Materials Chemistry and Physics 122 (2010) 574–581
19. K. Cai, M. Ode, H. Murakami Colloids and Surfaces, Influence of polyelectrolyte dispersants on the surface chemical properties of aluminum in aqueous suspension, Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 458–463
20. S. Shabana, S.H. Sonawane, V. Ranganathan, P.H. Pujjalwar, D.V. Pinjari, B.A. Bhanvase, P.R. Gogate, Muthupandian Ashokkumar, Ultrasonics Sonochemistry 36 (2017) 59–69
21. M. Nöske, S. Breitung-Faes, A. Kwade, Electrostatic Stabilization and Characterization of Fine Ground Silicon Particles in Ethanol, Silicon (2019) 11:3001–3010
22. A. Gupta, A. S. G. Khalil, M. Winterer, H. Wiggers, Stable Colloidal Dispersions of Silicon Nanoparticles for the Fabrication of Films using Inkjet Printing Technology, 2010 3rd International Nanoelectronics Conference (INEC), Hong Kong, 2010, 1018-1019
23. J. Rytk¨onen, R. Miettinen, M. Kaasalainen, V. Lehto, J. Salonen, A. N¨arv¨anen, Functionalization of Mesoporous Silicon Nanoparticles for Targeting and Bioimaging Purposes Journal of Nanomaterials, Volume 2012, Article ID 896562, 9 pages
24. K. Herynkova, P. Simakova, O. Cibulka, A. Fucíkova, M. Hubalek Kalbacov, Hydrophilic Luminescent Silicon Nanoparticles in Steric Colloidal Solutions: Their Size, Agglomeration, and Toxicity, Phys. Status Solidi C 2017, 1700195
25. Y.B. Zhou, B.Y. Qian, H.J. Zhang, Al particles size effect on the microstructure of the co-deposited Ni–Al composite coatings, Thin Solid Films 517 (2009) 3287–3291
26. N. Mohsen ADABI, Ahmad Ali AMADEH, Electrodeposition mechanism of Ni??Al composite coating, Transactions of Nonferrous Metals Society of China 24(2014) 3189-3195
27. S. Ghanbari, F. Mahboubi, Corrosion resistance of electrodeposited Ni–Al composite coatings on the aluminum substrate Materials and Design, 2011
28. N. Daemi, F. Mahboubi, H. Alimadadi, Effect of plasma nitriding on electrodeposited Ni–Al composite coating, Materials and Design 32 (2011) 971–975
29. B. Bostani, R. Arghavanian and N. Parvini-Ahmadi, Study on particle distribution, microstructure and corrosion behavior of Ni-Al composite coatings Materials and Corrosion 2012, 63, No. 4
30. X. Cui, W. Wei, H. Liub, W. Chena, Electrochemical study of codeposition of Al particle—Nanocrystalline Ni/Cu composite coatings Electrochimica Acta 54 (2008) 415–420
31. S. Chhangani, M.J.N.V. Prasad, Microstructure, texture and tensile behavior of pulsed electrodeposited Ni–Al composites produced using organic additive-free sulfamate bath loaded with Al nanoparticles Materials Characterization 136 (2018) 247–256
32. H. Liu, W. Chen, Electrodeposited Ni–Al composite coatings with high Al content by sediment co-deposition Surface & Coatings Technology 191 (2005) 341–350
33. M. ALIZADEH, A. TEYMURI, Structure, indentation and corrosion characterizations of high-silicon Ni-Si nano-composite coatings prepared by modified electrodeposition process Transactions of Nonferrous Metals Society of China 29(2019) 608-616
34. P. Fellner, P.K. Cong, Ni-B and Ni-Si composite electrolytic coatings, Surface and Coatings Technology 82 (1996) 317-319
35. Q. Long, Y. Zhong, T. Zheng, M. Peng, Behavior of Electrodeposited Fe/FeSi Composite in High Magnetic Fields, Journal of The Electrochemical Society, 166 (15) D857-D867 (2019)
36. Y. Zhong, P. Zhou, J. Zhou, H. Wang, L. Fan, L. Dong, T. Zheng, W. Shen Shanghai, Study on the pulse reverse electrodeposition of Fe-nano-Si composite coatings in magnetic field, Applied Surface Science 309 (2014) 278–284
37. Direct and pulsed current electroplating compositions and methods to produce improved zinc-iron coatings on steel, 202131013982
38. H. PARK and J. A. SZPUNAR; THE ROLE OF TEXTURE AND MORPHOLOGY IN OPTIMIZING THE CORROSION RESISTANCE OF ZINC-BASED ELECTROGALVANIZED COATINGS; corrosion science.

We Claim:

1. An electroplating composition comprising 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, boric acid in an amount of about 30 g/L, and aluminium or silicon particles at a concentration of about 0.5-10 g/L, wherein the electroplating composition has a pH of about 3.5.
2. The electroplating composition as claimed in claim 1, wherein the aluminium particles are present at a concentration of about 2.5-5 g/L.
3. The electroplating composition as claimed in claim 1, wherein the silicon particles are present at a concentration of about 5 g/L.
4. A method for preparing the electroplating composition as claimed in any one of claims 1-3, comprising:
a. adding boric acid to distilled water;
b. after dissolution of boric acid, adding zinc sulphate to the distilled water;
c. after dissolution of zinc sulphate, adding ferrous sulphate to the distilled water;
d. after dissolution of ferrous sulphate, adding zinc chloride to the distilled water to obtain an electrolyte solution;
e. after dissolution of zinc chloride, adding aluminium or silicon particles to a portion of the electrolyte solution to obtain an aluminium or silicon-containing solution;
f. stirring the aluminium or silicon-containing solution for about 30 minutes at a speed of about 300-400 rpm;
g. ultrasonicating the aluminium or silicon-containing solution after stirring to obtain an ultrasonicated solution;
h. adding the ultrasonicated solution to remaining portion of the electrolyte solution;
i. adding distilled water to obtain the electroplating composition;
j. adjusting pH of the electroplating composition to about 3.5;
k. stirring the electroplating composition for about 24 hours at a speed of about 300-400 rpm; and
l. ultrasonicating the electroplating composition.
5. The method as claimed in claim 4, wherein the aluminium or silicon-containing solution is stirred for about 30 minutes at a speed of about 350 rpm.
6. The method as claimed in claim 4 or 5, wherein the aluminium or silicon-containing solution is ultrasonicated in step g) at a frequency of about 25-35 kHz for about 30 minutes and the electroplating composition is ultrasonicated in step l) at a frequency of about 25-35 kHz for about 30 minutes.
7. The method as claimed in any one of claims 4-6, wherein the electroplating composition is stirred for about 24 hours at a speed of about 350 rpm.
8. A method for depositing the electroplating composition as claimed in any one of claims 1-3 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 180-200 mA/cm2 and at a stirring rate of about 350 rpm to provide a steel substrate comprising a zinc-iron-aluminium (Zn-Fe-Al) or a zinc-iron-silicon (Zn-Fe-Si) coating.
9. The method as claimed in claim 8, wherein the current density is about 190 mA/cm2.
10. The method as claimed in claim 8 or 9, wherein the electroplating composition comprises aluminium particles at a concentration of about 2.5-5 g/L.
11. The method as claimed in any one of claims 8-10, wherein the Zn-Fe-Al coating deposited by the method comprises about 0.30-0.35% by weight of aluminium.
12. The method as claimed in any one of claims 8-11, wherein the Zn-Fe-Al coating provided by the method exhibits a corrosion current density of about 0.6-1 µA/cm2.
13. The method as claimed in claim 8 or 9, wherein the electroplating composition comprises silicon particles at a concentration of about 5 g/L.
14. The method as claimed in any one of claims 8, 9, and 13, wherein the Zn-Fe-Si coating deposited by the method comprises about 0.95-1% by weight of silicon.
15. The method as claimed in any one of claims 8, 9, 13, and 14, wherein the Zn-Fe-Si coating provided by the method exhibits a corrosion current density of about 2.5 µA/cm2.
16. A method for depositing the electroplating composition as claimed in any one of claims 1-3 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 about 50-75% and a frequency of about 75-200Hz to provide a steel substrate comprising a zinc-iron-aluminium (Zn-Fe-Al) or a zin-iron-silicon (Zn-Fe-Si) coating.
17. The method as claimed in claim 16, wherein the electroplating composition comprises aluminium particles at a concentration of about 2.5 g/L.
18. The method as claimed in claim 17, wherein the pulsed current has a duty cycle of about 75% and a frequency of about 200 Hz.
19. The method as claimed in claim 17 or 18, wherein the Zn-Fe-Al coating deposited by the method comprises about 0.4% by weight of aluminium.
20. The method as claimed in any one of claims 17-19, wherein the Zn-Fe-Al coating provided by the method exhibits a corrosion current density of about 0.8 µA/cm2.
21. The method as claimed in claim 16, wherein the electroplating composition comprises silicon particles at a concentration of about 5 g/L.
22. The method as claimed in claim 21, wherein the pulsed current has a duty cycle of about 50% and a frequency of about 75 Hz.
23. The method as claimed in claim 21 or 22, wherein the Zn-Fe-Si coating deposited by the method comprises about 1% by weight of silicon.
24. The method as claimed in any one of claims 21-23, wherein the Zn-Fe-Si coating provided by the method exhibits a corrosion current density of about 2.31 µA/cm2.
25. A steel substrate comprising a zinc-iron-aluminium (Zn-Fe-Al) coating, wherein the coating comprises about 0.3-0.4% by weight of aluminium.
26. The steel substrate as claimed in claim 25, wherein the coating exhibits a mixed morphology having basal and pyramidal microstructure.
27. The steel substrate as claimed in claim 25 or 26, wherein the coating exhibits a corrosion current density of about 0.6-1 µA/cm2.
28. A steel substrate comprising a zinc-iron-silicon (Zn-Fe-Si) coating, wherein the coating comprises about 0.95-1% by weight of silicon.
29. The steel substrate as claimed in claim 28, wherein the coating exhibits a pyramidal morphology.
30. The steel substrate as claimed in claim 28 or 29, wherein the coating exhibits a corrosion current density of about 2.3-2.5 µA/cm2.