Description:FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[see section 10 and rule13]

“ZINC-BASED METAL MATRIX COMPOSITE COATINGS AND METHODS THEREOF”

NAME AND ADDRESS OF THE APPLICANTS:

APPLICANT 1 (A) NAME: Tata Steel Limited
(B) NATIONALITY: INDIA
(C) ADDRESS: Jamshedpur, 831001, Jharkhand, India

APPLICANT 2 (A) NAME: Council of Scientific and Industrial Research
(B) NATIONALITY: INDIA
(C) ADDRESS: Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110001

The following specification particularly describes the invention and the manner in which it is to be performed.

TECHNICAL FIELD
The present disclosure relates to the field of electroplating. Particularly, the present disclosure relates to electroplating compositions comprising zinc, silicon nanoparticles, a grain refiner and/or a surfactant; methods of preparing them; methods of depositing them on steel substrates and steel substrates obtained therefrom.

BACKGROUND OF THE DISCLOSURE
Electroplating is one of the most important surface coating techniques used to enhance the service life of steel. In particular, the development of zinc coatings has been a potential area of interest for protection of aerospace and automobile components formed out of steel. However, these coatings are found to undergo corrosion in aggressive media and at temperatures greater than 384 K. Several methods have been adopted to improve the properties of such zinc-based coatings. Electrodeposition is widely used for preparing metal matrix composites due to its low cost, ease of preparation, low working temperatures, low energy requirements and capability for coating components with complex geometries. In recent years, there has been a growing research attention on composite coatings for their excellent properties such as wear resistance and high temperature corrosion resistance. The composite coating layer is composed of metal or alloy matrix containing uniform distribution of compositing nanoparticles of desirable microstructure. The composite coating can be provided by a method employing a direct current electrodeposition to form a metal matrix coating on a steel substrate with superior corrosion resistance. The quality of the electrodeposited composite coatings depends on the operating conditions such as current density, bath composition, pH, particle concentration in solution, temperature, stirring rate, presence of additives and the size and form of particles, which markedly affect the content and distribution of incorporated particles in the coatings.

Various studies have been carried out to provide metal-based composite coatings, some of which are described briefly below.

U. S. Pat. No. 5,618,634, by Hosoda et al. describes an electroplated metal sheet and a plating solution containing at least one organic compound to co-deposit carbon up to 10 wt-% and provides improved post-painting corrosion resistance, press formability, and spot weldability but resists “powdering” during press forming.
U. S. Pat. No. 4,800,134, by Izaki et al. reports Zn or Zn alloy matrix comprising substantially water-insoluble chromate particles, and additional fine or colloidal particles of Al2O3 to yield improved corrosion resistance.

U. S. Pat. No. 4,411,742, by Donakowski et al. provides a process for co-depositing zinc and graphite and resultant surface had improved corrosion resistance.

U.S. Pat. No. 5,618,634, by Hosoda et al. discloses a composite plated coating with a coating weight of 0.5-200 g/m2 which contains 0.001-10 wt % of co-deposited carbon, wherein the electroplated metal sheet exhibited improved post-painting corrosion resistance, press form ability, and spot weldability.

U. S. Pat. No. 6,938,552 B2 by Tom et al. discloses zinc or zinc-alloy multi-layer plated wires coated with a corrosion resistant coating to enhance corrosion, abrasion, and erosion resistance which find application in extreme corrosion environments such as salt water.

U.S. Patent Application Publication No. 2015/0197857 A1 by Nakada et al. discloses a corrosion-resistant alloy coating film containing 10 to 98 wt % Ni, 1 to 50 wt % Cr, and 0.1 to 30 wt % Si.

U.S. Pat. No. 11,066,752 B2 by Gaydos et al., discloses an electrodeposition of multilayer zinc and manganese by imposing two different current densities.

U.S. Patent Application Publication No. 2007/0158619 A1 by Wang et al. discloses forming a composite coating on a substrate by employing an electroplating solution that includes metal cations of a metallic matrix to be deposited and carbon nanotubes dispersed in it.

U.S. Pat. No. 4,533,606 by Teng et al. discloses a zinc/silicon/phosphorus coating that improves the resistance of the substrate to corrosion, wear, galling and stress corrosion cracking. The electrodeposited coating comprises of 70% to about 99.5% by weight of zinc, and about 0.10% to about 10% by weight of silicon, and about 0.5% to about 20% by weight of phosphorus.

A research article, Ind. Eng. Chem. Res. 50 (2013) 12827-12837, discloses Zn-Si3N4 composite electrodeposited in the presence of surfactants SDS and CTAB wherein the latter exhibited highest corrosion resistance.

A research article, Int. J. Electrochem. Soc. 162 (2015) D480, discloses that the corrosion and electrochemical properties of Zn/SiO2 nanocomposite coatings were found to depend strongly on the concentration of SiO2 nanoparticles; low SiO2 concentration (<10%) had the least corrosion current compared to coating with high SiO2 concentration (>10%).

A research article, Int. J. Electrochem. Sci., 10 (2015) 3988, discloses that zinc composites containing Cr2O3 and SiO2 nanoparticles have good synergetic effect on the corrosion resistance, and an improvement in microhardness and thermal stability was observed post-annealing at temperatures of 250 ?C for 16 hours.

A research article, Journal of Physics: Conf. Series 1082 (2018) 012064, discloses that the corrosion resistance of Zn-SiC composite coating was improved as SiC particles act as inert barriers to the initiation of corrosion defects and modify the microstructure of Zn-SiC composite coating.

A research article, Surface Engineering and Applied Electrochemistry, 49 (2013) 161–167, discloses Zn–Ni–SiO2 multilayer nanocomposite coatings that exhibited more than 100 times higher corrosion resistance than the monolayer Zn–Ni–SiO2 composite coatings which were 1.5 times higher corrosion resistant than the monolayer Zn–Ni alloy coatings.

Although several studies have attempted to provide Zn-based composite coatings, there is still a need in the art to provide a suitable electroplating bath, electrolyte preparation process and suitable particle dispersion process 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 200-300 g/L, zinc chloride in an amount of about 5-100 g/L, boric acid in an amount of about 10-50g/L, silicon nanoparticles in an amount of about 0.5-10 g/L, and a surfactant and/or a grain refiner in an amount of about 1-5 g/L, wherein the electroplating composition has a pH of about 3 to 5.

The present disclosure also relates to a method for preparing the electroplating composition described herein, comprising: a) adding boric acid to hot distilled water; b) after dissolution of boric acid, adding a surfactant and/or a grain refiner to the distilled water; c) after dissolution of the surfactant and/or the grain refiner, adding zinc sulphate to the distilled water; d) after dissolution of zinc sulphate, adding zinc chloride to the distilled water; e) after dissolution of zinc chloride, adding silicon nano-powder to the distilled water to obtain an electrolyte; f) ultrasonicating the electrolyte for about 30-90 minutes to obtain an ultrasonicated solution; g) stirring the ultrasonicated solution at about 600-900 rpm for about 2-4 hours; and h) adjusting pH of the ultrasonicated solution to about 3 to 5 to obtain the electroplating composition.

The present disclosure provides a direct current method for depositing the electroplating composition on a steel substrate, comprising: a) providing the steel substrate as a cathode; b) depositing the electroplating composition on the steel substrate at a constant current with a current density of about 10-20 A/dm2 and at a temperature of about 35 to 45? to provide a steel substrate comprising a zinc-silicon (Zn-Si) composite coating.

The present disclosure further relates to a steel substrate comprising a zinc-silicon (Zn-Si) composite coating, wherein the coating comprises about 1-3% by weight of silicon and exhibits a corrosion rate of about 0.01-0.06 mm per year (mmpy).

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

Figure 2 shows the scanning electron microscopic (SEM) image of a Zn coating (deposited from an electroplating composition comprising no surfactant or grain refiner and no silicon particles) on a mild steel substrate exhibiting a closed packed morphology.

Figure 3, panel a shows the SEM image of a Zn coating (deposited from an electroplating composition comprising a surfactant and/or grain refiner) on a mild steel substrate exhibiting a polygonal morphology.

Figure 3, panel b shows the SEM image of a Zn-Si composite coating (deposited from an electroplating composition comprising Si nanoparticles and no surfactant or grain refiner) on a mild steel substrate exhibiting a closed packed polygonal morphology.

Figure 4, panel a shows the SEM image of a Zn-Si composite coating (deposited from an electroplating composition comprising both Si nanoparticles and a surfactant and/or grain refiner) on a mild steel substrate exhibiting a plate-like morphology.

Figure 4, panel b shows a cross-sectional SEM image showing the coating thickness of 6 to 7 µm.

Figure 5, panel a shows an Energy Dispersive X-ray analysis of Zn-Si nanocomposite coating (Inset: Zn, Si composition in atomic percentage).

Figure 5, panel b shows an X-ray fluorescence spectrum of Zn-Si nanocomposite coating (Inset: Zn, Si composition in atomic percentage).

Figure 6, panel a shows a representative Nyquist plot of: a) Zn coating, b) Zn coating deposited from an electroplating composition comprising SDS, c) Zn + Si coating deposited from an electroplating composition comprising no surfactant or grain refiner, d) Zn + Si coating deposited from an electroplating composition comprising SDS.

Figure 6, panel b shows a representative Tafel Polarization curves of: a) Zn coating, b) Zn coating deposited from an electroplating composition comprising SDS, c) Zn + Si coating deposited from an electroplating composition comprising no surfactant or grain refiner, d) Zn + Si coating deposited from an electroplating composition comprising SDS.

Figure 7 shows a representative corrosion rate of Zn-Si nanocomposite coating in mmpy using plating conditions and bath compositions 1 to 8 wherein conditions 6 & 7 yield superlative 0.01 mmpy corrosion rate.

Figure 8, panel (a) shows Salt Spray Test (SST) results of a Zn-Si composite coating deposited from an electroplating composition comprising Zn + Dextrin + Thiourea + Si (5 g/L).

Figure 8, panel (b) shows SST results of a Zn-Si composite coating deposited from an electroplating composition comprising Zn + Dextrin + Thiourea + Si (10 g/L).

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 salts), silicon nanoparticles, grain refiner(s) and/or surfactant(s). In some embodiments, the size of the silicon nanoparticles ranges from about 30 nm to about 100 nm, about 30 nm to about 70 nm or about 30 nm to about 50 nm, including values and ranges thereof.

The term “grain refiner” as used herein refers to an agent that reduces the grain size and improves the surface microstructure of the coating. The surfactants such as SDS and dextrin employed in the present invention act as grain refiners as well whereas agents such as thiourea act as a grain refiner but not as a surfactant. In some embodiments, the term “grain refiner” encompasses surfactants.

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

In some embodiments, the present disclosure provides an electroplating composition comprising zinc sulphate in an amount of about 200-300 g/L, zinc chloride in an amount of about 5-100 g/L, boric acid in an amount of about 10-50g/L, silicon nanoparticles in an amount of about 0.5-10 g/L, and a surfactant and/or grain refiner in an amount of about 1-5 g/L, wherein the electroplating composition has a pH of about 3 to 5.

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, silicon nanoparticles are present in the electroplating composition at a concentration of about 0.5, 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, the electroplating composition comprises zinc sulphate in an amount of about 200 g/L, zinc chloride in an amount of 30-100 g/L, boric acid in an amount of 25-50 g/L, silicon nanoparticles in an amount of about 0.5-10 g/L, and a surfactant and/or grain refiner in an amount of about 1-3 g/L.

In some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 200 g/L, zinc chloride in an amount of 30-100 g/L, boric acid in an amount of 25-50 g/L, silicon nanoparticles in an amount of about 0.5 g/L, and a surfactant and/or grain refiner in an amount of about 1 g/L.

In some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 200 g/L, zinc chloride in an amount of 30-100 g/L, boric acid in an amount of 25-50 g/L, silicon nanoparticles in an amount of about 10 g/L, and a surfactant and/or grain refiner in an amount of about 3 g/L.

In some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 200 g/L, zinc chloride in an amount of 30-100 g/L, boric acid in an amount of 25-50 g/L, silicon nanoparticles in an amount of about 5 g/L, and a surfactant and/or grain refiner in an amount of about 1 g/L.

In some embodiments, the surfactant or grain refiner is selected from dextrin, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS), gelatine, glycine, triethanolamine, thiourea, sorbitol, ethylenediamine tetraacetic acid (EDTA), benzylidine acetone, or a combination thereof. In some embodiments, the surfactant or grain refiner present in the electroplating composition is a combination of two or more surfactants/grain refiners. In these embodiments, the combined concentration of the surfactant and/or grain refiner ranges between 1-5 g/L of the electroplating composition.

In some embodiments, the grain refiner present in the electroplating composition is a combination of dextrin and thiourea. Accordingly, in some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 200 g/L, zinc chloride in an amount of 30-100 g/L, boric acid in an amount of 25-50 g/L, silicon nanoparticles in an amount of about 0.5-10 g/L, and a combination of dextrin and thiourea in an amount of about 1-3 g/L.

In some embodiments, the electroplating composition comprising dextrin and thiourea as grain refiners provides a Zn-Si composite coating that exhibits Salt Spray Test (SST) red rust life of more than 500 hours with outstanding self-passivation properties, such as about 500-600 hours of SST red rust life.

Also provided herein is a method of preparing the electroplating compositions of the present disclosure. In some embodiments, the method for preparing an electroplating composition of the present disclosure comprises: a) adding boric acid to hot distilled water; b) after dissolution of boric acid, adding a surfactant and/or grain refiner to the distilled water; c) after dissolution of the surfactant and/or grain refiner, adding zinc sulphate to the distilled water; d) after dissolution of zinc sulphate, adding zinc chloride to the distilled water; e) after dissolution of zinc chloride, adding silicon nano-powder (30-100 nm particle size) to the distilled water to obtain an electrolyte; f) ultrasonicating the electrolyte for about 30-90 minutes to obtain an ultrasonicated solution; g) stirring the ultrasonicated solution at about 600-900 rpm for about 2-4 hours; and h) adjusting pH of the ultrasonicated solution to about 3 to 5 to obtain the electroplating composition.

In the above method, the ultra-sonication is carried out at a frequency of about 40-60 kHz for about 30-90 minutes and stirring is carried out at a speed of about 600-900 rpm such as about 600, 650, 700, 750, 800, 850, or 900 rpm. In some embodiments, the ultra-sonication is carried out at a frequency of about 40-60 kHz for about 60 minutes. In some embodiments, stirring is carried out at about 800 rpm for about 3 hours.

The present disclosure also provides a direct current method for depositing the electroplating compositions described herein on steel substrates to provide substrates with zinc-silicon (Zn-Si) composite coatings.

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 at a constant current with a current density of about 10-20 A/dm2 and at a temperature of about 35 to 45? to provide a steel substrate comprising a zinc-silicon (Zn-Si) composite coating.

The inventors have found that the electroplating compositions and methods of depositing them according to the present disclosure provide a higher rate of deposition and provide coatings with lower corrosion rates. In some embodiments, the rate of deposition provided by the present method is about 5.5-6.5 µm/min, including values and ranges thereof compared to the rate of deposition of about 1µm/min of certain commercial coatings currently in use.

In some embodiments, the current density employed in the DC method of deposition is about 10, 15, or 20 A/dm2, including values and ranges thereof.

In some embodiments, the temperature of the electroplating composition is maintained at about 35?, 40?, or 45?, including values and ranges thereof.

In some embodiments, the DC method provides a Zn-Si composite coating comprising about 1-3% by weight of silicon, having a thickness of about 5-7 µm, and exhibiting a corrosion rate of about 0.01-0.06 mm per year (mmpy), and a Salt Spray Test (SST) red rust life of about 450-600 hrs.

The present disclosure provides a steel substrate comprising a zinc-silicon (Zn-Si) composite coating. In some embodiments, the steel substrate comprises about 5-7 µm thick Zn-Si composite coating.

In some embodiments, the steel substrate comprises a Zn-Si coating comprising about 1-3% by weight of silicon, including values and ranges therebetween. For example, in some embodiments, the steel substrate comprises a Zn-Si coating comprising about 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, or 3% by weight of silicon.

In some embodiments, the steel substrate comprising a Zn-Si coating exhibits a corrosion rate of about 0.01-0.06 mm per year (mmpy), including values and ranges thereof.

In some embodiments, the steel substrate comprising a Zn-Si coating exhibits about 450-600 hrs of SST red rust life, such as about 450 hrs, about 500 hrs, about 525 hrs, about 550 hrs, about 575 hrs, or about 600 hrs SST red rust life with excellent self-passivation properties compared to about 96-120 hours SST red rust life exhibited by Zn coating alone or Zn coating deposited in the presence of SDS as the surfactant and grain refiner.

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 comprising about 200-300 g/L of zinc sulphate, about 5-100 g/L of zinc chloride, about 10-50 g/L of boric acid, about 0.5-10 g/L of silicon nanoparticles, and about 1-5 g/L of a surfactant and/or grain refiner and having a pH of about 3 to 5 was prepared as shown in Figure 1. Specifically, about 10-50 g/L of boric acid was dissolved in hot distilled water. After dissolution of boric acid, about 1-5 g/L of a grain refiner was dissolved in the distilled water. The grain refiner can be a surfactant. The grain refiner can be a single agent such as a single surfactant, or a combination of two or more surfactants, or a combination of two or more grain refiners. After dissolution of the surfactant and/or grain refiner, about 200-300 g/L of zinc sulphate was dissolved in the distilled water. After dissolution of zinc sulphate, about 5-100 g/L of zinc chloride was dissolved in the distilled water. After dissolution of zinc chloride, about 0.5-20 g/L of silicon nanoparticles powder having a particle size of between 30-100 nm was dissolved in the distilled water to obtain an electrolyte. The electrolyte was sonicated for 60 minutes and followed by stirring at 800 rpm for 3 hours before plating. The pH of the bath was controlled at 3 to 5 using boric acid and adjusted with diluted sulphuric acid. The temperature of the electroplating composition varied between 35 to 60?.

Example 2: Deposition of the electroplating composition
Prior to deposition of the electroplating composition, the steel substrate was pre-treated. The pre-treatment of the steel substrate involved degreasing to remove organic impurities followed by basic cleaning using 350 g/l of Na2CO3, 250 g/l of NaOH at current densities of 1 to 5 A/dm2 for 2 to 5 minutes and final acid prickling for 10 to 30 seconds in 20 to 50 % of H2SO4.

After pre-treatment of the steel substrate, the electroplating composition was deposited on the steel substrate acting as a cathode at a constant current with a current density of 10 or 20 A/dm2 and at a temperature of about 35 to 45? to provide a steel substrate comprising a zinc-silicon (Zn-Si) composite coating. The plating duration was adjusted to achieve about 5 µm thick deposits.

Post-plating, the deposited samples were rinsed with distilled water, dried and stored in a vacuum desiccator. Electrochemical impedance and Tafel polarization studies were performed in 3.5 % sodium chloride solution after 30 min of open circuit potential stabilization. Impedance spectroscopic measurements were carried out in the frequency range 100 kHz to 10 mHz with sinusoidal amplitude of 5 mV. Charge transfer resistance (Rct) was measured in each sample from Nyquist plots and corrosion current (Icorr) and corrosion rate were extracted from Tafel plot. The microstructure and thickness of Zn + Si nanocomposite coating was studied using cross sectional SEM. Cross-sectional SEM images of Zn + Si nanocomposite deposited at 20 A/dm2 shows an average thickness of ~ 6 microns (Figure 2e).

Example 3
An aqueous electroplating bath was prepared containing no surfactant or grain refiner.
Zinc sulfate 250 g/l
Zinc chloride 50 g/l
Boric acid 15 g/l
pH 3
Temperature 35?
Current density 10 A/dm2

The zinc deposit obtained from this composition was porous with pitting and exhibited a corrosion rate of 0.1 mmpy for the 6.5 µm thick deposits.

Example 4
An aqueous electroplating bath containing no surfactant or grain refiner was prepared. Si nanoparticles were added into the electroplating composition without additional treatment.
Zinc sulfate 200 g/l
Zinc chloride 50 g/l
Boric acid 25 g/l
Si 0.5 g/l
pH 3
Temperature 35?
Current density 10 A/dm2

In the zinc deposit obtained from this composition, silicon nanoparticles were not distributed uniformly in the matrix. A corrosion rate of 0.08 mmpy and the matty white finish was observed for the 5.8 µm thick coatings.

Example 5
In the electroplating bath, the surfactant and/or grain refiner was added, and Si nanoparticles were added into the electrolyte without acid treatment.
Zinc sulfate 200 g/l
Zinc chloride 50 g/l
Boric acid 30 g/l
Si 0.5 g/l
Surfactant 1 g/l
Grain Refiner 1 g/l
pH 3
Temperature 35?
Current density 10 A/dm2

An elemental analysis of the zinc coatings obtained from this electroplating composition showed uniform distribution of Si nanoparticles exhibiting corrosion rate of 0.06 mmpy.

Example 6
In the electroplating bath, the surfactant and/or grain refiner was added and Si nanoparticles were added into the electrolyte after additional acid treatment. The acid treatment involves dispersion of silicon nanoparticles and etching with a solution mixture of 10% HF and 10% nitric acid for 15 minutes followed by ultracentrifugation and drying.
Zinc sulfate 200 g/l
Zinc chloride 50 g/l
Boric acid 25 g/l
Si 0.5 g/l
Surfactant 1 g/l
Grain Refiner 2 g/l
pH 3
Temperature 35?
Current density 10 A/dm2

An elemental analysis of the zinc coatings obtained from this electroplating composition showed that Si nanoparticles were distributed uniformly indicating a corrosion rate of 0.06 mmpy. Both, Examples 5 and 6, show a similar corrosion rate and morphology indicating a negligible effect of the acid treatment of Si nanoparticles.

Example 7
An electroplating bath having the following composition was prepared without any acid treatment of Si nanoparticles.
Zinc sulfate 200 g/l
Zinc chloride 30 g/l
Boric acid 40 g/l
Si 10 g/l
Surfactant 3 g/l
Grain Refiner 2 g/l
pH 4
Temperature 35?
Current density 10 A/dm2

A uniform distribution of silicon nanoparticles and a corrosion rate of 0.03 mmpy was observed. With an increase in the surfactant and/or grain refiner concentration, a higher coating thickness was observed resulting in 5-6 µm coating.

Example 8
An electroplating bath having the following composition was prepared without any acid treatment of Si nanoparticles.
Zinc sulfate 200 g/l
Zinc chloride 50 g/l
Boric acid 50 g/l
Si 10 g/l
Surfactant 3 g/l
Grain Refiner 0.5 g/l
pH 5
Temperature 35?
Current density 10 A/dm2

A uniform distribution of silicon nanoparticles and a corrosion rate of 0.01 mmpy was observed. The 5-6 µm thickness Zn-Si coating had a dull grey finish with a uniform surface coverage.

Example 9
An electroplating bath having the following composition was prepared without any acid treatment of Si nanoparticles.
Zinc sulfate 200 g/l
Zinc chloride 100 g/l
Boric acid 25 g/l
Si 10 g/l
Surfactant 3 g/l
Grain Refiner 1 g/l
pH 5
Temperature 45?
Current density 20 A/dm2

A uniform distribution of silicon nanoparticle and a corrosion rate of 0.01 mmpy was observed. The 5-6 µm thickness Zn-Si coating had a matte white finish without pitting with a uniform surface coverage.

Example 10
An electroplating bath having the following composition was prepared without any acid treatment of Si nanoparticles.
Zinc sulfate 200 g/l
Zinc chloride 50 g/l
Boric acid 25 g/l
Si 5 g/l
Surfactant 3 g/l
Grain Refiner 3 g/l
pH 4
Temperature 35o C
Current density 20 A/dm2

A poor distribution of silicon nanoparticles and a corrosion rate of 0.08 mmpy was observed. The Zn-Si coating exhibited cracks and had porous morphology.

Example 11: Salt Spray Test (SST) for corrosion
The coatings from the above examples were exposed to the salt spray test (SST) chamber for analyzing the corrosion properties of the coating. The corrosion properties are shown in Table 1. Zn with Si reinforcement in the presence of dextrin and thiourea showed excellent SST results improving the red rust hours from around 96 hours to more than 500 hours with outstanding self-passivation properties.

Table 1: SST results of the composite coated panels
Samples Current Density
A/dm2 Plating rate
(µm/min) White rust (hours) Red rust (hours)
Zn 20 5.71 <24 96-120
Zn+SDS 20 5.65 <24 120
Zn+Dextrin+Thiourea 20 5.67 <24 212
Zn+Si 5 g/L 20 5.61 <24 212
Zn+Si 10 g/L 20 6.40 <24 288
Zn+SDS+Si 5 g/L 20 6.03 <24 240-264
Zn+SDS+Si 10 g/L 20 5.87 <24 240-264
Zn+Dextrin+Thiourea+Si 5 g/L 20 6.38 <24 576
Zn+Dextrin+Thiourea+Si 10 g/L 20 6.35 <24 480

Zn with Si reinforcements in the presence of dextrin and thiourea forms a protective scaly surface to prevent further corrosion. Figure 8a and 8b show the panels for 5 g/L and 10 g/L of Si concentration, respectively, in the electrolytic bath.
, Claims:We Claim:
1. An electroplating composition comprising zinc sulphate in an amount of about 200-300 g/L, zinc chloride in an amount of about 5-100 g/L, boric acid in an amount of about 10-50g/L, silicon nanoparticles in an amount of about 0.5-10 g/L, and a surfactant and/or grain refiner in an amount of about 1-5 g/L, wherein the electroplating composition has a pH of about 3 to 5.
2. The electroplating composition as claimed in claim 1, wherein zinc chloride is present in an amount of about 30 g/L, about 50 g/L, or about 100 g/L.
3. The electroplating composition as claimed in claim 1 or 2, wherein:
a. the silicon nanoparticles are present in an amount of about 0.5 g/L and the surfactant and/or grain refiner is present in an amount of about 1 g/L,
b. the silicon nanoparticles are present in an amount of about 10 g/L and the surfactant and/or grain refiner is present in an amount of about 3 g/L, or
c. the silicon nanoparticles are present in an amount of about 5 g/L and the surfactant and/or grain refiner is present in an amount of about 1 g/L.
4. The electroplating composition as claimed in any one of claims 1-3, wherein the surfactant and/or grain refiner is selected from the group consisting of dextrin, thiourea, sodium dodecyl sulphate (SDS), and a combination thereof.
5. A method for preparing the electroplating composition as claimed in any one of claims 1-4, comprising:
a. adding boric acid to hot distilled water;
b. after dissolution of boric acid, adding a surfactant and/or grain refiner to the distilled water;
c. after dissolution of the surfactant and/or grain refiner, adding zinc sulphate to the distilled water;
d. after dissolution of zinc sulphate, adding zinc chloride to the distilled water;
e. after dissolution of zinc chloride, adding silicon nano-powder to the distilled water to obtain an electrolyte;
f. ultrasonicating the electrolyte for about 30-90 minutes to obtain an ultrasonicated solution;
g. stirring the ultrasonicated solution at about 600-900 rpm for about 2-4 hours; and
h. adjusting pH of the ultrasonicated solution to about 3 to 5 to obtain the electroplating composition.
6. The method as claimed in claim 5, wherein the electrolyte is ultrasonicated for about 60 minutes at a frequency of about 40-60 Hz.
7. The method as claimed in claim 5 or 6, wherein the ultrasonicated solution is stirred at about 800 rpm for about 3 hours.
8. 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 10-20 A/dm2 and at a temperature of about 35 to 45? to provide a steel substrate comprising a zinc-silicon (Zn-Si) composite coating.
9. The method as claimed in claim 8, wherein the current density is about 10 - 20 A/dm2.
10. The method as claimed in claim 8 or 9, wherein the method provides a deposition rate of about 5.5 - 6.5 µm/min.
11. The method as claimed in any one of claims 8-10, wherein the Zn-Si composite coating provided by the method exhibits a corrosion rate of about 0.01-0.06 mm per year (mmpy).
12. The method as claimed in any one of claims 8-11, wherein the Zn-Si composite coating provided by the method exhibits about 450-600 hrs of Salt Spray Test (SST) red rust life.
13. The method as claimed in any one of claims 8-12, wherein the Zn-Si coating deposited by the method has a thickness of about 5-7 µm.
14. The method as claimed in any one of claims 8-13, wherein the Zn-Si coating provided by the method comprises about 1-3% by weight of silicon.
15. A steel substrate comprising a zinc-silicon (Zn-Si) composite coating, wherein the coating exhibits a corrosion rate of about 0.01-0.06 mm per year (mmpy).
16. The steel substrate as claimed in claim 15, wherein the coating exhibits about 450-600 hrs of Salt Spray Test (SST) red rust life.
17. The steel substrate as claimed in claim 15 or 16, wherein the coating has a thickness of about 5-7 µm.
18. The steel substrate as claimed in any one of claims 15-17, wherein the Zn-Si coating comprises about 1-3% by weight of silicon.