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

COMPLETE SPECIFICATION
[see section 10 and rule13]

“CORROSION RESISTANT ZINC-NICKEL ALLOY COATINGS AND METHODS THEREOF”

Name and address of the applicant:
TATA STEEL LIMITED
Jamshedpur, 831001, Jharkhand, India

Nationality: Indian

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 and nickel salts; methods of preparing them; methods of depositing them on steel substrates and steel substrates obtained therefrom.

BACKGROUND OF THE DISCLOSURE
Three types of coatings are synthesized using electrodeposition namely pure metals, metal alloys and metal based composite coatings [1]. For pure metal deposition, one anode and one cathode are placed in the electrolyte solution and the 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 cathode surface. When current is passed between the anode and the cathode, pure metal is deposited on the cathode surface. For alloy deposition, the electrolyte solution is composed of the 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 different metals [1].

Zinc coating is generally used to provide sacrificial corrosion protection over steel substrate, but the zinc coated surfaces are prone to form white rust (a complex compound of zinc hydroxide) [2-4]. A zinc alloy electrodeposition has been widely studied to improve the mechanical as well as chemical properties (like white rust resistance) of the zinc coatings [5, 6]. Among the zinc alloys, zinc-nickel (Zn-Ni) alloys have been studied extensively because of their high corrosion resistance and other functional properties [7, 8].

A Zn-Ni deposition is an anomalous type of co-deposition where Zn (more active) is deposited preferentially over Ni [9]. However, an appreciable amount of Ni can be obtained in optimised plating conditions. It has been reported that a zinc-nickel electrodeposited coating of a certain thickness with a nickel content of 12-16 wt.% provides five to six times better corrosion resistance than pure zinc coatings of the same thickness [10]. The excellent corrosion resistance of the alloy comes due to the formation of a single-solid solution gamma phase (? -Ni2Zn11) [9, 11, 12].

Various baths (alkaline and acidic) have been reported to deposit Zn-Ni coatings. Nickel weight percentage in coatings deposited from an acidic bath is generally higher (12-16 wt. %) than those deposited from alkaline baths (5-9 wt. %) [13]. Higher Cathodic Current Efficiency (CCE) is observed in case of acidic baths whereas in alkaline baths, better deposit distribution is reported [14]. Different kind of additives like grain refiners, complexing agents, levellers are used to improve the properties of the coatings [15]. However, additives might have adverse effects. Some additives might decrease the CCE [16].

A few studies have shown that the pulse current electrodeposition shows better surface morphologies and better properties than the DC deposition [17, 18]. The coatings obtained using pulse current showed smooth deposition, better ductility, higher strength as well as better adhesion with the substrate [18].

Deposition kinetics and SST red rust life of coatings obtained from commercial acidic and alkaline baths is shown in Figure 1. An alkaline commercial bath has a SST red rust life of 400 hrs for 8µm coating thickness whereas, an acidic commercial bath has 450hrs of SST red rust life for a similar thickness. The deposition kinetics (0.5 µm/min) is similar for both the coatings. When coatings are deposited from these baths at a higher deposition rate (1µm/min for alkaline bath and 3.1µm/min for acidic bath), the SST red rust life is reduced to 200-240hrs (for both alkaline and acidic baths).

The Zn-Ni alloy coating is a promising coating that can provide excellent corrosion resistance to steel. However, there is still a need in the art to provide a suitable electroplating bath, an electrolyte preparation process and a deposition method to improve the deposition kinetics. Also, the complexing agents may have a negative impact on the deposition kinetics and the morphology of the coating. Thus, a bath without complexing agents is needed. The present disclosure attempts to address these needs.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to an electroplating composition comprising zinc sulphate in an amount of about 100-150 g/L, nickel sulphate in an amount of about 50-225 g/L, nickel chloride in an amount of about 10-100 g/L, and boric acid in an amount of about 10-50 g/L, wherein a ratio of nickel ions to zinc ions is in the range of about 1.1-1.7.

The present disclosure also relates to a method for preparing the electroplating composition described herein, comprising: a) heating water to about 60-70? to obtain a heated water; b) adding boric acid to the heated water to obtain a first solution; c) adding zinc sulphate to the first solution to obtain a second solution; d) adding nickel sulphate to the second solution to obtain a third solution; and e) adding nickel chloride to the third solution 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 50-150 mA/cm2, at a stirring rate of about 250-400 rpm and at a temperature of about 60? to provide a steel substrate comprising a zinc-nickel (Zn-Ni) coating.

The present disclosure further relates to a steel substrate comprising a zinc-nickel (Zn-Ni) coating, wherein the Zn-Ni coating comprises about 12-16% by weight of nickel.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the results of deposition kinetics vs SST red rust life study of commercial baths.

Figure 2 shows an exemplary schematic of a method of preparing an electroplating composition according to the present disclosure.

Figure 3 shows a schematic of hull cell deposition.

Figure 4 shows a schematic of bath setup and bath parameters.

Figure 5 shows a ratio of free Ni ions to free Zn ions in the electrolyte vs the Ni wt.% in the final coating.

Figure 6 shows results of a potentiodynamic polarization test of DC deposited coatings.

Figure 7 shows a variation in Ni wt.% of the coatings with the current density of DC deposition.

Figure 8 shows a variation in the corrosion current of the coatings with the stirring rate during the electrodeposition.

Figure 9 shows a variation in Ni wt.% of the coatings with the stirring rate during the electrodeposition.

Figure 10 shows the top surface microstructures of DC deposited Zn-Ni alloy coatings at different current densities for a fixed stirring rate of 350 rpm.

Figure 11 shows a comparison of Salt Spray Test (SST) red rust life and deposition kinetics of the present coatings vs commercial coatings.

Figure 12 shows the effect of different types of stirring on the SST red rust life.

Figure 13 shows the effect of different types of stirring on the SST white rust life.

Figure 14 shows the cross-sectional microstructure of a commercial Zn-Ni coating and the Zn-Ni coating provided by the present disclosure.

Figure 15 shows the appearance of the electroplating composition of the present disclosure after 6 months.

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 Ni salts) and boric acid.

The term “about” as used herein encompasses variations of +/-5% and more preferably +/-2.5%, as such variations are appropriate for practicing the present invention.

The present disclosure provides electroplating compositions for depositing zinc-nickel (Zn-Ni) alloy coatings on steel substrates. Further, the present disclosure provides a method for preparing electroplating compositions comprising Zn and Ni salts. 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-Ni coatings with improved surface microstructure, higher nickel content, and/or improved corrosion resistance.

In some embodiments, the present disclosure provides an electroplating composition comprising zinc sulphate in an amount of about 100-150 g/L, nickel sulphate in an amount of about 50-225 g/L, nickel chloride in an amount of about 10-100 g/L, and boric acid in an amount of about 10-50 g/L, wherein a ratio of nickel ions to zinc ions is in the range of about 1.1-1.7.

Zinc sulphate is present in the electroplating compositions in the amount of about 100-150 g/L, including values and ranges thereof, such as about 110-140 g/L, 120-150 g/L, 120-140 g/L, 120-130 g/L, 100 g/L, 110 g/L, 120 g/L, 125 g/L, 130 g/L, 140 g/L, or 150 g/L.

Nickel sulphate is present in the electroplating compositions in the amount of about 50-225 g/L, including values and ranges thereof, such as about 100-200 g/L, 120-180 g/L, 120-150 g/L, 130-180 g/L, 130-150 g/L, 130-140 g/L, 120 g/L, 130 g/L, 140 g/L, 150 g/L, 160 g/L, 170 g/L, or 180 g/L. Nickel chloride is present in the electroplating compositions in the amount of about 10-100 g/L, including values and ranges thereof, such as about 20-80 g/L, 30-60 g/L, 40-80 g/L, 40-60 g/L, 45-55 g/L, 30 g/L, 40 g/L, 50 g/L, 60 g/L, or 80 g/L.

In the electroplating compositions of the present disclosure, the amount of zinc sulphate, nickel sulphate, and nickel chloride is adjusted in such a way that the ratio of nickel ions to zinc ions is maintained at about 1.1-1.7, such as about 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, or 1.7, including values and ranges thereof. In some embodiments, the ratio of nickel ions to zinc ions is about 1.4-1.5 or about 1.45.

In some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 125 g/L, nickel sulphate in an amount of about 135 g/L, nickel chloride in an amount of about 50 g/L, and boric acid in an amount of about 30 g/L.

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

The electroplating compositions of the present disclosure do not include any complexing agent. The inventors found that the free Ni content in the electroplating composition increases with an increasing temperature and with the presence of nickel chloride in the electrolyte. In some embodiments, the Ni ion concentration in the electroplating composition is maintained between about 20-40g/L to provide a Ni content in the range of about 12-16 wt.% in the final coating.

The present disclosure also provides a method of preparing the electroplating compositions described herein. In some embodiments, the method for preparing an electroplating composition comprises: a) heating water to about 60-70? to obtain a heated water; b) adding boric acid to the heated water to obtain a first solution; c) adding zinc sulphate to the first solution to obtain a second solution; d) adding nickel sulphate to the second solution to obtain a third solution; and e) adding nickel chloride to the third solution to obtain the electroplating composition. After addition of nickel chloride, the volume of the electroplating composition is adjusted to a desired level and the pH of the composition is adjusted to about 3.5. An exemplary schematic of the method of preparing the electroplating compositions is shown in Figure 2.

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

In some embodiments, a method for depositing the electroplating composition on a steel substrate comprises: a) providing the steel substrate as a cathode; b) depositing the electroplating composition on the steel substrate at a constant current with a current density of about 50-150 mA/cm2, at a stirring rate of about 250-400 rpm and at a temperature of about 60? to provide a steel substrate comprising a Zn-Ni coating.

In some embodiments, the current density employed in the DC method of deposition is about 110-140 mA/cm2, including values and ranges thereof. In some embodiments, the current density employed in the DC method of deposition is about 120 mA/cm2.

In some embodiments, the electroplating composition is stirred during electrodeposition at a stirring rate of about 250-400 rpm, including values and ranges thereof. In some embodiments, the electroplating composition is stirred at a stirring rate of about 250-350 rpm. In some embodiments, the electroplating composition is stirred at a stirring rate of about 350 rpm. In some embodiments, the electroplating composition is stirred during electrodeposition by magnetic stirring. In some embodiments, the electroplating composition is stirred during electrodeposition by air agitation. In some embodiments, the electroplating composition is stirred during electrodeposition by air agitation at a rate of about 2-4 litres per minute (LPM). In an exemplary embodiment, the electroplating composition is stirred during electrodeposition by air agitation at a rate of about 3 LPM.

In some embodiments, the temperature of the electroplating composition is maintained at about 55?-65?, including values and ranges thereof, such as about 60?.

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 2-2.5 µm/min, including values and ranges thereof compared to the rate of deposition of about 0.5 µm/min of commercial coatings.

In some embodiments, the compositions and methods of the present disclosure provide a Zn-Ni alloy coating comprising about 12-16% by weight of nickel, having a thickness of about 4-4.5 µm, and exhibiting a corrosion current density of about 1.8-2.2 µA/cm2.

In some embodiments, the Zn-Ni coating provided by the present compositions and methods exhibits about 240-270 hrs of Salt Spray Test (SST) red rust life, including values and ranges thereof, compared to about 200-220 hrs SST red rust life exhibited by commercial coatings. In some embodiments, the Zn-Ni coating provided by the present compositions and methods exhibits a SST red rust life of about 260-270 hrs. In some embodiments, the Zn-Ni coating provided by the present compositions and methods exhibits a SST red rust life of about 240, 245, 250, 255, 260, 265, or about 270 hrs.

In some embodiments, the Zn-Ni coating provided by the present compositions and methods comprises ? Ni-Zn phases and exhibits a nodular morphology.

The present disclosure provides a steel substrate comprising a Zn-Ni alloy coating.

In some embodiments, the steel substrate comprises a Zn-Ni coating comprising about 12-16% by weight of Ni, including values and ranges therebetween. For example, in some embodiments, the steel substrate comprises a Zn-Ni coating comprising about 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, or 16 % by weight of nickel.

In some embodiments, the steel substrate comprises about 4-4.5 µm thick Zn-Ni coating.

In some embodiments, the steel substrate comprising a Zn-Ni coating exhibits a corrosion current density of about 1.8-2.2 µA/cm2, including values and ranges thereof, compared to the corrosion current density of about 2.5 µA/cm2 exhibited by commercial coatings.

In some embodiments, the steel substrate comprising a Zn-Ni coating exhibits a SST red rust life of about 240-270 hrs of SST red rust life, including values and ranges thereof, such as about 260-270 hrs or about 240, 245, 250, 255, 260, 265, or 270 hrs.

In some embodiments, the steel substrate comprises a Zn-Ni coating, wherein the coating comprises ? Ni-Zn phases and exhibits a nodular 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: Rate of deposition and corrosion current for various electroplating compositions
Various electroplating compositions were analysed for deposition kinetics and potentiodynamic polarization tests. The rate of deposition and the corrosion current was measured for every coating. The lowest corrosion current of 2.21 mA/cm2 was observed for an electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, 30 g/L boric acid having a pH of 3.5.

Example 2: Direct current (DC) deposition of the electroplating compositions
The DC deposition was performed galvanostatically using a Potentiostat (Make: AMETEK) in a two-electrode setup. Interstitial Free (IF) steel sheet was used as a cathode and pure Zn (99.5% pure) as an anode.

The electroplating composition was agitated using a magnetic stirrer (5 cm). The stirring rate was varied between 200-400 rpm. The current was supplied through a potentiostat. The steel samples, prior to deposition, 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 then deposition was carried out. After plating, the coated samples were rinsed with distilled water and dried.

Various characterizations were performed after coating. A Scanning Electron Microscope (Make: Nova-Nano) study was conducted to observe the coating morphology, SEM-EDS to obtain Zn and Ni wt.% in coating at different current densities and potentiodynamic polarization test (Make: Gamry) was conducted to obtain the corrosion potential (Ecorr) and corrosion current density (ICorr) values using Tafel extrapolation. Also, salt spray tests (SST) were done to obtain white rust and red rust life. X-Ray Fluorescence (XRF) (Shimadzu) was conducted to obtain elemental Zn and Ni present in the electrolyte.

Example 3: Hull cell study to determine the current density of deposition
A Hull cell study was conducted to determine the operating current density range. The electroplating setup is shown in Figure 3. The electroplating composition that provided the lowest corrosion current in the earlier example (electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, 30 g/L boric acid having a pH of 3.5) was used in the Hull cell study. During the Hull Cell experiments, the electrolyte was agitated using magnetic stirring. The current was supplied through a potentiostat (AMETEK make). Based on the limiting current density for the above-mentioned electroplating composition, the current density was varied between 50-150 mA/cm2 during the Zn-Ni alloy plating.

Example 4: The effect of the ratio of free Ni to free Zn ions
Free Ni and Zn ions in the electroplating compositions were calculated from the XRF analysis. As, no complexing agent was used in the electrolyte preparation, all the elemental Zn and Ni were in ions form. From there, free Ni to Zn ions ratio was calculated. Also, the Ni wt.% in the coatings were also calculated from EDS. Figure 5 shows that to obtain 12-16 wt.% Ni in the final coating, the ratio of free Ni to Zn ions should be in between 1.1-1.7 range. The electroplating bath that provided the lowest corrosion current has a free Ni to free Zn ion ratio of 1.45.

Example 5: The effect of current density on the corrosion current
For this study, the electroplating composition that provided the lowest corrosion current in the earlier example (electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, 30 g/L boric acid having a pH of 3.5) was used. The Zn-Ni alloy was electrodeposited at various current densities - 50-150 mA/cm2. The corrosion current variation with the current density for various coatings is shown in Figure 6. The results were further compared to the commercial Zn-Ni alloy coated steel sample where the coating was deposited from an acidic bath.

The corrosion current of the coatings was obtained using a Gamry software (Version: 4.35) by Tafel extrapolation method. Figure 6 shows that the corrosion current decreased with the increasing current density up to 120 mA/cm2, beyond that the corrosion current again increased with the increasing current density. The lowest corrosion current (1.91 µA/cm2) was observed at 120 mA/cm2.

The variation in the Ni wt.% with the current density for various coatings is shown in Figure 7. The results are further compared with the commercial Zn-Ni alloy coated steel sample where the coating was deposited from an acidic bath.

It is known in the art that the Zn-Ni alloy coatings with 12-16wt.% of Ni provide superior corrosion protection over other alloy compositions due to the gamma phase (? -Ni5Zn21) formation which is the more corrosion resistive phase than the other phases in Zn-Ni alloy systems [9-12]. For the electroplating composition comprising 125 g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid having a pH of 3.5, 12-16wt.% Ni was obtained at the current density of 110-140 mA/cm2.
Example 6: The effect of stirring rate on the corrosion current
For this study, the electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, having a pH of 3.5, was used. Figure 8 shows the effect of the stirring rate during the electrodeposition on the corrosion current. In this study, the stirring rate was varied between 200-400 rpm. A burnt deposition was observed at the lowest stirring rate (200 rpm) and accordingly, the corrosion current was higher. The corrosion current was higher for the highest stirring rate (400 rpm) as well. In between, for the stirring rate of 250-350 rpm, the corrosion current decreased with the increasing stirring rate. The corrosion current was the lowest at 350 rpm stirring rate.

The variation of Ni wt.% with the stirring rate was also studied. Ni wt.% in the coating decreased with an increasing stirring rate. Though highest Ni wt.% (18.29 wt.%) was noticed at the lowest stirring rate (200rpm) but the corresponding corrosion current was higher due to powdery deposit. The lowest Ni wt.% (10.29 wt.%) was obtained at 400 rpm causing an increase in the corrosion current. For the stirring rates between 200-400 rpm, the Ni wt.% was in between 12-16 wt.%.

It was observed that the bath agitation is an important parameter to produce uniform deposition. The coating uniformity and the Ni content in the final deposit can be controlled using agitation speed. A higher agitation speed led to lower Ni content in the final deposition due to disturbance in local cathode surface chemistry whereas, lower agitation speed led to powdery deposition (also called burnt deposition) by affecting the ionic concentration profile near the cathode thus impacting the overall limiting current density of the electrolyte. It was observed that the stirring rate of 350 rpm provided the desired Ni wt% and lower corrosion current.

Example 7: The effect of the current density on the microstructure
For this study, the electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, having a pH of 3.5, was used. The stirring rate was 350 rpm. The current density was varied between 60-150 mA/cm2. Figure 10 shows the variation in the coating morphology with the corresponding Ni wt.% at different current densities. XRD tests was also conducted to determine the phases present at different current densities. At lower current density, ? Ni-Zn and (?+d) Ni-Zn was observed whereas ? Ni-Zn phases were noticed at higher current densities (110-150 mA/cm2). Hexagonal type morphologies of Zn rich phases were observed at lower current densities (60-100 mA/cm2). With increasing current density, Ni content was increased, and the morphology changed to nodular. This nodular grains with ?-phase have high corrosion resistance compared to others. Finer grain was observed for lower current densities (60-120 mA/cm2). As the current density (130-150 mA/cm2) was increased, coarser grains started forming. Compact structures were seen at lower and medium current densities (60-130 mA/cm2) but beyond that, a porous coating was observed. Based on the above observations in terms of Ni content, microstructural morphology, preferred phases, structural fineness and compactness, coatings obtained at 120 mA/cm2 current density were considered the best.

Example 8: Salt Spray Test (SST) and Deposition Kinetics
The deposition kinetics and SST red rust life of the Zn-Ni alloy coating deposited from the electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, having a pH of 3.5, was compared with commercial coatings deposited from acidic and alkaline baths. It can be observed from Figure 11 that the deposition kinetics provided by the above electroplating composition was improved (2µm/min which is four times improvement) without affecting the SST life for the similar coating thickness of 4µm. In the case of commercial alkaline bath, 200hrs SST red rust life was observed and for commercial acidic bath, 220hrs SST red rust life was noticed at 0.5µm/min. On the other hand, 264hrs SST red rust life was seen for the above electroplating composition with the deposition rate of 2 µm/min. This shows that the electroplating composition and the deposition method of the present disclosure deposit a Zn-Ni coating on a steel substrate at 4 times faster rate of deposition compared to the commercially available solutions with superior corrosion resistance properties.

Example 9: The effect of different types of stirring
In this study, the effect of other modes of agitation which can be adopted easily for commercial production conditions like air agitation, solution circulation and cathode rod movement etc. was studied. The electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, and having a pH of 3.5 was used. Figure 12 shows the comparative resistance to red rust of the coatings produced through different modes of agitations. In all these coatings, the average coating thickness was obtained around 4µm with a similar deposition kinetics of 2µm/min. The commercial sample had 4µm coating thickness, but the rate of deposition was around 0.5µm/min. Among the three different types of commercially adoptable modes of agitation, air agitation at a rate of 3 LPM showed better result (264 hrs) compared to other two types of agitation (192 hrs for both cathode rod movement and solution circulation). It also showed marginally superior SST red rust life than the magnetic stirring (240hrs). Both, solution circulation and cathode rod movement showed lower SST life than magnetic stirring.

The SST white rust life comparison for different types of agitation is shown in Figure 13. White rust life depends upon the surface properties. For all the cases, Ni wt% was maintained between 12-16wt.%. For that reason, no change in terms of white rust life was observed using different types of agitation. For all the cases, white rust of the coating was observed within 24hrs of the SST test.

Example 10: Comparison of cross-sectional microstructure of the present coatings vs. the commercial Zn-Ni coating
Figure 14 shows the comparative cross-sectional view of the commercial sample having an average 3 µm coating thickness deposited at 0.4 µm/min rate and the Zn-Ni coating of the present disclosure with an average thickness of 4µm deposited at 2µm/min rate. The cross-sectional microstructure of the commercial sample shows multiple cracks through the thickness. On the other hand, the Zn-Ni alloy coating of the present disclosure (prepared from air agitation) was very compact, with no cracks observed throughout the cross-section. Hence, the present coating is less brittle compared to the commercial one.

Example 11: Ni and Zn concentration in the electrolyte
Tabel 1 shows the elemental Zn and Ni present in various electrolytes.
Table 1: Elemental Zn and elemental Ni present in electrolyte
System Name Elemental Zn (ppm) Elemental Ni (ppm)
Zn-Ni commercial Acidic Bath 38089 16284
Zn-Ni commercial Alkaline Bath 35089 15279
Zn-Ni bath of the present disclosure 30256 43786

In EMF series, the standard reduction potential of zinc is -0.76V and Ni is -0.25V. So, Zn is much more active compared to Ni. In commercial baths, different kinds of complexing agents are used to co-deposit Zn-Ni. So, in commercial baths, 2 types of Zn are present in the electrolyte - one in the ionic form and one in the complexed form. Free ions are deposited easily compared to complexed one. This explains why the deposition kinetics is slower in commercial baths. To improve the deposition kinetics, free ions concentrations need to be increased. The electroplating bath of the present disclosure is composed of only basic salts like ZnSO4 and NiCl2. No complexing agent was used. Figure 5 showed that to deposit 12-16 wt.% Ni, free Ni to Zn ions ration should be in between 1.1-1.7 range. The electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, and having a pH of 3.5 has free Ni to Zn ion ratio of 1.45. On the other hand, in commercial baths, Zn is in the complexed form; therefore, free Zn ions will be low. Also, the Ni ions are present very low concentration. Therefore, to obtain a free Ni to Zn ions ratio in the 1.1-1.7 range, free Zn ions should be at least in the 14770-9557 ppm range in an acidic bath and 13890-8987 ppm range in an alkaline bath. Compared to this, 3-4 times higher free Zn ions and Ni ions were present in the electroplating composition of the present disclosure. These high amount of free Zn and Ni ions contribute to the higher deposition kinetics provided by the electroplating compositions of the present disclosure compared to commercial baths.

Example 12: Stability study of the electroplating composition
The electrolyte/electroplating composition was prepared using only basic salts like zinc sulphate, nickel sulphate and nickel chloride. Boric acid was added as a buffering agent to maintain pH at 3.5. A bath stability study was conducted for 6 months. The color of the electrolyte was dark green when prepared. After 6 months, no color change was observed in the electrolyte. The electrolyte was transparent even after 6 months of preparation. After plating, the bath pH (3.5) did not change much, and no precipitation and color change were observed.

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19. Soroor Ghaziof, Wei Gao, Applied surdface science 2014.
20. M. M. Momeni, S. Hashemizadeh, M. Mirhosseini, A. Kazempour & S. A. Hosseinizadeh, Surface Engineering , Claims:FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[see section 10 and rule13]

“CORROSION RESISTANT ZINC-NICKEL ALLOY COATINGS AND METHODS THEREOF”

Name and address of the applicant:
TATA STEEL LIMITED
Jamshedpur, 831001, Jharkhand, India

Nationality: Indian

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 and nickel salts; methods of preparing them; methods of depositing them on steel substrates and steel substrates obtained therefrom.

BACKGROUND OF THE DISCLOSURE
Three types of coatings are synthesized using electrodeposition namely pure metals, metal alloys and metal based composite coatings [1]. For pure metal deposition, one anode and one cathode are placed in the electrolyte solution and the 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 cathode surface. When current is passed between the anode and the cathode, pure metal is deposited on the cathode surface. For alloy deposition, the electrolyte solution is composed of the 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 different metals [1].

Zinc coating is generally used to provide sacrificial corrosion protection over steel substrate, but the zinc coated surfaces are prone to form white rust (a complex compound of zinc hydroxide) [2-4]. A zinc alloy electrodeposition has been widely studied to improve the mechanical as well as chemical properties (like white rust resistance) of the zinc coatings [5, 6]. Among the zinc alloys, zinc-nickel (Zn-Ni) alloys have been studied extensively because of their high corrosion resistance and other functional properties [7, 8].

A Zn-Ni deposition is an anomalous type of co-deposition where Zn (more active) is deposited preferentially over Ni [9]. However, an appreciable amount of Ni can be obtained in optimised plating conditions. It has been reported that a zinc-nickel electrodeposited coating of a certain thickness with a nickel content of 12-16 wt.% provides five to six times better corrosion resistance than pure zinc coatings of the same thickness [10]. The excellent corrosion resistance of the alloy comes due to the formation of a single-solid solution gamma phase (? -Ni2Zn11) [9, 11, 12].

Various baths (alkaline and acidic) have been reported to deposit Zn-Ni coatings. Nickel weight percentage in coatings deposited from an acidic bath is generally higher (12-16 wt. %) than those deposited from alkaline baths (5-9 wt. %) [13]. Higher Cathodic Current Efficiency (CCE) is observed in case of acidic baths whereas in alkaline baths, better deposit distribution is reported [14]. Different kind of additives like grain refiners, complexing agents, levellers are used to improve the properties of the coatings [15]. However, additives might have adverse effects. Some additives might decrease the CCE [16].

A few studies have shown that the pulse current electrodeposition shows better surface morphologies and better properties than the DC deposition [17, 18]. The coatings obtained using pulse current showed smooth deposition, better ductility, higher strength as well as better adhesion with the substrate [18].

Deposition kinetics and SST red rust life of coatings obtained from commercial acidic and alkaline baths is shown in Figure 1. An alkaline commercial bath has a SST red rust life of 400 hrs for 8µm coating thickness whereas, an acidic commercial bath has 450hrs of SST red rust life for a similar thickness. The deposition kinetics (0.5 µm/min) is similar for both the coatings. When coatings are deposited from these baths at a higher deposition rate (1µm/min for alkaline bath and 3.1µm/min for acidic bath), the SST red rust life is reduced to 200-240hrs (for both alkaline and acidic baths).

The Zn-Ni alloy coating is a promising coating that can provide excellent corrosion resistance to steel. However, there is still a need in the art to provide a suitable electroplating bath, an electrolyte preparation process and a deposition method to improve the deposition kinetics. Also, the complexing agents may have a negative impact on the deposition kinetics and the morphology of the coating. Thus, a bath without complexing agents is needed. The present disclosure attempts to address these needs.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to an electroplating composition comprising zinc sulphate in an amount of about 100-150 g/L, nickel sulphate in an amount of about 50-225 g/L, nickel chloride in an amount of about 10-100 g/L, and boric acid in an amount of about 10-50 g/L, wherein a ratio of nickel ions to zinc ions is in the range of about 1.1-1.7.

The present disclosure also relates to a method for preparing the electroplating composition described herein, comprising: a) heating water to about 60-70? to obtain a heated water; b) adding boric acid to the heated water to obtain a first solution; c) adding zinc sulphate to the first solution to obtain a second solution; d) adding nickel sulphate to the second solution to obtain a third solution; and e) adding nickel chloride to the third solution 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 50-150 mA/cm2, at a stirring rate of about 250-400 rpm and at a temperature of about 60? to provide a steel substrate comprising a zinc-nickel (Zn-Ni) coating.

The present disclosure further relates to a steel substrate comprising a zinc-nickel (Zn-Ni) coating, wherein the Zn-Ni coating comprises about 12-16% by weight of nickel.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the results of deposition kinetics vs SST red rust life study of commercial baths.

Figure 2 shows an exemplary schematic of a method of preparing an electroplating composition according to the present disclosure.

Figure 3 shows a schematic of hull cell deposition.

Figure 4 shows a schematic of bath setup and bath parameters.

Figure 5 shows a ratio of free Ni ions to free Zn ions in the electrolyte vs the Ni wt.% in the final coating.

Figure 6 shows results of a potentiodynamic polarization test of DC deposited coatings.

Figure 7 shows a variation in Ni wt.% of the coatings with the current density of DC deposition.

Figure 8 shows a variation in the corrosion current of the coatings with the stirring rate during the electrodeposition.

Figure 9 shows a variation in Ni wt.% of the coatings with the stirring rate during the electrodeposition.

Figure 10 shows the top surface microstructures of DC deposited Zn-Ni alloy coatings at different current densities for a fixed stirring rate of 350 rpm.

Figure 11 shows a comparison of Salt Spray Test (SST) red rust life and deposition kinetics of the present coatings vs commercial coatings.

Figure 12 shows the effect of different types of stirring on the SST red rust life.

Figure 13 shows the effect of different types of stirring on the SST white rust life.

Figure 14 shows the cross-sectional microstructure of a commercial Zn-Ni coating and the Zn-Ni coating provided by the present disclosure.

Figure 15 shows the appearance of the electroplating composition of the present disclosure after 6 months.

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 Ni salts) and boric acid.

The term “about” as used herein encompasses variations of +/-5% and more preferably +/-2.5%, as such variations are appropriate for practicing the present invention.

The present disclosure provides electroplating compositions for depositing zinc-nickel (Zn-Ni) alloy coatings on steel substrates. Further, the present disclosure provides a method for preparing electroplating compositions comprising Zn and Ni salts. 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-Ni coatings with improved surface microstructure, higher nickel content, and/or improved corrosion resistance.

In some embodiments, the present disclosure provides an electroplating composition comprising zinc sulphate in an amount of about 100-150 g/L, nickel sulphate in an amount of about 50-225 g/L, nickel chloride in an amount of about 10-100 g/L, and boric acid in an amount of about 10-50 g/L, wherein a ratio of nickel ions to zinc ions is in the range of about 1.1-1.7.

Zinc sulphate is present in the electroplating compositions in the amount of about 100-150 g/L, including values and ranges thereof, such as about 110-140 g/L, 120-150 g/L, 120-140 g/L, 120-130 g/L, 100 g/L, 110 g/L, 120 g/L, 125 g/L, 130 g/L, 140 g/L, or 150 g/L.

Nickel sulphate is present in the electroplating compositions in the amount of about 50-225 g/L, including values and ranges thereof, such as about 100-200 g/L, 120-180 g/L, 120-150 g/L, 130-180 g/L, 130-150 g/L, 130-140 g/L, 120 g/L, 130 g/L, 140 g/L, 150 g/L, 160 g/L, 170 g/L, or 180 g/L. Nickel chloride is present in the electroplating compositions in the amount of about 10-100 g/L, including values and ranges thereof, such as about 20-80 g/L, 30-60 g/L, 40-80 g/L, 40-60 g/L, 45-55 g/L, 30 g/L, 40 g/L, 50 g/L, 60 g/L, or 80 g/L.

In the electroplating compositions of the present disclosure, the amount of zinc sulphate, nickel sulphate, and nickel chloride is adjusted in such a way that the ratio of nickel ions to zinc ions is maintained at about 1.1-1.7, such as about 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, or 1.7, including values and ranges thereof. In some embodiments, the ratio of nickel ions to zinc ions is about 1.4-1.5 or about 1.45.

In some embodiments, the electroplating composition comprises zinc sulphate in an amount of about 125 g/L, nickel sulphate in an amount of about 135 g/L, nickel chloride in an amount of about 50 g/L, and boric acid in an amount of about 30 g/L.

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

The electroplating compositions of the present disclosure do not include any complexing agent. The inventors found that the free Ni content in the electroplating composition increases with an increasing temperature and with the presence of nickel chloride in the electrolyte. In some embodiments, the Ni ion concentration in the electroplating composition is maintained between about 20-40g/L to provide a Ni content in the range of about 12-16 wt.% in the final coating.

The present disclosure also provides a method of preparing the electroplating compositions described herein. In some embodiments, the method for preparing an electroplating composition comprises: a) heating water to about 60-70? to obtain a heated water; b) adding boric acid to the heated water to obtain a first solution; c) adding zinc sulphate to the first solution to obtain a second solution; d) adding nickel sulphate to the second solution to obtain a third solution; and e) adding nickel chloride to the third solution to obtain the electroplating composition. After addition of nickel chloride, the volume of the electroplating composition is adjusted to a desired level and the pH of the composition is adjusted to about 3.5. An exemplary schematic of the method of preparing the electroplating compositions is shown in Figure 2.

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

In some embodiments, a method for depositing the electroplating composition on a steel substrate comprises: a) providing the steel substrate as a cathode; b) depositing the electroplating composition on the steel substrate at a constant current with a current density of about 50-150 mA/cm2, at a stirring rate of about 250-400 rpm and at a temperature of about 60? to provide a steel substrate comprising a Zn-Ni coating.

In some embodiments, the current density employed in the DC method of deposition is about 110-140 mA/cm2, including values and ranges thereof. In some embodiments, the current density employed in the DC method of deposition is about 120 mA/cm2.

In some embodiments, the electroplating composition is stirred during electrodeposition at a stirring rate of about 250-400 rpm, including values and ranges thereof. In some embodiments, the electroplating composition is stirred at a stirring rate of about 250-350 rpm. In some embodiments, the electroplating composition is stirred at a stirring rate of about 350 rpm. In some embodiments, the electroplating composition is stirred during electrodeposition by magnetic stirring. In some embodiments, the electroplating composition is stirred during electrodeposition by air agitation. In some embodiments, the electroplating composition is stirred during electrodeposition by air agitation at a rate of about 2-4 litres per minute (LPM). In an exemplary embodiment, the electroplating composition is stirred during electrodeposition by air agitation at a rate of about 3 LPM.

In some embodiments, the temperature of the electroplating composition is maintained at about 55?-65?, including values and ranges thereof, such as about 60?.

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 2-2.5 µm/min, including values and ranges thereof compared to the rate of deposition of about 0.5 µm/min of commercial coatings.

In some embodiments, the compositions and methods of the present disclosure provide a Zn-Ni alloy coating comprising about 12-16% by weight of nickel, having a thickness of about 4-4.5 µm, and exhibiting a corrosion current density of about 1.8-2.2 µA/cm2.

In some embodiments, the Zn-Ni coating provided by the present compositions and methods exhibits about 240-270 hrs of Salt Spray Test (SST) red rust life, including values and ranges thereof, compared to about 200-220 hrs SST red rust life exhibited by commercial coatings. In some embodiments, the Zn-Ni coating provided by the present compositions and methods exhibits a SST red rust life of about 260-270 hrs. In some embodiments, the Zn-Ni coating provided by the present compositions and methods exhibits a SST red rust life of about 240, 245, 250, 255, 260, 265, or about 270 hrs.

In some embodiments, the Zn-Ni coating provided by the present compositions and methods comprises ? Ni-Zn phases and exhibits a nodular morphology.

The present disclosure provides a steel substrate comprising a Zn-Ni alloy coating.

In some embodiments, the steel substrate comprises a Zn-Ni coating comprising about 12-16% by weight of Ni, including values and ranges therebetween. For example, in some embodiments, the steel substrate comprises a Zn-Ni coating comprising about 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, or 16 % by weight of nickel.

In some embodiments, the steel substrate comprises about 4-4.5 µm thick Zn-Ni coating.

In some embodiments, the steel substrate comprising a Zn-Ni coating exhibits a corrosion current density of about 1.8-2.2 µA/cm2, including values and ranges thereof, compared to the corrosion current density of about 2.5 µA/cm2 exhibited by commercial coatings.

In some embodiments, the steel substrate comprising a Zn-Ni coating exhibits a SST red rust life of about 240-270 hrs of SST red rust life, including values and ranges thereof, such as about 260-270 hrs or about 240, 245, 250, 255, 260, 265, or 270 hrs.

In some embodiments, the steel substrate comprises a Zn-Ni coating, wherein the coating comprises ? Ni-Zn phases and exhibits a nodular 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: Rate of deposition and corrosion current for various electroplating compositions
Various electroplating compositions were analysed for deposition kinetics and potentiodynamic polarization tests. The rate of deposition and the corrosion current was measured for every coating. The lowest corrosion current of 2.21 mA/cm2 was observed for an electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, 30 g/L boric acid having a pH of 3.5.

Example 2: Direct current (DC) deposition of the electroplating compositions
The DC deposition was performed galvanostatically using a Potentiostat (Make: AMETEK) in a two-electrode setup. Interstitial Free (IF) steel sheet was used as a cathode and pure Zn (99.5% pure) as an anode.

The electroplating composition was agitated using a magnetic stirrer (5 cm). The stirring rate was varied between 200-400 rpm. The current was supplied through a potentiostat. The steel samples, prior to deposition, 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 then deposition was carried out. After plating, the coated samples were rinsed with distilled water and dried.

Various characterizations were performed after coating. A Scanning Electron Microscope (Make: Nova-Nano) study was conducted to observe the coating morphology, SEM-EDS to obtain Zn and Ni wt.% in coating at different current densities and potentiodynamic polarization test (Make: Gamry) was conducted to obtain the corrosion potential (Ecorr) and corrosion current density (ICorr) values using Tafel extrapolation. Also, salt spray tests (SST) were done to obtain white rust and red rust life. X-Ray Fluorescence (XRF) (Shimadzu) was conducted to obtain elemental Zn and Ni present in the electrolyte.

Example 3: Hull cell study to determine the current density of deposition
A Hull cell study was conducted to determine the operating current density range. The electroplating setup is shown in Figure 3. The electroplating composition that provided the lowest corrosion current in the earlier example (electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, 30 g/L boric acid having a pH of 3.5) was used in the Hull cell study. During the Hull Cell experiments, the electrolyte was agitated using magnetic stirring. The current was supplied through a potentiostat (AMETEK make). Based on the limiting current density for the above-mentioned electroplating composition, the current density was varied between 50-150 mA/cm2 during the Zn-Ni alloy plating.

Example 4: The effect of the ratio of free Ni to free Zn ions
Free Ni and Zn ions in the electroplating compositions were calculated from the XRF analysis. As, no complexing agent was used in the electrolyte preparation, all the elemental Zn and Ni were in ions form. From there, free Ni to Zn ions ratio was calculated. Also, the Ni wt.% in the coatings were also calculated from EDS. Figure 5 shows that to obtain 12-16 wt.% Ni in the final coating, the ratio of free Ni to Zn ions should be in between 1.1-1.7 range. The electroplating bath that provided the lowest corrosion current has a free Ni to free Zn ion ratio of 1.45.

Example 5: The effect of current density on the corrosion current
For this study, the electroplating composition that provided the lowest corrosion current in the earlier example (electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, 30 g/L boric acid having a pH of 3.5) was used. The Zn-Ni alloy was electrodeposited at various current densities - 50-150 mA/cm2. The corrosion current variation with the current density for various coatings is shown in Figure 6. The results were further compared to the commercial Zn-Ni alloy coated steel sample where the coating was deposited from an acidic bath.

The corrosion current of the coatings was obtained using a Gamry software (Version: 4.35) by Tafel extrapolation method. Figure 6 shows that the corrosion current decreased with the increasing current density up to 120 mA/cm2, beyond that the corrosion current again increased with the increasing current density. The lowest corrosion current (1.91 µA/cm2) was observed at 120 mA/cm2.

The variation in the Ni wt.% with the current density for various coatings is shown in Figure 7. The results are further compared with the commercial Zn-Ni alloy coated steel sample where the coating was deposited from an acidic bath.

It is known in the art that the Zn-Ni alloy coatings with 12-16wt.% of Ni provide superior corrosion protection over other alloy compositions due to the gamma phase (? -Ni5Zn21) formation which is the more corrosion resistive phase than the other phases in Zn-Ni alloy systems [9-12]. For the electroplating composition comprising 125 g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid having a pH of 3.5, 12-16wt.% Ni was obtained at the current density of 110-140 mA/cm2.
Example 6: The effect of stirring rate on the corrosion current
For this study, the electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, having a pH of 3.5, was used. Figure 8 shows the effect of the stirring rate during the electrodeposition on the corrosion current. In this study, the stirring rate was varied between 200-400 rpm. A burnt deposition was observed at the lowest stirring rate (200 rpm) and accordingly, the corrosion current was higher. The corrosion current was higher for the highest stirring rate (400 rpm) as well. In between, for the stirring rate of 250-350 rpm, the corrosion current decreased with the increasing stirring rate. The corrosion current was the lowest at 350 rpm stirring rate.

The variation of Ni wt.% with the stirring rate was also studied. Ni wt.% in the coating decreased with an increasing stirring rate. Though highest Ni wt.% (18.29 wt.%) was noticed at the lowest stirring rate (200rpm) but the corresponding corrosion current was higher due to powdery deposit. The lowest Ni wt.% (10.29 wt.%) was obtained at 400 rpm causing an increase in the corrosion current. For the stirring rates between 200-400 rpm, the Ni wt.% was in between 12-16 wt.%.

It was observed that the bath agitation is an important parameter to produce uniform deposition. The coating uniformity and the Ni content in the final deposit can be controlled using agitation speed. A higher agitation speed led to lower Ni content in the final deposition due to disturbance in local cathode surface chemistry whereas, lower agitation speed led to powdery deposition (also called burnt deposition) by affecting the ionic concentration profile near the cathode thus impacting the overall limiting current density of the electrolyte. It was observed that the stirring rate of 350 rpm provided the desired Ni wt% and lower corrosion current.

Example 7: The effect of the current density on the microstructure
For this study, the electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, having a pH of 3.5, was used. The stirring rate was 350 rpm. The current density was varied between 60-150 mA/cm2. Figure 10 shows the variation in the coating morphology with the corresponding Ni wt.% at different current densities. XRD tests was also conducted to determine the phases present at different current densities. At lower current density, ? Ni-Zn and (?+d) Ni-Zn was observed whereas ? Ni-Zn phases were noticed at higher current densities (110-150 mA/cm2). Hexagonal type morphologies of Zn rich phases were observed at lower current densities (60-100 mA/cm2). With increasing current density, Ni content was increased, and the morphology changed to nodular. This nodular grains with ?-phase have high corrosion resistance compared to others. Finer grain was observed for lower current densities (60-120 mA/cm2). As the current density (130-150 mA/cm2) was increased, coarser grains started forming. Compact structures were seen at lower and medium current densities (60-130 mA/cm2) but beyond that, a porous coating was observed. Based on the above observations in terms of Ni content, microstructural morphology, preferred phases, structural fineness and compactness, coatings obtained at 120 mA/cm2 current density were considered the best.

Example 8: Salt Spray Test (SST) and Deposition Kinetics
The deposition kinetics and SST red rust life of the Zn-Ni alloy coating deposited from the electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, having a pH of 3.5, was compared with commercial coatings deposited from acidic and alkaline baths. It can be observed from Figure 11 that the deposition kinetics provided by the above electroplating composition was improved (2µm/min which is four times improvement) without affecting the SST life for the similar coating thickness of 4µm. In the case of commercial alkaline bath, 200hrs SST red rust life was observed and for commercial acidic bath, 220hrs SST red rust life was noticed at 0.5µm/min. On the other hand, 264hrs SST red rust life was seen for the above electroplating composition with the deposition rate of 2 µm/min. This shows that the electroplating composition and the deposition method of the present disclosure deposit a Zn-Ni coating on a steel substrate at 4 times faster rate of deposition compared to the commercially available solutions with superior corrosion resistance properties.

Example 9: The effect of different types of stirring
In this study, the effect of other modes of agitation which can be adopted easily for commercial production conditions like air agitation, solution circulation and cathode rod movement etc. was studied. The electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, and having a pH of 3.5 was used. Figure 12 shows the comparative resistance to red rust of the coatings produced through different modes of agitations. In all these coatings, the average coating thickness was obtained around 4µm with a similar deposition kinetics of 2µm/min. The commercial sample had 4µm coating thickness, but the rate of deposition was around 0.5µm/min. Among the three different types of commercially adoptable modes of agitation, air agitation at a rate of 3 LPM showed better result (264 hrs) compared to other two types of agitation (192 hrs for both cathode rod movement and solution circulation). It also showed marginally superior SST red rust life than the magnetic stirring (240hrs). Both, solution circulation and cathode rod movement showed lower SST life than magnetic stirring.

The SST white rust life comparison for different types of agitation is shown in Figure 13. White rust life depends upon the surface properties. For all the cases, Ni wt% was maintained between 12-16wt.%. For that reason, no change in terms of white rust life was observed using different types of agitation. For all the cases, white rust of the coating was observed within 24hrs of the SST test.

Example 10: Comparison of cross-sectional microstructure of the present coatings vs. the commercial Zn-Ni coating
Figure 14 shows the comparative cross-sectional view of the commercial sample having an average 3 µm coating thickness deposited at 0.4 µm/min rate and the Zn-Ni coating of the present disclosure with an average thickness of 4µm deposited at 2µm/min rate. The cross-sectional microstructure of the commercial sample shows multiple cracks through the thickness. On the other hand, the Zn-Ni alloy coating of the present disclosure (prepared from air agitation) was very compact, with no cracks observed throughout the cross-section. Hence, the present coating is less brittle compared to the commercial one.

Example 11: Ni and Zn concentration in the electrolyte
Tabel 1 shows the elemental Zn and Ni present in various electrolytes.
Table 1: Elemental Zn and elemental Ni present in electrolyte
System Name Elemental Zn (ppm) Elemental Ni (ppm)
Zn-Ni commercial Acidic Bath 38089 16284
Zn-Ni commercial Alkaline Bath 35089 15279
Zn-Ni bath of the present disclosure 30256 43786

In EMF series, the standard reduction potential of zinc is -0.76V and Ni is -0.25V. So, Zn is much more active compared to Ni. In commercial baths, different kinds of complexing agents are used to co-deposit Zn-Ni. So, in commercial baths, 2 types of Zn are present in the electrolyte - one in the ionic form and one in the complexed form. Free ions are deposited easily compared to complexed one. This explains why the deposition kinetics is slower in commercial baths. To improve the deposition kinetics, free ions concentrations need to be increased. The electroplating bath of the present disclosure is composed of only basic salts like ZnSO4 and NiCl2. No complexing agent was used. Figure 5 showed that to deposit 12-16 wt.% Ni, free Ni to Zn ions ration should be in between 1.1-1.7 range. The electroplating composition comprising 125g/L zinc sulphate, 135 g/L nickel sulphate, 50 g/L nickel chloride, and 30 g/L boric acid, and having a pH of 3.5 has free Ni to Zn ion ratio of 1.45. On the other hand, in commercial baths, Zn is in the complexed form; therefore, free Zn ions will be low. Also, the Ni ions are present very low concentration. Therefore, to obtain a free Ni to Zn ions ratio in the 1.1-1.7 range, free Zn ions should be at least in the 14770-9557 ppm range in an acidic bath and 13890-8987 ppm range in an alkaline bath. Compared to this, 3-4 times higher free Zn ions and Ni ions were present in the electroplating composition of the present disclosure. These high amount of free Zn and Ni ions contribute to the higher deposition kinetics provided by the electroplating compositions of the present disclosure compared to commercial baths.

Example 12: Stability study of the electroplating composition
The electrolyte/electroplating composition was prepared using only basic salts like zinc sulphate, nickel sulphate and nickel chloride. Boric acid was added as a buffering agent to maintain pH at 3.5. A bath stability study was conducted for 6 months. The color of the electrolyte was dark green when prepared. After 6 months, no color change was observed in the electrolyte. The electrolyte was transparent even after 6 months of preparation. After plating, the bath pH (3.5) did not change much, and no precipitation and color change were observed.

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