Claims:We Claim:
1. A method of providing a zinc (Zn)-based coating on a steel substrate, comprising:
a. treating the steel substrate with an aqueous fluxing solution comprising two or more chloride salts to obtain a treated steel substrate; and
b. dipping the treated steel substrate in a plating bath comprising molten zinc to provide a Zn-based coating on the steel substrate.
2. The method as claimed in claim 1, wherein the chloride salts are selected from zinc chloride, ammonium chloride, sodium chloride, lead chloride, cupric chloride, cobalt chloride, and stannous chloride.
3. The method as claimed in claim 1 or 2, wherein ratio of water to total salt in the aqueous fluxing solution is ranging from about 10:1 to about 2:1.
4. The method as claimed in claim 3, wherein the ratio of water to total salt in the aqueous fluxing solution is about 3:1.
5. The method as claimed in claim 1 or 2, wherein a total amount of the chloride salts in the aqueous fluxing solution ranges from about 100-600 gram per litre of water.
6. The method as claimed in any one of claims 1-5, wherein the chloride salts are zinc chloride and ammonium chloride.
7. The method as claimed in claim 6, wherein zinc chloride constitutes about 68-90% by weight of total chloride salts; and wherein ammonium chloride constitutes about 8-12% by weight of total chloride salts.
8. The method as claimed in any one of claims 5-7, wherein other chloride salts constitute about 2-20% by weight of total chloride salts.
9. The method as claimed in any one of claims 1-8, wherein the steel substrate is treated with the aqueous fluxing solution at a temperature of about 40-90?.
10. The method as claimed in any one of claims 1-9, wherein the steel substrate is cleaned or pickled or both cleaned and pickled prior to treatment with the aqueous fluxing solution.
11. The method as claimed in claim 10, wherein the steel substrate is cleaned by alkali cleaning, ultrasonic cleaning, or brush cleaning; and wherein the steel substrate is pickled by applying hydrochloric acid.
12. The method as claimed in claim 10 or 11, wherein the steel substrate is cleaned to a level of less than 0.6 mg/cm2 residual dirt.
13. The method as claimed in any one of claims 1-12, wherein the zinc-based coating is selected from a group comprising zinc (Zn) coating, zinc-aluminium (Zn-Al) coating, zinc- lanthanum (Zn-La) coating and zinc-aluminium-lanthanum (Zn-Al-La) coating.
14. The method as claimed in claim 13, wherein the zinc-based coating is zinc-aluminium-lanthanum (Zn-Al-La) coating.
15. The method as claimed in claim 1, wherein the plating bath contains molten zinc.
16. The method as claimed in claim 1, wherein the plating bath comprises molten zinc and metal selected from a group comprising aluminium and lanthanum or a combination thereof.
17. The method as claimed in claim 1 or 16, wherein the plating bath comprises molten zinc, aluminium and lanthanum.
18. The method as claimed in claim 17, wherein the ratio of Zn:Al:La in the plating bath is in the range of 3:1:0.02 to 49:1:0.02.
19. The method as claimed in any one of claims 1-18, wherein the plating bath comprises at least 90 wt % zinc.
20. The method as claimed in any one of claims 1-19, wherein the plating bath comprises at least 0.15 wt % aluminium.
21. The method as claimed in any one of claims 1-20, wherein the plating bath comprises about 2-20 wt % aluminium.
22. The method as claimed in any one of claims 1-21, wherein the plating bath comprises about 0.05-0.15 wt% lanthanum.
23. The method as claimed in any one of claims 1-22, wherein the plating bath comprises about 95 wt% zinc, about 5 wt% aluminium, and about 0.1 wt% lanthanum.
24. The method as claimed in any one of claims 1-23, wherein the plating bath has a temperature of about 400-500?.
25. The method as claimed in any one of claims 1-24, wherein the Zn-based coating provided by the method has a thickness of about 4-40 microns.
26. The method as claimed in any one of claims 1-25, wherein the Zn-based coating provided by the method has a corrosion current of about 1.5-4 A/cm2.
27. The method as claimed in any one of claims 1-26, wherein the Zn-based coating provided by the method has a corrosion rate of about 2-6 mils penetration per year (mpy).
28. The method as claimed in any one of claims 1-27, wherein the steel substrate is in the form of a continuous product or a batch product.
29. The method as claimed in any one of claims 1-28, wherein the steel substrate is a wire, sheet, plate, nail, pipe, hollow section, or thermos-mechanically treated (TMT) rebar.
30. A fluxing solution for preparing a steel substrate for a hot-dip coating, comprising two or more chloride salts selected from zinc chloride, ammonium chloride, sodium chloride, lead chloride, cupric chloride, cobalt chloride, and stannous chloride.
31. The fluxing solution as claimed in claim 30, wherein ratio of water to total salt in the aqueous fluxing solution ranges from about 10:1 to about 2:1.
32. The fluxing solution as claimed in claim 25, wherein a total amount of the chloride salts in the aqueous fluxing solution ranges from about 100-600 gram per litre of water.
33. The fluxing solution as claimed in any one of claims 30-32, wherein the fluxing solution comprises zinc chloride, ammonium chloride, and one or more chlorides selected from sodium chloride, lead chloride, cupric chloride, cobalt chloride, and stannous chloride.
34. The fluxing solution as claimed in any one of claims 30-33, wherein zinc chloride constitutes about 68-90% by weight of total chloride salts, ammonium chloride constitutes about 8-12% by weight of total chloride salts, and remaining chloride salts constitute about 2-20% by weight of total chloride salts. , Description:TECHNICAL FIELD
The present disclosure generally relates to the field of material science and metallurgy. In particular, the present disclosure relates to a method of providing a zinc (Zn)-based coating on a steel substrate; and a fluxing solution for preparing a steel substrate for a hot-dip coating.

BACKGROUND OF THE DISCLOSURE
Steel is a widely employed material for various applications. Unfortunately, steel has a tendency to corrode over time. A variety of methods have evolved over the past several centuries for controlling corrosion, with particular emphasis on methods to extend the life of metallic substances in corrosive environments. These methods typically include protective coatings which are principally used to upgrade the corrosion resistance of ferrous metals, such as steel and nonferrous metals, such as aluminium, and to avoid the necessity for using more costly alloys. However, such protecting coatings typically have several pitfalls. Protective coatings fall into two main categories, viz., (i) topical coating and (ii) sacrificial coating. Topical coatings, such as paints, is the largest of these categories, and it acts as a physical barrier against the environment. Sacrificial coatings, such as zinc or cadmium, are designed to preferentially corrode in order to save the base metal from attack.

Organic coatings including epoxy, phenolic, polyester, phthalic acid, fluorine and silicone have been proposed. However, such coatings have been generally unsatisfactory because they give only limited corrosion protection, are relatively soluble and/or result in a toxic waste disposal problem.

Phosphate conversion coatings are widely used to improve the corrosion performance of painted ferrous metals, particularly painted steel. The corrosion processes of painted steel involve high pH conditions at the paint-metal substrate interface. Since, phosphate coatings are unstable in an alkaline environment, phosphate steel are rinsed with solutions containing chromium or chromate ions to improve their alkaline stability. However, recent studies suggest the improvement is marginal. Although dry paint adhesion on chromated phosphate steel is good, wet paint adhesion is unacceptable. The bond between the paint-phosphate interface is weak when water or other corrosion species are present.

Chromate coating have been used to improve corrosion resistance of cold-rolled steel by minimizing red rusting and of galvanized steel by minimizing white rusting. Unfortunately, hexavalent chromium has carcinogenic properties. Because of its toxic nature, rinses containing chromate ions are undesirable for industrial usage.

To improve corrosion resistance and paint adhesion of cold-rolled or galvanized steel sheet, organic polymeric coatings containing a silane or inorganic coatings including a combination of silane and silicate have been explored. To improve alkaline corrosion resistance and paint adhesion of phosphate cold-rolled or galvanized steel sheet, a two-step process including rinsing the sheet in an alkaline waterglass solution to form a silicate coating and subsequently rinsing the silicate coated sheet in an aqueous silane containing solution have been explored. However, the disadvantage of these coatings is that there is a chance of higher dross formation. Further, normal pad wiping may not work for these coatings.

Building materials, structures, white goods and automobiles made of iron and steel are coated with molten zinc for protection against corrosion. Cathodic protection (CP) reduces corrosion by changing the thermodynamics of the steel, i.e., the chemical potential of the steel is changed to make it more inert. More electropositive material like Zn and Al have a tendency to give sacrificial protection to base steel where the coating material is likely to be damaged and the base metal remains free from corrosion. A series of long term exposures test reported that galvanized coating delays the onset of corrosion in marine environment, but does not prevent it completely. An eleven-year exposure programme in marine environment revealed that the zinc coating suffered 2 to 3 mil losses in thickness of the original zinc layer due to corrosion. Zinc is an amphoteric metal, which is stable in a specific range of pH 6-12 only and corrosion behaviour of galvanized steel in presence of chlorides is controlled by the medium pH. Ductility of the coating of pure zinc coated material is also very poor due to presence of thick brittle zeta phase. Further, use of alloy elements like copper and cadmium are harmful for ductility of the coating. Addition of nickel in zinc coating reduces the hydrogen evolution reaction as well increases the corrosion resistance property against chloride. The minimum requirement of nickel in a zinc-nickel alloy is 12 wt % for drastic improvement in corrosion resistance against chloride ions. However, making alloys making with such composition is expensive and difficult due to wide difference of their melting temperatures.

Zinc-aluminium alloys such as Galfan, which mainly consists of 95 wt % zinc and 5 wt % aluminium, impart higher corrosion protection to steel, improve its formability as well as its paintability compared to traditional hot-dip zinc coating. Though Galfan alloy was developed more than thirty years ago, their application for the coating of continuous products such as wires, tubes and sheets can only be performed by a limited number of or rather sophisticated and relatively expensive processes. Such processes include double-dip process whereby regular galvanising precedes Galfan coating, and the hot process whereby a furnace with a reducing atmosphere is used before Galfan application. Numerous attempts to apply Galfan by the traditional and more affordable Cook-Norteman flux process on continuous lines, have failed.

Thus, there has been a long-felt need in the industry to develop a low cost, nontoxic, relatively insoluble, corrosion resistant coating for ferrous metals that is environmentally safe and that can be disposed of inexpensively. Further, considering the popularity of flux galvanizing and its relatively low manufacturing cost, it seems very attractive to develop a method wherein Galfan coating would become possible on continuous lines as well as in batch operations. The present disclosure attempts to address said need of the art and provides for effective means and methods of coating ferrous metals such as steel substrates on continuous lines as well as in batch operations to prevent the corrosion of the substrate.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to a method of providing a zinc (Zn)-based coating on a steel substrate, comprising treating the steel substrate with an aqueous fluxing solution comprising two or more chloride salts to obtain a treated steel substrate; and dipping the treated steel substrate in a plating bath comprising molten zinc to provide a Zn-based coating on the steel substrate. The Zn-based coatings, such as Zn-Al or Zn-La-Al coatings, provided by the methods of the present disclosure are low cost, non-toxic, stable, hard, compact, impermeable, relatively insoluble, and corrosion resistant.

The present disclosure also relates to a fluxing solution for preparing a steel substrate for a hot-dip coating, comprising two or more chloride salts selected from zinc chloride, ammonium chloride, sodium chloride, lead chloride, cupric chloride, cobalt chloride, and stannous chloride.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
For the purpose that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure where:

Figure 1 depicts the pictorial view of pilot scale Zincal coating (a) bath and (b) process.

Figure 2 depicts the pictorial view of (a) commercial GI and (b) pilot scale Zincal coated wire coil.

Figure 3 depicts SEM and EDX depth analysis of commercial (a) GI and (b) Zincal coated steel substrate.

Figure 4 depicts the Tafel test results of GI and Zincal coated steels.

Figure 5 depicts the electrochemical impedance test results of GI and Zincal coated steels.

Figure 6 depicts appearance of wire surfaces after different exposure times in salt spray. Upper panel, from left to right: GI after 24 hours, Zincal after 24 hours, GI after 196 hours and Zincal after 196 hours. Lower panel, left to right: Zincal after 508 hours, Zincal after 610 hours, and Zincal after 658 hours.

Figure 7 depicts appearance of wire surfaces after wrapping for different exposure times in salt spray. Upper panel, left to right: Zincal - 0 hours and GI - 0 hours. Middle panel, left to right: Zincal – 120 hours and GI – 120 hours. Lower panel, left to right: Zincal – 300 hours, Zincal – 400 hours and Zincal – 658 hours.

DETAILED DESCRIPTION OF THE DISCLOSURE
The present disclosure overcomes the drawbacks of the art and provides a method of providing anticorrosion coating on the surface of ferrous metals and a fluxing solution for preparing a steel substrate for a hot-dip coating.

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.

Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” 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. Similarly, terms such as “include” or “have” or “contain” and all their variations are inclusive and 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.

The term “about” or “approximately” as used herein encompasses variations of +/-10% and more preferably +/-5%, as such variations are appropriate for practicing the present invention.

As used herein, the term “zinc-based coating” or “Zn-based coating” are used interchangeably and refer to a coating provided on a steel substrate that comprises zinc as the major element of the coating. In some embodiments, the Zn-based coating comprises zinc as the sole element. In some embodiments, Zn-based coatings comprise zinc or an alloy of zinc such as, but not limiting to, zinc alloyed with aluminium (e.g., Zn-Al coating) and/or lanthanum (e.g., Zn-Al-La coating).

The present disclosure relates to a simple and cost-effective method for providing a zinc-based anticorrosion coating on the surface of ferrous metal, particularly steel. The anticorrosion Zn-based coating provided on the surface of ferrous metal, such as steel, by the method of the present disclosure is stable, hard, compact, impermeable, non-toxic, relatively insoluble and resistant to corrosion in different aggressive corrosive environments.

In particular, the present disclosure has developed a suitable fluxing solution for pre-treating the surface of ferrous metal such as steel, prior to subjecting the ferrous metal to a single hot-dip process for coating its surface with a Zn-based coating. Coating the Zn-based coating on a steel substrate which has been pre-treated with the fluxing solution of the present disclosure significantly improves the properties of the coating vis-à-vis conventional galvanized coatings. In particular, treatment with the fluxing solution of the present disclosure applies a protective layer on the surface of the steel substrate. This improves the properties of the Zn-based coating applied subsequently on the steel substrate and prevents corrosion of the base metal effectively without any adverse effect to the properties of the base metal. The fluxing solution is an aqueous mixture of two or more chloride salts.

In some embodiments, the method of the present disclosure comprises cleaning oil or dirt, pickling and fluxing of the surface to be coated. The treated surfaces can then be galvanised by single immersion in a molten zinc-based bath. The process of the present disclosure is especially suited for galvanisation of continuous products such as steel wire, tube, sheet, etc as well as batch products such as nails or a plate.

In some embodiments, the method of providing a zinc (Zn)-based coating on a steel substrate comprises:
a. treating the steel substrate with an aqueous fluxing solution comprising two or more chloride salts to obtain a treated steel substrate; and
b. dipping the treated steel substrate in a plating bath comprising molten zinc to provide a Zn-based coating on the steel substrate.

In some embodiments, the chloride salts employed in the fluxing solution of the present disclosure are selected from a group comprising zinc chloride, ammonium chloride, sodium chloride, lead chloride, cupric chloride, cobalt chloride, and stannous chloride or any combination thereof.

In an exemplary and non-limiting embodiment of the present disclosure, the aqueous fluxing solution comprises about 500 ppm to about 1000 ppm of marvan additive.

In an exemplary embodiment, the fluxing solution is an aqueous mixture of two or more chloride salts selected from a group comprising zinc chloride, ammonium chloride, sodium chloride, lead chloride, cupric chloride, cobalt chloride and stannous chloride optionally along with an additive such as but not limiting to a commercial grade of marvan from MERK.

The fluxing solution of the present disclosure can be prepared by using all grades of water such as tap water, distilled water and double distilled water, and all grades of salts.

In some embodiments, total amount of the chloride salts in the aqueous fluxing solution ranges from about 100-600, 200-600, 300-600, 400-600, or 500-600 gram per litre of water. In some embodiments, the total chloride salt content of the fluxing solution ranges from about 500 to 550 g/L.

In some embodiments, ratio of water to total chloride salts in the aqueous fluxing solution ranges from about 10:1 to about 2:1. In some embodiments, ratio of water to total salt in the aqueous fluxing solution is about 10:1, about 9:1, about 8:1, about 7:1, about 6:1, about 5:1, about 4:1, about 3:1 or about 2:1. In a preferred and non-limiting embodiment, the ratio of water to total chloride salt in the aqueous fluxing solution is about 3:1.

In some embodiments, the chloride salts employed in the method of the present disclosure are zinc chloride and ammonium chloride.

In some embodiments, zinc chloride is the major constituent of the fluxing solution and is present in an amount ranging from about 68-90% by weight of the total chloride salts in the fluxing solution.

In some embodiments, the fluxing solution comprises at least 7 wt % of ammonium chloride.

In some embodiments, ammonium chloride constitutes about 8-12% by weight of the total chloride salts in the fluxing solution.

In some embodiments, the chloride salts employed in the fluxing solution comprise zinc chloride, ammonium chloride and optionally other chlorides, wherein the zinc chloride constitutes about 68-90% by weight of the total chloride salts, ammonium chloride constitutes about 8-12% by weight of the total chloride salts, and the other chloride salts constitute about 2-20% by weight of total chloride salts.

In some embodiments, the total chloride salt content of the fluxing solution ranges from about 500 to 550 g/L and the fluxing solution comprises about 75-80 wt% zinc chloride, about 8-15 wt% ammonium chloride, about 2-5 wt% ferric chloride, about 2-5 wt% sodium chloride, and about 2-5 wt% potassium chloride.

In some embodiments, the steel substrate employed in the method of the present disclosure is treated with the aqueous fluxing solution at a temperature ranging from about 40? to about 90?, preferably ranging from about 70? to about 85?, more preferably ranging from about 70? to about 75?.

In some embodiments, the steel substrate employed in the method of the present disclosure is cleaned and pickled prior to treatment with the aqueous fluxing solution.

In some embodiments, the steel substrate is cleaned by alkali cleaning, ultrasonic cleaning, or brush cleaning. Cleaning the steel substrate helps in removing any oil and/or dirt from the surface of the substrate.

In some embodiments, the steel substrate is cleaned to a level of less than 0.6 mg/cm2 residual dirt.

In some embodiments, pickling of the steel substrate is performed by acid pickling. In an exemplary embodiment, the steel substrate is pickled by applying hydrochloric acid.

In some embodiments, the method of providing a zinc (Zn)-based coating on a steel substrate comprises:
a. optionally cleaning and/or pickling the steel substrate,
b. treating the steel substrate with an aqueous fluxing solution, at a temperature ranging from about 40? to about 90?, comprising two or more chloride salts selected from a group comprising zinc chloride, ammonium chloride, sodium chloride, lead chloride, cupric chloride, cobalt chloride, and stannous chloride or any combination thereof, to obtain a treated steel substrate; and
c. hot-dipping the treated steel substrate in a plating bath comprising molten zinc and optionally other alloying elements to provide an anticorrosion Zn-based coating on the surface of the steel substrate.
In some embodiments, the zinc-based coating coated on the steel substrate is selected from a group comprising but not limiting to zinc (Zn) coating, zinc-aluminium (Zn-Al) coating, zinc- lanthanum (Zn-La) coating, zinc-aluminium-lanthanum (Zn-Al-La) coating, zinc-magnesium (Zn-Mg) coating, zinc-aluminium-strontium (Zn-Al-Sr) coating, zinc-aluminium-chromium (Zn-Al-Cr) coating, and the like.

In some embodiments, the plating bath in the method of the present disclosure comprises molten zinc. In some embodiments, the plating bath in the method of the present disclosure comprises molten zinc in combination with one or more metals such as but not limiting to aluminium and lanthanum or a combination thereof. In some embodiments, the plating bath in the method of the present disclosure comprises molten zinc, aluminium and lanthanum.

In some embodiments, the plating bath employed in the method of the present disclosure comprises at least 90 wt % of zinc. In some embodiments, the plating bath comprises 90 wt %, 91 wt %, 92 wt %, 93 wt %, 94 wt %, 95 wt %, 96 wt %, 97 wt %, 98 wt %, 99 wt % or 100 wt % of zinc.

In some embodiments, the plating bath employed in the method of the present disclosure comprises at least 0.15 wt % of aluminium.

In some embodiments, the plating bath employed in the method of the present disclosure comprises about 2 wt% to about 20 wt % of aluminium.

In some embodiments, the plating bath employed in the method of the present disclosure comprises at least 0.05 wt % of lanthanum.

In some embodiments, the plating bath employed in the method of the present disclosure comprises about 0.05 wt% to about 0.15 wt% lanthanum.

In some embodiments, the zinc-based coating coated on the steel substrate is zinc-aluminium-lanthanum (Zn-Al-La) coating.

In some embodiments, the method of the present disclosure comprises providing a zinc-aluminium-lanthanum (Zn-Al-La) coating on a steel substrate, comprising:
a. treating the steel substrate with an aqueous fluxing solution comprising two or more chloride salts to obtain a treated steel substrate; and
b. dipping the treated steel substrate in a plating bath comprising molten zinc, aluminium, and lanthanum (Zn-Al-La) to provide a Zn-Al-La coating on the steel substrate.

In some embodiments, the method of the present disclosure comprises providing a zinc-aluminium-lanthanum (Zn-Al-La) coating on a steel substrate, comprising:
a. optionally cleaning the surface of the steel substrate by alkali cleaning, ultrasonic cleaning, or brush cleaning,
b. optionally pickling the surface of the steel substrate,
c. treating the steel substrate with an aqueous fluxing solution comprising two or more chloride salts to obtain a treated steel substrate; and
d. hot-dipping the treated steel substrate in a plating bath comprising molten zinc, aluminium, and lanthanum (Zn-Al-La) to provide a Zn-Al-La coating on the steel substrate.

In some embodiments, the ratio of Zn:Al:La in the plating bath employed in the method of the present disclosure is in the range of 3:1:0.02 to 49:1:0.02. In a preferred embodiment, the treated steel substrate is coated with Zn:Al:La coating in a ratio of 19:1:0.02 at low temperature.

In some embodiments, the plating bath employed in the method of the present disclosure comprises about 95 wt% zinc, about 5 wt% aluminium, and about 0.1 wt% lanthanum.

In an exemplary embodiment, steel substrate with 95 wt% Zn - 5wt% Al - 0.1wt% La defect free alloy coating is herein termed as Zincal. Said coating has extremely good corrosion resistance properties.

In some embodiments, the plating bath employed in the method of the present disclosure has a temperature ranging from about 400? to about 500?.

In an embodiment, the controlled bath temperature during the method of the present disclosure is about 400°C, about 410°C, about 420°C, about 430°C, about 440°C, about 450°C, about 460°C, about 470°C, about 480°C, about 490°C or about 500°C. In an exemplary and non-limiting embodiment of the present disclosure, the most suitable bath temperature is 40°C above the melting point of that alloy.

Once the steel substrate is coated with the zinc-based coating in a plating bath by the hot dip process, any extra liquid metal can be removed from the surface of the steel substrate by any conventionally acceptable wiping process.

In an embodiment, all types of wiping processes are suitable for the alloy coating as per the present disclosure. Wiping process that can be employed for wiping the Zn-based coating (including the Zn-Al-La alloy coating) includes but is not limited to force air, nitrogen, cooled air, charcoal, pad or asbestos rope, preferably force air. After wiping, the coated ferrous substrate can be cooled in water, oil or cooled air, preferably in cooled air.

In some embodiments, the Zn-based coating provided by the method of the present disclosure has a thickness ranging from about 4 microns to about 40 microns.

In an exemplary embodiment, the coated surfaces of the present disclosure exhibited excellent corrosion resistance. The corrosion resistance against aggressive saline environment increased up to 4 times than pure GI coating for same coating thickness and the corrosion resistance increased up to 8 times against industrial environment, for the same coating thickness. The resistance was found to increase 13 times against white rust formation, for the same coating thickness. The tensile and ductility parameters were also found to be much better for the alloy coated steel rather than for pure zinc coated steel.

In some embodiments, the Zn-based coating provided by the method of the present disclosure has a corrosion current ranging from about 1.5 A/cm2 to about 4 A/cm2.

In some embodiments, the Zn-based coating provided by the method of the present disclosure has a corrosion rate of about 2 mils penetration per year to about 6 mils penetration per year (mpy).

In some embodiments, the steel substrate employed for coating is in the form of a continuous product or a batch product.

In an exemplary embodiment, Figure 1 illustrates the pilot scale line set up for Zincal coating on steel substrate and more specifically on the medium and low carbon steel wire surface. The line consists of phosphate bonded ceramic bath with electrical heating facility to maintain the desirable bath temperature.

In an exemplary and non-limiting embodiment of the present disclosure, the steel substrate that is provided with n anticorrosion coating by the method of the present disclosure is in a form selected from a group comprising a wire, sheet, plate, nail, pipe, structure hollow section or rebar.

In an embodiment, the sheet / sheet metal includes cold rolled and hot rolled steel sheet. The cold rolled steel sheet includes CQ (commercial quality), EDD (extra deep drawable), IF (interstitial free), IF-HS (interstitial free high strength) and DP (Dual phase) steel sheets. The wires on which the coating may be applied include high, medium and low carbon wires and all forms of drawn and normalized wires. The pipes and hollow sections on which the coating may be applied, includes all sizes and compositions, and the rebar includes TMT (thermos mechanically treatment) and TMT-CRS (corrosion resistance steel) or any other form of steel material that is capable of undergoing corrosion.

The present disclosure also pertains to a fluxing solution for preparing a steel substrate for a hot-dip coating, comprising two or more chloride salts selected from a group comprising zinc chloride, ammonium chloride, sodium chloride, lead chloride, cupric chloride, cobalt chloride, and stannous chloride.

In some embodiments, total amount of the chloride salts in the aqueous fluxing solution ranges from about 100-600 gram per litre of water.

In some embodiments, ratio of water to total salt in the aqueous fluxing solution is ranging from about 10:1 to about 2:1. In an embodiment, ratio of water to total salt in the aqueous fluxing solution is about 10:1, about 9:1, about 8:1, about 7:1, about 6:1, about 5:1, about 4:1, about 3:1 or about 2:1. In a preferred and non-limiting embodiment, the ratio of water to total salt in the aqueous fluxing solution is about 3:1.

In some embodiments, the chloride salts employed in the method of the present disclosure are zinc chloride and ammonium chloride.

In some embodiments, zinc chloride constitutes about 68-90% by weight of the total chloride salts in the fluxing solution.

In some embodiments, ammonium chloride constitutes about 8-12% by weight of the total chloride salts in the fluxing solution.

In some embodiments, the chloride salts employed in the fluxing solution comprise zinc chloride, ammonium chloride and optionally other chlorides, wherein the zinc chloride constitutes about 68-90% by weight of the total chloride salts, ammonium chloride constitutes about 8-12% by weight of the total chloride salts, and the other chloride salts constitute about 2-20% by weight of total chloride salts.

In some embodiments, the other chlorides employed in addition to zinc chloride and ammonium chloride are selected from a group comprising sodium chloride, lead chloride, cupric chloride, cobalt chloride, and stannous chloride or any combination thereof.

In some embodiments, the total chloride salt content of the fluxing solution ranges from about 500 to 550 g/L and the fluxing solution comprises about 75-80 wt% zinc chloride, about 8-15 wt% ammonium chloride, about 2-5 wt% ferric chloride, about 2-5 wt% sodium chloride, and about 2-5 wt% potassium chloride.

In some embodiments, the aqueous fluxing solution further comprises an additive.

In an exemplary and non-limiting embodiment of the present disclosure, the aqueous fluxing solution comprises about 500 ppm to about 1000 ppm of marvan additive.

In some embodiments, the present disclosure provides for a fluxing solution for a zinc-aluminium-lanthanum (Zn-Al-La) alloy coating on steel surfaces, which prevents corrosion of the base metal effectively and does not affect the properties of the base metal.

In an exemplary embodiment, a cleaned steel substrate is treated with the fluxing solution of the present disclosure and then the flux treated steel substrate is dipped in a molten bath comprising zinc optionally along with one or more other metals such as aluminium and lanthanum at about 420°C - 460°C. After coating, the substrate was cooled in water or normal air. A defect free or a nearly defect free coating is observed on the surface of the steel thus obtained. The coating that was formed was compact and had superior resistance property towards corrosion.

In some embodiments, the properties of Zincal coated ferrous metal (steel) was compared with conventional galvanised steel by scanning electron microscope (SEM) analysis, Energy Dispersive X-Ray Analysis (EDX), Tafel test, Electrochemical impedance test and Salt fog test.

In some embodiments, during the method of the present disclosure the Zn-Al-La alloy coating reacts with steel surface under controlled temperature and controlled process conditions to form a Zn-Al alloy coating on the surface of steel with alternative layer of aluminium rich and zinc rich phase which is ductile, stable, impermeable and insoluble. The said lamellar Zn-5wt%Al-0.1wt%La alloy coating on the surface of steel has superior anticorrosion properties.

In an embodiment, the Tafel test results demonstrate the dissolution/corrosion of zinc coated material in an aggressive corrosive environment. The zinc-based coating provided by the method of the present disclosure provides superior resistance against corrosion than conventional galvanised steel.

In an embodiment, the electrochemical impedance test results indicate that the anticorrosion coating on ferrous metal provided by the method of the present disclosure provides superior resistance against corrosion than the uncoated steel, with increased resistivity and less phase angle shift. Higher the resistivity and less phase angle shift of steel surfaces indicate better resistance against electrochemical corrosion of steel.

In an embodiment, the surface appearance of the galvanised and Zincal coated steel substrates were assessed post their exposure in a salt fog test. The results demonstrated that the Zincal coated steel has more resistivity and less weight loss after exposure in aggressive chloride environment when compared to the conventional pure zinc coated steel. This showcases that Zincal coated steel has superior (excellent) resistance against corrosion when compared to pure zinc coated galvanised steel.

ADVANTAGES
The method of providing a zinc (Zn)-based coating on a steel substrate and the fluxing solution of the present disclosure have several advantages/benefits, including, but not limiting to the following:
a. The method of the present disclosure is simple and cost effective.
b. The anticorrosion coating provided on the surface of ferrous metal such as steel by the method of the present disclosure is stable, hard, compact, impermeable, non-toxic, relatively insoluble and resistant to corrosion in different aggressive corrosive environments.
c. The coating on the surface of the ferrous metal provided by the method of the present disclosure not only prevents the base metal from corrosion but also prevents the ferrous metal from any other damaging reactions caused by the environment in which the ferrous metals are used. It also does not affect the properties of the base metal.
d. The method of the present disclosure is suited for galvanisation of continuous products such as steel wire, tube or sheet, etc.
e. Zinc-aluminium alloys such as Galfan, which mainly consists of 95 wt % zinc and 5 wt % aluminium, impart higher corrosion protection to steel, improve its formability as well as its paintability compared to traditional hot-dip zinc coating. However, attempts to apply Galfan by the traditional and affordable processes on continuous lines, have failed. The method of the present disclosure makes it possible to apply Galfan coating in a cost-effective manner on continuous lines as well as in batch operations.

Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based on the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein.

The foregoing description of the specific embodiments fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments in this disclosure have been described in terms of preferred embodiments, 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.

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. As used in this specification and the appended claims, the singular forms “a,” “an” and “the” includes both singular and plural references unless the content clearly dictates otherwise. 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. As such, the terms “a” (or “an”), “one or more”, and “at least one” can be used interchangeably herein.

Numerical ranges stated in the form ‘from x to y’ include the values mentioned and those values that lie within the range of the respective measurement accuracy as known to the skilled person. If several preferred numerical ranges are stated in this form, of course, all the ranges formed by a combination of the different end points are also included.

As regards the embodiments characterized in this specification, it is intended that each embodiment be read independently as well as in combination with another embodiment. For example, in case of an embodiment 1 reciting 3 alternatives A, B and C, an embodiment 2 reciting 3 alternatives D, E and F and an embodiment 3 reciting 3 alternatives G, H and I, it is to be understood that the specification unambiguously discloses embodiments corresponding to combinations A, D, G; A, D, H; A, D, I; A, E, G; A, E, H; A, E, I; A, F, G; A, F, H; A, F, I; B, D, G; B, D, H; B, D, I; B, E, G; B, E, H; B, E, I; B, F, G; B, F, H; B, F, I; C, D, G; C, D, H; C, D, I; C, E, G; C, E, H; C, E, I; C, F, G; C, F, H; C, F, I, unless specifically mentioned otherwise.

Any discussion of documents, acts, materials, devices, articles and the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

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. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

All references, articles, publications, general disclosures etc. cited herein are incorporated by reference in their entireties for all purposes. However, mention of any reference, article, publication etc. cited herein is not, and should not be taken as, an acknowledgment or any form of suggestion that they constitute valid prior art or form part of the common general knowledge in any country in the world.

Further, while the instant disclosure is susceptible to various modifications and alternative forms, specific aspects thereof has been shown by way of examples and drawings and are described in detail below. However, it should be understood that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and the scope of the invention.

EXAMPLES
The present disclosure is further described with reference to the following examples, which are only illustrative in nature and should not be construed to limit the scope of the present disclosure in any manner.

Example 1: Preparation of Zn-Al-La alloy coating on the surface of ferrous metal
A cleaned steel substrate was first treated in a conventional zinc-ammonium chloride fluxing solution comprising 80wt% zinc chloride and 20 wt% ammonium chloride and then the flux treated steel substrate was dipped in molten zinc containing about 2 wt% of aluminium and about 0.05wt% of lanthanum at about 460 °C.

A defective coating was observed on the surface of the steel thus obtained, wherein the coating comprised of many black spots. The coating that was formed was not compact and had an inferior resistance property towards corrosion with a corrosion rate of about 12.513mpy.

Example 2: Preparation of Zn-Al-La alloy coating on the surface of ferrous metal
A cleaned steel substrate was first treated in a conventional zinc-ammonium chloride fluxing solution comprising 80wt% zinc chloride and 20 wt% ammonium chloride and then the flux treated steel substrate was dipped in molten zinc containing about 5 wt% of aluminium and about 0.1wt% of lanthanum at about 460 °C. After coating, the substrate was cooled in water.

A defective coating was observed on the surface of the steel thus obtained, wherein the coating comprised of many black spots. The coating that was formed was not compact and had an inferior resistance property towards corrosion with a corrosion rate of about 8.563mpy.

Example 3: Preparation of Zn-Al-La alloy coating on the surface of ferrous metal
A cleaned steel substrate was first treated in a conventional zinc-ammonium chloride fluxing solution comprising 80wt% zinc chloride and 20 wt% ammonium chloride and then the flux treated steel substrate was dipped in molten zinc containing about 20 wt% of aluminium and about 0.1wt% of lanthanum at about 460 °C. After coating, the substrate was cooled in water.

A defective coating was observed on the surface of the steel thus obtained, wherein the coating comprised of many black spots. The coating that was formed was not compact and had an inferior resistance property towards corrosion with a corrosion rate of about 7.863mpy.

Example 4: Preparation of Zn-Al-La alloy coating on the surface of ferrous metal
A cleaned steel substrate was first treated in a modified fluxing solution comprising 78 wt% zinc chloride, 10 wt% ammonium chloride, 4 wt% ferric chloride, 4 wt% sodium chloride, and 4 wt% potassium chloride and then the flux treated steel substrate was dipped in molten zinc containing about 5 wt% of aluminium and about 0.1wt% of lanthanum at about 460 °C. After coating substrate was cooled in water.

A nearly defect free coating was observed on the surface of the steel thus obtained, wherein the coating comprised of less black spots. The coating that was formed was not compact and had an inferior resistance property towards corrosion with a corrosion rate of about 6.263mpy.

Example 5: Preparation of Zn-Al-La alloy coating on the surface of ferrous metal
A cleaned steel substrate was first treated in a modified fluxing solution comprising 78 wt% zinc chloride, 10 wt% ammonium chloride, 4 wt% ferric chloride, 4 wt% sodium chloride, and 4 wt% potassium chloride and then the flux treated steel substrate was dipped in molten zinc containing about 5 wt% of aluminium and about 0.1wt% of lanthanum at about 420 °C. After coating, the substrate was cooled in water.

A nearly defect free coating was observed on the surface of the steel thus obtained, wherein the coating comprised of very few defect spots. The coating that was formed was compact and had superior resistance property towards corrosion with a corrosion rate of about 6.023mpy.

Example 6: Preparation of Zn-Al-La alloy coating on the surface of ferrous metal
A cleaned steel substrate was first treated in a modified fluxing solution comprising 78 wt% zinc chloride, 10 wt% ammonium chloride, 4 wt% ferric chloride, 4 wt% sodium chloride, and 4 wt% potassium chloride and then the flux treated steel substrate was dipped in molten zinc containing about 20 wt% of aluminium and about 0.1wt% of lanthanum at about 420 °C. After coating, the substrate was cooled in water.

A nearly defect free coating was observed on the surface of the steel thus obtained, wherein the coating comprised of few defect spots. The coating that was formed was compact and had superior resistance property towards corrosion with a corrosion rate of about 4.433mpy.

Example 7: Preparation of Zn-5wt%Al-0.1wt%La alloy coating (Zincal)
A cleaned steel substrate was first treated in a modified fluxing solution comprising 78 wt% zinc chloride, 10 wt% ammonium chloride, 4 wt% ferric chloride, 4 wt% sodium chloride, and 4 wt% potassium chloride and then the flux treated steel substrate was dipped in molten zinc containing about 5 wt% of aluminium and about 0.1wt% of lanthanum at about 420 °C. After coating, the substrate was cooled in normal air. Figure 1 illustrates the pilot scale line set up for Zincal coating on steel substrate and more specifically on the medium and low carbon steel wire surface, wherein Figure 1(a) illustrates the bath and Figure 1(b) illustrates the process for continuous production of Zincal coating on steel substrate. The line consists of phosphate bonded ceramic bath with electrical heating facility to maintain the desirable bath temperature.

A defect free coating was observed on the surface of the steel thus obtained, wherein the coating comprised of layer of zinc and aluminium rich phases. The coating that was formed was compact and had an excellent resistance property towards corrosion with a corrosion rate of about 2.023mpy. The said defect free coating having extremely good corrosion resistance is herein referred to as Zincal.

Example 8: Analysis of Zincal and galvanized metal
The properties of Zincal coated ferrous metal (steel) obtained in Example 7 was compared with conventional galvanised steel. Figure 2 illustrates the wire bundle of the commercial GI coated steel and pilot scale zincal coated steel. Both steel substrates were subjected to scanning electron microscope (SEM) analysis, Energy Dispersive X-Ray Analysis (EDX), Tafel test, Electrochemical impedance test and Salt fog test.

Figure 3 illustrates the microstructure of commercial GI and Zincal coated steel substrate via., the SEM image and EDX depth analysis. It can be seen from Figure 3 that the GI coating consists of pure zinc layer at the outer coating layer followed by an iron-zinc intermetallic phase at the steel coating interface whereas the zincal coating consists of alternative layer of zinc rich and Al rich phases. The Zn-Al-La alloy reacts with steel surface under controlled temperature and controlled process conditions to form a Zn-Al alloy coating on the surface of steel with alternative layer of aluminium rich and zinc rich phase which is ductile, stable, impermeable and insoluble.

Figure 4 illustrates the results of the Tafel test, wherein it is demonstrated that the Zincal coated steel provides much superior (excellent) resistance against corrosion with corrosion current (Icorr) of about 1.70 A/cm2 and corrosion rate of about 2.023 mpy when compared to conventional galvanised steel which had Icorr of about 6.87 A/cm2 and corrosion rate of about 8.68 mpy. Lesser Icorr and lesser corrosion rate of steel indicate better resistance against electrochemical corrosion of steel.

Figure 5 illustrates the results of electrochemical impedance test of the zincal coated steels and conventional galvanised steel. The test results indicate the charge transfer resistivity and phase angle shift of the steel surface in corrosive environment under different frequencies. More the resistivity and less phase angle shift of steel surfaces indicate better resistance against electrochemical corrosion of steel. As per Figure 5, the anticorrosion coating on ferrous metal provided by the method of the present disclosure provides superior resistance (impedance) against corrosion than the galvanized steel, with increased resistivity and less phase angle shift. Thus, the Zincal coated steel has a much superior anti-corrosion performance when compared to conventional galvanised steel.

Figures 6 & 7 illustrate the surface appearance of the galvanised and Zincal coated steel substrates after exposure in salt fog test for a time period ranging from about 24 h to about 658 h. The figures depict the weight loss of both the coated steel substrates after exposure in aggressive chloride environment, wherein it is demonstrated that the Zincal coated steel has more resistivity and less weight loss when compared to the conventional pure zinc coated steel. This implies that Zincal coated steel has superior (excellent) resistance against corrosion when compared to pure zinc coated galvanised steel.