Claims:1. A process (100) of preparing Zn-Ni-Fe alloy coated steel substrate, the process (100) comprising:
cleaning surface of a steel substrate;
pickling the surface of the steel substrate with an acid solution;
annealing the steel substrate in controlled atmosphere to obtain annealed steel substrate;
electroplating Ni alloy to the annealed steel substrate in Ni alloy plating bath to obtain a Ni coated steel substrate;
dipping the Ni coated steel substrate in molten Zinc bath to obtain Zn-Ni coated substrate;
annealing the Zn-Ni coated substrate to obtain a Zn-Ni-Fe alloy coated metal substrate.
2. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the steel substrate is any one of steel wire, steel plate, steel nails, steel TMT rebar, steel tube, and steel sheet.
3. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the cleaning is performed by aqueous alkali solution such of NaOH or similar alkalis containing a surface-active agent, ultrasonic cleaning, or brush cleaning.
4. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the acid solution used for pickling is selected from any one of hydrochloric acid, and sulfuric acid.
5. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the annealing operation of the steel substrate is performed in a temperature range of 700°C - 860°C.
6. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 5, wherein the annealing operation of the steel substrate is performed in a temperature range of 780°C - 830°C.
7. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein nitrogen or hydrogen gas is used to control the atmosphere during the annealing operation of the steel substrate.
8. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the Ni alloy plating bath is an electrolytic cell comprising an electrolytic solution in which solution a cathode, which is the annealed steel substrate, and a nickel anode are immersed.
9. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the electrolytic solution comprises nickel sulphate, nickel chloride, ammonium chloride and boric acid.
10. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the nickel sulfate content in the electrolytic solution is varied in the range of 150 g/L to 300 g/L.
11. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the nickel chloride content in the electrolytic solution is varied in the range of 30 g/L to 150 g/L.
12. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the boric acid content in the electrolytic solution is varied in the range of 15 g/L -30 g/L.
13. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the NH4Cl content in the electrolytic solution is varied between 8 g/L - 12 g/L.
14. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the electrolytic solution is maintained in temperature range between 20 - 60 °C.
15. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 14, wherein the electrolytic solution is maintained at a temperature of 45°C.
16. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the pH of the electrolytic solution is maintained in the range of 2 to 5.
17. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 16, wherein the electrolytic solution is maintained at the pH of 3.5.
18. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the Ni coating thickness on the Ni coated steel substrate is in the range of 0.25 to 3 microns.
19. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein after electroplating on the steel substrate in the Ni alloy plating bath, the steel substrate is removed from the Ni alloy plating bath and is rinsed in distilled water tank to remove contaminants and is then dried in hot air to obtain Ni coated steel substrate.
20. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the molten Zinc bath comprises pure Zinc or Zn-Al alloy having the Al content between 0-0.2 %.
21. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the molten zinc bath is maintained in a temperature between 440 - 470 °C.
22. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 21, wherein the molten zinc bath is maintained at a temperature of 460°C.
23. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein after dipping the Ni coated steel substrate into the molten Zinc bath, the Ni coated steel substrate now with zinc coating is passed through at least one wiping process to control the thickness of the Zinc coating on the Zn-Ni coated steel substrate.
24. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the annealing operation of Zn-Ni coated steel substrate is performed for 60 - 120 secs.
25. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the annealing of Zn-Ni coated steel substrate is performed in a temperature range of 500°C - 600°C.
26. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the Zn-Ni-Fe alloy coating thickness on the steel substrate is in the range of 4 to 40 microns.
27. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 1, wherein the Zn-Ni-Fe alloy coating on the Zn-Ni-Fe alloy coated steel substrate comprises about 80-88% by weight of Zinc, 10.5-15.5% by weight of Nickel, and 1.4-2.6% by weight of Iron.
28. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 27, wherein the Zn-Ni-Fe alloy coating on the Zn-Ni-Fe alloy coated steel substrate comprises about 84-85% by weight of Zinc, 12.5-13.5% by weight of Nickel, and 2.4-2.6% by weight of Iron
29. A process (100) of preparing Zn-Ni-Fe alloy coated steel substrate, the process (100) comprising:
cleaning surface of a steel substrate with aqueous alkali solution of NaOH;
pickling the surface of the steel substrate with hydrochloric acid solution;
annealing the steel substrate in in nitrogen-controlled atmosphere in a temperature range of 700°C - 860°C to obtain annealed steel substrate;
electroplating Ni alloy to the annealed steel substrate in Ni alloy plating bath kept at pH between 2 - 5 and temperature between 20 - 60 °C to obtain a Ni coated steel substrate;
dipping the annealed Ni coated steel substrate in molten Zinc bath maintained in a temperature between 440 - 470 °C to obtain Zn-Ni coated steel substrate;
annealing the Zn-Ni coated steel substrate inn a temperature range of 500°C - 600°C for 60 - 120 secs to obtain a Zn-Ni-Fe alloy coated steel substrate having about 80-88% by weight of Zinc, 10.5-15.5% by weight of Nickel, and 1.4-2.6% by weight of Iron.
30. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 29, wherein the steel substrate is any one of steel wire, steel plate, steel nails, steel TMT rebar, steel tube, and steel sheet.
31. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 29, wherein the Ni alloy plating bath is an electrolytic cell comprising an electrolytic solution in which solution a cathode, which is the steel substrate, and a nickel anode are immersed, wherein the electrolytic solution comprises nickel sulphate, nickel chloride, ammonium chloride and boric acid.
32. The process (100) of preparing Zn-Ni-Fe alloy coated steel substrate as claimed in claim 29, wherein the Zn-Ni-Fe alloy coating thickness is in the range of 4 to 40 microns.
33. A process (100) of preparing ternary alloy coated substrate, the process (100) comprising:
cleaning surface of a substrate;
pickling the surface of the substrate with an acid solution;
annealing the substrate in controlled atmosphere to obtain annealed substrate;
electroplating Ni alloy to the annealed substrate in Ni alloy plating bath to obtain a Ni coated substrate;
dipping the Ni coated substrate in molten Zinc bath to obtain Zn-Ni coated substrate;
annealing the Zn-Ni coated substrate to obtain a ternary alloy coated substrate.
34. The process (100) of preparing ternary alloy coated substrate as claimed in claim 33, wherein the substrate is a steel substrate, an aluminum substrate, stainless steel substrate, aluminum-magnesium substrate, titanium substrate, titanium alloy substrate, copper substrate and copper alloy substrate.
, Description:FIELD OF INVENTION
[0001] The present invention relates to a process of preparing a Zn-Ni-Fe alloy coated steel substrate, and more particularly to the process of coating Zn-Ni-Fe ternary alloy onto a steel substrate for preparing Zn-Ni-Fe alloy coated steel substrate.

BACKGROUND
[0002] Metals such as steel, aluminum are widely employed for various applications such as, automobile parts or construction materials. Unfortunately, these metals tend to corrode over time. A variety of methods have been widely used for a long time for protecting such metals and alloys to extend the life of substance in corrosive environment. These methods typically include protective coatings which are used principally to upgrade the corrosion resistance of ferrous metals, such as steel and nonferrous metals, such as aluminum. Protective coating fall into multiple categories. The largest of these categories is the topical coating such as a paint, that acts as a physical barrier against the environment. The second category consists of sacrificial coatings, such as zinc or cadmium, that are designed to preferentially corrode to save the base metal from attack. Further, there are proposed organic coatings including epoxy, phenolic, polyester, phthalic acid, fluorine and silicone. However, these coatings were found to be unsatisfactory because they gave only limited corrosion protection, and/or are relatively soluble and/or result in a toxic waste disposal problem.
[0003] Protective coating such as phosphate conversion coating is used to improve the corrosion performance of painted ferrous substrate, particularly painted steel. The corrosion process of painted steel involves 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 and 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.
[0004] Protective coating such as chromate coating is 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 their toxic nature, rinses containing chromate ions are undesirable for industrial usage.
[0005] 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 tends 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.
[0006] An eleven-year exposure program 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, it stable in a specific range of pH 6-12 and corrosion behavior of galvanized steel in presence of chlorides is controlled by the medium pH. Ductility of the coating of pure zinc coated material is very poor due to presence of thick brittle phase zeta. Alloying elements like copper and cadmium is harmful for ductility of the coating. Addition of nickel in zinc coating reduces the hydrogen evolution reaction as well increase 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. Alloy making with such composition is cost prohibitive and difficult due to wide difference of their melting temperatures.
[0007] As evidenced by the methods mentioned above, there has been a long-felt need to develop a low cost, nontoxic, relatively insoluble, corrosion resistant coating for ferrous metals, ferrous alloys, non-ferrous metals, and non-ferrous alloys that is environmentally safe and that can be disposed of inexpensively.
OBJECTIVE OF INVENTION
[0008] It is an object of the invention to solve the problems of the prior art and to provide a process of Zn-Ni-Fe ternary alloy coating on a surface of a metal substrate to prepare Zn-N-Fe ternary alloy coated metal substrate with right proportion of Ni, which prevents corrosion of the base metal effectively.
[0009] Another objective of the present invention is to develop the process of Zn-Ni-Fe ternary alloy coating on a metal substrate surface which does not affect the properties of the base metal.
[0010] Another objective of present invention to the process of Zn-Ni-Fe ternary alloy coating on a metal substrate surface which is cost effective and simple to produce.
[0011] It is further another objective of the present invention to provide Zn-Ni-Fe ternary coating on the metal substrate which is low cost, non-toxic, stable, hard, compact, impermeable, relatively insoluble and resistant to corrosion.
SUMMARY OF INVENTION
[0012] This summary is provided to introduce concepts related to a process of preparing a ternary alloy coated substrate such as Zn-Ni-Fe alloy coated steel substrate. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
[0013] In one aspect of the present invention, a process of preparing Zn-Ni-Fe alloy coated steel substrate is provided. The process comprises cleaning surface of a steel substrate. The process also comprises pickling the surface of the steel substrate with an acid solution. The process further comprises annealing the steel substrate in controlled atmosphere to obtain annealed steel substrate. The process comprises electroplating Ni alloy to the annealed steel substrate in Ni alloy plating bath to obtain a Ni coated steel substrate. The process also comprises dipping the Ni coated steel substrate in molten Zinc bath to obtain Zn-Ni coated substrate. The process further comprises annealing the Zn-Ni coated substrate to obtain a Zn-Ni-Fe alloy coated metal substrate.
[0014] In an embodiment, the steel substrate is any one of steel wire, steel plate, steel nails, steel TMT rebar, steel tube, and steel sheet.
[0015] In an embodiment, the cleaning is performed by aqueous alkali solution such of NaOH or similar alkalis containing a surface-active agent, ultrasonic cleaning, or brush cleaning.
[0016] In an embodiment, the acid solution used for pickling is selected from any one of hydrochloric acid, and sulfuric acid.
[0017] In an embodiment, the annealing operation of the steel substrate is performed in a temperature range of 700°C - 860°C. In an embodiment, the annealing operation of the steel substrate is performed in a temperature range of 780°C - 830°C.
[0018] In an embodiment, nitrogen or hydrogen gas is used to control the atmosphere during the annealing operation of the steel substrate.
[0019] In an embodiment, the Ni alloy plating bath is an electrolytic cell comprising an electrolytic solution in which solution a cathode, which is the annealed steel substrate, and a nickel anode are immersed.
[0020] In an embodiment, the electrolytic solution comprises nickel sulphate, nickel chloride, ammonium chloride and boric acid.
[0021] In an embodiment, the nickel sulfate content in the electrolytic solution is varied in the range of 150 g/L to 300 g/L. In an embodiment, the nickel chloride content in the electrolytic solution is varied in the range of 30 g/L to 150 g/L. In an embodiment, the boric acid content in the electrolytic solution is varied in the range of 15 g/L -30 g/L. In an embodiment, the NH4Cl content in the electrolytic solution is varied between 8 g/L - 12 g/L. In an embodiment, the electrolytic solution is maintained in temperature range between 20 - 60 °C.
[0022] In an embodiment, the electrolytic solution is maintained at a temperature of 45°C.
[0023] In an embodiment, the pH of the electrolytic solution is maintained in the range of 2 to 5. In an embodiment, the electrolytic solution is maintained at the pH of 3.5.
[0024] In an embodiment, the Ni coating thickness on the Ni coated steel substrate is in the range of 0.25 to 2 microns.
[0025] In an embodiment, after electroplating on the steel substrate in the Ni alloy plating bath, the steel substrate is removed from the Ni alloy plating bath and is rinsed in distilled water tank to remove contaminants and is then dried in hot air to obtain Ni coated steel substrate.
[0026] In an embodiment, the molten Zinc bath comprises pure Zinc or Zn-Al alloy having the Al content between 0-0.2 %.
[0027] In an embodiment, the molten zinc bath is maintained in a temperature between 440 - 470 °C.
[0028] In an embodiment, the molten zinc bath is maintained at a temperature of 460°C.
[0029] In an embodiment, after dipping the Ni coated steel substrate into the molten Zinc bath, the Ni coated steel substrate now with zinc coating is passed through at least one wiping process to control the thickness of the Zinc coating on the Zn-Ni coated steel substrate.
[0030] In an embodiment, the annealing operation of Zn-Ni coated steel substrate is performed for 60 - 120 secs.
[0031] In an embodiment, the annealing of Zn-Ni coated steel substrate is performed in a temperature range of 500°C - 600°C.
[0032] In an embodiment, the Zn-Ni-Fe alloy coating thickness on the steel substrate is in the range of 4 to 40 microns.
[0033] In an embodiment, the Zn-Ni-Fe alloy coating on the Zn-Ni-Fe alloy coated steel substrate comprises about 80-88% by weight of Zinc, 10.5-15.5% by weight of Nickel, and 1.4-2.6% by weight of Iron.
[0034] In an embodiment, the Zn-Ni-Fe alloy coating on the Zn-Ni-Fe alloy coated steel substrate comprises about 84-85% by weight of Zinc, 12.5-13.5% by weight of Nickel, and 2.4-2.6% by weight of Iron.
[0035] In another aspect of the present invention, a process of preparing Zn-Ni-Fe alloy coated steel substrate. The process comprises cleaning surface of a steel substrate with aqueous alkali solution of NaOH. The process also comprises pickling the surface of the steel substrate with hydrochloric acid solution. The process further comprises annealing the steel substrate in in nitrogen-controlled atmosphere in a temperature range of 700°C - 860°C to obtain annealed steel substrate. The process comprises electroplating Ni alloy to the annealed steel substrate in Ni alloy plating bath kept at pH between 2 - 5 and temperature between 20 - 60 °C to obtain a Ni coated steel substrate. The process also comprises dipping the annealed Ni coated steel substrate in molten Zinc bath maintained in a temperature between 440 - 470 °C to obtain Zn-Ni coated steel substrate. The process comprises annealing the Zn-Ni coated steel substrate inn a temperature range of 500°C - 600°C for 60 - 120 secs to obtain a Zn-Ni-Fe alloy coated steel substrate having about 80-88% by weight of Zinc, 10.5-13.5% by weight of Nickel, and 1.4-2.6% by weight of Iron.
[0036] In an embodiment, the steel substrate is any one of steel wire, steel plate, steel nails, steel TMT rebar, steel tube, and steel sheet.
[0037] In an embodiment, the Ni alloy plating bath is an electrolytic cell comprising an electrolytic solution in which solution a cathode, which is the steel substrate, and a nickel anode are immersed, wherein the electrolytic solution comprises nickel sulphate, nickel chloride, ammonium chloride and boric acid.
[0038] In an embodiment, the Zn-Ni-Fe alloy coating thickness is in the range of 4 to 40 microns.
[0039] In yet another aspect of the present invention, a process of preparing ternary alloy coated substrate is provided. The process comprises cleaning surface of a substrate. The process also comprises pickling the surface of the substrate with an acid solution. The process further comprises annealing the substrate in controlled atmosphere to obtain annealed substrate. The process comprises electroplating Ni alloy to the annealed substrate in Ni alloy plating bath to obtain a Ni coated substrate. The process also comprises dipping the Ni coated substrate in molten Zinc bath to obtain Zn-Ni coated substrate. The process further comprises annealing the Zn-Ni coated substrate to obtain a ternary alloy coated substrate.
[0040] In an embodiment, the substrate is a metal substrate including, a steel substrate, an aluminum substrate, stainless steel substrate, aluminum-magnesium substrate, titanium substrate, titanium alloy substrate, copper substrate and copper alloy substrate.
[0041] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0042] Figure 1 illustrates a flowchart of a process of preparing a Zn-Ni-Fe alloy coated steel substrate, according to an embodiment of the present invention;
[0043] Figure 2 illustrates pictures of (a) galvanized steel substrate, (b) Electro Ni-hot dip steel substrate and (c) Zn-Ni-Fe alloy coated steel substrate, according to an embodiment of the present invention;
[0044] Figure 3 illustrates SEM micrograph and EDX depth point analysis of (a) galvanized steel substrate, (b) Electro Ni-hot dip steel substrate and (c) Zn-Ni-Fe alloy coated steel substrate, according to an embodiment of the present invention;
[0045] Figure 4 illustrates potentiodynamic polarization test results of galvanized steel substrate and Zn-Ni-Fe alloy coated steel substrate, according to an embodiment of the present invention;
[0046] Figure 5 illustrates open circuit potential measurement with exposures in saline atmosphere of galvanized steel substrate and Zn-Ni-Fe alloy coated steel substrate, according to an embodiment of the present invention;
[0047] Figure 6 illustrates weight loss (g/m2) of galvanized steel substrate and Zn-Ni-Fe alloy coated steel substrate with exposure time in aggressive chloride environments, according to an embodiment of the present invention; and
[0048] Figure 7 illustrates charge transfer resistivity of galvanized steel substrate and Zn-Ni-Fe alloy coated steel substrate, according to an embodiment of the present invention.
[0049] The drawings referred to in this description are not to be understood as being drawn to scale except if specifically noted, and such drawings are only exemplary in nature.

DETAILED DESCRIPTION
[0050] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0051] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0052] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0053] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0054] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0055] According to an aspect of the present invention, a process of coating a ternary alloy onto a substrate to obtain a ternary alloy coated substrate is provided. The substrate comprises a ferrous metal substrate such as a steel substrate, stainless steel substrate, and non-ferrous metal substrate such as an aluminum substrate, aluminum-magnesium substrate, titanium substrate, titanium alloy substrate, copper substrate and copper alloy substrate. For simplicity and clarity purposes one process of coating Zn-Ni-Fe ternary alloy onto a steel substrate for preparing Zn-Ni-Fe ternary alloy coated steel substrate is being described in detail herein. However, it will be apparent to those skilled in the art that description is equally applicable to all the other processes of preparing ternary alloy coated substrates using different substrates as starting materials, without limiting the scope of the invention.
[0056] Figure 1 illustrates an exemplary process (100) of coating Zn-Ni-Fe ternary alloy onto a steel substrate for preparing Zn-Ni-Fe ternary alloy coated steel substrate. In one example, the steel substrate may be a continuous product. In another example, the steel substrate may be stand-alone product, without any limitations.
[0057] In the illustrated example, the steel substrate may be any one of steel wire, steel plate, steel nails, steel TMT rebar, steel tube, steel sheet or any other form of steel material that is capable of undergoing corrosion. The steel sheet includes cold rolled and hot rolled steel sheet. The cold rolled steel sheet includes CQ, EDD (extra deep drawable), IF (interstitial free), IF-HS (interstitial free high strength), and DP (Dual phase). 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).
[0058] Referring to Figure 1, the process (100) for preparing Zn-Ni-Fe alloy coated steel substrate is illustrated. At step (102), surface of the steel substrate that is to be coated is subjected to cleaning (cleaning step). At step (104), the surface cleaned steel substrate obtained in the cleaning step is subjected to picking with acid solution (pickling step). At step (106), the pickled steel substrate obtained in the pickling step is annealed in controlled atmosphere to obtain annealed steel substrate (first annealing step). At step (108), the annealed steel substrate is electroplated with Ni alloy in Ni alloy plating bath to obtain a Ni coated metal substrate (electroplating step). At step (110), the Ni coated metal substrate obtained in the electroplating step is dipped in molten Zinc bath to obtain Zn-Ni coated metal substrate (hot dip step). At step (112), the Zn-Ni coated metal substrate obtained in the hot dip step is annealed to obtain a Zn-Ni-Fe alloy coated metal substrate (second annealing step).
[0059] < Cleaning step >
[0060] In the cleaning step, surface of the steel substrate that is to be coated is cleaned by any one of alkali cleaning, ultrasonic cleaning, or brush cleaning. In the preferred embodiment, the surface of the steel substrate is cleaned using aqueous alkali solution such of NaOH or similar alkalis containing a surface-active agent. The cleaning is done to degrease or remove contaminants such as dirt present on the surface of the steel substrate.
[0061]
[0062] In the pickling step, the surface cleaned steel substrate obtained in the cleaning step is subjected to pickling with an acid. In the preferred embodiment, hydrochloric acid (HCL) is used to pickle the steel substrate. Alternatively, sulfuric acid (H2SO4) may be used to pickle the surface of the steel substrate, without limiting the scope of the invention. The picking step removes impurities, such as stains, inorganic contaminants, rust or scale from the steel substrate.
[0063]
[0064] In the first annealing step, the pickled steel substrate obtained in the pickling step is annealed in controlled atmosphere to obtain annealed steel substrate. In the preferred embodiment, the annealing operation of the steel substrate is performed in a temperature range of 700°C - 860°C and preferably between 780°C to 830°C depending on the grade of steel substrates used. In the preferred embodiment, nitrogen is used to control the atmosphere during the annealing operation of the steel substrate. In another embodiment, hydrogen gas is used to control the atmosphere during the annealing operation of the steel substrate, without limiting the scope of the invention.
[0065]
[0066] In the electroplating step, the annealed steel substrate which is obtained in the first annealing step is electroplated with Nickel (Ni) alloy in a nickel alloy plating bath to obtain Ni coated steel substrate. In the preferred embodiment, the nickel alloy plating bath is an electrolytic cell comprising an electrolytic solution in which a cathode, which is the steel substrate, and a nickel anode are immersed.
[0067] In the preferred embodiment, the electrolytic solution is an aqueous solution comprising nickel sulphate, nickel chloride, ammonium chloride and boric acid. The nickel sulfate content in the electrolytic solution is varied in the range of 150 g/L to 300 g/L. The nickel chloride content in the electrolytic solution is varied in the range of 30 g/L to 150 g/L. The boric acid content in the electrolytic solution is varied in the range of 15 g/L -30 g/L. The NH4Cl content in the electrolytic solution is varied between 8 g/L - 12 g/L.
[0068] In the preferred embodiment, the temperature of the electrolytic solution is maintained in a range between 20 - 60 °C. More preferably, the temperature of the electrolytic solution is maintained at a temperature of 45°C which is most suitable operating temperature for electrolytic plating of Ni on the steel surface. In the preferred embodiment, the pH of the electrolytic solution is maintained in the range of 2 to 5. More preferably, the pH for is maintained at 3.5 which is most suitable pH for electrolytic plating of Ni on the steel surface. In the preferred embodiment, the cathode current density is in the range 2 to 50 A/dm2.
[0069] The time duration of electroplating is selected according to the thickness of Ni coating required on the steel substrate. In the preferred embodiment, the Ni coating thickness on the Ni coated steel substrate is in the range of 0.25 to 3 microns. Further the Ni coating may be performed to a level of less than 0.6 mg/cm2 residual dirt.
[0070] After electroplating on the steel substrate in the Ni alloy plating bath, the steel substrate is removed from the Ni alloy plating bath and is wiped to remove all electrolyte contaminant using various wiping process. In the preferred embodiment, the steel substrate removed from the Ni alloy plating bath is wiped by rinsing the Ni coated steel substrate in distilled water tank and then drying in hot air to obtain Ni coated metal substrate.
[0071]
[0072] In the hot dip step, the Ni coated steel substrate obtained in the electroplating step is dipped in molten Zinc bath to obtain Zn-Ni coated steel substrate. In the preferred embodiment, the molten Zinc bath comprises pure Zinc or Zn-Al alloy having the Al content between 0-0.2 %. In the preferred embodiment, the molten zinc bath is maintained in a temperature between 440 - 470 °C.
[0073] After galvanizing the Ni coated steel substrate in the molten zinc bath, the steel substrate is removed from the molten zinc bath and is passed through at least one wiping process such as force air, nitrogen, cooled air, charcoal, pad or asbestos rope etc. to control the thickness of the Zinc coating. In the preferred embodiment, force air wiping process is used to control the thickness of the Zinc coating on the steel substrate after molten zinc coating. After wiping, the now Zinc-Nickel coated steel substrate is cooled in water, oil or cooled air to obtain Zn-Ni coated steel substrate. In the preferred embodiment, the most preferable cooling method for this type of coated steel substrate is cooled air.
[0074]
[0075] In the second annealing step, the Zn-Ni coated steel substrate obtained in the hot dip step is annealed to obtain a Zn-Ni-Fe coated steel substrate. In the preferred embodiment, the annealing of Zn-Ni coated steel substrate is performed in a temperature range of 480°C - 600°C. More preferably at a temperature of 500°C - 540°C. In the preferred embodiment, the annealing operation of Zn-Ni coated steel substrate is performed for 5 - 120 secs. Most suitable annealing temperature and time for around 25 µm coating is 540 °C and 80 seconds respectively, In the preferred embodiment, the annealing is done in an induction heating furnace. During annealing process, the Zn-Ni alloy reacts with steel surface under the said controlled temperature and the said controlled process conditions to form a Zn-Ni-Fe alloy coating on the surface of steel which is ductile, stable, impermeable and insoluble. Cooling of the Zn-Ni-Fe coated steel substrate may be done after annealing step with air or water.
[0076] The Zn-Ni-Fe ternary alloy coating on the Zn-Ni-Fe alloy coated metal substrate obtained from the above process (100) comprises about 80-88% by weight of Zinc, 10.5-15.5% by weight of Nickel, and 1.4-2.6% by weight of Iron. Preferable coating composition ranges are 84-85% by weight of Zinc, 12.5-13.5% by weight of Nickel, and 2.4-2.6% by weight of Iron. The Zn-Ni-Fe alloy coating thickness on the steel substrate obtained from the process (100) is in the range of 4 to 40 microns. The said Zn-Ni-Fe alloy coating on the surface of steel has superior anticorrosion properties and is termed as electrodeposition-hot dip-annealing coating (EHAG coating).
[0077] Following examples further illustrates the details of the present invention. However, the present invention is comprised of but not limited to these illustrations.

EXAMPLES
[0078] Experiments were conducted to prepare coatings on the surface of steel substrate under different process conditions, and properties of these different coatings were observed.
EXAMPLE 1: Preparation of Zn coating on the surface of steel substrate
[0079] A cleaned and pickled steel sample was first treated in a control atmosphere in an annealing furnace then dipped in molten zinc containing about 0.2 wt.% of aluminum at 460 °C.
[0080] Defective coating was observed on the surface of steel, 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-Ni two layers coating on the surface of steel substrate
[0081] A cleaned and pickled steel sample was first treated in a control atmosphere in an annealing furnace then plated with just a flash layer (0.2µm) of Ni coating onto steel sample and then dipped in molten zinc containing about 0.2 wt.% of aluminum at 460 °C. After coating sample was cooled in water.
[0082] Defective coating was observed on the surface of steel, 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-Ni two layers coating on the surface of steel substrate
[0083] A cleaned and pickled steel sample was first treated in a control atmosphere in an annealing furnace then plated with just a thin layer (0.2µm) of Ni coating onto steel sample and then dipped in molten zinc containing about 0.2 wt.% of aluminum at 460 °C. After coating sample was cooled in air.
[0084] Defective coating was observed on the surface of steel, 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-Ni two layers coating on the surface of steel substrate
[0085] A cleaned and pickled steel sample was first treated in a control atmosphere in an annealing furnace then plated with a slightly thicker (0.5-1µm) layer of Ni coating onto steel sample and then dipped in molten zinc containing about 0.2 wt.% of aluminum at 460 °C. After coating sample was cooled in air.
[0086] Less defective coating was observed on the surface of steel, 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-Ni two layers coating on the surface of steel substrate
[0087] A cleaned and pickled steel sample was first treated in a control atmosphere in an annealing furnace then plated with a slightly thicker (1.5-3µm) layer of Ni coating onto steel sample and then dipped in molten zinc containing about 0.2 wt.% of aluminum at 460 °C. After coating sample was cooled in air.
[0088] Defective free coating was observed on the surface of steel, wherein the coating comprised of compact, brighter and free from any bare spot. The coating that was formed was compact and had superior resistance property towards corrosion with a corrosion rate of about 4.05mpy.
EXAMPLE 6: Preparation of Zn-Ni-Fe alloy coating on the surface of steel substrate
[0089] The annealing operation was done of the two layers Zn-Ni coated steel substrate prepared as per above examples to obtain ternary Zn-Ni-Fe alloy coated steel substrate, here the annealing operation conducted under temperature 500 °C for 60 seconds. The sample was cooled in normal air after annealing operation.
[0090] The brighter coating was observed on the surface of steel, wherein the coating comprised of ternary alloy layer at the steel coating interface with pure zinc layer at the top. 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-Ni-Fe alloy coating on the surface of steel substrate
[0091] The annealing operation was done of the two layers Zn-Ni coated steel substrate prepared as per above examples to obtain ternary Zn-Ni-Fe alloy coated steel substrate, here the annealing operation was conducted under temperature 600 °C for 120 seconds. The sample was cooled in normal air after annealing operation.
[0092] The dull coating was observed on the surface of steel, wherein the coating comprised of ternary alloy layer throughout the depth of the coating with very high amount of Fe (~20wt%) in the coating. The coating that was formed was compact and dull, inferior in resistance property towards corrosion with a corrosion rate of about 7.433mpy.
EXAMPLE 8: Preparation of perfect Zn-Ni-Fe alloy (EHAG) coating on the surface of steel substrate
[0093] The annealing operation was done of the two layers Zn-Ni coated steel substrate prepared as per above examples to obtain ternary Zn-Ni-Fe alloy coated steel, here the annealing operation was conducted under temperature 540 °C for 80 seconds. The sample was cooled in normal air after annealing operation.
[0094] The mat finish coating was observed on the surface of steel, wherein the coating comprised of ternary alloy layer throughout the depth of the coating with Ni content around 13wt% and Fe content around 2wt% in the coating. The coating that was formed was compact and mat color in appearance, excellent in resistance property towards corrosion with a corrosion rate of about 1.53mpy.
[0095] Figure 2 illustrates pictorial views of three different coated steel substrates a) galvanized steel substrate, (b) Electro Ni-hot dip steel substrate and (c) Zn-Ni-Fe alloy coated steel substrate. It is evident hot dip zinc coat ability aspect is improved after putting Ni layer on the steel substrate by electroplating process. The mat finish appearance of EHAG coated substrate (of example 8) is obtained.
[0096] Referring to Figure 3 the depth microstructural image (SEM micrograph) and EDX depth point (SEM-EDX) analysis of (a) galvanized steel substrate, (b) Electro Ni-hot dip steel substrate and (c) Zn-Ni-Fe alloy coated steel substrate are illustrated. The EHAG coating consists of ternary alloy of zinc, nickel and iron throughout the depth of the coating whereas the pure zinc coating consists of only Zn layer throughout the depth of the coating. The best EHAG coating composition is 13wt% nickel, 2.5wt% iron and remaining balance composition is zinc.
[0097] Referring to Figure 4, the potentiodynamic polarization test results of galvanized steel substrate and Zn-Ni-Fe alloy coated steel substrate are illustrated. The test results as shown in figure 4 demonstrates that the EHAG coated steel provides much superior (excellent) resistance against corrosion with Icorr of about 1.40 A/cm2 and corrosion rate of about 1.53 mpy as compared to conventional galvanized steel which had Icorr of about 6.87 A/cm2 and corrosion rate of about 8.68 mpy.
[0098] Referring to Figure 5, the open circuit potential measurement with exposures in saline atmosphere of galvanized steel substrate and Zn-Ni-Fe alloy coated steel substrate is illustrated. It is evident that EHAG coated steel substrate provide stable open circuit potential value. Further it is also evident that open circuit potential of pure zinc coated steel substrate comes only after exposure of 700 hours, whereas open circuit potential of EHAG coated steel substrate did not even come after exposure for more than 2200 hours. It indicates EHAG coated steel substrate is capable to provide much superior performance.
[0099] Referring to Figure 6, weight loss (g/m2) of galvanized steel substrate and Zn-Ni-Fe alloy coated steel substrate with exposure time in aggressive chloride environments is illustrated, wherein it is demonstrated that the EHAG coated steel has more resistivity and less weight loss when compared to the conventional pure zinc coated steel. Thus, implying that EHAG coated steel substrate has superior (excellent) resistance against corrosion when compared to pure zinc coated steel.
[00100] Referring to Figure 7, charge transfer resistivity of galvanized steel substrate and Zn-Ni-Fe alloy coated steel substrate is illustrated, wherein it is demonstrated that the EHAG coated steel substrate shows excellent resistance against charge transfer than pure zinc coated steel substrate.
[00101] The potentiodynamic polarization test was performed for all coated steels in simulated chloride solution using Gamry DC105 system. In case of Tafel test sample, counter electrode (platinum) and reference electrode (saturated calomel electrode) were used as electrodes. In case of OCP, test sample and reference electrode were used as electrodes. Surface area exposed in corrosion medium was 10 mm2 for Tafel and cyclic anodic polarization study. The scan rate and immersion time for all these tests were 2 mV/s and 15 minutes respectively. The pH of the electrolyte solution was kept constant value of 5.
[00102] The present invention relates to the process (100) for preparing Zn-Ni-Fe coated steel substrate. The EHAG coated steel substrate provides excellent resistance against charge transfer and superior (excellent) resistance against corrosion. The process (100) of coating the Zn-Ni-Fe alloy onto the substrate is a cost-effective process which provides a low cost, non-toxic, stable, hard, compact, impermeable and relatively insoluble corrosion resistant Zn-Ni-Fe ternary coating for ferrous metals. The ternary alloy coating such as Zn-Ni-Fe coating on the steel substrate, which mainly consists of 85 wt. % zinc,13 wt. % nickel and 2 wt.% iron, impart higher corrosion protection to steel, improve its formability as well as its paint ability compared to traditional hot-dip zinc coating.
[00103] In the above description, the term metal includes both ferrous metal such as steel, cast iron etc., and non-ferrous metals such as aluminum, aluminum alloys, nickel, nickel alloys etc., without limiting the scope of the invention.
[00104] Furthermore, the terminology used herein is for describing embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[00105] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[00106] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.