JP7689818B2 - Functional chromium alloy plating based on trivalent chromium electrolyte - Google Patents

Description

The present disclosure provides an electrolyte solution for electrodeposition of chromium alloys, a method for forming the electrolyte solution, and a method for electrodeposition of chromium alloys.

Chrome plating is an electrodeposition process that provides a chromium coating on a substrate. Hard chrome plating provides a chromium coating that typically has a thickness of about 10 microns or more, thereby providing the coated substrate with high machinability and abrasion resistance. Another type of chrome plating is decorative chrome plating, which provides a chromium coating that typically has a thickness ranging from about 0.1 to about 0.5 microns. Chromium plating is often performed using a bath that includes chromic acid and a catalyst based on a fluoride, sulfate, or organic acid. Chromic acid has chromium(VI), a hexavalent form of chromium that is environmentally undesirable and has high disposal costs.

Trivalent chromium, with its benign properties and lower waste disposal costs, is an alternative to hexavalent chromium. Although there has been some success in implementing trivalent chromium for thin decorative plating, it remains difficult to achieve thicker or more functional coatings. In addition, trivalent chromium-based baths used for decorative plating typically contain boric acid as a buffering agent.

Therefore, there is a need to improve the chrome plating process and the formulation of the solution used in chrome plating.

The present disclosure provides an electrolyte solution for electrodeposition of chromium alloys, a method for forming the electrolyte solution, and a method for electrodeposition of chromium alloys.

At least one electrolyte solution for electroplating includes a trivalent chromium salt, an oxalate compound, an iron salt, aluminum sulfate, an alkali metal sulfate, and an alkali metal halide.

At least one electrolyte solution for electroplating includes a trivalent chromium salt in an amount ranging from about 0.3 moles per liter to about 0.9 moles per liter of electrolyte solution. The electrolyte solution further includes an oxalate compound in an amount ranging from about 0.2 moles per liter to about 1.2 moles per liter of electrolyte solution. The electrolyte solution further includes an iron salt in an amount ranging from about 0.005 moles per liter to about 0.2 moles per liter of electrolyte solution. The electrolyte solution further includes aluminum sulfate in an amount ranging from about 0.05 moles per liter to about 0.5 moles per liter of electrolyte solution. The electrolyte solution further includes an alkali metal sulfate in an amount ranging from about 0.1 moles per liter to about 2.0 moles per liter of electrolyte solution. The electrolyte solution further includes an alkali metal halide in an amount ranging from about 0.1 moles per liter to about 0.5 moles per liter of electrolyte solution.

At least one method of plating chromium on a substrate using an electrolyte solution is provided. The method includes dissolving a trivalent chromium salt in an aqueous medium in an amount ranging from about 0.3 moles per liter to about 0.9 moles per liter. The method further includes dissolving an oxalate compound in an amount ranging from about 0.2 moles per liter to about 1.2 moles per liter. The method further includes dissolving an iron salt in an amount ranging from about 0.005 moles per liter to about 0.2 moles per liter. The method further includes dissolving aluminum sulfate in an amount ranging from about 0.05 moles per liter to about 0.5 moles per liter. The method further includes dissolving an alkali metal sulfate in an amount ranging from about 0.1 moles per liter to about 2.0 moles per liter. The method further includes dissolving an alkali metal halide in an amount ranging from about 0.1 moles per liter to about 0.5 moles per liter. The method further includes passing an electric current through the electrolyte solution between the cathode and the anode to deposit a chromium-iron alloy on the substrate.

At least one method is provided for plating chromium on a substrate using an electrolyte solution. The method includes introducing a cathode and an anode into an electrolyte solution including a trivalent chromium salt, an oxalate compound, an iron salt, aluminum sulfate, an alkali metal sulfate, and an alkali metal halide. The method further includes passing an electric current through the electrolyte solution between the anode and the cathode to deposit a chromium-iron alloy layer on the substrate.

At least one substrate is provided that includes a chromium-iron alloy coating. The chromium-iron alloy coating has a chromium content ranging from about 40% to about 90% by weight, an iron content ranging from about 8% to about 18% by weight, and a carbon content ranging from about 5% to about 50% by weight.

The above-described features, functions, and advantages may be realized alone in various aspects or may be combined in yet other aspects, the details of which may be seen with reference to the following description and the accompanying drawings.

In order that the above-mentioned features of the present disclosure may be understood in detail, a more detailed description of the present disclosure than that summarized above may be made by reference to the embodiments, some of which are illustrated in the accompanying drawings. It should be noted, however, that the present disclosure may admit of other equally effective embodiments, and therefore, that the accompanying drawings show only typical embodiments of the present disclosure and should not be considered as limiting the scope of the present invention.

FIG. 1 is a flow diagram illustrating a method for forming an electrolyte solution according to one or more aspects of the present disclosure. FIG. 1 is a flow diagram illustrating a method for forming a chromium alloy coating on a substrate by electrodeposition according to one or more aspects of the present disclosure. 3A-3C are images of chromium-iron alloy plated substrates formed by the process of FIG. 2, each plated at a different current density. 3A-3C are images of chromium-iron alloy plated substrates formed by the process of FIG. 2, each plated at a different pH. 3A-3C are images of chromium-iron alloy plated substrates formed by the process of FIG. 2, each plated at a different current density. 3A-3C are images of chromium-iron alloy plated substrates formed by the process of FIG. 2, each plated at a different pH. 3A-3C are images of chromium-iron alloy plated substrates formed by the process of FIG. 2, each plated at a different current density. 3A-3C are images of chromium-iron alloy plated substrates formed by the process of FIG. 2, each plated at a different pH. 3A-3C are images of chromium-iron alloy plated substrates formed by the process of FIG. 2, each plated at a different temperature.

For ease of understanding, the same reference numbers have been used where possible to indicate identical elements common to the figures. In addition, elements of one embodiment may be advantageously adapted for use in other embodiments described herein.

DETAILED DESCRIPTION According to aspects of the present disclosure, various methods and formulations are provided for chromium plating a substrate that can obtain a structurally robust, reliable, and cost-effective chromium layer (e.g., chromium coating) formed on the substrate while using direct current and a boric acid-free trivalent chromium solution. Thus, the methods and formulations described herein can be advantageously used for hard chromium plating to form a hard chromium layer (e.g., a robust functional chromium layer greater than 10 microns). However, the present disclosure is not limited to hard chromium plating, and the methods and formulations described herein can also be advantageously used to effectively and efficiently perform decorative chromium plating to form a decorative chromium layer (e.g., a chromium layer ranging from 0.25 microns to 1.0 microns).

The present disclosure further provides an electrolyte solution for the electrodeposition of chromium-iron alloys and a method of forming the chromium-iron alloys. In at least one embodiment, the electrolyte solution of the present disclosure is aqueous. In at least one embodiment, the electrolyte solution includes an iron salt, such as, for example, ferrous sulfate or ferric chloride. It has been discovered that one or more of these iron salts present in the electrolyte solution provide for the deposition of a thick chromium-iron alloy layer on a substrate, such as a steel substrate, without the use of hexavalent chromium and pulse plating. The electrolyte solution of the present disclosure further includes at least one complexing agent, such as an oxalate compound, e.g., sodium oxalate, that forms a complex with the trivalent chromium ions as well as the ferrous ions.

The electrolyte solutions of the present disclosure provide controllable deposition of chromium-iron alloys on substrates.
In at least one embodiment, the substrate is a steel substrate, a copper substrate, a brass substrate, a copper-coated substrate, a nickel-coated substrate, or other metal or metal alloy containing substrate. In at least one embodiment, the electrolyte solutions of the present disclosure, when used at a pH ranging from about 2 to about 4 and at a temperature ranging from about 30° C. to about 60° C., provide sufficiently hard chromium-iron alloy deposits that are believed to be comparable to common hexavalent chromium based coatings. In at least one embodiment, the iron content of the chromium-iron alloys of the present disclosure is from about 1% iron by weight to about 20% iron by weight, based on the combined weight of chromium and iron in the alloy.

The disclosed chromium-iron alloy coatings provide substrates such as steel substrates with similar machinability and wear resistance as typical hexavalent hard chromium plated substrates. The disclosed chromium-iron alloys can be disposed in aircraft, spacecraft, marine, engine and blow-flap components, waste wash structures, thermal structural components for high performance supersonic, hypersonic, and space re-entry vehicle structures, automotive components, buildings such as steel bridges, and propulsion structures such as power generation turbines, car engines, alternative energy applications, and related technologies. As one particular example, the disclosed alloys can be disposed in steel-based landing gear and/or the underside of aircraft.

In at least one embodiment, the chromium-iron coating of the present disclosure is formed using a single-bath technique. The deposition vessel is a laboratory-scale glass beaker or a large polypropylene tank for commercial-scale plating. In at least one embodiment, the deposition vessel is an electrolyte solution prepared by mixing all of the electrolyte solution components simultaneously or stepwise starting with the chromium salt and complexing agent followed by the iron salt. An anode (e.g., graphite), as described below, is introduced into the beaker containing the electrolyte solution. The deposition process is regulated by applying a direct current, thereby producing a chromium-iron alloy coating. The thickness of the chromium-iron alloy coating can be controlled by the duration of time that the direct current is applied to the electrolyte solution electrodes. In at least one embodiment, the total thickness of the chromium-iron alloy coating is about 1 micron to about 100 microns, e.g., about 10 microns to about 50 microns, e.g., about 20 microns to about 40 microns, e.g., about 30 microns.

Varying the thickness and composition of the chromium-iron alloy coating can be controlled by the current density and time scale of the deposition process of the present disclosure.

The electrolyte solution of the present disclosure includes a metal salt. The metal salt used herein can include anhydrous and/or hydrated forms of the metal salt. In at least one embodiment, the metal salt includes one or more of a trivalent chromium salt and an iron salt. The electrolyte solution of the present disclosure further includes at least one complexing agent, such as an alkali metal oxalate compound, e.g., sodium oxalate or potassium oxalate. The complexing agent, such as an alkali metal oxalate compound, coordinates to the iron ions in the electrolyte solution and promotes controllable deposition of iron on the substrate upon application of a current density to the electrolyte solution.

The electrolyte solution of the present disclosure further comprises at least one buffering agent, such as an aluminum salt, e.g., aluminum sulfate or an aluminum halide. The buffering agent of the present disclosure maintains a desired pH of the electrolyte solution and does not substantially interfere with the deposition of chromium and iron on the substrate, as described in more detail below.

The electrolyte solution of the present disclosure further comprises at least one ionic conductivity control agent, such as an alkali metal salt, e.g., sodium sulfate or potassium sulfate. The ionic conductivity control agent of the present disclosure maintains the desired conductivity of the electrolyte solution and does not substantially interfere with the deposition of chromium and iron on the substrate.

The electrolyte solution of the present disclosure further includes at least one alkali metal halide, such as sodium fluoride or potassium fluoride. The alkali metal halide of the present disclosure provides the electrolyte solution with wetting and etching properties and can aid in chrome adhesion during chrome plating.

Optionally, the electrolyte solution of the present disclosure further comprises at least one surfactant, such as sodium lauryl sulfate, sodium lauryl ether sulfate, or potassium lauryl sulfate. The surfactant of the present disclosure reduces pitting and gassing during chrome plating.

In at least one embodiment, the pH of the electrolyte solutions of the present disclosure is between about 1 and about 6, e.g., between about 1.5 and about 4, e.g., the pH is 2 or 4. In at least one embodiment, the pH of the electrolyte solutions of the present disclosure is controlled by the addition of one or more bases, such as a sodium hydroxide (NaOH) solution, to increase the pH of the solution, or by the addition of one or more acids, such as a sulfuric acid ( _H2SO4 ) solution, to decrease the pH of the solution. Chromium salts, iron salts, complexing agents, buffers, acids, and bases _can be obtained from any suitable commercial source, such as MERCK-India or Sigma-Aldrich Co. LLC (St. Louis, MO).

FIG. 1 is a flow diagram illustrating a method 100 for forming an electrolyte solution according to one or more aspects of the present disclosure. As shown in FIG. 1, in step 110, the method 100 includes dissolving a trivalent chromium salt in a medium, such as water or an aqueous solution, to form a first electrolyte solution. The trivalent chromium salt is a source of trivalent chromium. In at least one embodiment, the trivalent chromium salt _includes a chromium (III) halide, a chromium (III) sulfate (e.g., _Cr2 ( _SO4 ) ₃ , _Cr2 ( _SO4 ) _3.12H2O , and/or other chromium (III) sulfates), and/or other chromium (III) salts. The chromium (III) halide can include, for example, _a chromium (III) chloride (e.g., _CrCl3 , _CrCl35H2O , _CrCl3.6H2O , and/or other _chromium (III) chlorides).

In at least one embodiment, the concentration of the trivalent chromium salt in the electrolyte of the present disclosure ranges from about 0.1 moles per liter (mol/L) to about 1 mol/L of electrolyte solution, e.g., from about 0.3 mol/L to about 0.9 mol/L, e.g., from about 0.2 mol/L to about 0.7 mol/L, e.g., from about 0.4 mol/L to about 0.7 mol/L, e.g., from about 0.5 mol/L to about 0.6 mol/L. In at least one embodiment, the amount of trivalent chromium salt dissolved is about 0.1 mol/L, 0.2 mol/L, 0.3 mol/L, 0.4 mol/L, 0.5 mol/L, 0.6 mol/L, 0.7 mol/L, 0.8 mol/L, 0.9 mol/L, or 1 mol/L of electrolyte solution, as appropriate (any value can form an upper or lower endpoint). At concentrations above 1 mol/L, it becomes difficult to dissolve the trivalent chromium salt in the electrolyte, and solubility problems may arise.

In at least one embodiment, the trivalent chromium salt is dissolved by stirring at ambient temperature, room temperature, about 25° C., or at a temperature ranging from about 10° C. to about 40° C., e.g., from about 20° C. to about 30° C. In at least one embodiment, the temperature at which step 110 is performed can be about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40° C., as appropriate (any value can form an upper or lower endpoint). In at least one embodiment, stirring can be performed for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or 30 minutes, as appropriate, or until all the trivalent chromium salt is dissolved (any value can form an upper or lower endpoint).

In step 120, the method 100 further includes, for example, dissolving an oxalate compound in water or an aqueous solution (e.g., the first solution) to form an electrolyte solution, such as a second electrolyte solution. The oxalate compound includes an oxalate that can function as a complexing agent. In at _least one embodiment, the oxalate compound includes an _{alkali metal oxalate (e.g., sodium oxalate (Na2C2O4 )} , _potassium oxalate _{( K2C2O4} ), and/or other alkali metal oxalates) and/or an oxalate acid ₍ e.g., oxalic acid ( _H2C2O4 ) and/or other oxalate _acids ).

In at least one embodiment, the concentration of the oxalate compound in the electrolyte of the present disclosure is in the range of about 0.1 mol/L to about 2.0 mol/L of the electrolyte solution, e.g., in the range of about 0.2 mol/L to about 1.2 mol/L, e.g., in the range of about 0.1 mol/L to about 0.9 mol/L, e.g., in the range of about 0.2 mol/L to about 0.7 mol/L, e.g., in the range of about 0.4 mol/L to about 0.7 mol/L, e.g., in the range of about 0.5 mol/L to about 0.6 mol/L. In at least one embodiment, the amount of oxalate compound dissolved is about 0.1 mol/L, 0.2 mol/L, 0.4 mol/L, 0.6 mol/L, 0.8 mol/L, 1.0 mol/L, 1.2 mol/L, 1.4 mol/L, 1.6 mol/L, 1.8 mol/L, or 2.0 mol/L of the electrolyte solution, as appropriate (any value can form an upper or lower endpoint).

In at least one embodiment, the oxalate compound is placed in a solution (e.g., a solution obtained from step 110 or another step performed prior to step 120) to dissolve the oxalate compound and form a complex of oxalate and trivalent chromium. The solution can be heated to a higher temperature ranging from about 70° C. to about 80° C. and stirred for about 1 hour to about 3 hours. In at least one embodiment, the solution is cooled (e.g., to ambient temperature, room temperature, about 25° C., or a temperature ranging from about 20° C. to about 30° C.). Alternatively, the oxalate compound can be dissolved without heating, in which case the complex of oxalate and trivalent chromium is formed in 3 to 4 days. Advantageously, the electrolyte solution can be prepared even faster by heating the solution in step 120 to a temperature ranging from about 70° C. to about 80° C. In at least one embodiment, the stirring is performed for about 1 hour, 1 hour 15 minutes, 1 hour 30 minutes, 1 hour 45 minutes, 2 hours, 2 hours 15 minutes, 2 hours 30 minutes, 2 hours 45 minutes, or 3 hours, as appropriate (any of these values can form upper or lower endpoints). Additionally, in at least one embodiment, the temperature at which step 120 is performed is about 50° C., 55° C., 60° C., 65° C., 70° C., 75° C., or 80° C., as appropriate (any of these values can form upper or lower endpoints).

In step 130, the method 100 further includes, for example, dissolving an iron salt in water or an aqueous solution (e.g., the second electrolyte solution) to form an electrolyte solution (e.g., the third electrolyte solution). In at least one embodiment, the iron salt is a divalent iron salt, a trivalent iron salt, or a combination thereof. In at least one embodiment, the iron salt is ferrous sulfate heptahydrate, ferric chloride, or a combination thereof. In at least one embodiment, the iron salt is a divalent iron salt. In at least one embodiment, the divalent iron salt includes ferrous sulfate, ferrous chloride, ferrous acetate, and/or other divalent iron salts. Each of these divalent iron salts can include its corresponding hydrated form. For example, ferrous sulfate has the formula _FeSO4.xH2O , where x is an integer (e.g., 0, 1, 2, 4 _, 5, 6, or 7). Thus, in at least one embodiment, the iron(II) sulfate is anhydrous iron(II) sulfate, iron(II) sulfate monohydrate, iron(II) sulfate dihydrate, iron(II) sulfate tetrahydrate, iron(II) sulfate pentahydrate, iron(II) sulfate hexahydrate, iron(II) sulfate heptahydrate, or iron(II) sulfate having another hydration state. In at least one embodiment, the iron salt is a trivalent iron salt. In at least one embodiment, the trivalent iron salt includes iron(III) sulfate, iron(III) chloride, iron(III) acetate, and/or other trivalent iron salts. Each of these trivalent iron salts can include its corresponding hydrated form. For example, iron(III) sulfate has the formula _Fe2 ( _SO4 ) _3.xH2O , where x is an integer (e.g., 0, 1, 2, 4 _, 5, 6, or 7). Thus, in at least one embodiment, the iron(III) sulfate is anhydrous iron(III) sulfate, iron(III) sulfate monohydrate, iron(III) sulfate dihydrate, iron(III) sulfate tetrahydrate, iron(III) sulfate pentahydrate, iron(III) sulfate hexahydrate, iron(III) sulfate heptahydrate, or iron(III) sulfate having another hydration state. In a further example, the iron(III) chloride has the formula _FeCl3.xH2O , where x is an integer (e.g., 0, 1, 2 _, 4, 5, 6, or 7). Thus, in at least one embodiment, the iron(III) chloride is anhydrous iron(III) chloride, iron(III) chloride monohydrate, iron(III) chloride dihydrate, iron(III) chloride tetrahydrate, iron(III) chloride pentahydrate, iron(III) chloride hexahydrate, iron(III) chloride heptahydrate, or iron(III) chloride having another hydration state.

In at least one embodiment, the concentration of the iron salt in the electrolyte of the present disclosure ranges from about 0.005 mol/L to about 0.2 mol/L of electrolyte solution, such as from about 0.01 mol/L to about 0.2 mol/L, such as from about 0.02 mol/L to about 0.2 mol/L, for example from about 0.1 mol/L to about 0.2 mol/L. In at least one embodiment, the amount of iron salt dissolved is about 0.005 mol/L, 0.01 mol/L, 0.02 mol/L, 0.03 mol/L, 0.04 mol/L, 0.05 mol/L, 0.06 mol/L, 0.07 mol/L, 0.08 mol/L, 0.09 mol/L, 0.1 mol/L, or 0.2 mol/L of electrolyte solution, as appropriate (any value can form an upper or lower endpoint). The inventors have discovered that at concentrations greater than 0.2 mol/L, the deposited chromium-iron alloy coating can become soft and susceptible to corrosion.

In at least one embodiment, the iron salt is dissolved by stirring at ambient temperature, room temperature, about 25° C., or at a temperature ranging from about 20° C. to about 30° C. In at least one embodiment, stirring is performed for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or 30 minutes (any value can form an upper or lower endpoint), as needed, or until all the iron salt is dissolved. In at least one embodiment, the temperature at which step 130 is performed is performed at about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40° C. (any value can form an upper or lower endpoint), as needed.

In step 140, the method 100 further includes dissolving a metal salt, for example, in water or an aqueous solution (e.g., the third electrolyte solution) to form an electrolyte solution (e.g., the fourth electrolyte solution). The metal salt is a source of metal ions that dissolves to provide metal ions, such as aluminum ions, which can act as a buffer and provide ionic strength with high valency metal ions (e.g., Al ³⁺ ) in the solution. In at least one embodiment, the metal salt includes a Group 13 metal salt, for example, an aluminum halide and/or other metal salt, such as an aluminum salt (e.g., aluminum sulfate (Al ₂ (SO ₄ ) ₃ ), aluminum chloride (AlCl ₃ ), and/or other aluminum salts).

In at least one embodiment, the concentration of the metal salt in the electrolyte of the present disclosure ranges from about 0.01 mol/L to about 1.0 mol/L of the electrolyte solution, e.g., from about 0.05 mol/L to about 0.8 mol/L, e.g., from about 0.1 mol/L to about 0.7 mol/L, e.g., from about 0.2 mol/L to about 0.5 mol/L, e.g., from about 0.2 mol/L to about 0.3 mol/L. In at least one embodiment, the amount of metal salt dissolved is about 0.01 mol/L, 0.05 mol/L, 0.1 mol/L, 0.2 mol/L, 0.3 mol/L, 0.4 mol/L, 0.5 mol/L, 0.6 mol/L, 0.8 mol/L, or 1.0 mol/L of the electrolyte solution, as appropriate (any value can form an upper or lower endpoint). At concentrations above 1.0 mol/L, it becomes difficult to dissolve the metal salt in the electrolyte and solubility problems may arise.

In at least one embodiment, the metal salt is dissolved by stirring at ambient temperature, room temperature, about 25° C., or at a temperature ranging from about 20° C. to about 30° C. In at least one embodiment, stirring is performed for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or 30 minutes (any value can form an upper or lower endpoint), as needed, or until all the metal salt is dissolved. In at least one embodiment, the temperature at which step 140 is performed is performed at about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40° C. (any value can form an upper or lower endpoint), as needed.

In step 150, the method 100 further includes dissolving an alkali metal salt, for example, in water or an aqueous solution (e.g., the fourth electrolyte solution) to form an electrolyte solution (e.g., the fifth electrolyte solution). The alkali metal salt can increase the conductivity of the electrolyte solution. In at least one embodiment, the alkali metal salt includes an alkali metal sulfate (e.g., sodium sulfate ( _Na2SO4 ), _{potassium sulfate (K2SO4 )} , and/or other alkali metal sulfates).

In at least one embodiment, the concentration of the alkali metal salt in the electrolyte of the present disclosure ranges from about 0.1 mol/L to about 2.0 mol/L, such as from about 0.5 mol/L to about 2.0 mol/L, such as from about 1.0 mol/L to about 1.5 mol/L, for example from about 1.3 mol/L to about 1.4 mol/L. In at least one embodiment, the amount of oxalate compound dissolved is about 0.1 mol/L, 0.2 mol/L, 0.4 mol/L, 0.6 mol/L, 0.8 mol/L, 1.0 mol/L, 1.2 mol/L, 1.4 mol/L, 1.6 mol/L, 1.8 mol/L, or 2.0 mol/L of the electrolyte solution, as appropriate (any value can form an upper or lower endpoint). At concentrations above 2.0 mol/L, it becomes difficult to dissolve the alkali metal salt in the electrolyte, and solubility problems may arise.

In at least one embodiment, the alkali metal salt is dissolved by stirring at ambient temperature, room temperature, about 25°C, or at a temperature ranging from about 10°C to about 40°C, e.g., from about 20°C to about 30°C. In at least one embodiment, the temperature at which step 150 is performed can be about 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, or 40°C, as appropriate (any value can form an upper or lower endpoint). In at least one embodiment, stirring can be performed for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or 30 minutes, as appropriate, or until all of the alkali metal salt is dissolved (any value can form an upper or lower endpoint). In at least one embodiment, the alkali metal salt is dissolved by stirring for 15 minutes at ambient temperature, room temperature, about 25°C.

At step 160, the method 100 further includes dissolving an alkali metal halide, for example, in water or an aqueous solution (e.g., the fifth electrolyte solution) to form an electrolyte solution (e.g., the sixth electrolyte solution). The alkali metal halide can provide the electrolyte solution with wetting and etching properties and can aid in chrome adhesion during chrome plating. In at least one embodiment, the alkali metal halide includes an alkali metal fluoride (e.g., sodium fluoride (NaF), potassium fluoride (KF), and/or other alkali metal fluorides) and/or other alkali metal halides.

In at least one embodiment, the concentration of the alkali metal halide in the electrolyte of the present disclosure ranges from about 0.1 mol/L to about 2.0 mol/L, such as from about 0.1 mol/L to about 0.5 mol/L, such as from about 0.2 mol/L to about 0.5 mol/L, for example from about 0.3 mol/L to about 0.4 mol/L. In at least one embodiment, the amount of oxalate compound dissolved is about 0.1 mol/L, 0.2 mol/L, 0.3 mol/L, 0.4 mol/L, 0.5 mol/L, 0.6 mol/L, 0.8 mol/L, or 1.0 mol/L of the electrolyte solution, as appropriate (any value can form an upper or lower endpoint). At concentrations above 1.0 mol/L, it becomes difficult to dissolve the alkali metal halide in the electrolyte, and solubility problems can arise.

In at least one embodiment, the alkali metal halide is dissolved by stirring at ambient temperature, room temperature, about 25° C., or at a temperature ranging from about 10° C. to about 40° C., e.g., from about 20° C. to about 30° C. In at least one embodiment, the temperature at which step 160 is performed is about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40° C., as appropriate (any value can form an upper or lower endpoint). In at least one embodiment, stirring is performed for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or 30 minutes, as appropriate, or until all the alkali metal halide is dissolved (any value can form an upper or lower endpoint). In at least one embodiment, the alkali metal halide is dissolved by stirring at ambient temperature, room temperature, about 25° C. for 15 minutes.

Optionally, in step 170, the method 100 further includes dissolving a surfactant, for example, in the water or aqueous solution (e.g., the sixth electrolyte solution) to form an electrolyte solution (e.g., the seventh electrolyte solution). The surfactant can prevent or reduce pitting corrosion and reduce gassing (e.g., chlorine gas, hydrogen gas, etc.) during chrome plating. In at least one embodiment, the surfactant includes _sodium lauryl sulfate ( _NaC12H25SO4 ), _sodium lauryl ether sulfate (CH3( _{CH2 ) 11 ( OCH2CH2 ) nOSO3Na} ), potassium _lauryl sulfate ( _{KC12H25SO4 )} , and/or other surfactants.

In at least one embodiment, the concentration of the surfactant in the electrolyte of the present disclosure ranges from about 0.0001 mol/L to about 0.01 mol/L. In at least one embodiment, the amount of surfactant dissolved can be about 0.0001 mol/L, 0.0002 mol/L, 0.0004 mol/L, 0.0006 mol/L, 0.0008 mol/L, 0.0010 mol/L, 0.0020 mol/L, 0.0040 mol/L, 0.0060 mol/L, 0.0080 mol/L, or 0.0100 mol/L of the electrolyte solution (either value can form an upper or lower endpoint, as appropriate). For example, in at least one embodiment, the amount of sodium lauryl sulfate, sodium lauryl ether sulfate, or potassium lauryl sulfate ranges from about 0.1 g to about 1 g per liter of electrolyte solution. At concentrations above 0.01 mol/L, surfactants can cause excessive foaming and uneven deposition in the bath.

Optionally, in step 180, the method 100 further includes adjusting the pH of the electrolyte solution using one or more aqueous acidic or basic solutions, such as potassium hydroxide (KOH), sodium hydroxide (NaOH), and/or sulfuric acid ( _H2SO4 ). The volume _of the aqueous acidic or basic solution added to the electrolyte solution is sufficiently small so that the concentration of other components of the electrolyte solution (complexing agents, buffers, etc.) is not substantially affected. Alternatively, solid potassium hydroxide and/or solid sodium hydroxide are added directly to the electrolyte solution, and/or concentrated sulfuric acid is added directly to the electrolyte solution. In at least one embodiment, the pH of the electrolyte solution is adjusted to a target pH of about 1 to about 7, such as about 1 to about 5, such as about 1.5 to about 4, for example 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5. In at least one embodiment, the pH of the electrolyte solution of the present disclosure is adjusted (as described in more detail below) before passing an electric current through the electrolyte solution. In at least one embodiment, the pH of the electrolyte solution of the present disclosure is maintained within a target pH range while an electric current is passed through the electrolyte solution.

Optionally, in step 190, a time is provided to reach equilibrium. In at least one embodiment, the solution is left to reach equilibrium for a time ranging from 1 hour to 2 days. The time provided to reach equilibrium can be about 1 hour, 3 hours, 6 hours, 9 hours, 12 hours, 15 hours, 18 hours, 21 hours, 24 hours, 27 hours, 30 hours, 33 hours, 36 hours, 39 hours, 42 hours, 45 hours, or 48 hours (either value can form an upper or lower endpoint, as appropriate).

In at least one embodiment, method 100 is performed in the order presented. Alternatively, method 100 is performed in a different order. In at least one embodiment, some steps are performed in order and other steps are performed in a different order. For example, steps 110 and 120 are performed in order and steps 130, 140, 150, 160, 170, 180, and 190 are performed in a different order after steps 110 and 120. In another embodiment, steps 110 and 120 are performed in a different order and steps 130, 140, 150, 160, 170, 180, and 190 are performed in order. In another embodiment, steps 110, 120, 130, 140, 150, and 160 are performed in order and steps 170, 180, and 190 are performed in a different order. One set of steps can be performed before another set of steps. For example, steps 110 and 120 can be performed in any order, and after steps 110 and 120 are performed, steps 130, 140, 150, and 160 are performed in any order. As one of ordinary skill in the art would understand, other orders are contemplated. Additionally, one or more of steps 170, 180, and 190 are omitted in some aspects.

2 is a flow diagram illustrating a method 200 for forming a chromium alloy coating on a substrate by electrodeposition, according to one or more embodiments of the present disclosure. In step 210, an electrolyte solution is prepared, such as by method 100 of FIG. 1. In step 220, the method 200 further includes adjusting and/or maintaining the pH of the electrolyte solution at a target pH or target pH range. In at least one embodiment, the target pH is a pH ranging from about 1 to about 4. The pH can be maintained at about 1.2, 1.4, 1.5, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, or 4.0 (any value can form an upper or lower endpoint, as appropriate).

At step 230, method 200 further includes adjusting and/or maintaining the temperature of the electrolyte solution (e.g., the electrolyte solution formed by step 210). In at least one embodiment, the temperature is adjusted/maintained at a target temperature of about 20° C. to about 70° C., e.g., about 20° C. to about 40° C., e.g., about 20° C. to about 35° C., e.g., 20° C., 25° C., or 30° C., using any suitable heating or cooling device. In at least one embodiment, the temperature of the electrolyte solution is adjusted prior to passing a current through the electrolyte solution. In at least one embodiment, the temperature of the electrolyte solution is maintained during the passage of a current through the electrolyte solution to maintain the appearance of the deposited layer. Maintaining the temperature within a desired range facilitates reproducible results in terms of appearance and alloy composition.

The method further includes, in step 240, introducing the cathode and anode containing substrate into the electrolyte solution, and, in step 250, passing a current through the electrolyte solution between the cathode and the anode to deposit the chromium alloy on the cathode substrate. In at least one embodiment, the cathode substrate is, for example, a steel substrate, an iron-based alloy substrate, a copper substrate, a brass substrate, a nickel substrate, a copper-coated substrate (e.g., copper-coated steel or copper-coated iron-based alloy), or a nickel-coated substrate (e.g., nickel-coated steel or nickel-coated iron-based alloy).

In at least one embodiment, the anode comprises a carbonaceous electrode material. For example, the carbonaceous anode can be a graphite anode or other anode comprising carbon. In at least one embodiment, the graphite anode is used for a chloride-based electrolyte solution (e.g., an electrolyte solution including one or more compounds having a chloride, such as chromium (III) chloride), a sulfate-based electrolyte solution (e.g., an electrolyte solution including one or more compounds having a sulfate, such as chromium (III) sulfate), or a chloride and sulfate-based electrolyte solution (e.g., an electrolyte solution including one or more compounds having a chloride and one or more other compounds having a sulfate). Advantageously, the graphite anode or other carbonaceous anode promotes desirable deposition rates (e.g., ranging from about 1 micron to about 2 microns per minute) while minimizing outgassing and the formation of undesirable by-products. Alternatively, a platinum anode or a platinized titanium anode can be used for a sulfate-based electrolyte solution (e.g., an electrolyte solution including one or more compounds having a sulfate, such as chromium (III) sulfate). For example, platinum anodes or platinized titanium anodes can be used when the electrolyte solution does not contain chloride-containing compounds such that chlorine gas is not produced, or when the electrolyte solution has less chloride such that chlorine gas production is reduced (e.g., there is no need to use a carbonaceous anode to reduce chlorine gas production).

In at least one embodiment, passing a current between the cathode and the anode is performed using a direct current. In at least one embodiment, a direct current having a current density of about 50 mA/cm ² to about 600 mA/cm ² is used, for example about 100 ^mA /cm ² to about 500 mA/cm ² , for example about 100 mA/cm ² to about 400 mA/cm ² , for example about 200 mA/cm ² to about 400 mA/cm ^{2 , for example 200 mA/cm 2 ,} 250 mA/cm 2 , or 300 mA/cm ² . The value of the current density can be adjusted depending on the distance between the cathode and the anode. In at least one embodiment, the current density depends on the separation distance between the cathode and the anode, and is about 50mA/ ^cm2 , 100mA/ ^cm2 , 150mA/ ^cm2 , 200mA/ ^cm2 , 250mA/ ^cm2 , 300mA/ ^cm2 , 350mA/ ^cm2 , 400mA/cm2, 450mA/ ^cm2 , or 500mA/ ^cm2 , as appropriate (any value can form an upper or lower endpoint, as appropriate ⁾ . For example, a current density ranging from about 200mA/ ^cm2 to about 400mA/ ^cm2 can be applied when the cathode and anode are separated by about 3 cm. The inventors have discovered that a current density within the above range achieves a reasonable rate of deposition, e.g., 0.5 to 1 micron/min, while minimizing the formation of undesirable hexavalent chromium by-products.

In response to passing a current between the cathode and the anode, chromium and iron are deposited on the cathode substrate. Step 250 is performed until a chromium alloy coating layer having a desired thickness is formed on the substrate. In at least one embodiment, the chromium alloy coating layer is a chromium-iron alloy having about 1 wt. % iron to about 60 wt. % iron, e.g., about 1 wt. % iron to about 20 wt. % iron, e.g., about 1 wt. % iron to about 5 wt. % iron, or about 10 wt. % iron to about 20 wt. % iron, based on the total weight of the alloy. For example, the chromium-iron alloy can have about 1 wt. %, 2 wt. %, 10 wt. %, 11 wt. %, or 12 wt. % iron. Further, the chromium-iron alloy has about 80% chromium to about 99% chromium by weight, for example about 85% chromium to about 95% chromium by weight, for example about 99% chromium, 98% chromium, 90% chromium, 89% chromium, or 88% chromium by weight, based on the total weight of the alloy.

In response to performing step 250, chromium is deposited on the substrate. In at least one embodiment, chromium and carbon are co-deposited on the substrate. In at least one embodiment, step 250 forms a chromium layer (e.g., a chromium coating) or a chromium-carbon layer (e.g., a chromium carbide coating) having a desired thickness (e.g., a thickness greater than about 5 microns) on the substrate. In at least one embodiment, a chromium layer having a thickness greater than about 5 microns can have a machinability greater than about 800 HV.

EMBODIMENTS Clause 1. An electrolyte solution for electroplating comprising: a trivalent chromium salt; an oxalate compound; an iron salt; aluminum sulfate; an alkali metal sulfate; and an alkali metal halide.

Clause 2. The electrolyte solution of clause 1, wherein the trivalent chromium salt is present in an amount ranging from about 0.3 mol per liter to about 0.9 mol per liter of electrolyte solution; the oxalate compound is present in an amount ranging from about 0.2 mol per liter to about 1.2 mol per liter of electrolyte solution; the iron salt is present in an amount ranging from about 0.005 mol per liter to about 0.2 mol per liter of electrolyte solution; the aluminum sulfate is present in an amount ranging from about 0.05 mol per liter to about 0.5 mol per liter; the alkali metal sulfate is present in an amount ranging from about 0.1 mol per liter to about 2.0 mol per liter of electrolyte solution; and the alkali metal halide is present in an amount ranging from about 0.1 mol per liter to about 0.5 mol per liter of electrolyte solution.

Clause 3. The electrolyte solution of clause 1 or 2, wherein the iron salt is a divalent iron salt including one or more of iron (II) sulfate, iron (II) chloride, iron (II) acetate, and hydrates thereof.

Clause 4. The electrolyte solution of any one of clauses 1 to 3, wherein the iron salt is a trivalent iron salt including one or more of iron sulfate (III), iron chloride (III), iron acetate, and hydrates thereof.

Clause 5. The electrolyte solution of any one of clauses 1 to 4, wherein the trivalent chromium salt is selected from chromium (III) halides, chromium (III) sulfate, or a combination thereof.

Clause 6. The electrolyte solution of any one of clauses 1 to 5, wherein the oxalate compound is selected from sodium oxalate, potassium oxalate, an oxalate acid, or a combination thereof.

Clause 7. The electrolyte solution of any one of clauses 1 to 6, wherein the alkali metal sulfate is selected from sodium sulfate, potassium sulfate, or a combination thereof.

Clause 8. The electrolyte solution of any one of clauses 1 to 7, wherein the alkali metal halide is selected from sodium fluoride, potassium fluoride, or a combination thereof.

Clause 9. The electrolyte solution of any one of clauses 1 to 8, wherein the pH of the electrolyte solution is in the range of about 1 to about 4.

Clause 10. The electrolyte solution of any one of clauses 1 to 9, further comprising sodium lauryl sulfate, sodium lauryl ether sulfate, or a combination thereof.

Clause 11. An electroplating electrolyte solution comprising: a trivalent chromium salt in an amount ranging from about 0.3 mol per liter to about 0.9 mol per liter; an oxalate compound in an amount ranging from about 0.2 mol per liter to about 1.2 mol per liter; an iron salt in an amount ranging from about 0.005 mol per liter to about 0.2 mol per liter; aluminum sulfate in an amount ranging from about 0.05 mol per liter to about 0.5 mol per liter; an alkali metal sulfate in an amount ranging from about 0.1 mol per liter to about 2.0 mol per liter; and an alkali metal halide in an amount ranging from about 0.1 mol per liter to about 0.5 mol per liter.

Clause 12. The electrolyte solution of clause 11, wherein the iron salt is a divalent iron salt including one or more of iron sulfate (II), iron chloride (II), iron acetate (II), and hydrates thereof.

Clause 13. The electrolyte solution of clause 11 or 12, wherein the iron salt is a trivalent iron salt including one or more of iron sulfate (III), iron chloride (III), iron acetate, and hydrates thereof.

Clause 14. The electrolyte solution of any one of clauses 11 to 13, wherein the trivalent chromium salt is selected from chromium (III) halides, chromium (III) sulfate, or a combination thereof.

Clause 15. The electrolyte solution of any one of clauses 11 to 14, wherein the oxalate compound is selected from sodium oxalate, potassium oxalate, an oxalate acid, or a combination thereof.

Clause 16. The electrolyte solution of any one of clauses 11 to 15, wherein the alkali metal sulfate is selected from sodium sulfate, potassium sulfate, or a combination thereof.

Clause 17. The electrolyte solution of any one of clauses 11 to 16, wherein the alkali metal halide is selected from sodium fluoride, potassium fluoride, or a combination thereof.

Clause 18. The electrolyte solution of any one of clauses 11 to 17, wherein the pH of the electrolyte solution is in the range of about 1 to about 4.

Clause 19. The electrolyte solution of any of clauses 11 to 18, further comprising sodium lauryl sulfate, sodium lauryl ether sulfate, or a combination thereof.

Clause 20. A method of plating chromium on a substrate using an electrolyte solution, comprising dissolving in an aqueous medium a trivalent chromium salt in an amount ranging from about 0.3 mol per liter to about 0.9 mol per liter; dissolving an oxalate compound in an amount ranging from about 0.2 mol per liter to about 1.2 mol per liter; dissolving an iron salt in an amount ranging from about 0.005 mol per liter to about 0.2 mol per liter; dissolving aluminum sulfate in an amount ranging from about 0.05 mol per liter to about 0.5 mol per liter; dissolving an alkali metal sulfate in an amount ranging from about 0.1 mol per liter to about 2.0 mol per liter; dissolving an alkali metal halide in an amount ranging from about 0.1 mol per liter to about 0.5 mol per liter; and passing a current through the electrolyte solution between a cathode and an anode to deposit chromium on the substrate.

Clause 21. The method of clause 20, wherein the cathode is a steel substrate, a copper substrate, a brass substrate, a nickel substrate, a copper-coated substrate, or a nickel-coated substrate.

Clause 22. The method of clause 20 or 21, wherein the anode is a platinum material, a platinized titanium material, or a carbonaceous electrode material.

Clause 23. The method of any of clauses 20 to 22, wherein the current has a current density in the range of about 150 to about 600 mA/ ^cm2 by passing a direct current between the anode and the cathode.

Clause 24. The method of any of clauses 20 to 23, wherein the current density is in the range of about 200 to about 400 mA/ ^cm2 .

Clause 25. The method of any of clauses 20 to 24, wherein the electrolyte solution is maintained at a temperature in the range of about 20°C to about 60°C Celsius.

Clause 26. The method of any of clauses 20 to 25, further comprising adjusting the pH of the electrolyte solution to a pH in the range of about 1.5 to about 4.

Clause 27. The method of any one of clauses 20 to 26, wherein the iron salt is a divalent iron salt including one or more of iron sulfate (II), iron chloride (II), iron acetate (II), and hydrates thereof.

Clause 28. The method of any one of clauses 20 to 27, wherein the iron salt is a trivalent iron salt including one or more of iron sulfate (III), iron chloride (III), iron acetate, and hydrates thereof.

Clause 29. The method of any one of clauses 20 to 28, wherein the trivalent chromium salt is selected from chromium(III) halides, chromium(III) sulfate, or a combination thereof.

Clause 30. The method of any one of clauses 20 to 29, wherein the oxalate compound is selected from sodium oxalate, potassium oxalate, an oxalate acid, or a combination thereof.

Clause 31. The method of any one of clauses 20 to 30, wherein the alkali metal sulfate is selected from sodium sulfate, potassium sulfate, or a combination thereof.

Clause 32. The method of any one of clauses 20 to 31, wherein the alkali metal halide is selected from sodium fluoride, potassium fluoride, or a combination thereof.

Clause 33. The method of any of clauses 20 to 32, further comprising dissolving sodium lauryl sulfate, sodium lauryl ether sulfate, or a combination thereof in an amount ranging from about 0.1 grams per liter to about 1 gram per liter of electrolyte solution.

Clause 34. A method of plating chromium on a substrate using an electrolyte solution, the method comprising: introducing a cathode and an anode into an electrolyte solution containing a trivalent chromium salt, an oxalate compound, an iron salt, aluminum sulfate, an alkali metal sulfate, and an alkali metal halide; and passing an electric current through the electrolyte between the anode and the cathode to deposit a chromium layer on the substrate.

Clause 35. The method of clause 34, wherein the cathode is a steel substrate, a copper substrate, a brass substrate, a nickel substrate, a copper-coated substrate, or a nickel-coated substrate.

Clause 36. The method of clause 34 or 35, wherein the anode is a platinum material, a platinized titanium material, or a carbonaceous electrode material.

Clause 37. The method of any of clauses 34 to 36, wherein the current has a current density in the range of about 100 to about 600 mA/ ^cm2 by passing a direct current between the anode and the cathode.

Clause 38. The method of any of clauses 34 to 37, wherein the current density is in the range of about 200 to about 400 mA/ ^cm2 .

Clause 39. The method of any of clauses 34 to 38, wherein the electrolyte solution is maintained at a temperature in the range of about 20°C to about 60°C Celsius.

Clause 40. The method of any of clauses 34 to 39, further comprising adjusting the pH of the electrolyte solution to a pH in the range of about 1.5 to about 4.

Clause 41. The method of any one of clauses 34 to 40, wherein the iron salt is a divalent iron salt including one or more of iron sulfate (II), iron chloride (II), iron acetate (II), and hydrates thereof.

Clause 42. The method of any one of clauses 34 to 41, wherein the iron salt is a trivalent iron salt including one or more of iron sulfate (III), iron chloride (III), iron acetate (III), and hydrates thereof.

Clause 43. The method of any one of clauses 34 to 42, wherein the trivalent chromium salt is selected from chromium (III) halides, chromium (III) sulfate, or a combination thereof.

Clause 44. The method of any one of clauses 34 to 43, wherein the oxalate compound is selected from sodium oxalate, potassium oxalate, an oxalate acid, or a combination thereof.

Clause 45. The method of any one of clauses 34 to 44, wherein the alkali metal sulfate is selected from sodium sulfate, potassium sulfate, or a combination thereof.

Clause 46. The method of any one of clauses 34 to 45, wherein the alkali metal halide is selected from sodium fluoride, potassium fluoride, or a combination thereof.

Clause 47. The method of any of clauses 34 to 46, further comprising dissolving sodium lauryl sulfate, sodium lauryl ether sulfate, or a combination thereof in an amount ranging from about 0.1 grams per liter to about 1 gram per liter of electrolyte solution.

Clause 48. A substrate comprising a chromium-iron alloy coating having a chromium content ranging from about 40% to about 90% by weight, an iron content ranging from about 8% to about 18% by weight, and a carbon content ranging from about 5% to about 50% by weight.

Clause 49. The substrate of clause 48, wherein the substrate comprises one or more of steel, copper, brass, or nickel.

Clause 50. The substrate of clause 48 or 49, wherein the coating has a thickness of about 1 micron to about 100 microns.

The following non-limiting examples are provided to further illustrate the aspects described herein. However, these examples are not intended to be comprehensive or to limit the scope of the aspects described herein.

Example 1
The ingredients of Example 1 were mixed in stages, first with chromium chloride hexahydrate and sodium oxalate, followed by the metal salts. The pH of Example 1 was .about.2.2.

Example 2
The ingredients of Example 2 were mixed stepwise, first with chromium chloride hexahydrate and sodium oxalate, followed by metal salts. The ingredients of Example 2 were similar to those of Example 1, except that the amount of ferrous sulfate heptahydrate was increased 10-fold compared to the ingredients of Example 1. The pH of Example 1 was ∼2.2.

Example 3
The ingredients of Example 3 were mixed in stages, first with chromium chloride hexahydrate and sodium oxalate, followed by the metal salts. The ingredients of Example 3 were similar to those of Example 1, except that in Example 3, ferrous sulfate heptahydrate was replaced with ferric chloride. The pH of Example 1 was about 2.2.

3 is an image 300 of chromium-iron alloy plated substrates 310, 320, 330, and 340 formed by the process of FIG. 2, each plated at a different current density using the electrolyte solution of Example 1. For each chromium-iron alloy plated substrate 310, 320, 330, and 340, the plating parameters were a plating time of about 1 hour while maintaining the electrolyte solution at a temperature of 30° C. and a pH of about 2.2. Chromium-iron alloy plated substrate 310 was plated at a current density of 100 mA/cm ² , resulting in a chromium-iron alloy layer having a thickness of 12 μm. Chromium-iron alloy plated substrate 320 was plated at a current density of 200 mA/cm ² , resulting in a chromium-iron alloy layer having a thickness of 15 μm. Chromium-iron alloy plated substrate 330 was plated at a current density of 250 mA/cm ² , resulting in a chromium-iron alloy layer having a thickness of 28 μm. The chromium-iron alloy plated substrate 340 was plated at a current density of 300 mA/ ^cm2 , resulting in a chromium-iron layer having a thickness of 32 μm. X-ray fluorescence was performed on a portion of each coating to determine the chromium and iron content in weight percent. It is noted that X-ray fluorescence does not detect carbon, so in at least one embodiment, the chromium-iron alloys described herein also contain carbon, but the carbon does not show up in the X-ray fluorescence results.
The results are shown in Table I.

As shown in Table I, any current density ranging from 100 mA/ ^cm2 to about 300 mA/ ^cm2 deposits a chromium-iron alloy layer. Current densities ranging from about 200 mA/ ^cm2 to about 280 mA/ ^cm2 provide a thick chromium-iron alloy layer with low iron content. Additionally, a current density of 100 mA/ ^cm2 provides the thinnest chromium-iron alloy with high iron content for current densities ranging from about 200 mA/ ^cm2 to about 300 mA/ ^cm2 .

4 is an image 400 of chromium-iron alloy plated substrates 410, 420, 430, 440, and 450 formed by the process of FIG. 2, each plated at a different pH using the electrolyte solution of Example 1. For each chromium-iron alloy plated substrate 410, 420, 430, 440, and 450, the plating parameters were direct current at a current density of 250 mA/ ^cm2 for a plating time of about 1 hour while maintaining the electrolyte solution at a temperature of 30° C. Chromium-iron alloy plated substrate 410 was plated at a pH of 1.0, resulting in an uneven chromium-iron alloy deposit that was not measured but was believed to be less than 10 μm. Chromium-iron alloy plated substrate 420 was plated at a pH of 2.0, resulting in an uneven chromium-iron alloy deposit that was not measured but was believed to be less than 10 μm. The chromium-iron alloy plated substrate 430 was plated at a pH of 2.5 resulting in a chromium-iron alloy layer having a thickness of 20 μm. The chromium-iron alloy plated substrate 440 was plated at a pH of 3.0 resulting in a chromium-iron alloy layer having a thickness of 28 μm. The chromium-iron alloy plated substrate 450 was plated at a pH of 3.5 resulting in a chromium-iron alloy layer having a thickness of 45 μm. X-ray fluorescence was performed on a portion of each coating to determine the chromium and iron content in weight percent. The results are shown in Table II.

As shown in Table II, any pH ranging from 2.5 to 3.5 will deposit a chromium-iron alloy layer. The pH range of 2.5 to 3.0 advantageously provides a thick chromium-iron alloy layer with the desired iron content. A pH of 3.5 provides a thicker chromium-iron alloy layer with a lower iron content relative to the pH range of 2.5 to 3.0.

5 is an image 500 of chromium-iron alloy plated substrates 510, 520, 530, and 540 formed by the process of FIG. 2, each plated at a different current density using the electrolyte solution of Example 2. For each chromium-iron alloy plated substrate 510, 520, 530, and 540, the plating parameters were a plating time of about 1 hour while maintaining the electrolyte solution at a temperature of 30° C. and a pH of about 2.2. Chromium-iron alloy plated substrate 510 was plated at a current density of 100 mA/cm ² , resulting in a chromium-iron alloy layer having a thickness of 30 μm. Chromium-iron alloy plated substrate 520 was plated at a current density of 200 mA/cm ² , resulting in a chromium-iron alloy layer having a thickness of 40 μm. Chromium-iron alloy plated substrate 530 was plated at a current density of 250 mA/cm ² , resulting in a chromium-iron alloy layer having a thickness of 70 μm. The chromium-iron alloy plated substrate 540 was plated at a current density of 300 mA/ ^cm2 , resulting in a chromium-iron layer having a thickness of 50 μm. X-ray fluorescence was performed on a portion of each coating to determine the chromium and iron content in weight percent. The results are shown in Table III.

As shown in Table III, any current density ranging from 100 mA/ ^cm2 to about 300 mA/ ^cm2 deposits a chromium-iron alloy. Current densities ranging from about 100 mA/ ^cm2 to about 250 mA/ ^cm2 provide thick chromium-iron alloy layers. Additionally, a current density of 300 mA/ ^cm2 reduces the thickness of the chromium-iron alloy compared to the thickness of the chromium-iron alloy deposited at a current density of 250 mA/ ^cm2 .

6 is an image 600 of chromium-iron alloy plated substrates 610, 620, 630, 640, 650, and 660 formed by the process of FIG. 2, each plated at a different pH using the electrolyte solution of Example 2. For each chromium-iron alloy plated substrate 610, 620, 640, 650, and 660, the plating parameters were direct current at a current density of 250 mA/ ^cm2 for a plating time of about 1 hour while maintaining the electrolyte solution at a temperature of 30° C. Chromium-iron alloy plated substrate 610 was plated at a pH of 1.5, resulting in an uneven chromium-iron alloy deposit. Chromium-iron alloy plated substrate 620 was plated at a pH of 2.0, resulting in an uneven chromium-iron alloy deposit. Chromium-iron alloy plated substrate 630 was plated at a pH of 2.5, resulting in a chromium-iron alloy layer having a thickness of 50 μm. Chromium-iron alloy plated substrate 640 was plated at a pH of 3.0 resulting in a chromium-iron alloy layer having a thickness of 70 μm. Chromium-iron alloy plated substrate 650 was plated at a pH of 3.5 resulting in a chromium-iron alloy layer having a thickness of 50 μm. Chromium-iron alloy plated substrate 660 was plated at a pH of 4.0. X-ray fluorescence was performed on a portion of each coating to determine the chromium and iron content in weight percent. The results are shown in Table IV.

As shown in Table IV, any pH ranging from 1.5 to 4.0 will deposit a chromium-iron alloy layer. A pH range of 2.0 to 4.0 advantageously provides a thick chromium-iron alloy layer with a desired high chromium content. A pH of 3.0 provides a thicker chromium-iron alloy layer with good quality deposit. At a pH of 4, the deposit exhibited burning at the edges, and at a pH of 2 or less, the deposit was patchy.

7 is an image 700 of chromium-iron alloy plated substrates 710, 720, 730, 740, 750, and 760 formed by the process of FIG. 2, each plated at a different current density using the electrolyte solution of Example 3. For each chromium-iron alloy plated substrate 710, 720, 720, 730, 740, and 760, the plating parameters were a plating time of about 1 hour while maintaining the electrolyte solution at a temperature of 30° C. and a pH of about 2.2. Chromium-iron alloy plated substrate 710 was plated at a current density of 100 mA/cm ² , resulting in an uneven chromium-iron alloy deposit believed to be less than 10 μm thick. Chromium-iron alloy plated substrate 720 was plated at a current density of 150 mA/cm ² , resulting in an uneven chromium-iron alloy deposit believed to be less than 10 μm thick. The chromium-iron alloy plated substrate 730 was plated at a current density of 200 mA/ ^cm2 , resulting in a chromium-iron alloy layer having a thickness of 10 μm. The chromium-iron alloy plated substrate 740 was plated at a current density of 300 mA/ ^cm2 , resulting in a chromium-iron layer having a thickness of 30 μm. The chromium-iron alloy plated substrate 750 was plated at a current density of 400 mA/ ^cm2 , resulting in a chromium-iron layer having a thickness of 50 μm. The chromium-iron alloy plated substrate 760 was plated at a current density of 500 mA/ ^cm2 , resulting in a deposit exhibiting edge burning.

As shown in Table V, any current density ranging from 200 mA/ ^cm2 to about 400 mA/ ^cm2 deposits a chromium-iron alloy. Current densities ranging from about 300 mA/ ^cm2 to about 400 mA/ ^cm2 provide thicker chromium-iron alloy layers. Furthermore, at a current density of 500 mA/ ^cm2 , the thickness of the chromium-iron alloy is increased compared to the thickness of the chromium-iron alloy deposited at a current density of 400 mA/ ^cm2 , but the edge deposit is burned (black powder).

8 is an image 800 of chromium-iron alloy plated substrates 810, 820, 830, 840, 850, 860, and 870 formed by the process of FIG. 2, each plated at a different pH using the electrolyte solution of Example 3. For each chromium-iron alloy plated substrate 810, 810, 820, 850, 860, and 870, the plating parameters were direct current at a current density of 250 mA/ ^cm2 for a plating time of about 1 hour while maintaining the electrolyte solution at a temperature of about 30° C. Chromium-iron alloy plated substrate 810 was plated at a pH of 1.0, resulting in a chromium-iron alloy deposit having a thickness of 20 μm. Chromium-iron alloy plated substrate 820 was plated at a pH of 1.5, resulting in a chromium-iron alloy layer having a thickness of 24 μm. Chromium-iron alloy plated substrate 830 was plated at a pH of 2.0, resulting in a chromium-iron alloy layer having a thickness of 20 μm. Chromium-iron alloy plated substrate 840 was plated at a pH of 3.0, resulting in a chromium-iron alloy layer having a thickness of 16 μm. Chromium-iron alloy plated substrate 860 was plated at a pH of 3.5, resulting in a chromium-iron alloy layer having a thickness of 16 μm. Chromium-iron alloy plated substrate 870 was plated at a pH of 4.0, resulting in a chromium-iron alloy deposit with a small uneven thickness. X-ray fluorescence was performed on portions of the coatings deposited at pH 2 and pH 2.5 to determine the chromium and iron content in weight percent. The results are shown in Table VI.

As shown in Table VI, any pH ranging from 1.0 to 4.0 will deposit a chromium-iron alloy. A pH ranging from 1.0 to 2.5 advantageously provides a thicker chromium-iron alloy layer than the higher pHs. Furthermore, a pH ranging from 1.5 to 2.0 provides the thickest chromium-iron alloy layer.

9 is an image 900 of chromium-iron alloy plated substrates 910, 920, 930, and 940 formed by the process of FIG. 2, each plated at a different temperature using the electrolyte solution of Example 3. For each chromium-iron alloy plated substrate 910, 920, 930, and 940, the plating parameters were direct current at a current density of 250 mA/ ^cm2 for a plating time of about 1 hour while maintaining the electrolyte solution at a pH of about 2.1. Chromium-iron alloy plated substrate 910 was plated at a temperature of 30° C., resulting in a chromium-iron alloy layer having a thickness of 40 μm. Chromium-iron alloy plated substrate 920 was plated at a temperature of 40° C., resulting in a chromium-iron alloy layer having a thickness of 30 μm. Chromium-iron alloy plated substrate 930 was plated at a temperature of 50° C., resulting in a chromium-iron alloy layer having a thickness of 30 μm. The chromium-iron alloy plated substrate 940 was plated at a temperature of 60° C., resulting in a chromium-iron alloy layer having a small thickness. The results are shown in Table VII.

As shown in Table VII, any temperature ranging from about 30° C. to about 50° C. deposits a chromium-iron alloy layer. An operating temperature of 30° C. provides the thickest chromium-iron alloy layer, but the deposit at 40° C. is aesthetically superior.

Using the electrolyte solution of Example 3, chromium-iron-alloy coatings were deposited on Taber Wear panels by direct current at a current density of 300 mA/ ^cm2 for a plating time of about 2 hours, while maintaining the electrolyte solution at a temperature of about 40° C. and a pH of about 2.5. The alloy composition of the coating, measured by SEM-EDS, was chromium-41 wt.%, iron-10 wt.%, and carbon-49 wt.%. Machinability values were measured on the coupons "as plated" and after baking the coupons at about 190° C. for 23 hours. The resulting chromium-iron-alloy coatings were evaluated for wear resistance. Wear resistance measurements were performed using the Taber wear test specified by ASTM D 4060. The grinding wheels used were CS10 with a load of 500 g per wheel and grinding was performed for 10,000 cycles at a rotation speed of 60 rpm. The Taber abrasion index was less than 10. The average of five machinability measurements of the "as plated" chromium-iron alloy layer was 1056 HV. The average of five machinability measurements of the "baked" chromium-iron alloy layer was 1187 HV.

Using the electrolyte solution of Example 1, a chromium-iron-alloy coating was deposited on by direct current at a current density of 250 mA/ ^cm2 for a plating time of about 2 hours while the electrolyte solution was maintained at a temperature of about 35° C. and a pH of about 2.5. The alloy composition of the coating, as measured by SEM-EDS, was chromium-80 wt%, iron-12 wt%, and carbon-8 wt%. Machinability values were measured on the "as plated" coupons as well as after baking the coupons at about 190° C. for 23 hours. The average of five machinability measurements of the "as plated" chromium-iron alloy layer was 1147 HV. The average of five machinability measurements of the "baked" chromium-iron alloy layer was 1249 HV.

Overall, the present disclosure provides improvements to electrolyte solutions for the electrodeposition of chromium-iron alloys, methods for forming chromium-iron alloys, and methods for the electrodeposition of chromium-iron alloys.

The description of the various aspects of the present disclosure is provided for illustrative purposes and is not intended to be exhaustive or to be limited to the disclosed aspects. Numerous modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described aspects. The terms used herein have been selected to best explain the principles of the aspects, practical applications or technical improvements to the technology found in the marketplace, or to enable others skilled in the art to understand the aspects disclosed herein. While the above is directed to aspects of the present disclosure, other and further aspects of the present disclosure may be devised without departing from the basic scope of the present disclosure.

Claims (16)

Trivalent chromium salts;
Oxalate compounds;
Iron salts;
Aluminum sulfate;
alkali metal sulfates; and alkali metal halides ,
The trivalent chromium salt is present in an amount ranging from 0.3 moles per liter to 0.9 moles per liter of electrolyte solution;
the oxalate compound is present in an amount ranging from 0.2 moles per liter to 1.2 moles per liter of electrolyte solution;
The iron salt is present in an amount ranging from 0.005 moles per liter to 0.2 moles per liter of electrolyte solution;
Aluminum sulfate is present in an amount ranging from 0.05 moles per liter to 0.5 moles per liter;
The alkali metal sulfate is present in an amount ranging from 0.1 moles per liter to 2.0 moles per liter of electrolyte solution;
An electrolyte solution for electroplating, wherein the alkali metal halide is present in an amount ranging from 0.1 moles per liter to 0.5 moles per liter of electrolyte solution. 2. The electrolyte solution of claim 1 , wherein the iron salt is a divalent iron salt comprising one or more of iron (II) sulfate, iron (II) chloride, iron (II) acetate, and hydrates thereof. 3. The electrolyte solution of claim 1 or 2 , wherein the iron salt is a trivalent iron salt comprising one or more of iron (III) sulfate, iron (III) chloride, iron (III) acetate, and hydrates thereof. 4. The electrolyte solution of claim 1, wherein the trivalent chromium salt is selected from chromium (III) halides, chromium (III) sulfate, or a combination thereof. 5. The electrolyte solution of claim 1 , wherein the oxalate compound is selected from sodium oxalate, potassium oxalate, an oxalate acid, or a combination thereof. 6. The electrolyte solution of claim 1 , wherein the alkali metal sulfate is selected from sodium sulfate, potassium sulfate, or a combination thereof. 7. The electrolyte solution of claim 1 , wherein the alkali metal halide is selected from sodium fluoride, potassium fluoride, or a combination thereof. 8. The electrolyte solution of claim 1, wherein the pH of the electrolyte solution is in the range of 1 to 4 . 9. The electrolyte solution of claim 1, further comprising sodium lauryl sulfate, sodium lauryl ether sulfate, or a combination thereof. A method (200) for plating chromium on a substrate using an electrolyte solution, comprising:
introducing the cathode and the anode into an electrolyte solution containing a trivalent chromium salt, an oxalate compound, an iron salt, aluminum sulfate, an alkali metal sulfate, and an alkali metal halide (240); and passing an electric current through the electrolyte between the anode and the cathode to deposit a chromium layer on the substrate (250).
Including,
The trivalent chromium salt is present in an amount ranging from 0.3 moles per liter to 0.9 moles per liter of electrolyte solution;
the oxalate compound is present in an amount ranging from 0.2 moles per liter to 1.2 moles per liter of electrolyte solution;
The iron salt is present in an amount ranging from 0.005 moles per liter to 0.2 moles per liter of electrolyte solution;
Aluminum sulfate is present in an amount ranging from 0.05 moles per liter to 0.5 moles per liter;
The alkali metal sulfate is present in an amount ranging from 0.1 moles per liter to 2.0 moles per liter of electrolyte solution;
The process wherein the alkali metal halide is present in an amount ranging from 0.1 moles per liter to 0.5 moles per liter of electrolyte solution . 11. The method (200) of claim 10 , wherein the cathode is a steel substrate, a copper substrate, a brass substrate, a nickel substrate, a copper-coated substrate, or a nickel-coated substrate. 12. The method (200) of claim 10 or 11 , wherein the anode is a platinum material, a platinized titanium material, or a carbonaceous electrode material. 13. The method (200) of any one of claims 10 to 12 , wherein the electric current has a current density in the range of 10 to 60 mA/ ^cm2 by passing a direct current between the anode and the cathode. 14. The method (200) of claim 13 , wherein the current density is in the range of 20 to 40 mA/ ^cm2 . 15. The method (200) of claim 13 or 14 , wherein the electrolyte solution is maintained at a temperature in the range of 20 °C to 60 °C. 16. The method (200) of any one of claims 10 to 15 , further comprising adjusting the pH of the electrolyte solution to a pH in the range of 1.5 to 4 .

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