Deposition of chromium oxides from a trivalent chromium solution

Abstract: A water-soluble composition of matter free of an added buffering agent, for electrolytically depositing a chromium oxide coating on a metal substrate is disclosed comprising a mixture of a trivalent chromium compound, a weak chelating agent, an optional conductivity enhancing cation, an optional depolarizer, and an optional surfactant. A process is also described as well as a product obtained by the process.

Description: BACKGROUND OF THE INVENTION Field of the Invention The field of the invention comprises a soluble composition of matter and process for electrolytically depositing a chromium oxide coating on a metal substrate from a bath containing a trivalent chromium compound. Description of the Related Art The majority of tin mills produce Electrolytic Chromium Coated Steel (ECCS) from chromium plating baths based on hexavalent chromium. Although the chromium layer provides protection for the steel or tin layer or zinc layer on the steel, the surface of the chromium is not especially suitable for applying other coatings since it is difficult to get other materials to adhere to it. Accordingly, the chromium metal is converted into a chromium oxide to promote adhesion. Strong oxidizing solutions such as hexavalent chromium solutions make a thin oxide on chromium automatically. A second step may be used for better control. One of the difficulties with employing hexavalent chromium compounds in electrolytic coating baths for this process is that it is considered carcinogenic, teratogenic and toxic. As a result, use of these baths present occupational and environmental problems. Employing safe equipment such as ventilating and recovery systems to prevent atmospheric and water pollution as well as safe operating procedures that require highly trained and skilled operators minimzes or avoids these problems. Trivalent chromium compounds substantially eliminate or minimize occupational and environmental problems associated with hexavalent chromium. Trivalent chromium solutions, however, do not form oxide while plating the metal using prior art processes. The prior art in one instance, teaches that the electrolytic deposition of chrome oxides from trivalent chromium baths proceeds in two steps, the first of which involves electrolytic deposition of chromium metal from a trivalent chromium bath, the second, a conversion of the chromium metal coating to a chromium oxide compound. Specifically in this regard, Lavezzari, United States Patent No. 4,520,077 describes both a two-step and a so-called "one-step" process for depositing chromium metal and chromium oxide from a trivalent chromium bath. The reaction deposits chromium metal and afterwards trivalent chromium in the bath also reacts to form a chromium hydroxide on the deposited chromium metal. A subsequent dehydration or oxidation process converts the chromium hydroxide to a chromium oxide. The patentee specifies that the electrolytically deposited trivalent chromium film transforms chemically into chromium hydroxide by an optimal combination of at least the electrolytic bath composition, temperature, types of anodes, and cathodic current density. The one-step process of Lavezzari is directed to the formation of a two layer coating of chromium metal with a chromium hydroxide top coat in a single bath. The two-stage process deposits a chromium metal first coat in one bath and a chromium hydroxide coating in a second bath utilizing the same chemistry. In both the one-stage and two-stage processes, a boron oxide such as boric acid is added to the coating bath. Shahin, United States Patent No. 5,294,326 describes a composition for applying chromium metal from a trivalent chromium electrolytic coating bath which requires boric acid anywhere from about 50 grams per liter of the bath up to its solubility limit in the bath. McMullen et al., United States Patent No. 4,450,052 also describe conventional trivalent chromium plating baths for the deposition of chromium metal which also contain boric acid. Lashmore et al., United States Patent 4,804,446; Lashmore, United States Patent 4,461,680 and Huba et al., United States Patent No. 3,706,641 all describe electrodeposition of chromium metal from a trivalent chromium metal electrolyte which also employ boric acid as a component in the bath. Benaben et al., United States Patent No. 4,612,091 describe a chromium electroplating bath based on trivalent chromium which is not chelated whereas, Tardy et al., United States Patent No. 4,460,438 describe a composition and a process for the electrolytic deposit of chromium from a trivalent chromium bath obtained by the reduction of chromic acid in a sulfuric medium by means of an excess of a reducing alcohol such as methanol. Ward et al., United States Patent No. 4,169,022 describes a method of depositing a protective chromite layer on a substrate comprising providing an anode and as a cathode the substrate to be coated in an electrolyte comprising Cr III ions in a concentration of not more than 1 Molar and a weak complexing agent for Cr III ions and passing an electric current between the anode and cathode at a cathode current density of not more than 2000 amps per square meter and a temperature of not more than 35°C for a period of not more than 3 minutes. The reference discloses the use of conductivity enhancing agents in col. 2, lines 24 to 29, which contain cations (e.g. ammonium, alkali, and alkaline earth elements) and "anions such as halide ...". Subsequently, however, the reference cautions against the use of one halide "such as fluoride ion" because "it has an exhaltant effect on Cr metal depositions" (cf. col. 3, lines 17 to 21). The high speed electrolytic coating of steel, or other metals used on an industrial scale, requires high current densities. Industry presently uses current densities somewhere in the range of about 86.4 Adm -2 (800 amps per square foot (ASF)) and seeks the advantage of a composition and a process for forming chromium oxide coatings on steel or other metals at this or higher current densities. Higher current densities would increase production rates or line speeds if bath compositions were available that would allow plating at these conditions. Industry also seeks the advantage of directly obtaining chromium oxide coatings from trivalent chromium compositions which have high surface area and may chemically bond to the coating so that other coatings such as organic coatings e.g., epoxy coatings, phenolic coatings and buff-vinyl coatings would adequately adhere to the chromium oxide substrate. Manufacturers also want to obtain the advantage of a composition and a method for electrolytically depositing chromium coatings from trivalent chromium compositions at plating efficiencies of from about 30 to about 40 percent or greater, and at current densities from about 54 Adm -2 to about 108 Adm -2 (about 500 to about 1000 ASF). The coating industry also wants the advantage of a composition and a process for depositing chromium oxide coatings in an amount up to or greater than about 18.6 mgdm -2 (2 mg/ft 2 ) and especially coating weights greater than about 3.72 mgdm -2 (0.4 mg/ft 2 ) that will provide excellent adhesion of coatings such as organic coatings e.g., epoxy coatings, phenolic coatings and buff-vinyl coatings and other coatings known in the art. These and other advantages are obtained according to the present invention which comprises a composition and process which substantially obviates one or more of the limitations and disadvantages of the described prior compositions, processes and products. The present invention comprises a water soluble composition and a process for electrolytically depositing chromium oxide coatings directly from trivalent chromium in which the foregoing and other disadvantages are overcome. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the composition and process, particularly pointed out in the written description and claims hereof. Summary of the Invention To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention comprises a water-soluble composition of matter free of an added buffering agent for electrolytically depositing a chromium oxide coating on a metal substrate comprising a mixture of a trivalent chromium compound, a weak chelating agent, an optional conductivity enhancing cation, a depolarizer comprising a compound containing a bromide ion, and an optional surfactant. In one embodiment, the conductivity enhancing cation comprises an alkali metal cation, the depolarizer comprises a bromide salt, and the weak chelating agent comprises a formic acid anion. In yet another embodiment of the invention, a water-soluble composition of matter that is free of an added buffering agent is provided for electrolytically depositing a chromium oxide coating on a metal substrate comprising: a trivalent chromium compound in an amount from about 0.03 to about 0.5 moles; a weak chelating agent in an amount from about 0.04 to about 0.7 moles; a conductivity enhancing cation in an amount from about 0.3 to about 5.0 moles; a depolarizer in an amount from about 0.01 to about 0.15 moles; and an optional surfactant. The ratio of chromium to chelating agent ranges from about 0.3 to about 3.0 mole ratio. Specifically, the conductivity enhancing cation may comprise a potassium cation, the depolarizer comprises a compound having a bromide ion, and the weak chelating agent may comprise a formic acid anion. In one embodiment, the trivalent chromium compound comprises basic chromium (III) sulfate. The invention also comprises a process of coating a metallic substrate employing the foregoing compositions, such as a substrate comprising a steel, chromium or tin substrate wherein the chromium oxide coated on these substrates is optionally coated with an organic coating such as an epoxy coating, a phenolic coating or a buff-vinyl coating. Detailed Description Tin Free Steel (TFS) also known as ECCS, and tin plated steel has a chromium oxide top layer to reduce corrosion and increase adhesion of paints, lacquers or organic coatings to the substrate. Chromium oxides form spontaneously or readily deposit from hexavalent chromic acid plating solutions; however, chromium oxides do not form when chromium is plated out of trivalent chromium baths which contain buffering agents such as boron oxides, described by Lavezzari as catalysts, in United States Patent No. 4,520,077. It has been discovered, according to the present invention, that boric acid and similar boron oxide compounds act as buffering agents to stabilize the pH of the composition during plating. The buffering agents help to stabilize the pH of the bath which is somewhere around 2.5 and promote the deposition of chrome metal in the electrolytic coating process. The buffering agents substantially minimize or eliminate any increases in pH that occur in the cathode film of the cell. The present invention, however, eliminates added buffers or boron oxide compounds from the trivalent chromium oxide composition in order to make the pH increase faster in the cathode film. This faster increase in pH allows for the direct formation of chrome oxides on the cathode. It was found that the trivalent chromium at higher pH's formed oligomers unlike hexavalent chromium compounds, and readily plate on most metallic surfaces directly to form a chromium oxide during the plating process. The composition of the present invention allows electrolytic deposition of coatings of chrome oxides on a conductive substrate wherein said substrate comprises not only iron or steel, but also coatings or substrates comprising chromium, nickel, tin, zinc, copper, aluminum, magnesium or titanium. These metals, as that term is used herein, include the alloys of the metals. As noted above, the metals also include metal coatings on a substrate. For example, the substrates can comprise a metal or an alloy as described above or a non-metal where either is coated with one or more of the foregoing metals. For example, a metallized ceramic or plastic or other non-metallic substrate that has an electrically conductive area can be coated according to the invention. The invention therefore comprises coating these substrates with the composition and by the process of the invention to obtain novel products as well. One of the preferred trivalent chromium compounds employed according to the present invention comprises basic chromium (III) sulfate (chrome tan) which has the formula CrOH SO 4 .Na 2 SO 4 .xH 2 O and contains 17.2 percent of chromium. Other trivalent chromium compounds employed according to the invention, and that are known in the art include those disclosed by Barclay et al., United States Patent No. 4,062,737 such as chromium (III) thiocyanate complexes; Benaben et al., United States Patent No. 4,612,091 who describe the use of trivalent chromium ions in a solution with a low pH; U.S. Patent No. 3,954,574; U.S. Patent No. 4,141,803; U.S. Patent No. 4,167,460; the trivalent chromium chloride salts disclosed by Lashmore et al., United States Patent No. 4,804,446; and the chromium complexes described by Tardy et al., United States Patent No. 4,460,438. Other specific trivalent chromium salts employed comprise chromium (III) formate, chromium (III) acetate, chromium (III) bromide hexahydrate, chromium (III) chloride hexahydrate, chromium (III) iodate, hydrate, chromium (III) nitrate, chromium (III) oxalate, chromium (III) orthophosphate, chromium (III) sulfate, hexamine chromium (III) chloride, hexaurea chromium (III) fluosilicate, chromium (III) fluoride tetrahydrate, chromium (III) iodide nonahydride, chromium (III) nitrate hexammonate, chromium (III) potassium oxalate, and the various art known equivalents thereof as well as, combinations thereof, especially the two, three component or four component combinations. The composition also includes an optional conductivity enhancing cation, especially an ammonium or alkali metal cation such as a sodium, potassium or lithium cation but especially a potassium cation. Employing a depolarizer in the composition substantially reduces or substantially eliminates the tendency of trivalent chromium compounds to oxidize at the anode to hexavalent chromium, the depolarizer comprising a compound containing a bromide ion as a depolarizer since it oxidizes more readily at the anode than the trivalent chromium ion because of its lower oxidizing potential. In theory, an iodide salt could also be used, although this would also result in liberation of iodine at the anode. Fluoride and chloride salts also oxidize at the anode and result in the evolution of halogen gases during the coating process. Additionally, using the proper anodes minimizes the oxidation of trivalent chromium to hexavalent chromium, such as carbon anodes as described by Benaben et al. in the United States Patent No. 4,612,091 or nickel-chromium, or platinum anodes as well as lead, graphite, platinized titanium and the like as described by Lashmore in United States Patent No. 4,461,680. The composition also includes a weak chelating agent such as a formic acid anion, typically a formate salt such as an alkali metal formate, e.g., potassium formate. Other useable chelating agents include either glycolic acid, ammonium formate, acetic acid, ferrous ammonium sulphate, propionic acid, polycarboxylic acids, especially the lower molecular weight dicarboxylic acids and the hydroxycarboxylic acids such as citric acid and the like and the various esters and salts of the foregoing acids including the low molecular weight alkyl alcohol esters, i.e., those having from 1 to about 4 carbon atoms and the various isomeric forms thereof and the alkali and ammonia and amine salts thereof, especially the lower molecular weight alkyl amine salts as that term is described herein. Various mixtures, especially the two component, three component, or four component mixtures of these compounds may also be employed. The chelating agent may comprise any of the various classes of weak chelating agents and specific compounds disclosed in Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Volume 5, pages 339-368. Chelating agents that are preferred comprise the aminocarboxylic acids and the hydroxycarboxylic acids. Some specific aminocarboxylic acids included in this respect comprise hydroxy-ethylethylenediamine-triacetic acid, nitrilotriacetic acid, N-dihydroxy-ethylglycine, and ethylenebis(hydroxy-phenylglycine). Tetra (lower alkyl) ammonium hydroxy compounds may also be employed where the lower alkyl group has from about 2 to about 6 carbon atoms such as tetrabutyl ammonium hydroxide. The chelating agents also include carboxylic acids that comprise tartaric acid, gluconic acid and 5-sulfosalicylic acid. The amino carboxylic acids used as chelating agents include lysine, alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, methionine, glycine, serine, threonine, cystenine, tyrosine, asparagine, glutamine, aspartic acid, glutamic acid, arginine, histidine and the like including the so-called rare amino acids, e.g., gamma-amino butyric acid, gamma-methyleneglutamic acid, 5-hydroxy lysine and the like. Salts and esters, as those terms are defined herein, of these acids may also be used. Mixtures of chelating agents may also be used, e.g., two or three or four component mixtures. The composition may include an optional surfactant such as an ethoxylated diamine as described by Shahin, United States Patent No. 5,294,326. Generally, the surfactants comprise the nonionic surfactants known in the art, and as described in Kirk-Othmer ( supra ), used in an amount up to about 300 ppm of the coating bath. The exact nature of the surfactant is not critical to the performance of the bath of the present invention, although better coating results are obtained when a small amount of surfactant is present. The coating process can be carried out over a pH range of from about 1 to about 4 and especially from about 2.2 to about 2.8. The coating temperature will be anywhere from about 20 to about 90 degrees Celsius and especially from about 30 to about 70 degrees Celsius. The current density may be anywhere from about 5.4 Adm -2 (50 ASF) to about 140 Adm -2 (1300 ASF) and especially from 32.4 to about 108 Adm -2 (300 to about 1000 ASF). Current density depends upon line speed in production. The following example is illustrative. An electrochemical method produced an adherent chromium oxide deposit on steel and chromium by cathodic treatment in a trivalent chromium solution in a cell having a carbon anode, employing the following composition: Basic chromium (III) sulfate 120 g/l Potassium chloride 250 g/l Potassium bromide 15 g/l Potassium formate 51.2 g/l Wetting agent 100 ppm The coating bath conditions comprise a pH of 2.5, a coating temperature of about 49°C., and a current density of 15 A/dm 2 . The coating cell employed comprised a beaker containing 1.4 liters of solution and 3 graphite anodes with a 0.95 cm diameter rod substrate arranged to provide a coated length of 5 cm. The coating process proceeded while maintaining constant temperature with stirring of the solution to prevent temperature gradients. Between 50 to about 100 mg of oxide as chromium metal/m 2 deposits in 1 to 5 seconds both on steel and chromium metal. The chromium oxide formation obtained by the present invention follows as a second step after depositing trivalent or hexavalent chromium on a steel substrate and especially TFS or ECCS and tin plated steel employed for tin cans as well as other metal substrates as noted herein. The trivalent chromium oxide process replaces the hexavalent process for coating tin and is also used for applying chromium oxides on steel and trivalent chromium whether decorative or functional, as a passivation coating. The chromium oxide coatings obtained optionally have organic coatings applied to them such as epoxy coatings, phenolic coatings and buff-vinyl coatings, especially chromium oxide coatings applied to steel, chromium or tin. Organic coatings successfully applied to the chromium oxide coatings obtained according to the invention comprised commercially available epoxy phenolic coatings, clear epoxy coatings, and buff vinyl coatings. Baking the coatings after application completed the process. A tape test on the epoxy coated panel showed that the coating had good adhesion to the oxide. The tape test comprised scribing a one inch by four inch coupon having the coating applied to it, and immersing the scribred coupon in a 1.5 wt.% sodium chloride/1.5 wt.% citirc acid water solution for four days. After air drying the coupon at room temperature for several days, a clear transparent tape, 3M 610, is firmly applied to the scored surface and rapidly removed after which, the tape is placed on a white paper background. By observing any coating removed an operator rates the adhesion visually as acceptable or unacceptable.

Claims: A water-soluble composition of matter free of an added buffering agent for electrolytically depositing a chromium oxide coating on a metal substrate comprising a mixture of a trivalent chromium compound, a weak chelating agent, an optional conductivity enhancing cation, a depolarizer comprising a compound containing a bromide ion, and an optional surfactant. The composition of claim 1 wherein: said weak chelating agent comprises a formic acid anion; said conductivity enhancing cation comprises an alkali metal cation; and said depolarizer comprises a bromide salt. The composition of claim 2 where said alkali metal cation comprises a potassium cation and said depolarizer comprises potassium bromide. The composition of claim 3 wherein said trivalent chromium compound comprises basic chromium (III) sulfate. The water-soluble composition of any one of claims 1 to 4 comprising: said trivalent chromium compound in an amount from about 0.03 to about 0.5 moles; said weak chelating agent in an amount from about 0.04 to about 0.7 moles; said conductivity enhancing cation in an amount from about 0.3 to about 5.0 moles; said depolarizer in an amount from about 0.01 to about 0.15 moles; and an optional surfactant. A process of depositing a chromium oxide coating on a metal substrate comprising electrolytically depositing on said substrate a composition as claimed in claims 1 to 5. The process of claim 6 wherein said substrate comprises iron, steel, or coatings or substrates comprising chromium, nickel, tin, zinc, copper, aluminum, magnesium or titanium. The process of claim 6 wherein said substrate comprises steel and said chromium oxide coating is optionally coated with an organic coating wherein said organic coating is an epoxy coating, a phenolic coating or a buff-vinyl coating. The process of claim 6 wherein said substrate comprises chromium, and said chromium oxide coating is optionally coated with an organic coating wherein said organic coating is an epoxy coating, a phenolic coating or a buff-vinyl coating. The process of claim 6 wherein said substrate comprises tin, and said chromium oxide coating is optionally coated with an organic coating wherein said organic coating is an epoxy coating, a phenolic coating or a buff-vinyl coating. The process of claim 6 where said coating is applied at a current density of from about 32.4 to about 140.4 Adm -2 (about 300 to about 1300 ASF).

Inventors: JONES ALLEN RAY SR, SHAHIN GEORGE EDWARD, JONES, ALLEN RAY, SR., SHAHIN, GEORGE EDWARD
Applicants: ATOTECH USA INC, ATOTECH USA, INC.
Application year: 1996
Main classification: C25D11
IPC codes: C25D11