International Lead Zinc Research Organization, Inc.

Bnf Metals Technology Centre

John J Ward

Clive Barnes

Direj Vallabhbhai Patel

An electrolyte solution and the method of using same for electrodeposition of black chromium on a substrate, said solution comprising water, trivalent chromium ions, ions of a metal selected from the group consisting of iron and cobalt, a weak complexing agent for the trivalent chromium ions, and phosphate ions. The preferred complexing agents are hypophosphite or formate.

Electrodeposition of black chromium

A method of depositing a protective chromite conversion coating is described. The chromite coatings produced by the method contain no Cr.sup.VI. The electrolyte used in the method comprises Cr.sup.III ions in a concentration of not more than 1 molar and a weak complexing agent for Cr.sup.III ions. The electrolyte, preferably, also contains conductivity salts. The method involves using a cathode current density of not more than 2000 amps per square meter and a temperature of not more than 35.degree. C. for a period of not more than 3 minutes. The chromite conversion coatings can be improved by aging and can be subsequently painted or lacquered. The Cr.sup.III electrolytes used are much less corrosive than Cr.sup.VI electrolytes and thus the substrates which can be coated include materials which cannot readily be chromate coated because they are reactive towards Cr.sup.VI electrolytes.

Electrolytic formation of chromite coatings

A nickel electroplating bath comprises in aqueous solution at a pH of from 4 to 7, nickel ions at a Molar concentration of at least 0.25, chloride ions at a Molar concentration of at least 0.25, and a weak complexant for the nickel selected from formate, acetate, citrate, glutamate, anions and lactones of sugar acids and anions and lactones of acids having the formula X(C.sub.n H.sub.2n)COOH where X is OH or NH.sub.2 and n is 1 to 5, preferably 1 to 2, present at a Molar concentration of from 0.5 to 4.0 times that of the nickel. These baths have good throwing power and can be operated at convenient temperatures and pH values and at lower nickel ion concentrations than hitherto.

Nickel plating

A method whereby a surface of chromium metal electrodeposited on a zinc substrate from a trivalent chromium electrolyte is treated by contacting the surface with an aqueous solution at a pH of from 5 to 12 and containing a dissolved metal salt of a weak acid which does not form a soluble complex with zinc. Preferred salts are carbonates, phosphates and silicates of an alkali metal, e.g. sodium or potassium. Also disclosed is the product of said process comprising an article having a zinc substrate, a microporous electrodeposited chromium coating from 0.025 to 2.5 microns in thickness on said substrate, and a water-insoluble basic salt of zinc within the micropores.

Treatment of chromium electrodeposit

An electrolyte bath and a method for using such a bath comprising trivalent chromium ions dissolved in an aqueous solution containing sulphide. The bath may also contain a weak complexing agent such as hypophosphite or glycine. The electrolyte according to the invention permits the electrodeposition of chormium from electrolytes having low solids content without adversely affecting plating rates.

Trivalent chromium plating baths

A method of producing chromium conversion coatings is described. The coatings are novel in that they are chromite i.e. Cr.sub.2 O.sub.3 coatings rather than the conventional chromate i.e. CrO.sub.3 coatings. The invention includes a novel electrolyte for depositing chromite layers. The electrolyte is an aqueous solution containing Cr.sup.III ions, a weak complexing agent for Cr.sup.III ions and a poison for the electrodeposition of chromium metal. The electrolyte preferably also contains conductivity salts and may include other additives such as fluoride ion and boric acid. Examples of poisons are Cr.sup.VI ions, peroxide, nitrate, polyamines, phosphates and formaldehyde. The chromite conversion coatings can be improved by aging and can be subsequently painted or lacquered. The electrolytes of the present invention are much less corrosive than Cr.sup.VI electrolytes and thus the substrates which can be coated include materials which cannot readily be chromate coated because they are reactive towards Cr.sup.VI electrolytes.

Chromite coatings, electrolytes, and electrolytic method of forming the

An aqueous trivalent chromium plating bath having low temperature stability comprising trivalent chromium ions preferably in a concentration of at least 0.2 M, sulphate ions preferably in a concentration of at least 0.3 M, a weak complexing agent for the chromium ions in a concentration of at least 0.1 M, and fluoride ions in a concentration of at least 0.025 M. The bath preferably also contains chloride ions in a concentration of at least 0.1 M.

Trivalent chromium electroplating baths and method

Levelling agents for use in zinc electroplating solutions are described comprising thallous ions or a combination of peptone and thiourea. The thallous ions are preferably added to a concentration of at least 1 mg/l and the peptone-thiourea combination added to a concentration of at least 100 mg/l peptone and 50 mg/l thiourea.

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Wantage, United Kingdom

John J Ward