Surface treatment of parts of ferrous alloys

Claim 1

A process for the treatment of the surface of a ferrous alloy, comprising the steps of a. depositing a copper-tin alloy on the said surface by electrolysis in a bath containing monovalent copper ions, tetravalent tin ions, an alkali metal hydroxide, an alkali metal cyanide, alkali metal carbonates, alkali metal stannites, and an alkali metal citrate, and b. subjecting the coated surface to a diffusion heat treatment at a temperature between 500° and 580°C., the mass ratio of tin metal to copper metal in the bath being between 1.5 and 4, the alkali metal citrate content of the bath being between 0.1 and 0.3 gram of the said citrate per gram of copper metal in the bath, the concentration of alkali metal stannite in the bath being between 15 and 30 mg per liter, the ratio of free alkali metal hydroxide to free alkali metal cyanide being between 1 and 1.5, the concentration of free alkali metal hydroxide of the bath being between 12 and 20 grams per liter, the concentration of alkali metal carbonates in the bath being between 30 and 200 grams of carbonates per liter, the pH of the bath being between 13.5 and 14, this pH being maintained between these two values by varying the alkali metal hydroxide content of the bath, the temperature of the bath being between 50° and 60°C., the cathodic current density being between 3 and 10A/dm 2 , the anodic current density being between 1.5 and 7A/dm 2 , the diffusion heat treatment being effected in a neutral or reducing atmosphere for a time between 1 and 3 hours.