DE2350994

1435518 Electrolytically hardening a film formed on a chromium containing iron alloy INTERNATIONAL NICKEL Ltd 9 Oct 1973 [12 Oct 1972] 47048/72 Headings C7B and C7U The hardness of a porous film, formed on the surface of a Cr-containing Fe alloy by treatment in aqueous chromic and sulphuric acids, is increased by cathodic electrolysis in an electrolyte containing chromic acid and an agent (other than SiF 6 - or F- ions) capable of promoting the cathodic deposition of an oxidic Cr deposit in preference to metallic Cr. The bath may contain 0.25-7.5 mole/litre CrO 3 with a CrO 3 : agent mole ratio of 20-500:1. The agent may be Ct (from Cr Cl 3 ), NO 3 -(HNO 3 ), PO 4 3-(H 3 PO 4 ) or acetate (HAc). Current densities of 0.05-17.2 A/dm2 are disclosed for 2-30 minutes at 20-60‹C. The alloy treated may contain 18-20% Cr and 8-12% Ni. The deposit may be chromic oxide, hydrated chromic oxide and/or chromic hydroxide.