procedures for the application of a chromidueberzuges by kontaktabscheidung with, where appropriate, zusaetzlicher aeusserer cmh on metallkoerpern

摘要

An article having a melting point of at least 700 DEG C. and containing at least 50 mol per cent of at least one of the metals copper, silver, gold, molybdenum, tungsten, rhenium, iron, cobalt, nickel or the six platinum group metals is chromized by electrolysis as cathode at a cathode current density not exceeding 1 amp./sq.dm. with a chromium anode of a fused salt bath consisting of one or more alkali metal fluorides and 0.5 to 50 mol per cent total of one or more alkali metal chromous fluorides at a temperature of at least 700 DEG C. but below the melting point of the article and in an oxygen-free atmosphere e.g. vacuum or an inert gas. Typically the bath is prepared under vacuum by completing a circuit between a metal cathode and a chromium in a melt consisting of 1600 gms of a ternary entectic formed on a mol per cent ratio basis of 45:45:10 of KF:LiF:NaF, to which is added 150 gms of potassium chromic fluoride. The self-exciting electrolytic reduction of the chromic fluoride to the chromous state is assisted by applying a small voltage to provide a potential across the bath of the order of 0.03 to 0.05 volt, the bath being held at a temperature in the range of 670 to 700 DEG C. and a current of around 0.5 A being passed for 1/2 hour. The bath containing the chromous fluoride is then electrolyzed under an argon atmosphere with a steel strip article cathode and a chromium anode at 900 DEG C. with a small but increasing applied voltage after a short self-exciting treatment period to maintain current of 50 mA, providing a cathode current density of about 0.5 A/sq.dm, to give a total treatment period of about 5 hours, resulting in a chromized layer of 0.8 mil on both sides of the article. Similar treatments with differing resulting thicknesses of chromized layer are described for nickel, platinum, and molybdenum strip, and reference is made to treating iron electroplated with nickel.