Method of nitriding ferrous metal parts having improved corrosion resistance

摘要

In a nitriding method intended to confer upon ferrous metal parts, in addition to surface properties resulting directly from nitriding, corrosion resistance comparable to that obtained when the nitriding treatment is followed by an oxidizing treatment, in particular in a salt bath, the parts are treated by immersion for an appropriate time in a molten salt bath containing in the known manner essentially alkali metal carbonates and cyanates and a small quantity of a sulfur-containing substance. The parts are held at a positive potential relative to a counter-electrode in contact with the bath so that a high current flows through the bath from the parts to the counter-electrode and the concentration of cyanides formed by secondary reaction is kept below 6%. It is preferable to use a constant average current; the typical current densities are from 300 amperes to 800 amperes per m2, the typical temperature ranges are from 450 DEG C. to 650 DEG C. and the typical treatment times are from 10 minutes to 150 minutes.