Black oxide coating for stainless steels

Abstract

Abstract A method of blackening ferritic and/or martensitic stainless steels which comprises treating said steels by (a) immersing said steel in a fused alkali metal hydroxide-salt bath, which as conventionally produced may initially contain from 3 to 8% water of hydration, for a period of 1-30 minutes, at a temperature of 540*-720*F., and (b) continually adding water during the heat treatment in step (a); said water can be introduced in the form of steam to replenish the original water of hydration of the fused bath and the amount of water added ranges from an effective amount to about 15% by weight of the fused salt mixture. This addition of water in the ranges 1-15% and 3-8% replenishes and reinvigorates the molten salt bath so that the bath likewise has a satisfactory water content based on the molten salt mixture. A preferred fused salt mixture is the eutectic mixture of sodium and potassium hydroxide, and an additive agent such as an alkali metal molybdate may be added to the fused salt mix.

Inventors

• 👤 Thomas J Smollett
• 👤 Bozidar Stipanovic

Bibliographic Details