Black oxide coating treatment is widely used as one kind of surface treatments for investment castings. Such surface treatment can help to an anti-rust layer to increase the lifetime and release the stress of the castings caused during hardening process. Surface of investment castings after black oxide will be smooth and glossy.
Normaly used black oxide coating treatment includes alkaline hot process and normal temperature oxide process.
*Alkaline hot oxide process is a kind of traditional black oxide coating treatment. Processes include 1)clearing, 2)removing oil, 3)acid pickling, 4)clearing, 5)oxidizing in the mixture of caustic soda, sodium nitrite and water, 6)clearing, 7)saponification, 8)clearing & drying, 9)applying oil.
*Black oxide coating in normal temperature condition is the process used recently. No requirement for hot process help to save power costs. The production time is shorted, thus to improve the work efficiency. With simple equipment requirements and low production costs, this process can be widely used for production, even for the ductil iron casting surface treatment. But it's not good to use this process for the low carbon steel investment castings. The processes include 1)clearing, 2)degreasing, 3)water clearing, 4)acid pickling, 5) water clearing, 6)black oxidizing, 7)water clearing, 8)drying, 9)applying oil.
Defects for black oxide coating treatment are piebaldness, light colour, and red colour. To solve these problems, products have to be reblack oxided again.