[Blackening]: usually immerse the metal workpiece in a strong oxidizing chemical solution (sodium hydroxide and sodium nitrite aqueous solution) and form a beautiful, dense and antirust black iron oxide film on the surface after a certain time.
[Phosphating]: the process in which metal workpieces (iron and steel or aluminum and zinc parts) are immersed in phosphating solution (some acid phosphate based solutions) and deposited on the surface to form a layer of crystalline phosphate conversion film insoluble in water.
The purpose of blackening is to prevent rust.
Blackening treatment purpose: the dense oxide film generated by blackening can prevent rust on the metal surface; Increase the beauty and luster of metal surface; Eliminate internal stress during quenching. Phosphating is mainly used for corrosion protection and primer for paint. The main purpose of phosphating is to provide protection for the base metal surface and prevent the metal surface from corrosion to a certain extent; Used for priming before painting to improve the adhesion and anti-corrosion ability of paint film layer; It plays the role of antifriction and lubrication in metal cold working process.
There are great differences between blackening and phosphating in process and reagent selection. Blackening is mainly made by boiling sodium hydroxide and sodium nitrite at 147 ~ 152 ℃. Its alkalinity is strong. The main components of phosphating are phosphoric acid and main salts, such as zinc, calcium, manganese, etc. The temperature varies from 5 to 95 degrees. The corrosion resistance of phosphating is much better than blackening.