Phosphating

the creation of a conversion coating of insoluble phosphates (phosphate coating) on metal articles. When covered by paint or varnish, the conversion coating inhibits corrosion. Phosphating is used mainly on low-alloy and carbon steels and on cast iron. The phosphate coating (thickness, 2–5 micrometers) retains lubricant, a feature that acts to lower the coefficient of friction. Owing to their high electrical resistivity, the coatings can withstand potential differences of 300–500 volts, and they are stable at temperatures up to 400°–500°C.

Phosphating is carried out by immersing the article in a solution of iron, manganese, zinc, and cadmium phosphates that has been heated to 90°–100°C. Industry produces manganese ferric phosphate mixture in a form designed expressly for phosphating. The process usually takes approximately one hour. After being dried, the article is passivated in a weak chromate solution. Phosphating can also be carried out electrochemically with alternating or direct current; here, the process takes 15–20 min.