Phosphating

Phosphating, also known as phosphate coating or conversion coating, is a chemical process that converts the surface of metal—primarily iron and steel—into a crystalline phosphate layer. The coating provides corrosion protection and improves adhesion for paints and lubricants.

Common types include zinc phosphate coatings, iron phosphate coatings, and manganese phosphate coatings. Zinc phosphate is widely used in automotive and general metal finishing; iron phosphate is typical for steel as a base for primers; manganese phosphate offers higher hardness and is used to improve wear resistance and lubricity. The process can be carried out as hot phosphating (high-temperature baths) or cold phosphating (ambient temperature).

Typical process steps include surface preparation (degreasing and cleaning, sometimes pickling), immersion in a phosphating bath, thorough rinsing, drying, and, in many cases, sealing or applying a follow-up coating. Seals often involve chromate or non-chromate treatments to further enhance corrosion protection.

The phosphate layer is a thin, insoluble, crystalline film that adheres to the metal surface and provides a roughened substrate that improves paint adhesion and can reduce the spread of corrosion. It does not replace the need for protective coatings but serves as an efficient pretreatment.

Environmental and safety considerations include the handling of acidic phosphate baths and potential use of chromium-based sealants. Modern practices increasingly use trivalent chromium or non-chromate sealants to reduce environmental impact.