fosforcoating

Fosforcoating, or phosphate coating, is a chemical conversion coating applied to metal surfaces, most commonly steel, to create a porous layer of metal phosphate. This layer improves the adhesion of paints and primers, reduces corrosion under coatings, and can provide lubricity for certain applications.

The coating is formed by reacting the metal surface with phosphate-containing solutions, resulting in a thin, insoluble phosphate film. The bath composition determines the resulting coating: zinc phosphate, manganese phosphate, iron phosphate, or, for aluminum, aluminum phosphate. Zinc phosphate coatings are widely used in automotive and appliance industries for paint adherence; manganese phosphate coatings are harder and more lubricious, suitable for fasteners and sliding parts; iron phosphate coatings serve as a base for primers and provide a uniform finish.

Process steps typically include surface preparation through degreasing and descaling, rinsing, immersion in the phosphate conversion bath at controlled temperatures and times, thorough rinsing, sealing or neutralizing to reduce porosity, and finally drying and application of primer or paint. Sealing treatments can involve non-chromate formulations to improve corrosion resistance.

The coating itself is not a primary corrosion barrier but enhances corrosion resistance in combination with a topcoat and improves coating quality by reducing micro-porosity. It also improves paint adhesion and can impart lubricity in certain manganese phosphate applications. Limitations include the need for careful bath control, waste management concerns related to phosphates and metals, and regulatory requirements; modern processes often use environmentally friendlier sealants and technologies.

In industry, fosforcoating is a standard pretreatment step in automotive, construction, and general metal fabrication, contributing to longer service life and better paint performance.