Phosphate coatings are used on steel parts for corrosion resistance, lubricity, or as a foundation for subsequent coatings or painting. It serves as a conversion coating in which a dilute solution of phosphoric acid and phosphate salts is applied via spraying or immersion, chemically reacts with the surface of the part being coated to form a layer of insoluble, crystalline phosphates.
The main types of phosphate coatings are manganese, heat stable and zinc. Manganese phosphates are used both for corrosion resistance and lubricity and are applied only by immersion. Iron phosphates are typically used as a base for further coatings or painting and are applied by immersion or by spraying. Zinc phosphates are used for rust proofing a lubricant base layer, and as a paint/coating base and can also be applied by immersion or spraying.
The three types produced at Hohman Plating are:
- Zinc Phosphate: Zinc Phosphate has a large crystalline structure. The large crystal is an excellent medium for holding oil and adhering to paint. The phosphate also improves lubricity. Appearance is gray.
- Heat Stable Zinc Phosphate: Heat Stable Zinc Phosphate has a smaller crystalline structure than Zinc Phosphate. This smaller crystal provides for a smoother finish for paint. Corrosion resistance and resistance to detergents are also higher when compared to Zinc Phosphate. Appearance is gray.
- Manganese Phosphate: Used primarily on friction and bearing surfaces to prevent metal-to-metal contact and reduce wear. This coating is also used as a base for dry film lubricants. The crystal is smaller and smoother than zinc phosphate. Appearance is black. Hohman Plating utilizes both Rack and Bulk (basket or barrel) dip methods to apply these coatings.