What Does Phosphate Pigment Mean?
A phosphate pigment is an inorganic compound primarily containing thin layers of phosphate (PO43-) ions that firmly adhere to the surface of a metallic substrate. These coatings are classified as a type of conversion coating, where the phosphoric acid solution containing ions of other metals (e.g., magnesium, zinc or iron) chemically react with the metal's surface to produce a layer of insoluble crystalline metal-phosphates. This layer acts as an effective barrier that protects the underlying metal from corrosion.
A phosphate pigment is also known as a phosphate coating.
Corrosionpedia Explains Phosphate Pigment
Phosphate pigments are one of the most common types of conversion coatings. The phosphate coating gives rise to a film on the metal's surface that is a direct result of a chemical reaction between the substrate and the phosphate pigment compounds. This is different from conventional anti-corrosion coatings that are applied to the substrate without altering its chemical state.
Phosphate pigments are, however, porous in nature and therefore cannot provide corrosion resistance on their own. Rather, they are used as a form of surface preparation prior to applying other paints and coatings. The porosity of the coating allows other materials (such as zinc and magnesium) to seep into the phosphate coating and become permanently and mechanically interlocked upon drying.