Phosphate coatings are often used to provide corrosion resistance, however, phosphate coatings on their own do not provide this because the coating is porous. Therefore, oil or other sealers are used to achieve corrosion resistance. Zinc and manganese coatings are used to help achieve bigger corrosion resistance.
Most phosphate coatings serve as a surface preparation for further coating and/or powder or wet painting, a function it performs effectively with excellent adhesion and electric isolation.
The porosity allows the additional materials to seep into the phosphate coating and become mechanically interlocked after drying. The dielectric nature will electrically isolate anodic and cathodic areas on the surface of the part, minimizing underfilm corrosion that sometimes occurs at the interface of the paint/coating and the substrate.