Zinc phosphate (Zn3(PO4)2) is an inorganic chemical compound used as a corrosion resistant coating on metal surfaces either as part of an electroplating process or applied as a primer pigment (see also red lead). It has largely displaced toxic materials based on lead or chromium, and by 2006 it had become the most commonly used corrosion inhibitor. Zinc phosphate coats better on a crystalline structure than bare metal, so a seeding agent is often used as a pre-treatment. One common agent is sodium pyrophosphate.
Zinc Phosphate used in Paints
It enhances paint adhesion by giving the paint something to bond to securely. It reduces paint-to-metal reactions and forms a barrier between the substrate and the paint. Zinc phosphate also reduces general surface corrosion on metal substrates and reduces underfilm delamination in the event of damage to the paint film.
There are three types of zinc phosphate coatings.
- Iron phosphate is a lightweight amorphous coating. It generally has a coating weight between 15-100 mg/ft2 and typically used as paint base for low-corrosion environments.
- A modified zinc phosphate coating is a medium weight crystalline phosphate coating. It has a coating weight between 150-350 mg/ft2. It is typically used as paint base for high-corrosion environments. Vanchem’s primary product is called FOSTEX ZN.
- A heavy zinc phosphate coating produces a very heavy coating weight, typically between 700-3,500 mg/ft2. It is used for unpainted applications.
Why is it used in Paint
Zinc phosphate is widely used in automotive and military applications. It is used to a lesser extent in general industry where high paint performance may not be required. Since neither iron phosphate nor green technologies can out-perform zinc phosphate, zinc phosphate will be in use for many years to come. The big change will come in the future when new technologies perform as well as zinc phosphate combined with an economic incentive of cost reduction. At that point, we will see a major shift to green technologies.
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