The role of zinc powder in zinc-rich paint

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Zinc powder is the most important electrochemical rust-inhibiting pigment. A primer prepared with a large amount of zinc powder is called a zinc-rich primer. Zinc-rich paints are divided into two types: inorganic zinc-rich paints that use silicates as binders, and organic zinc-rich primers that use epoxy resins as binders. Compared to other metals, zinc has unique properties: it is lighter than iron, possesses good ductility, and can be alloyed with iron. Most importantly, it has high electrochemical activity, with a standard electrode potential of (-0.76V), making it more reactive than iron (-0.44V). Zinc can be melted and processed to purify it into high-purity zinc powder in the form of fine particles, which is used as the most important rust-inhibiting pigment in anti-rust paints. When the coating is eroded, zinc powder acts as the anode and is corroded first, while the base steel serves as the cathode and remains protected. The oxide products formed by zinc acting as a sacrificial anode serve to seal the coating, thereby enhancing the coating’s protection of the substrate. In zinc-rich coatings, zinc powder is gradually consumed during this protective process, but at a slow rate. The formation of its corrosion products reduces the potential difference between the coating and the substrate; when the paint film is damaged, new zinc metal is exposed, causing the potential difference to increase immediately, thereby providing strong cathodic protection. Therefore, rusting in zinc-rich paint does not spread from the damaged area to surrounding areas.