Zinc powder is the most important electrochemical antirust pigment. A primer made with a large amount of zinc powder is called a zinc-rich primer. Zinc-rich paint is divided into an inorganic zinc-rich primer with silicate as binder and epoxy zinc-rich primer with epoxy resin as a binder. Compared with other metals, zinc has its unique characteristics. It is lighter than iron and has good ductility. It can be alloyed with iron. The most important thing is its electrochemical activity. Its standard electrode potential is -0.76V, which is livelier than Iron (-0.44V). Zinc can be melted and purified into fine particles of high-purity zinc powder, which is used as the most important anti-rust pigment in anti-rust paint.
When the coating film is corroded, the zinc powder is corroded as the anode first, and the substrate steel is the cathode, which is protected. The oxidation product formed by the zinc as the sacrificial anode has a sealing effect on the coating and can still strengthen the coating protection to the substrate. In zinc-rich coatings, zinc powder is gradually consumed during the protection process, but the speed is very slow. The formation of its corrosion products reduces the potential difference between the coating and the substrate. When the paint film is damaged, new metallic zinc is exposed, and the potential difference increases immediately, resulting in strong cathodic protection. Therefore, the rust of the zinc-rich primer will not spread from the damage to the surroundings.
In the zinc-rich primer, the content of zinc powder has good cathodic protection.
Zinc-rich primer can effectively protect the steel structure in a heavy corrosive environment, because it is well resistant to salt spray, humidity and heat, and can effectively prevent corrosion in a humid marine environment for a long time.
Zinc-rich primer is widely used in various constructions and is suitable for offshore installations, power plants, petrochemical plants and other aspects.
The water-based inorganic zinc-rich primer has the characteristics of non-toxic and low environmental pollution; the inorganic zinc-rich primer has the advantages of heat resistance, solvent resistance, chemical resistance, etc.
Like all paint coatings, zinc-rich paint is a surface coating combined with steel, with a mechanical strength of hundreds of pounds per square inch (psi). Zinc-rich coatings are organic ,which consist of epoxy resins, chlorinated hydrocarbons and other polymers; or inorganic based on organic alkyl silicates. The dry film thickness of organic or inorganic coatings is 2.5 to 3.5 mils (64-90 µm).
One of the common features of all zinc coatings is the cathodic protection provided. However, zinc-rich paint coatings are different from other paint coatings because of the presence of binding materials for binding zinc particles. In order for cathodic protection to be possible, the concentration of zinc powder must be high enough to provide conductivity between the zinc particles and the steel. This is another reason why constant stirring and a homogeneous mixture are important during application. If the zinc particles are encapsulated in a binder and the binder is non-conductive, there are still some questions about whether cathodic protection is completely possible.
The properties of inorganic and organic zinc-rich coatings are different. The inorganic zinc-rich coating, adhering to steel with good chemical reactivity, has good solvent resistance, and can withstand temperatures up to about 700℉(375℃). Inorganic zinc-rich coatings are not easily crushed, peeled or blistered and are easier to weld than organic materials, having a simpler cleaning effect. The density of the inorganic zinc-rich coating is about half of the zinc density per mil of the hot-dip galvanized coating.
The performance of organic zinc-rich coatings depends on the solvent system. Multiple coats can be applied within 24 hours without cracking. Coatings rich in organic zinc have a high temperature resistance of 200-300 F, so they do not have the same temperature resistance as inorganic zinc. They are also degraded by ultraviolet light (sunlight) and are not as effective as inorganic substances in terms of corrosion resistance.