A brief discussion on the corrosion prevention mechanism of zinc-rich coatings

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A brief discussion on the corrosion prevention mechanism of zinc-rich coatings: It is generally believed that the main corrosion prevention functions of zinc-rich coatings include electrochemical protection, shielding effect, self-repairing of the coating film, and passivation. 1. Loss of the protective effect of zinc powder: Zinc-rich coatings contain a large amount of zinc powder; the zinc particles are in close contact with each other as well as with the steel surface. Zinc has a more negative electrical potential than iron, so in an electrolyte solution, zinc atoms easily lose electrons and become the anode, while iron becomes the cathode. In the anode area, zinc is corroded as it loses electrons, while in the cathode area, the steel surface gains electrons and thus is protected. For the coating to conduct electricity and act as a sacrificial anode, it is necessary for the entire coating to have good electrical conductivity, so that an electrochemical circuit can be formed between the entire coating and the steel substrate; only in this way can the zinc-rich coating provide electrochemical protection. 2. Shielding effect The shielding effect of zinc-rich coatings relies primarily on the formation of insoluble salts and corrosion products. Traditional zinc-rich coatings use spherical zinc particles, which transfer electrons through point contact with one another and also maintain point contact with the steel substrate. Due to the point contact, the conductivity of the coating is poor. To increase its electrical conductivity, a large amount of zinc powder must be added; at this point, the volume concentration of zinc powder far exceeds that of the binder, and the binder is not sufficient to fill the gaps between the coating layers, resulting in a porous coating structure. These days, flake-shaped zinc powder is commonly used to enhance the physical shielding effect. The addition of flake-shaped zinc powder increases the penetration distance and creates numerous tiny areas within the coating, which reduces the difference in thermal expansion coefficients between the coating and the metal substrate. This leads to a lower rate of shrinkage during coating hardening and reduced stress within the coating, thereby preventing cracking and peeling. It also improves the coating’s adhesion and resistance to medium penetration, thus enhancing its corrosion resistance. The corrosion products of zinc vary depending on the corrosive medium; they include zinc oxide, zinc hydroxide, basic zinc carbonate, basic zinc oxide, zinc sulfate, etc. The formation of these substances causes volume expansion, which fills the gaps in the coating layer and thereby prevents further contact between the iron surface and harmful substances such as oxygen and water, serving as a physical barrier. At the same time, these corrosion products help to bind the coating layers together more tightly, increasing resistance and slowing down the rate of electrochemical corrosion; as a result, the rate of zinc powder consumption is reduced, and the durability of the coating is improved. 　　3. Self-healing property of the coating: When a part of the coating is damaged and the underlying metal is exposed, corrosion current can flow to the exposed steel area. Zinc deposits then form a protective layer in that area, thereby slowing down further corrosion. 　　4. Passivation: The coating provides good passivation for steel. During the film-forming process, as water evaporates, the pH value of the coating changes, and the electrode potential of the steel substrate is not always in a state of electrochemical protection; therefore, the coating offers effective passivation for the steel substrate. Therefore, by taking advantage of the anti-corrosion mechanism of zinc-rich coatings, we have solved many problems related to surface corrosion of metals.