Is Higher Zinc Content Better in Epoxy Zinc-Rich Primers?

The Role of Zinc in Epoxy Zinc-rich Primer Corrosion Protection

Epoxy zinc-rich primer needs to achieve a certain amount of zinc in dry film to ensure its electrochemical protection performance. Zinc powder is not in direct contact with zinc powder or base steel because at least one layer of monomolecular resin film covers the contact surface. In this way, the efficiency of current transfer depends on the conductivity and the thickness of the contact surface. Only when PVC (pigment volume concentration)/CPVC (critical pigment volume concentration) ≥ 1, can zinc powder be arranged tightly, and the thickness of the contact surface is small enough to form good conductivity. However, the higher the zinc powder content, the better. Excessive zinc powder content will cause the coating to form a porous structure, which will affect the adhesion and film density of the coating, speed up the consumption rate of zinc powder, and affect the long-term corrosion shielding performance of the coating.

Zinc-rich Primer Based on Film-forming Resin

Epoxy zinc-rich primer coating has relatively loose requirements for surface treatment and construction environment, good mechanical properties of the coating, and good electrochemical protection performance. Alcohol-soluble inorganic zinc-rich coatings and water-soluble inorganic zinc-rich coatings have higher requirements for surface treatment and construction environment. Under strict control of construction quality, better corrosion resistance than epoxy zinc-rich can be obtained, and they also have excellent temperature resistance. Epoxy-zinc-rich coatings use epoxy resin and epoxy curing agent as film-forming resins. Epoxy resin is usually E-20, E-44 or a combination of the two, and the curing agent is usually a polyamide curing agent. There are also polyamide adducts and cashew oil-modified phenolic amides. The quality of the curing agent has a great influence on the adhesion, compatibility and corrosion resistance of epoxy zinc-rich coatings. The use of polyamide adducts and high-quality cashew oil-modified phenolic amides as curing agents can obtain films with excellent performance, while using low-quality polyamide curing agents can result in poor film performance.

The Use of High Zinc Content in Epoxy Zinc-rich Primers

Especially in epoxy zinc-rich primer, when the zinc powder content is ≥80%, as the zinc powder content increases, the salt spray resistance does not necessarily increase, but in a matching coating system test, the zinc powder content value that reflects the best corrosion resistance of the matching system zinc-rich primers is higher than the best zinc powder content value in the test of a single zinc-rich coating. The reason is that the surface coating shields the external corrosive medium, which makes the corrosion of the matching system zinc-rich primer slower than that of the single zinc-rich coating directly exposed to the environment, effectively avoiding the rapid corrosion of the high zinc coating caused by high porosity. In the salt spray resistance test of a single coating, inorganic zinc-rich coatings are much better than epoxy zinc-rich primers, but in a matching coating system test, there is not much difference in the salt spray resistance of systems with the same zinc powder content. When the zinc powder content is high, if it is close to 90%, the salt spray resistance performance of inorganic zinc-rich is better.

The Zinc Powder Content of Epoxy Zinc-rich Primer Needs to Be Considered Comprehensively

The higher the zinc powder content, the better the protection of the paint film, that is, the better the corrosion resistance performance. The higher the zinc powder content, the more suitable it is for harsh painting environments. However, the higher the zinc powder content, the better is not always the case, it depends on the situation. Excessive zinc powder content will cause the coating to form a porous structure, which will affect the adhesion and film density of the coating, speed up the consumption rate of zinc powder, and affect the long-term corrosion shielding performance of the coating.

Mining machinery and underground equipment that frequently endure collisions; places with severe salt spray corrosion such as the sea, beach, and sea water; relatively humid places such as swimming pool steel structures; chemical industry equipment such as gas cylinders, oil tanks, and pipelines.

For other projects, ordinary epoxy zinc-rich primer with lower zinc content can achieve similar effects, which not only guarantees quality but also saves cost in raw material procurement.