1.What effect does low temperature have on corrosion resistance?
Corrosive medium state: Water is easy to condense into ice or frost, liquid water decreases, oxygen solubility decreases, and the electrochemical corrosion rate decreases.
Coating state changes: Most organic coatings have limited molecular chain movement, reduced flexibility, increased brittleness, and if there are tiny cracks, low temperature may cause crack expansion.
Corrosion resistance performance:
Short-term corrosion resistance is good, the substrate rust rate is slow, and the main risk is that the coating loses its barrier properties due to brittle cracking.
Differences between different resins: After a freeze-thaw cycle at -20℃, the adhesion of ordinary PE coatings may drop from 5N/cm at room temperature to below 3N/cm, and the risk of crack expansion increases; while fluorocarbon coatings have strong molecular chain rigidity and good frost resistance, and the adhesion decreases by only 10%-15%, and crack expansion is extremely slow.
2.What effect does room temperature have on corrosion resistance?
Corrosive medium state: Water exists in liquid state, oxygen solubility is moderate, and electrochemical corrosion reaction proceeds stably.
Coating state change: The coating is in the best stable state - the molecular chain moves smoothly, the adhesion and barrier properties are maintained at the designed level, and the coating itself ages slowly.
Corrosion resistance performance:
Corrosion resistance is the most stable and meets the design life of the coating itself.
Corrosion is mainly concentrated in coating defects, manifested as "local pitting", and the expansion speed is very slow.
3.What effect does high temperature have on corrosion resistance?
Corrosive medium state: Water evaporates quickly, but under high humidity and high temperature, liquid water is still sufficient, and the oxygen diffusion rate is accelerated, and the electrochemical corrosion rate is 2-3 times that of normal temperature; at the same time, high temperature will accelerate the penetration of salt.
Changes in coating state:
The thermal motion of the molecular chain of the organic coating intensifies. If the cross-linking density is low, "thermal oxidation aging" may occur - the coating changes color, powders, and the barrier property decreases.
The adhesion between the coating and the substrate may decrease due to the difference in thermal expansion coefficients, and even "blistering" may occur.
4.What effect does low humidity have on corrosion resistance?
Corrosive medium state: There is very little liquid water in the air, and it is difficult to form a continuous water film. Oxygen and salt cannot be effectively dissolved, and the corrosion reaction is almost stagnant.
Coating state change: The coating may shrink slightly due to drying, but the molecular chain stability is not affected, and the adhesion and barrier properties remain good.
Corrosion resistance performance: The corrosion resistance is the best, and it is the scene with the slowest corrosion rate in all environments: even if the coating is scratched, it will take several years for the substrate rust to expand significantly.
The only risk is that the coating is slightly powdered due to long-term drying, which mainly occurs in ordinary PE coatings containing low weathering additives.
5.What effect does high humidity have on corrosion resistance?
Corrosive medium state: Moisture in the air can easily form a continuous water film on the surface of the coating, and can dissolve CO₂, SO₂, and salt in the air to form an "electrolyte solution", which accelerates the electrochemical corrosion of the substrate.
Coating state changes:
Water will penetrate into the coating. If the cross-linking density of the coating is low, "swelling" may occur, resulting in decreased adhesion.
High humidity will accelerate the hydrolysis of the coating, reduce the molecular weight of the coating, loosen the structure, and suddenly reduce the barrier properties.
Corrosion resistance performance:
Corrosion is concentrated in the "coating defect + high humidity" area, manifested as large-area blistering and pitting expansion: ordinary PE coatings may blister in 6 months in a high humidity environment, and the rust area in 1 year is 20%; while HDP or PVDF coatings have high cross-linking density and strong hydrolysis resistance, and the rust area in 1 year is usually less than 5%.