1.How to determine the critical humidity value?
Relative humidity is a key indicator: when the ambient humidity is greater than 60%, a continuous water film is easily formed on the steel surface, providing "electrolyte" for electrochemical corrosion;
If the humidity is greater than 85% and lasts for more than 48 hours, the thickness of the water film is sufficient to accelerate ion migration, and the risk of red rust outbreak increases sharply.
Special case: If there is condensed water, even if the short-term humidity does not reach 85%, the locally condensed liquid water may also trigger red rust.
2.How to determine the critical value of oxygen concentration?
When the ambient oxygen concentration is greater than 5% (the oxygen concentration in the air is about 21%, which is much higher than the critical value), the cathode reaction requirements can be met;
If the oxygen in the closed environment is not isolated, it will still slowly corrode when combined with the water film, and eventually red rust will break out.
3.How to determine the critical value of corrosive media?
Chloride ion: When the Cl⁻ concentration in the surface water film is greater than 50 mg/L, it will destroy the passive film on the steel surface, causing pitting and quickly expanding into red rust;
Sulfur dioxide: When the SO₂ concentration in the air is greater than 0.5 ppm (common in industrial plants), it dissolves in the water film to form sulfurous acid, making the water film pH less than 6, accelerating the dissolution of the anode iron;
Salt spray environment: According to the GB/T 10125 salt spray test standard, in neutral salt spray, unprotected steel usually develops red rust spots within 2 hours and breaks out within 24 hours.
4.How to determine the critical conditions of material state?
Surface damage threshold
If there are scratches, dents or burrs left over from processing on the steel surface, when the damage depth is greater than 50 μm, it will become a corrosion "active point". Even if the environmental corrosion is weak, red rust may break out first; if the surface oil and oxide scale are not cleaned up, they will form a local battery with the water film, accelerating local corrosion.
Critical value of composition and organization
High carbon steel is more prone to rust than low carbon steel: carbide iron matrix forms micro-batteries. When the carbide content is greater than 5%, the outbreak time of red rust is shortened by more than 30%;
If there are pores or loose organization in the casting, moisture and impurities will accumulate, becoming a "lurking point" for red rust, which can break out prematurely when the humidity is greater than 70%.
5.How to determine the critical conditions for failure of the protection system?
Failure threshold of galvanized layer
The galvanized layer is a sacrificial anode protection. When the thickness of the zinc layer is less than 5 μm, or the local non-plating area is greater than 0.5 mm², the steel substrate is exposed and red rust will spread from the non-plating point;
If white rust has appeared on the galvanized layer and the coverage area is greater than 30%, it means that the zinc layer is severely corroded and the remaining zinc layer is insufficient to protect the substrate. Red rust may break out within 1-2 weeks.
Coating failure threshold
Coating integrity is destroyed: When the scratch penetrates the coating and the length is greater than 5 mm, or the coating blister area is greater than 1 cm², red rust will break out from the damaged area within 1 month;
Coating aging: When the coating thickness drops to less than 20 μm due to wear and powdering, and the ambient humidity is greater than 70%, red rust will break out completely.