2026-04-30
Studies have shown that with increasing chromium content in steel, its corrosion resistance increases. When the chromium content in steel reaches or exceeds 12%, a sharp change in corrosion resistance occurs under atmospheric conditions: the steel stops corroding, stops rusting - this phenomenon is called the transition from an active state to a passive state, from an active state to a passive state. Simply put, the passive state is actually a state in which the reaction between the stainless steel and the surrounding corrosive environment is slowed down, meaning the steel becomes insensitive to the environment.
Research shows that with a chromium content of ≥ 12%, steel acquires stainless properties due to the spontaneous formation of an extremely thin, colorless, transparent and very smooth oxide film enriched in chromium on the surface. The formation of this film prevents the steel from rusting. This film is called passive film.
Further research also showed that in oxidizing acidic environments, for example in nitric acid, with increasing chromium content in steel, the corrosion rate decreases, and when a sufficiently high chromium content is reached, the steel becomes corrosion resistant.
In oxidizing environments, the reason for the corrosion resistance of stainless steel is also the formation of a passive film on the surface. Similarly, the transition of steel in an acidic environment from no corrosion resistance to corrosion resistance is also called active to passive transition, active to passive state.
The oxide film is not only extremely thin, colorless, transparent and capable of spontaneous formation, but also very stable. Passive film is a continuous, non-porous, sparingly soluble, relatively hard and firmly adhered to the surface oxide (hydroxide), which, even if damaged during operation, is capable of quickly self-healing.
Research also shows that the chemical composition, structure and properties of the passive film vary depending on the chemical composition of the stainless steel, processing method and operating conditions. With increasing chromium content in steel, the passive film passes from the crystalline state to the amorphous one. Since amorphous film contains fewer defects, has a uniform surface structure and is easier to enrich with chromium, it has higher strength and corrosion resistance compared to conventional crystalline film.
