Why is stainless steel not rusted or easily rusted?

December 18, 2023

Many people wonder why stainless steel does not rust or rusts slowly. The definition of "rust" is "corrosion or rust." Rust is the discoloration or corrosion induced by the chemical or electrochemical interaction of steel's surface with oxygen, moisture, acid, alkali, salt, and other chemicals in the atmosphere. The surface substance formed is "rust," which is iron oxide.

The presence of more than 12.5% chromium in the matrix is responsible for stainless steel's resistance to corrosion. Chromium may quickly develop a dense passivation coating on the steel surface in the oxidative corruption and inability to corrode medium to protect the metal matrix. When the chromium concentration exceeds 12.5%, a thick and stable passivation coating is created, anti-rust performance improves dramatically, and rust resistance is considerably improved. This is why stainless steel has a 12% chromium content. For the reasons stated above.

Stainless steel is, as the name suggests, stainless steel. In this context, "stainless" is relative to carbon steel, not absolute. Stainless steel is not easily rusty, but it is less likely to be corroded and rusted than carbon steel under the same conditions and environment.

When chromium is added to stainless steel, together with the appropriate quantity of alloying elements such as Ni, Mo, V, and so on, the anti-rust performance improves. As a result, austenitic stainless steel outperforms martensitic stainless steel in terms of anti-rust ability. Second, the corrosion resistance level of stainless steel is also affected by the carbon, chromium, and surface treatment condition. The anti-rust performance of stainless steel declines as the carbon content increases, thus 3Cr13 has a weaker anti-rust performance than 2Cr13. The anti-rust efficacy improves as the steel content increases. The rust prevention capacity is also affected by the surface treatment technique and condition. The chrome-plated knife and scissors surface, electrolytic polishing, mirror polishing, abrasive belt, and hand polishing have all reduced anti-rust effectiveness.

Because chromium is included in the composition, stainless steel does not rust. When the chromium content exceeds 12%, a passivation layer forms on the surface of the stainless steel. This passivation coating may isolate oxygen and prevent iron and oxygen from being present in the stainless steel composition. Contact weakened it.

The stainless steel passive film has three properties. The thickness of the passive film is very thin, only a few microns; the specific gravity of the passive film is greater than that of the substrate; and the chromium concentration of the passive film is more than three times that of the substrate. Because of these three features, the passivation coating is thin, thick, and extremely anti-corrosive, resulting in a high corrosion resistance.

The stainless steel application environment is quite complex, and depending exclusively on the passivation coating will not provide complete corrosion protection. To enhance the passivation coating, molybdenum (Mo), copper (Cu), nitrogen (N), and other elements must be added to stainless steel. Forming to increase corrosion resistance.

Furthermore, stainless steel's corrosion resistance is relative. Under certain situations, stainless steel's corrosion resistance is lost. As a result, stainless steel is not completely rust-free. This must be said unequivocally.