What Prevents Stainless Steel Corrosion?

Stainless steel tanks are commonly utilized in manufacturing operations requiring cleanliness and purity.

FREMONT, CA: Stainless steel employs the passivation principle, which causes metals to become "passive," or unreactive to oxidation from corrosive substances found in the atmosphere and process fluids. Stainless steel is coated with a thin layer of metal oxides, referred to as the passive film.

Stainless steel is predominantly ferrous in composition, alloyed with a minimum of 10.5 percent chromium. Chromium oxides are the metal oxides that produce the passive film. Additional alloying elements such as carbon, nickel, manganese, and molybdenum are present. Carbon is the primary alloying element used in the production of steel from pure iron. Steel becomes more complex and more robust when a particular percentage of this element is added. Nickel and manganese are metallurgical stabilizers that promote an austenitic structure. Stainless steel's austenitic nature prevents it from hardening during heat treatment. This allows stainless steel to withstand elevated temperatures while retaining mechanical qualities such as flexibility.

Manganese has only about half the nickel effect and is frequently used in place of nickel to make lower-cost grades. On the other hand, molybdenum serves the same purpose as chromium. Because molybdenum is a larger atom than chromium, it is more effective at strengthening steel, particularly at elevated temperatures. Additionally, it improves the material's corrosion resistance. The use of Molybdenum has the disadvantage of making stainless steel ferritic, which is characterized by being more brittle. Additionally, austenitic stainless steels are more robust at low temperatures than ferritic stainless steels. More nickel is added to counteract this.

Passivation

An essential material, stainless steel, is exposed to air to generate metal oxides on its surface. To enhance the production of the passive coating, the stainless steel is carefully cleaned in acidic passivation baths containing nitric acid. Exogenous iron or free iron compounds are eliminated to prevent interference with the passive layer formation. A neutralization bath follows this with aqueous sodium hydroxide. Descaling is also used to remove oxide layers generated by high-temperature milling processes like hot-forming, welding, and heat treatment, among others.