Stainless steel alloys are designed to naturally self-passivate when exposed to environments containing sufficient oxygen. This process forms a protective chromium-rich oxide surface layer, which is crucial for the corrosion resistance of these alloys. Under normal conditions, such as exposure to air or aerated water, stainless steel will naturally self-passivate.

In some cases, passivation treatments may be specified, but it is important to assess whether they are necessary. For stainless steel to be passivated, the surface must be clean and free from contamination and scale resulting from welding operations.

Scale may need to be removed through pickling or mechanical abrasion. It is essential to understand that pickling and passivation are not the same. Pickling typically involves using mixtures of nitric acid and hydrofluoric acid, while traditional passivation relies solely on nitric acid. Nitric acid can also help remove light surface iron contamination and facilitate the passivation of the cleaned steel surface.

An alternative to nitric acid treatments is the use of citric acid. Citric acid  provides the necessary oxidizing conditions for passivation and is a safer method with environmental benefits, including reduced fume emissions and easier waste acid disposal. Passivation treatments using citric acid typically specify solution strengths of 4-10%, as outlined in ASTM A967.

Various specifications exist for passivation treatments of stainless steel. The American standards, such as ASTM A380 and ASTM A967 (based on the US Defense Department standard QQ-P-35C), have traditionally been used. In 1997, an alternative British Standard, EN 2516, was published, focusing on the passivation of corrosion-resisting steels and decontamination of nickel-base alloys.

Metal Glossary | Metals Definitions | Heat Treatment of Metals | Passivation | Annealing | Quenching | Tempering | Heat Treatment of Steel | Heat Treating Definition | Heat Treating Stainless Steel | Technic of Metals Heat Treatment | Elements in Annealed State | Bright Anneaing | ASTM A380 | ASTM A967 | EN 2516 | 304 | 304L | 321 | 316L | 317L | 310S | 410 | 410S

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