page_title: Stainless steel
section_title: Corrosion resistance
hierarchical_section_title: Stainless steel / Corrosion resistance
caption_attribution_description: English: Alminium brass (top), copper-nickel 70/30 (middle) and stainless steel 304 (bottom) samples after two weeks in 3.5% NaCl.Magyar: Alumínium-sárgaréz (felső sor), 70/30 kupronikkel (középső sor) és 304-es rozsdamentes acél minták felülete, miután két hétig álltak 3,5%-os NaCl-oldatban.
context_page_description: Stainless steel is a family of iron-based alloys that contain a minimum of approximately 11% chromium, a composition that prevents the iron from rusting, as well as providing heat-resistant properties. Different types of stainless steel include the elements carbon, nitrogen, aluminium, silicon, sulfur, titanium, nickel, copper, selenium, niobium, and molybdenum. Specific types of stainless steel are often designated by a three-digit number, e.g., 304 stainless.
  Stainless steel's resistance to ferric oxide formation results from the presence of chromium in the alloy, which forms a passive film that protects the underlying material from corrosion attack, and can self-heal in the presence of oxygen. Corrosion resistance can be increased further, by:
  increasing the chromium content to levels above 11%;
  addition of 8% or higher amounts of nickel; and
  addition of molybdenum.
  The addition of nitrogen also improves resistance to pitting corrosion and increases mechanical strength. Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure.
context_section_description: Unlike carbon steel, stainless steels do not suffer uniform corrosion when exposed to wet environments. Unprotected carbon steel rusts readily when exposed to a combination of air and moisture. The resulting iron oxide surface layer is porous and fragile. In addition, as iron oxide occupies a larger volume than the original steel, this layer expands and tends to flake and fall away, exposing the underlying steel to further attack. In comparison, stainless steels contain sufficient chromium to undergo passivation, spontaneously forming a microscopically thin inert surface film of chromium oxide by reaction with the oxygen in the air and even the small amount of dissolved oxygen in the water. This passive film prevents further corrosion by blocking oxygen diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal. This film is self-repairing, even when scratched or temporarily disturbed by an upset condition in the environment that exceeds the inherent corrosion resistance of that grade.
  The resistance of this film to corrosion depends upon the chemical composition of the stainless steel, chiefly the chromium content. It is customary to distinguish between four forms of corrosion: uniform, localized (pitting), galvanic, and SCC (stress corrosion cracking). Any of these forms of corrosion can occur when the grade of stainless steel is not suited for the working environment.
  The designation "CRES" refers to corrosion-resistant steel. Most, but not all, mentions of CRES refer to stainless steel—non-stainless steel materials can also be corrosion-resistant.
caption: Stainless steel (bottom row) resists salt-water corrosion better than aluminium-bronze (top row) or copper-nickel alloys (middle row)
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