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Corrosion and surface treatment

What is the main reason stainless steel resists rusting in air and water?

  1. AAdding Ni makes the standard electrode potential of the alloy as noble as gold or platinum, so it hardly dissolves
  2. BA dense oxide scale several millimeters thick forms on the surface and seals it off from water and oxygen
  3. CIt contains about 11% or more Cr, forming a passive film a few nanometers thick that re-forms by itself
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Answer: C. It contains about 11% or more Cr, forming a passive film a few nanometers thick that re-forms by itself

Because stainless steel contains roughly 11% Cr or more (10.5% by the standard definition), a passive film consisting mainly of Cr oxide, only a few nanometers thick, forms on the surface and prevents further dissolution of the metal. What matters is that in environments containing oxygen the film spontaneously re-forms even when scratched. Passivity is therefore readily maintained in oxidizing environments such as nitric acid, but breaks down easily in oxygen-poor environments, chloride-rich environments and reducing acids (such as hydrochloric acid). The potential of the alloy as a whole does not become as noble as gold or platinum; the main role of Ni is to stabilize the austenitic structure. A thick oxide scale several millimeters thick forms by high-temperature oxidation, cracks and spalls easily, and is entirely different from a passive film.

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More Corrosion and surface treatment questions

  1. When SUS304 (JIS; AISI 304) is used in chloride-rich seawater, which correctly pairs the localized corrosion likely to occur with a countermeasure?
  2. Which statement about stress corrosion cracking is correct?
  3. Which of the following statements about the corrosion protection of steel sheet is correct?
  4. In a steel pipe, a corrosion current in which iron dissolves as Fe → Fe²⁺ + 2e⁻ flowed at an average of 0.10 A for one year (365 days). Which value is closest to the mass of iron dissolved? Take the molar mass of Fe as 55.85 g/mol and the Faraday constant as 96485 C/mol.
  5. When the corrosion current density of iron is uniformly 1.0 µA/cm² over the whole surface, which value is closest to the rate at which the surface recedes? Assume iron dissolves as Fe²⁺, with a molar mass of 55.85 g/mol, a density of 7.87 g/cm³, a Faraday constant of 96485 C/mol, and one year as 3.15×10⁷ s.