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What causes pitting on stainless steel?

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What causes pitting on stainless steel?

May 21, 2019

Concept of stainless steel pitting

The excellent corrosion resistance of stainless steel is due to the formation of invisible oxide film on the surface of the steel, making it dull. The passivation film is formed by the reaction of steel with oxygen when exposed to the atmosphere or by contact with other oxygen-containing environments. If the passivation film is destroyed, the stainless steel will continue to corrode. In many cases, the passivation film is only damaged locally on the metal surface, and the role of corrosion is to form small holes or pits, which produce irregular pit-like corrosion on the material surface.

Factors causing pitting corrosion of stainless steel

The pitting corrosion is most likely due to the presence of chloride ions combined with the depolarizer. The pitting of passive metals, such as stainless steel, is usually caused by the local destruction of passive films by some aggressive anions. Protection of a dull state with high corrosion resistance usually requires an oxidizing environment, but this is also the condition for pitting. Pitting medium is the presence of heavy metal ions such as Fe3+, Cu2+, and Hg2+ in CI-, Br-, l-, and C104- or chloride solutions containing Na+, Ca2+, and alkaline earth metal ions such as H2O2, O2, etc.

The pitting rate increases with temperature. For example in the concentration of 4% ~ 10% sodium chloride solution, pitting was achieved at 90 ℃ has caused the most weight loss; For thinner solutions, the maximum occurs at higher temperatures.

A method for preventing pitting of stainless steel

1. Avoid concentration of halogen ions.

2. Ensure the uniformity of oxygen or oxidizing solution, stir the solution and avoid static state.

3. Increase oxygen concentration or remove oxygen.

4. Increase the PH value. An obviously alkaline chloride solution causes less pitting or no corrosion at all, as opposed to neutral or acidic chloride solutions.

Work at the lowest possible temperature.

6. Add passivation agent into corrosive medium. Low concentrations of nitrates or chromates are effective in many media (inhibiting ion preferential adsorption on metal surfaces, thus preventing chloride ion adsorption and corrosion).

7. Cathodic corrosion protection. There is evidence that stainless steel with cathodic protection electrically interposed with mild steel, aluminum or zinc does not cause pitting corrosion in seawater.

Austenitic stainless steels containing 2% ~ 4% molybdenum have good pitting resistance. The use of molybdenum-containing austenitic stainless steel can significantly reduce pitting or general corrosion in corrosive media such as sodium chloride solution, sea water, sulfite, sulfuric acid, phosphoric acid and formic acid.

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