Pitting corrosion is the most common type of corrosion in stainless steel. In the surface of the local place to appear to the depth of the development of corrosion holes, the rest of the surface does not corrode or corrosion is very slight, this form is called small hole corrosion, referred to as pitting.
The concept of pitting corrosion
It is well known that stainless steel has excellent corrosion resistance because there is an invisible oxide film on the surface, making it passivated. If the oxide film is destroyed, stainless steel will continue to corrode. In many cases, the passivation film is destroyed only in localized places on the surface of the metal, and the corrosion is caused by the formation of tiny holes or pits, which produce irregularly distributed pitting corrosion on the surface of the material.
Factors causing pitting corrosion of stainless steel
1. Chemical environment
The appearance of pitting corrosion is likely to be the presence of chloride ions that are chemically compatible with the depolarizing agent. Stainless steel and other passivated metals pitting corrosion is often caused by some aggressive anions on the local damage to the passivation film. An oxidizing environment is usually required to protect passivates with high corrosion resistance, but this is precisely the condition in which pitting occurs. Pitting medium is in the CI-, Br-, l-, C104- solution in the presence of Fe3 +, Cu2 +, Hg2 + and other heavy metal ions or Na +, Ca2 + alkali and alkaline earth metal ions containing H2O2, O2 and other Na +, Ca2 + alkali and alkaline earth metal ions of chloride solution.
During sheet metal fabrication, the processing environment may introduce these aggressive ions. For example, if the water used for cleaning the sheet metal after fabrication contains high levels of chloride ions from improper water treatment, it can increase the risk of pitting corrosion on the stainless - steel surface. Also, in some industrial settings where sheet metal parts are used, exposure to chemical solutions with these aggressive ions can cause pitting.
2. Temperature
The pitting rate increases with temperature. For example, in the concentration of 4% to 10% sodium chloride solution, at 90 ℃ to reach the weight loss caused by pitting corrosion is the largest; for more dilute solution, the maximum value occurs at a higher temperature. In sheet metal fabrication, heat - related processes such as welding and heat treatment can raise the temperature of the stainless - steel sheet metal. If proper cooling measures are not taken or if the fabricated parts are exposed to high - temperature environments for extended periods, the likelihood of pitting corrosion increases.
Summary
The stainless steel surface oxide film and cause pitting corrosion phenomenon requires the formation of environmental conditions have similarities, so that the presence of aggressive elements in a particular environment to destroy the surface oxide film, after a long time, the oxide film can not be repaired, the erosion has been carried out, resulting in the formation of pitting corrosion phenomenon, and the temperature is to affect the rate of pitting corrosion is one of the main conditions.
Methods to prevent stainless steel pitting
1. Control the chemical environment
- Avoid halogen ion concentration: During sheet metal fabrication, ensure that the water and cleaning agents used do not contain high levels of halogen ions. For example, use deionized water for cleaning the fabricated stainless - steel sheet metal parts to reduce the risk of introducing chloride ions.
- Manage chemical exposure: When using sheet metal parts in various applications, be aware of the chemical environment they will be exposed to. If possible, choose less corrosive chemicals or implement protective measures such as coatings to prevent direct contact with aggressive substances.
2. Optimize processing conditions
- Ensure the uniformity of the oxygen or oxidizing solution: In processes like pickling or passivation during sheet metal fabrication, stir the solution and avoid the solution in a static state. This helps to ensure a uniform distribution of the oxidizing agent on the stainless - steel surface, promoting a more consistent passivation layer formation.
- Improve the concentration of oxygen or remove oxygen: Depending on the specific situation, adjust the oxygen level. For example, in some storage or transportation conditions of fabricated sheet metal parts, controlling the oxygen content in the environment can help prevent pitting. In some cases, adding an inert gas to displace oxygen can be beneficial.
3. Adjust environmental factors
- Increase the pH value: Compared with neutral or acidic chlorides, significantly alkaline chloride solutions cause less pitting, or none at all (hydroxide ions act as corrosion inhibitors). In sheet metal fabrication facilities, maintaining a proper pH level in the working environment or in any cleaning solutions can help reduce the risk of pitting corrosion.
- Work at the lowest possible temperature: Minimize the use of high - temperature processes during sheet metal fabrication. If high - temperature operations such as welding are necessary, implement rapid cooling methods to reduce the time the stainless - steel is exposed to elevated temperatures.
4. Add protective agents
- Add passivation agent: In some corrosive media, add low concentrations of nitrates or chromates. This is effective in many media as these agents inhibit ions preferentially adsorbed on the metal surface, thus preventing corrosion caused by chloride ion adsorption. When storing or transporting fabricated stainless - steel sheet metal parts, appropriate passivation agents can be used in the storage environment.
- Use cathodic corrosion protection: There is evidence that with mild steel, aluminum or zinc electric cornering combined cathodic protection of stainless steel in seawater does not cause pitting. In applications where sheet metal parts are used in harsh environments such as near the sea, consider implementing cathodic protection systems.
Note: Containing molybdenum 2% to 4% of austenitic stainless steel has good pitting resistance. The use of molybdenum austenitic stainless steel can significantly reduce pitting or general corrosion in corrosive media such as sodium chloride solution, seawater, sulfurous acid, sulfuric acid, phosphoric acid and formic acid. When selecting materials for sheet metal fabrication, considering this type of stainless steel can enhance the corrosion resistance of the final product.