Mechanism and Development of Pitting Corrosion in Stainless Steel
1. Introduction to Pitting Corrosion
The figure below demonstrates an example of pitting corrosion observed in SUS304 stainless steel. After removing the resin film from a drilled hole, the material was immersed in an NaCl solution. The corrosion occurred at the exposed area and propagated into the interior of the stainless steel substrate.
This localized corrosion is identified as pitting corrosion. It typically forms rust on the outside of the stainless steel surface, while the interior develops a cavity due to metal loss (Figure 34).
![Figure 34: Pitting Corrosion in SUS304 Stainless Steel]
2. Mechanism of Pitting Corrosion
The mechanism of pitting corrosion is illustrated in Figure 35. The experiment was conducted by immersing SUS304 stainless steel in a neutral NaCl solution (pH 7).
The process begins with the passive dissolution of the protective passive film. The dissolution reaction is as follows:
O₂ + 2H₂O + 4e⁻ → 4OH⁻
Once the passive film is compromised, the exposed metal surface undergoes further electrochemical reactions.
-
The anodic reaction (metal dissolution):
Fe → Fe²⁺ + 2e⁻ -
The cathodic reaction (oxygen reduction):
O₂ + 4H⁺ + 4e⁻ → 2H₂O
The anodic area within the pit becomes highly active, resulting in an accumulation of metal ions (e.g., Fe²⁺). These ions react with Cl⁻ from the solution to form aggressive compounds such as FeCl₂, which lower the local pH and promote further corrosion.
When the local pH drops below 4, the passive film cannot reform, and the pit continues to grow. This autocatalytic process is a hallmark of pitting corrosion.
3. Key Factors Influencing Pitting Corrosion
Pitting corrosion occurs due to localized reductions in pH, even in the absence of chloride ions (Cl⁻). However, Cl⁻ is widely considered an essential factor that accelerates the process by breaking down the passive film.
The following conditions are critical:
- The electrochemical potential at the pit site exceeds a critical threshold.
- Once initiated, the corrosion process is self-sustaining and difficult to stop, particularly under conditions of low pH or high chloride concentrations.
4. Evaluation of Pitting Corrosion
The JIS G 0592 standard specifies methods for measuring the repassivation potential of stainless steel to evaluate its resistance to pitting corrosion. This method uses electrochemical techniques to assess the material's ability to reform a passive film after localized breakdown.
By dynamically analyzing the electrochemical behavior of stainless steel in chloride environments, the method provides a reliable way to evaluate material performance in corrosive conditions.
5. Additional Theories and Mechanisms
Figure 35 also highlights the hydrolysis mechanism, where metal ions accumulate in the pit and further reduce pH through hydrolysis. This mechanism explains the autocatalytic nature of pitting growth.
Another theory, the iR-drop mechanism, suggests that the potential difference between the pit mouth and interior can destabilize the passive film, leading to further dissolution.
Finally, the micro-galvanic mechanism proposes that the pit acts as an anodic site while the surrounding surface serves as a cathode, driving localized corrosion.
Conclusion
Pitting corrosion is a localized but critical form of corrosion that can severely compromise the integrity of stainless steel. While various theories and mechanisms have been proposed, no single explanation fully accounts for all aspects of pitting corrosion.
Understanding and evaluating pitting corrosion through standards like JIS G 0592 and advanced electrochemical methods is essential for ensuring the durability and reliability of stainless steel in aggressive environments. HARRIS LEE
Tags:
#StainlessSteel #PittingCorrosion #MaterialScience #CorrosionMechanism #Engineering #JISStandards #ElectrochemicalTesting #Durability
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