Pitting
Pitting corrosion is a highly localized form of corrosion that creates small cavities or holes in metal by depassivating a small anodic area while the surrounding surface remains passive, driving intense galvanic corrosion that penetrates the metal mass. It is dangerous due to its ability to cause catastrophic failure with minimal total mass loss.
In shop floor maintenance, pitting corrosion is tracked via inspections such as visual checks, ultrasonic wall thickness measurements, or borescopy to measure pit depth and density. This data triggers maintenance tasks when the deepest pit approaches wall perforation limits. Pitting is often modeled using Extreme Value (Gumbel) theory to predict failure probability over time, enabling proactive asset reliability management in ServiceGrid workflows.
How is pitting quantified for reliability modeling?
By the pitting factor (ratio of deepest pit depth to average uniform corrosion penetration) and maximum pit depth (d_max), which feeds into Extreme Value distributions (e.g., Gumbel) to calculate the probability of perforation f(t) = P[D - d_max(t) < 0].
Why is pitting more dangerous than uniform corrosion?
It concentrates attack on microscopic areas, reducing shear resistance 51% more than general corrosion after 50 years, and can cause catastrophic failure with minimal total mass loss.
What environments trigger pitting in stainless steel?
Presence of chlorides (Cl-), such as from saltwater evaporation, thiosulfates, and warm or humid conditions that break down the chromium-rich passive oxide film.
How does pitting relate to stress corrosion cracking (SCC)?
Pits induce local dynamic plastic strain, acting as precursors where SCC cracks always initiate in susceptible alloy and environment combinations.