Meaning
Localized chemical degradation occurring within shielded or restricted geometric areas results in rapid metal loss without visible warning. The initiation of occluded cell corrosion creates a stagnant micro-environment where the local chemistry becomes significantly more aggressive than the bulk fluid. This process governs crevice regions, gaskets, and under-deposit zones, terminating where the metal surfaces are fully exposed to bulk flow.
Chemical Mechanism
Restricted fluid flow prevents oxygen replenishment inside the crevice while oxygen reduction continues on the exposed outer surfaces. In systems suffering from occluded cell corrosion, the accumulation of hydrogen ions and chloride ions inside the cavity accelerates the dissolution rate. The local pH drops, creating an acidic environment.
This autocatalytic process drives rapid localized pitting.
Structural Threat
Severe localized metal loss can lead to rapid wall perforation in process piping and storage vessels. Because the damage is hidden within crevices or beneath deposits, routine visual inspections fail to detect the deterioration before failure occurs. This degradation mechanism can cause high-pressure process leaks that risk personnel safety.
Weld joints and flange faces are particularly vulnerable. It requires advanced ultrasonic or radiographic inspection to identify.
Preventative Design
Eliminating crevices through continuous welding instead of lap joints protects the equipment from stagnant fluid accumulation. Regular cleaning programs prevent deposit formation.
