The Most Dangerous Corrosion
Pitting corrosion is the leading cause of unexpected failure in stainless steel and aluminum structures. Unlike uniform corrosion that gradually thins a surface, pitting creates deep, narrow holes that can penetrate pressure boundaries with negligible overall weight loss. A pipeline wall might lose only grams of metal yet develop a through-wall pit that causes catastrophic leakage. Detection is difficult because pits are small and often covered by corrosion product caps.
Initiation: Breaking the Passive Film
Pitting begins when aggressive ions — primarily chloride — penetrate the passive film at vulnerable sites: MnS inclusions, grain boundaries, or mechanical scratches. The chloride ion displaces oxygen in the oxide lattice, creating a local active site where metal dissolution begins. Whether this metastable pit grows into a stable pit depends on the competition between dissolution (driven by potential and chloride) and repassivation (driven by chromium and molybdenum content).
The PREN Approach
Metallurgists developed the Pitting Resistance Equivalent Number to rank alloys: PREN = %Cr + 3.3 x %Mo + 16 x %N. Each element contributes differently — molybdenum is 3.3 times more effective than chromium per weight percent, and nitrogen is 16 times more effective. This simple formula predicts critical pitting temperature within a few degrees for most austenitic and duplex stainless steels, making it an invaluable alloy selection tool.
Growth and Propagation
Once a stable pit forms, its internal chemistry becomes self-sustaining: dissolved metal ions hydrolyze to produce acid (pH drops to 1-2), chloride ions migrate in to maintain electrical neutrality, and oxygen is consumed faster than it can diffuse in. This autocatalytic process drives the pit deeper at a rate proportional to the square root of time. This simulation models both initiation probability and growth kinetics to help you select appropriate alloys for chloride environments.