Module 4.2 — The corrosion mechanism behind Module 1.1's dissolved oxygen warnings, and why pitting is one of the most dangerous corrosion patterns despite low total metal loss.
Click any element to see its role in pit formation. The area mismatch between a tiny anode and a huge cathode is what drives such rapid, concentrated penetration.
Module 1.1 established that dissolved oxygen is aggressive toward carbon steel and explained why deaeration and oxygen scavenging (Module 2.6) exist. This module explains the specific mechanism: oxygen pitting, one of the most classically destructive corrosion patterns in fossil plant systems, and a direct application of the anode/cathode framework from Module 4.1.
Pitting typically begins at a small, localized surface imperfection — a scratch, a break in a protective oxide film, a deposit, or a minor manufacturing defect — where dissolved oxygen can initiate localized attack. Once a tiny anode forms at that point, oxygen reduction on the vast surrounding surface (the cathode reaction from Module 4.1's electrochemistry) drives continued dissolution at the anode site specifically, rather than spreading the attack out. The reaction is self-reinforcing: metal dissolving at the pit releases iron ions, which react with water and available oxygen to form corrosion products that can cap the pit opening as a tubercle, creating a stagnant pocket underneath.
Once a tubercle forms, the water trapped beneath it becomes isolated from the bulk system's normal chemistry. Metal dissolution inside the pit produces metal ions that hydrolyze, generating acidity locally — meaning the pit interior can become significantly more acidic than the bulk water, even on a well-controlled alkaline AVT or phosphate system. This local acidification accelerates further metal dissolution, and the tubercle cap can also restrict oxygen from re-entering the pit even as it protects the pit from beneficial bulk water chemistry — a genuinely self-sustaining, worsening local environment.
An operating unit under normal AVT or phosphate chemistry control, with functioning deaeration and scavenging, is relatively well protected against oxygen pitting. An idle unit is a different story entirely — no continuous flow, no active deaeration, and often no active chemical feed, while oxygen from air in-leakage or simply atmospheric contact can slowly saturate stagnant water. This is exactly why plants use lay-up procedures: either draining and drying equipment completely (dry lay-up) or filling with treated, oxygen-scavenged, alkaline water and maintaining it in a sealed, monitored condition (wet lay-up) during extended outages, specifically to prevent the stagnant, oxygenated conditions that invite pitting.