Internal Corrosion Mechanisms
What the exam expects
Internal corrosion is an electrochemical process on the wetted surface of pipe or equipment. You must be able to name the mechanism, describe the anodic and cathodic reactions, and identify the controlling variable. The core mechanisms are:
- CO2 (sweet) corrosion: carbonic acid from dissolved CO2; iron carbonate scale can be protective or non-protective depending on pH, temperature, and flow.
- H2S (sour) corrosion: iron sulfide films, often protective but prone to pitting and to sulfide stress cracking in hard steels.
- Microbially influenced corrosion (MIC): sulfate-reducing bacteria produce H2S locally; often appears as shallow-to-deep pitting under deposits.
- Oxygen corrosion: typically from ingress during startup, hydrotest, or poorly managed water; fast and localized.
- Galvanic, crevice, under-deposit, erosion-corrosion, and acid (low pH) attack.
Traps in questions
- Confusing general with localized attack. Exam items often describe a pinhole leak and expect "localized pitting from MIC" rather than "general CO2 corrosion."
- Assuming a protective film always exists. Films are protective only within a pH/temperature/velocity window.
- Mixing up sweet (CO2) and sour (H2S) terminology.
- Forgetting that erosion-corrosion requires a velocity threshold and is flow-dependent, not chemistry-only.
Worked example
A produced-water line shows isolated deep pits with black deposits and a sulfide smell. General wall loss is minimal. The correct interpretation is localized MIC or sour pitting, not general CO2 corrosion, because the pattern is discrete and the deposit chemistry points to sulfide. The rule of thumb: pattern + deposit chemistry + water chemistry together identify the mechanism; a single number (corrosion rate) does not.