Internal Corrosion: Mechanisms and Control
What you must know
Internal corrosion occurs in pipelines, vessels, and equipment where a fluid contacts metal. The candidate must understand the dominant mechanisms: CO2 (sweet) corrosion, H2S (sour) corrosion and sulfide stress cracking, oxygen corrosion, galvanic and crevice attack under deposits, erosion-corrosion from flow, and MIC. Know that CO2 corrosion produces iron carbonate films whose protectiveness depends on pH, temperature, and flow; H2S can form iron sulfide films and, at sufficient partial pressure, cause cracking.
Understand the role of water wetting: internal corrosion requires an aqueous phase contacting the metal. In multiphase flow, water dropout and stratification are key. Know the corrosion-influencing parameters: partial pressures of CO2 and H2S, pH, temperature, flow regime, and velocity (which can strip protective films or cause erosion-corrosion).
Know mitigation: chemical inhibition (continuous or batch), internal coatings and linings, material selection (corrosion-resistant alloys), and control of operating parameters. Monitoring includes coupons, electrical resistance and linear polarization probes, and fluid analysis.
Traps
Questions often assume a dry gas line cannot corrode; the correct answer hinges on water condensation and dew point. Another trap is confusing CO2 and H2S mechanisms: CO2 is general/localized corrosion driven by carbonic acid; H2S adds cracking risk and can form protective or non-protective sulfide films. Watch for "sweet" vs. "sour" terminology.
Rule of thumb
No water, no internal aqueous corrosion. If the operating temperature is below the water dew point, expect a water phase and design mitigation for it.