Environmental Variables and Operating Conditions
What the exam expects
Internal corrosion is driven by the interaction of chemistry, physical conditions, and operating practice. Candidates must know which variables matter and how each shifts risk:
- Water chemistry: pH, dissolved CO2 and H2S partial pressures, chlorides, oxygen, and organic acids.
- Temperature: affects reaction kinetics and film stability; iron carbonate films become more protective at higher temperature within a range.
- Flow regime and velocity: low velocity allows deposits and under-deposit corrosion; high velocity causes erosion-corrosion and film removal. Stratified and slug flow create wetting variability.
- Water cut and wettability: dry gas is low risk; the first free water is the highest-risk transition.
- Pressure: changes gas solubility and therefore partial pressures.
- Operating practice: shutdowns, startups, hydrotests, and stagnant periods introduce oxygen and allow bacterial growth.
Traps in questions
- Treating water cut as a simple linear risk. The risk jump occurs at the water-wetting threshold, not proportionally.
- Ignoring partial pressure and using total pressure instead.
- Assuming higher temperature always increases corrosion; film formation can reduce it.
- Forgetting that stagnant conditions are a hazard, not a safe condition.
Worked example
A gas line at 5% water cut with no free water is described as low risk. A question asks what changes if water cut rises to 30% with stratified flow. The answer is that the line has crossed into continuous water wetting with potential under-deposit and preferential weld corrosion, raising localized risk sharply. Rule of thumb: find the water-wetting threshold first; mitigation and monitoring decisions follow from it.