Corrosion Inhibitor Program for a Produced Water Injection System
A produced water injection system handles 15,000 m³/day of deaerated water at 50 °C, with 12,000 mg/L chloride, 2 mg/L dissolved oxygen (target <20 ppb), and traces of H2S. The carbon steel injection lines have shown pitting at weld heat-affected zones and occasional iron spikes in water analysis. The operator currently doses a generic filming amine inhibitor at 10 ppm, with no monitoring beyond periodic water sampling. The chemical vendor proposes doubling the dose to 20 ppm to solve the problem.
1. Evaluate whether doubling the inhibitor dose is the right response to the observed pitting and iron spikes.
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Doubling the dose is not a defensible first response. The evidence points to localized attack at weld HAZs and possible oxygen ingress, not simply insufficient bulk inhibitor concentration:
- Pitting at weld HAZs suggests preferential attack at metallurgical discontinuities, which may require stripe or targeted treatment rather than higher bulk dose.
- Iron spikes indicate episodic corrosion, often linked to oxygen ingress or under-deposit attack, not steady-state inhibitor shortfall.
- 2 mg/L dissolved oxygen is far above the <20 ppb target; oxygen is a major corrosion driver and will consume inhibitor. Higher dose may be wasted on oxygen rather than protecting steel.
- 10 ppm generic filming amine may be inadequate or poorly matched to the water chemistry (high chloride, H2S traces).
Doubling dose without diagnosing the cause risks cost without benefit and may mask a real problem.
2. Identify the likely root causes and the diagnostic steps you would take.
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Likely root causes:
- Oxygen ingress - 2 mg/L DO vs. 20 ppb target indicates a deaeration or seal/blanket failure.
- Inhibitor mismatch - generic filming amine may not be effective at high chloride or in the presence of H2S; poor film persistence at weld HAZs.
- Under-deposit or localized attack - iron spikes suggest episodic events, possibly deposits or flow-induced attack.
- Weld HAZ metallurgy - hardness or microstructure may make HAZs more susceptible.
Diagnostic steps:
- Oxygen monitoring at multiple points to locate ingress.
- Corrosion monitoring - coupons and ER probes at welds and downstream.
- Water chemistry - iron, chloride, H2S, pH, and inhibitor residual.
- Inspection - UT or visual at weld HAZs; deposit analysis.
- Inhibitor performance testing (e.g., rotating cylinder or jet impingement) with the actual water chemistry.
3. Propose a revised chemical treatment and monitoring program.
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Revised program:
- Fix oxygen ingress first - repair deaeration, seals, and blanketing; target <20 ppb DO. Oxygen control is the highest-leverage action.
- Re-qualify the inhibitor - select a filming amine or imidazoline qualified for high-chloride, low-H2S produced water; confirm film persistence at weld HAZs via lab testing.
- Optimize dose - establish dose via lab and field trials, not by doubling blindly; consider continuous plus batch treatment.
- Targeted protection - consider stripe or batch treatment at weld HAZs.
- Monitoring - install ER probes and coupons at welds and downstream; track inhibitor residual, iron, oxygen, and chloride; set alarm limits.
- Review - periodic program review with the vendor, with performance metrics tied to iron spikes and probe readings.