Internal Corrosion of a Crude Oil Storage Tank Bottom
A 100,000-barrel API 650 storage tank stores sour crude oil with 1.5 wt% sulfur and 0.5 wt% water. The tank has a fixed roof and is equipped with a side-entry mixer. The bottom is 1/4-inch thick A36 steel with a fiberglass-reinforced plastic (FRP) liner. After 5 years, ultrasonic thickness measurements show localized wall loss of 0.15 inches in a 2-foot diameter area near the sump, with pitting and undercutting. The liner appears intact except for a 1-inch disbonded area near the sump. The tank operates at ambient temperature. Water accumulation is expected at the bottom. The operator wants to avoid tank replacement and extend service life by at least 10 years.
1. Identify the likely corrosion mechanisms and explain why the liner failed. Recommend a repair and mitigation plan.
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Mechanisms: (1) Under-deposit corrosion from water and sediment accumulation, particularly at the sump where water collects. (2) Microbially influenced corrosion (MIC) due to sulfate-reducing bacteria (SRB) in the water phase, indicated by pitting and undercutting. (3) Possible galvanic corrosion if the liner disbondment allowed electrolyte contact with steel. The liner failure likely initiated at a defect (e.g., holiday or mechanical damage) near the sump, allowing water to migrate under the liner, leading to disbondment and concentrated corrosion. The FRP liner is not impermeable to water vapor and can blister if applied over poorly prepared surface. Recommendation: (1) Drain and clean the tank. (2) Remove the damaged liner and inspect the steel; if remaining thickness is less than 0.1 inch, repair with welded patch or use a composite wrap. (3) Re-line with a high-performance coating (e.g., glass-flake epoxy) after proper surface preparation (NACE No. 2/SSPC-SP10). (4) Improve water draw-off by installing a larger sump or water-jetting system. (5) Implement a corrosion inhibitor program for the water phase (e.g., film-forming amine) and biocide treatment for SRB. (6) Monitor with coupons and ultrasonic measurements annually.
2. Calculate the remaining life of the tank bottom based on current corrosion rate and determine if the repair plan is adequate.
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Current thickness = 0.25 - 0.15 = 0.10 inch. The corrosion rate over 5 years is 0.15/5 = 0.03 inch/year (30 mpy). If left untreated, remaining life to reach minimum thickness (assume 0.05 inch for structural integrity) is (0.10 - 0.05)/0.03 = 1.67 years. After repair, if the corrosion rate is reduced to 2 mpy (0.002 inch/year) by mitigation, remaining life = (0.10 - 0.05)/0.002 = 25 years, exceeding the 10-year goal. However, the repair must address the localized pitting; if the pit depth is 0.15 inch, the remaining ligament is 0.10 inch, which may be acceptable if the area is small. The plan is adequate if the liner is fully replaced and water management is improved. Also, consider adding a cathodic protection system for the tank bottom (e.g., impressed current with mixed metal oxide anodes) to protect any exposed steel.