Epoxy Coating Failure on Buried Pipeline
A 12-inch diameter carbon steel pipeline is buried in clay soil with a resistivity of 1500 ohm-cm. The external coating is a fusion-bonded epoxy (FBE) applied 10 years ago. A recent close-interval survey shows coating conductance increased from 5 to 50 microsiemens. Excavations reveal disbonded coating with corrosion products underneath, but no visible holidays. Soil pH is 6.5. The pipeline operates at 60°C. What is the most likely failure mechanism, and what steps should be taken?
1. Identify the most likely failure mechanism and explain why.
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The most likely mechanism is cathodic disbondment exacerbated by elevated temperature and clay soil. FBE coatings are susceptible to disbondment when cathodic protection (CP) is applied, especially at temperatures above 50°C. The clay soil restricts oxygen diffusion, but moisture and ions penetrate the coating, leading to a local alkaline environment at the coating-steel interface due to CP current. This causes bond degradation and disbondment. The absence of visible holidays suggests the coating is intact but disbonded, allowing corrosion under the coating. The increase in coating conductance indicates a loss of barrier properties. The 60°C operating temperature accelerates the disbondment process. Soil resistivity of 1500 ohm-cm is moderately corrosive, but the key factor is the combination of CP, temperature, and coating type.
2. What actions should be taken to mitigate the problem and prevent recurrence?
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- Assess extent: Perform a direct current voltage gradient (DCVG) survey and excavation to determine the extent of disbondment. 2. Recoating: Remove disbonded coating and apply a compatible repair coating, such as a liquid epoxy or a heat-shrink sleeve, ensuring surface preparation to Sa 2.5. 3. CP adjustment: Optimize cathodic protection levels to avoid overprotection (which accelerates disbondment) while maintaining adequate protection. Use close-interval surveys and coupon checks. 4. Temperature mitigation: If possible, reduce operating temperature or use a high-temperature coating for future applications. 5. Soil treatment: Consider backfill with lower resistivity material or install a drainage system to reduce moisture. 6. Monitoring: Implement a coating integrity management program with periodic surveys. 7. Root cause analysis: Verify coating application quality and consider alternative coating systems for high-temperature service.