Standby Service Water Basins – Protective Coating failure
Underwater Coating Application to evaluate corrosion and repair coating defects on carbon Steel (Nuclear Power Project – applicable to any industry with coated carbon steel in immersion service)
Underwater coating repair in commercial nuclear plants is an accepted maintenance approach. Some of the early applications have now been in service for over 15 years. Surface area covered ranges from several square inches to several thousand square feet over both steel and concrete substrates. One of the first completely documented underwater coating projects took place in 1986. Thousands of repairs were performed in a BWR Mark I suppression pool. Additional repairs were also performed in the Condensate Storage Tank. The condition of these repairs has been periodically monitored and they continue to perform well while the principal coating continues to degrade.
Three case studies are presented that help to illustrate the effectiveness of the underwater repair technique.
Case Study #1: The Torus at a Mark I BWR was drained and completely recoated in 1986. During the following refueling outage, an underwater coating inspection was performed to assess the performance of the newly installed coating system. The inspection revealed extensive blistering and significant loss of adhesion over large areas. Fractured blisters and coating delamination exposed the steel substrate in numerous locations and pitting of the steel substrate had occurred.
Underwater coating repair was used to reestablish the protective corrosion barrier and prevent metal loss at the Torus pressure boundary. Coating repair application proved to be extremely difficult due to the lack of adhesion in the existing coating. Ultimately, the repair coating was used to tack down loose coating and protect exposed areas on the Torus shell.
Inspection during several subsequent outages has shown that the repairs are performing as expected. No significant defects were noted in repair areas and pull testing showed adhesion values were much higher in the repair coatings than in the principal coating.
Case Study #2: The Torus at a Mark I BWR was inspected underwater during the plant’s first refueling outage in 1990. The inspection revealed extensive blistering and moderate loss of adhesion. In addition, it was necessary to remove instrumentation that had been installed prior to start-up to monitor operating conditions. The removal of strain gauges, pressure transducers, and associated wiring and piping exposed large areas of steel substrate on the pressure boundary.
Numerous small coating defects were repaired as well as several large areas in excess of 100 square feet. Recent follow up inspections confirmed that repair-coating performance is excellent. No significant substrate corrosion was noted.
Case Study #3: Underwater coating repair was performed in the Torus at a Mark I BWR in 1996. At the next refueling outage, the Torus was drained for a recoat. The painting contractor used abrasive blasting to remove the old coating and provide a suitable surface profile. During the surface preparation, the contractor reported that blasting easily removed the original coating but left the repaired areas intact.