Abstract
Historically, the Bureau of Reclamation observed coating service lives of 50 to 80 years when lining its water conveyance structures with coal tar enamel. Changes to regulations have largely eliminated coal tar enamel as a field coating option, and existing coal tar enamel is beginning to show signs of degradation or has already been repaired or recoated. Reclamation has been working to find an appropriate alternative to coal tar enamel.
Elastomeric polyurethanes have superior flexibility and abrasion resistance, with expected service lives between 20 to 40 years. However, adhesion and delamination problems observed both in the laboratory and in the field currently render them unsuitable for most of Reclamation's needs.
Rigid polyurethanes also have good flexibility and abrasion resistance, but develop blisters during application to the cold steel of buried pipes. Blisters develop as a result of the reduced reaction rate (curing) at the steel interface while the bulk material cures at higher temperatures. These blisters are explained by the heat sink phenomena, which occurs with fast set products.
100 percent solids epoxies are abrasion resistant, have good adhesion, but are usually brittle in nature. Reclamation has some field experience with 100 percent solids epoxies as penstock or outlet works linings, but have used them primarily for maintenance and repairs of coal tar enamel and elastomeric polyurethanes. The life expectancy of 100% solids epoxy is 15 to 30 years.
Reclamation's coatings research is ongoing, but questions remain whether commercially available products can match the service life that coal tar enamel provided inside penstocks while suiting modern application technologies, methods, and logistical challenges.
