Long-Term Weathering Effects on Aromatic Polyurethane-Coated Pipe: Fid-Bau Portal

Long-Term Weathering Effects on Aromatic Polyurethane-Coated Pipe

Croll, Stuart G. / Bell, Graham E. C. / Keil, Brent D.

Polyurethane for coating of steel water pipe is used as an effective corrosion control system in the water and wastewater markets. As use has increased, questions have arisen regarding the ability of polyurethane to maintain its integrity for buried application while stored above ground and exposed to the elements. Polyurethane used for steel water pipe will typically be specified to meet AWWA C222, which requires the use of aromatic polyurethanes that result in a resilient and flexible coating and lining system. Aromatic polyurethanes can however be susceptible to surface degradation from ultraviolet rays and can chalk or discolor when left in the sun for long periods. Historic testing in the coating industry that is related to weathering effects focuses on color changes or color shift for architectural purposes, but for buried pipe, discoloration is not a concern. Potential loss of the coating film mechanical and chemical properties due to thermal cycling above and below the glass transition temperature of the material during the exposure are of greater interest. Testing was performed on samples of polyurethane applied to 96-in. diameter steel pipe manufactured and coated per AWWA C222 in Saginaw, Texas. Multiple test samples were removed for testing from the pipe, which had been stored in Saginaw for more than 24 months. The pipe was monitored over a period of time during the summer months to get an indication of the temperature fluctuations the coating was exposed to on a routine basis. Hardness and adhesion testing was performed on the coating as well. The samples were subjected to nano-indentation tests to determine any modulus shift and testing to determine chemical changes from the exterior of the coating to the steel surface. This paper will discuss the results of the testing of this aged polyurethane coating and discuss possible changes in the physical properties of the coating and its capability to provide its intended corrosion protection and service after 24 months in the Texas sun. Recommendations to address extended exposure of buried aromatic polyurethane coatings will also be discussed.