Exposure: Polyurethane Coating Response to Cyclic Heat Load and Ultraviolet Impact in the Texas Environment

The external polyurethane coating on welded steel pipe was selected for its properties as an excellent means to control corrosion, to minimize cathodic protection current requirements, and to help the pipe achieve a trouble-free, 100-year design life when buried in an earthen environment. For long, cross-country water transmission pipeline projects, it is not unheard of to have procurement segments in excess of 10-15 mi and diameters in the 84 in. to 108 in. range. For such projects, it is anticipated that pipe manufacturing and coating production will far outpace a contractor's field installation and will subsequently lead to short- and long-term storage of pipe whether at a staging yard or strung out along pipeline alignment. Due to the magnitude of these types of projects, it is anticipated this stored pipe may experience extended time above ground in extreme temperature and in the presence of solar light (radiation), including ultraviolet radiation (UV), which imparts thermal energy. A study was conducted at the plants of two steel pipe manufacturers in the Dallas Fort Worth (DFW) metroplex from June to September 2013. Sample pieces of large-diameter (72 in. to 96 in. diameter) polyurethane-coated steel pipe had been exposed to elements for periods of time, from one to two years, were monitored for surface temperatures during the peak of summer. Pipe surface temperatures measured in this study exceeded 150 degrees F. This paper will describe the test set-up and procedures, present results of field and subsequent laboratory measurements, and examine how the polyurethane coatings responded following exposure to the Texas sun.