Gloss and Color Changes during Exposure of Polyurethane Coatings for Steel Water Pipes and Their Possible Link to Long-Term Corrosion Protection

During large pipeline projects, pipe sections can be kept above ground for many months before burial. Thus, their corrosion protective polymer coating, commonly an aromatic polyurethane, may be exposed to ultraviolet solar radiation and high temperatures that were not anticipated for its ultimate service, in the ground. The gloss of such coatings deteriorates very rapidly and the color of the coating changes a great deal quickly which can lead to concerns about their remaining corrosion protective ability. Previous evaluation at 6-month intervals of field exposure in Texas and Florida showed that these changes in appearance occurred during the first 6 months. An expanded study wherein the coatings were evaluated at much shorter intervals, in Texas, shows that color changes rather faster than the gloss, within one month approximately, whereas the gloss diminishes over 3 months. Interestingly, both the color and gloss properties change in a pattern described very well by an exponential trend. This pattern of deterioration can be derived easily using a very simple assumption that ultraviolet light, water molecules, etc. from the environment cause their damage at random times and locations. This approach shows there may be links between properties such as weight loss and gloss loss and, in this case, that corrosion protection would diminish very little in these very thick polyurethane films. In fact, the most recent results from extended field test exposure sites in Florida and Texas show that the corrosion protection of these coatings remains intact.