Changes during the Weathering of Polyurethane Water Pipeline Coatings

Aromatic polyurethane coatings are commonly used to protect steel water pipelines from corrosion while they are buried underground. However, during construction of a large pipeline project, coated sections of pipe may be stored above ground for extended periods and the polymer coating can suffer rapid and substantial changes in outward appearance due to sunlight, moisture, pollution, and heat. These changes in surface appearance can cause concerns about the ability of the coatings to function as a long term corrosion barrier although that protection is imparted by the whole thickness of a coating. The purpose of this investigation is to study the effect of UV, moisture and heat on four polyurethane coating formulations on both the visibly obvious appearance changes and on the likely corrosion protection. Candidate polyurethane coatings were subjected to laboratory weathering testing that causes very rapid degradation compared to natural environments. Quantitative testing of corrosion protection by electrochemical impedance spectroscopy has been done in concert with measurement of coating thickness loss and appearance changes (color, gloss). Appearance change is obvious and rapid but corrosion protection remains at very high levels. A longer term program is underway in which the same coatings are exposed to natural weathering in Florida and Texas so that the changes to the coatings in the real world may be correlated to the much more rapid results possible from accelerated laboratory testing.