Field and laboratory studies were car ried out to investigate the effect of UV degradation on stockpiles of FBE Coated Pipeline. In addition, three commercially available UV protector paints were examined as potential protection of the underlying FBE system 1A pipeline coating from UV degradation. This study was done under actual field exposure for two years and using laboratory tests simulating weathering conditions using a Q-SUN Xenon Arc Test Chamber to simulate the damage caused by full-spectrum sunlight and rain. Coating flexibility by laboratory testing, and Photooxidation Index (POI), Carbonyl Index (CI) and changes in the polymer chemical structure by Attenuated Total Reflection – Fourier Transform Infrared (ATR-FTIR) were examined at different time intervals. The results indicated that although the tested UV protector coatings were effective in preventing the underlying FBE coating from chemical/oxidative degradation and chalking, maintaining the color of FBE gloss; the underlying FBE coating exhibited a reduction in flexibility after nine months of field exposure and 250 hours of accelerated laboratory test conditions. No further decrease in the FBE coating integrity was noted with increasing exposure/testing time.
Fusion bonded epoxy (FBE) is the primary coating used in North America for new pipeline construction and is strongly featured in the Middle East1. Pre-coated FBE pipe is routinely stockpiled and stored in large quantities at several locations close to pipeline right-of-ways. Once stockpiled, most of the pipes remain un-used until an inspection and remediation program is initiated2. Integrity projects purchase FBE coated pipe in advance and stockpile it until it is required. In addition, project delays and left-over pipe from completed projects can cause stockpiling for future use for undetermined periods of time. Under ultraviolet (UV) exposure in the presence of air, photooxidative degradation of FBE coatings occurs due to photometrical reactions arising from UV absorption. The main absorbers of UV radiation responsible for initiating photochemical reactions are carbonyl groups present on the polymer backbone3. The absorbed UV energy can break existing chemical bonds, form free radicals and produce new carbonyl groups in the main chain by reacting with oxygen. Therefore, the rate of carbonyl growth which correlates well with the rate of crosslink scission can be a relative measure of the photooxidation rate for a given FBE coating system4. Once photooxidation degradation begins, a rapid breakdown of the coating will follow as chemical degradation can transform into substantial changes in the physical, mechanical and corrosion protection properties, ultimately leading to failure of the coating. The extent and degree of polymer degradation (visibly observable as chalking) depend on the intensity and duration of the UV exposure. Generally, the top of a stockpiled FBE coated pipe (12 o’clock position) suffers the most coating breakdown, less on the sides (3 or 9 o’clock positions) and little to none on the bottom (6 o’clock position)2. Other elements in outdoor environments such as moisture and wind can also contribute to the degradation of stockpiled FBE coating. The outer layer of chalked FBE, which protects against further degradation, can be removed by rain and wind leading to alternate chalking causing a noticeable reduction in the coating thickness. Shorter wavelength UV rays have more destructive power than the longer wavelengths, but they cannot penetrate inside the coating and therefore cause most damage to the surface layer of coatings5. It is reported that UV light with a wavelength range of 290 nm to 400 nm has particular impacts on aesthetic changes, chemical breakdown and deterioration of mechanical properties3,6. Coating formulations can also affect the susceptibility of FBE coatings to UV degradation. To reduce or eliminate the effects of UV degradation, protective materials are usually applied preferentially to FBE coated pipe stored in the outer layers of the stockpiles. Whitewash UV protector paint has been proven to preserve and retain the original dry film thickness and conserve many of the initial physical characteristics of the underlying FBE coating except for the flexibility2. However, in the previous studies, the investigation of chemical changes in the polymer structure of stockpiled FBE coatings under UV exposure has not been conducted. In addition, few details were reported regarding the correlation between the real-world weathering conditions and accelerated weathering conditions in laboratory settings, this correlation would be helpful for future coating and protective material selection and evaluation for stockpiling.
