United States Department of Transportation
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TRIS Online Accession Number:01642980
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Edition:Final Report
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Abstract:Gas Technology Institute (GTI) and Edison Welding Institute (EWI) created both laboratory and infield girth weld samples to evaluate the effects of weld geometry and hydrogen off-gassing on the performance of protective coatings. Laboratory made plate welds were used to tightly control geometric differences and in-field welds were created to mimic real world welding conditions and hydrogen offgassing rates. These welds were then coated and tested with accelerated corrosion techniques to evaluate the coatings' effectiveness. Simulated girth welds investigated geometric effects on the performance of a liquid applied coating. Welds were created, coated, and testing in a salt-fog environment to accelerate corrosion. Undercuts up to 0.03 inches were found to have no significant effect on a coatings' resistance to corrosion. On the contrary the undercut tended to add to the coating thickness and therefore increased corrosion resistance. Increasing cap height of a weld was found to thin the coating making it more susceptible to chipping but no more susceptible to corrosion. If applying proper coating procedures, especially surface profiling, the weld geometries investigated here had no strong negative effects on a liquid applied two part epoxy coating's performance. Since fusion bonded epoxy (FBE) coatings are applied in a different manner, these results cannot be extended from liquid to FBE coatings. If the FBE provides the same wetting of the undercut and similar coating thickness on the cap height one would expect similar results. In-field welds were created to test the effects of hydrogen off-gassing on coating performance. Two different welding mediums were used, one with a high hydrogen content and one with a low hydrogen content. These different welds were then held for 2 or 5 hours to vary the amount of time allowed for hydrogen off-gassing and then coated in either FBE or a liquid 2 part epoxy. All Tech Report variables were held constant. Cross-sectional analysis of coated 24-inch diameter pipes showed no increase of voids above the welded area, indicating there was little off-gassing in these samples. Cathodic Disbondment Testing, per ASTM G-95, was performed to evaluate the coating's adhesion properties. No detectable adhesion differences were found that could be attributed to the hydrogen off-gassing from the weld, instead the results were more dependent on the coating thickness. Within the scope/boundary of the completed research, a hold time of two hours is sufficient to minimize any hydrogen off-gassing effects. Within the parameters of the in-field welds and simulated welds no major detrimental effects were found from hydrogen off-gassing and weld geometries. However, the higher cap height did make coatings more susceptible to damage when handling. This confirms previous GTI research which indicated that coatings often accrue damage during handling. GTI and EWI, taking into consideration the survey and testing results produced a recommendation to be distributed to various stakeholders in the pipeline industry. The summary document to be disturbed is located in the Recommendation section of this report.
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