CoatingsPro Magazine

MAR 2017

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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68 MARCH 2017 COATINGSPROMAG.COM coating thicknesses evaluated using a magnetic dr y film thickness gage appeared to be consistent between the areas near the corrosion and samples removed from the panels that were not display ing corrosion. e thicknesses of the organic coating materials on the samples were also examined using a digital micro- scope because the layer of galvanizing can influence the measurements of some of the older magnetic gages. e small sections of the larger samples were cut and mounted in epoxy and then polished to view the coating thick- ness in a cross-section. e coating thicknesses were consistent in the examined areas. T he consistenc y of the appli- cation of the coating mater ia ls removed one of the possible factors contr ibuting to the cor rosion of the roof ing mater ia l, but the ga lvanizing layer had not been eva luated. It was thought that the application of the ga lvanized layer may be lower than specif ied because the meta l roof ing mater ia l was cor rod ing so quick ly. T he paint was str ipped f rom sma l l coupons of severa l samples, and then the coupons were weighed and sched- u led for remova l of the ga lvanized layer in an acid bath. A f ter remova l of the paint layer, spots on the ga lva- nizing that were not associated w ith the spangle patter n were noted. T hese spots were not associated w ith any damage in the coating because the sma l l coupons were removed f rom areas on the ind iv idua l samples that d isplayed no v isible mechanica l damage to the coating or cor rosion. T hese var iations in the ga lvanized coating were photog raphed, and then the eva luation of the weight (thick- ness) of the ga lvanized layer was completed. Photog raphy was a cr itica l step since the spots wou ld no longer be v isible af ter remova l of the ga lva- nized layer w ith the acid. T he galvanized layer was deter- mined to be w ithin the ex pected weight for the grade of ga lvanized sheet mater ia l specified. T he presence of the spots, though, indicated that there was a defect in the ga lvanized coating. T hese spots were tested on separate areas of panels and were found to be iron, w ith little to no zinc, which should have been present as a protective layer. T he sma ll, random areas likely became active areas of corrosion in a ga lvanic cell. T hese highly active sma ll spots likely led to severe corrosion, which rapid ly spread from the active sites more quick ly than on a steel sur face w ith a consis- tent layer of ga lvanizing. T he panels w ith no corrosion did not display the spots when the coating was str ipped from the sur face. e spots in the galvanizing were random, and it was presumed that the Magnified at 50x, the roof samples showed various-sized defects. Photos of the sample panels were crucial to the investigation, as the spots would no longer be visible after the layer of galvanizing was removed. Holey Roof Decking

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