Coating Failure at Weld Seams
A new above-ground storage tank is being coated with a high-build epoxy. The inspector notices that after the first full coat, the coating over weld seams (both shell welds and nozzle welds) is showing small blisters and poor adhesion. The welds were ground smooth but not to a feathered edge; they have a slight ridge. The surface preparation was SSPC-SP 10, and the coating was applied within the recoat window. The ambient conditions were within specification. The coating is 12 mils thick over the welds, while the specified DFT is 8-10 mils. The inspector suspects the issue is related to weld preparation and coating thickness.
1. What are the likely causes of the blistering and poor adhesion at the weld seams?
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The blistering and poor adhesion at the weld seams are likely caused by a combination of factors: 1) Inadequate weld preparation: the welds were ground but not feathered to a smooth transition. The slight ridge creates a sharp edge where the coating is thinner or stressed, leading to potential discontinuity and adhesion failure. 2) Excessive coating thickness: the DFT over the welds is 12 mils, exceeding the specified 8-10 mils. High-build epoxies can suffer from solvent entrapment and internal stress when applied too thick, especially over edges. The thick coating may not cure properly, leading to blisters. 3) Possible contamination: weld seams can have residues from welding (e.g., slag, spatter, or anti-spatter compounds) that are not fully removed by blast cleaning, especially in crevices. Even after SSPC-SP 10, the surface profile may not be uniform over welds. 4) The blistering could also be due to osmotic blistering if soluble salts are present, but the scenario does not mention salt testing. However, the localized nature at welds suggests surface irregularities and thickness are primary. The combination of a ridge and excessive thickness creates a stress concentration and potential for solvent or air entrapment.
2. What steps should be taken to rectify the situation and prevent recurrence?
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Rectification: 1) Remove the affected coating by abrasive blast cleaning or power tool cleaning to bare metal, extending at least 2 inches beyond the blistered areas. 2) Re-prepare the welds: grind the welds to a smooth, feathered edge (minimum 1/8 inch radius) to eliminate sharp ridges. Ensure all weld spatter and slag are removed. 3) Re-blast the area to SSPC-SP 10 and check for surface profile and cleanliness (including soluble salts if specified). 4) Apply the coating according to the product data sheet, carefully controlling thickness over welds. Use a stripe coat if recommended, and ensure the DFT over welds does not exceed the maximum. 5) Allow proper cure before inspection. Prevention: 1) Specify and verify weld preparation requirements (e.g., SSPC-SP 10 plus grinding to a feathered edge) before coating. 2) Train applicators to apply stripe coats and control thickness over welds. 3) Inspect welds after surface preparation for smoothness and cleanliness. 4) Use a coating with better edge coverage or specify a stripe coat. 5) Measure DFT over welds specifically and ensure it is within specification.