Role of Smart-Release Pigments in Preventing Corrosion Driven Cathodic Disbondment of Organically Coated Hot Dip Galvanised Steel


 The role of smart-release corrosion inhibitive pigments in preventing cathodic delamination of organically coated hot-dip galvanised steel is investigated. The pigments consisted of hydrotalcite exchanged with a range of inorganic and organic anionic species and were dispersed in a model PVB coating. A scanning Kelvin probe technique was used to determine cathodic delamination rates, and the inhibition efficiencies obtained for inorganic ions increased in the order CO32-  MoO42-  NO3-  VO43-  WO42-  PO43-  CrO42-. The inhibition efficiencies for organic-based pigments increased in the order triazole <phenylphosphonate <trans-cinnamate <benzoate <salicylate <benzotriazole. The inhibition efficiency afforded by the best performing organic inhibitor, benzotriazole (BTA), rivalled that of HT containing stored chromate anions. Findings are consistent with HT-BTA acting to sequester anions from the underfilm electrolyte, releasing BTA- which subsequently strongly adsorbs on the underfilm metal surface but can also form an insoluble Zn-BTA precipitate at the coating-defect boundary.