High resolution analytical microscopy of damage progression within a polyester powder coating after cyclic corrosion testing

Progressive surface defect formation leading to crack initiation and propagation have been identified as a major route to failure of polyester power coatings exposed to cyclic corrosion testing. Corrosion damage was characterized at the microstructural level using analytical scanning electron microscopy with focused ion beam milling. A mechanism of crack initiation and propagation is proposed based on the initial microstructural features and how they develop during exposure to the cyclic corrosion testing. Cracks are initiated during environmental exposure within, or adjacent to, near-surface pigment particles or surface-breaking particles and small voids. Such cracks propagate along pigment-polymer interfaces and between pigment particles in a brittle manner. Pigment clusters that span the coating thickness are favoured in developing cracks that propagate through the coating to the substrate. Corrosion stains on the surface of the coating are associated with such through-thickness penetrating cracks. Furthermore, excess volume generated by corrosion product (rust) generation at the substrate results in tensile stresses that localise crack initiation and propagation resulting in blister formation. • Surface-breaking pigments and voids in polyester powder coatings form sites for crack initiation during service. • Cracks propagate through coatings via the brittle polymer matrix and brittle polymer-pigment interfaces. • Substrate corrosion and blistering initiates when cracks span the coating thickness connecting substrate to the environment.