Blistering of color coated steel: Use of broad ion beam milling to examine degradation phenomena and coating defects

Broad ion beam (BIB) cross-section preparation is emerging as a preferred technique to polish delicate multiphase materials. This paper presents an examination of coating degradation phenomena obtained with BIB sample preparation and scanning electron microscopy (SEM) characterization. Early-stage humidity-induced coating degradation included oxidization of zinc and break-up of metal/coating and corrosion inhibitor particle/coating bonding. Ongoing corrosion product formation and accumulation of water within the coating finally led to evolution of macroscopic blisters. These degradation stages were successfully observed from pristine cross-sections, even when blisters did not yet appear on the coating surface, and the technique significantly facilitated distinguishing zinc corrosion-based blistering from other coating defects.