Corrosion of PVC coated steel during thinning of 0·5M NaCl electrolyte layer

Abstract

The present work is concerned with the corrosion behaviour of polyvinylchloride (PVC) coated steel during the thinning of a layer of aerated 0.5M NaCl solution. Galvanic current and ac impedance were measured from the steel galvanically coupled with copper and the steel respectively, during both the immersion of the specimen in bulk electrolyte and the thinning of the electrolyte layer. It was observed that the galvanic current increased up to the transition from rate control by diffusion limited oxygen reduction to anodic metal dissolution, and then decreased during the thinning of the electrolyte layer when the PVC coating on the specimen became detached or was very thin or coating degradation had proceeded sufficiently. The increment in the galvanic current was considerably reduced and the decrement was markedly slowed where there was a thin PVC coating or a highly degraded coating on the steel surface, as compared with the uncoated steel surface. These results suggest that during the thinning of the electrolyte layer the rate of corrosion Of PVC coated steel is enhanced by increased oxygen reduction up to the transition time, whereas during the thinning the rate of corrosion is retarded because anodic dissolution is decreased due to precipitation of corrosion products into the PVC coating after the transition time. Corrosion resistance due to the PVC coating was markedly lowered in subsequent immersion-thinning cycles, indicating the occurrence of the degradation of the PVC coating during the thinning of the electrolyte layer.