1992 Volume 41 Issue 11 Pages 741-750
Corrosion of zinc under an organic coating was studied with a specimen of zinc in galvanic contact with steel exposed into environment separately inside an artificial defect introduced. The corrosion tests consisted of repeated 24 hour cycle of immersion in NaCl solution followed by drying in the air. Corrosion current decayed shortly in the drying period of early cycles, while it remained larger than 2nA in the whole drying period of later cycles, when underfilm corrosion of zinc began to start with such wet situation at the coating-zinc interface. Qdry, the ratio of electricity integrated for all the drying periods to the total one, depended significantly on test conditions and increased with higher NaCl concentration, shorter immersion time (or longer drying time) and higher humidity. The Qdry value more than 20% made high probability of initiation of underfilm corrosion of zinc, combined with the total electricity, Qt, more than 21C/cm2, equivalent to 10μm depth of anodic dissolution of free surface zinc.
