A thin oxide film on NZ alloy (Zr-1Sn-0.3Nb-0.3Fe-0.1Cr) was formed to investigate the correlation of microstructure and corrosion behavior at the initial stage by directly identifying defect-enriched sites. The sites of high conductivity are revealed and labelled by silver deposition, and in the further investigation of microstructure secondary phase precipitates (SPPs) interconnected with micro cracks are found below. Enrichment of oxygen-18 and deuterium in SPPs after isotopic tracing by secondary corrosion highlights their influence on carrier diffusion. Thus, the percolation for corrosion media transportation is reached locally by SPPs interconnected with micro cracks.