Influence of oxide film to stress corrosion cracking of zirconium in boiling nitric acid solution

Kato, Chiaki   ; Ishijima, Yasuhiro ; Motooka, Takafumi; Yamamoto, Masahiro 

Zirconium has excellent corrosion resistance in nitric acid solutions. However, it has been known that zirconium has stress corrosion cracking (SCC) susceptibility in concentrated HNO with nobler corrosion potential. In this study, we investigated the oxide film growth of zirconium related SCC initiation with various potentials in boiling nitric acid solutions. Electrochemical tests and corrosion tests with various applied potentials conducted in boiling 3, 6 and 9 mol/dm HNO. The potentials in the corrosion tests were set at 1.16, 1.4 and 1.5 V vs. saturated KCl-Ag/AgCl electrode (SSE). These were in the region of trans-passive state of zirconium in boiling nitric acid solution. The test durations were 10, 100 and 500 h. After the corrosion tests, cross-sectional observations of oxide films were conducted. From the results of electrochemical tests of zirconium, passivity region of anodic polarization curves was observed from rest potential to about 1.5 V in boiling 3 mol/dm HNO. Rapid increase of current density was observed at the potential attributed to transition from passivity to trans-passive region. The transition potential in boiling 3, 6 and 9 mol/dm HNO was 1.57, 1.46 and 1.38 V vs. SSE, respectively. The potential was shifted to nobler with decreasing nitric acid concentration. The corrosion tests with various applied potentials indicated that the surfaces of the samples in trans-passive region covered with thick black oxide films. The existence of these oxide films coincided to SCC occurrence. Besides, the oxide film was extremely thin and hardly grew in the passive state potential with no SCC. These results show that SCC of zirconium was initiated by thick oxide film formation. Thus SCC occurrence of zirconium is considerably little in extremely slow oxide film growth rate under the passivity potential.