The kinetics of stress corrosion cracking(SCC) of a high purity Al-Zn-Mg alloy was studied as a function of applied potential, temperature and kind of corrosive media. The SCC tests were carried out in the range of applied potential from -2150 to -550 mV$_{\mbox{SHE}}$ and at the temperatures between 288 and 318K. 3.5wt.% NaCl solution (pH = 1), air and transformer oil were used as a corrosive medium.
The relative fracture strain and the relative fracture energy, relative to inert atmosphere increased with applied potential up to -1150mV$_{\mbox{SHE}}$ and then decreased. Activation energy due to SCC in NaCl solution was found to be 18-20 KJ/mole for the specimen aged at 180℃ for 0.5h (underaged state). Intergranular brittle fracture mode was also observed in oil, probably caused by the water in oil.
Stress corrosion(SC) fracture mode altered from the mixed fracture, composed of intergranular brittle fracture and dimpled rupture at the subsurface to pure dimpled fracture in the interior of specimen.
Average SC crack propagation rate ($&γuml;$) under assumption of volume constancy and strain rate constancy over the gauge section in corrosive medium, tested by constant strain rate method. Average SC crack propagation rate was calculated to be $1.3 ×10^{-4}$ (mm/sec), $8×10^{-5}$ (mm/sec) and $2.5×10^{-5}$ (mm/sec) at the applied potential of -550, -750 and -1150 mV$_{\mbox{SHE}}$ respectively, tested at the strain rate of $8.3×10^{-7}$/sec with the underaged specimen. It is suggested on the basis of the observation of potential dependence of relative fracture energy and SCC fracture surfaces that SCC is caused by hydrogen embrittlement.
