| 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 -550 to -2150 mV_(SHE) and at the temperatures between 288 and 318 K. 3.5 wt.% NaCl solution (PH=1), air and transformer oil were used as corrosive medium. The percentage elongation and the fracture energy due to SCC increased with decreasing applied potential from -550 to -1150 mV_(SHE) and then decreased. The SCC fractography correspondingly revealed a decreased intergranular-brittle fracture fraction with decreasing applied potential up to -1150 mV_(SHE) and then increased one with decreasing applied potential up to -2150 mV_(SHE). Average SC crack propagation rate, γ, was calculated by introducing a new function related to the σ-εcurve in the inert environment. It was also observed to be 100×10^(-6), 39×10^(-6) and 2.1×10^(-4) ㎜ sec^(-1) at the applied potential of -550, -750 and -1150 mV_(SHE), respectively, tested under the strain rate 8.3×10^(-7) sec^(-1) with the underaged specimen. The observed values are in good accord with the calculated values. The intergranular-brittle fracture mode was also observed in oil, probably caused by the water in oil. Stress-corrosion (SC) fracture mode varied from a simply corroded fracture near the surface/subsurface, followed by an intergranular-brittle fracture over the mixed fracture, composed of the intergranular-brittle fracture and a dimpled rupture, to the pure dimpled rupture in the interior of specimen. It is suggested, on the basis of the observation of potential dependence of the fracture energy and SCC fractography, that SCC is caused by hydrogen-assisted embrittlement. Activation energy for hydrogen-assisted embrittlement in the NaCl solution was found to be 18-20 kJ mole^(-1) for the specimen aged at 180℃ for 0.5h (underaged stage). |
|
