Abstract
The characteristics of stress corrosion cracking (SCC) of pure Al and homogenized Al-Cu single crystals were investigated with reference to crack morphology and faceting dissolution at various potentials in 1mol/kg-NaCl solution, and then an electrochemical role of NaCl+H2O2 solution which has been used as a SCC testing solution was examined in connection with pitting potential. Fracture time in 1mol/kg-NaCl solution decreased exponentially with an increase of potential above the pitting potential (Vc′) determined without stressing. Cracks are classified into the following three types according to applied potentials: well-type cracks which appear near Vc′, similar cracks which issue from bottoms of pits with increase of potential and only pits which develope above -0.40V (vs. SCE). Cracks always grow due to (100) faceting dissolution. As growth rates of pits become larger than the rate of (100) faceting dissolution above -0.40V, ductile fracture occurs by reduction of area due to pitting. When potentiostated, pure Al also suffers from SCC similar to Al-Cu alloys at potentials near Vc′, but the susceptible potential range is narrow. The critical potential of SCC seems to correspond to the pitting potential under straining (Vcδ′). Spontaneous corrosion potential in 1mol/kg-NaCl+0.09mol/kg-H2O2 solution is higher than Vcδ′ for several minutes, and decreases slowly with time to a constant value below Vcδ′. This constant corrosion potential value is higher than the protection potential for Al-Cu alloys and lower than that for pure Al. Therefore, the cracks of Al-Cu alloys keep growing due to (100) faceting dissolution and those of pure Al stops growing due to pit repassivation.
