In the first, mechanisms of stress-corrosion(SC) crack initiation and propagation have been studied by measuring effects of hydrogen recombination poisoner(HRP) on both stress-corrosion cracking(SCC) behavior and corrosion behavior, and by clarifying the relation between pre-exposure embrittlement (PEE) and SCC. SCC tests were carried out in constant load apparatus equipped with linear variable differential transformer (LVDT). SC crack initiation and propagation periods were differentiated by measuring the displacement in gauge section with time by means of LVDT. When the 3.5 wt.% NaCl solution contained HRP(added in the form of $Ca_3P_2$ or $Na_2S$ㆍ$9H_2O$), the anodic dissolution rate decreased probably due to adsorption of hydride and/or its reduced phase, and the hydrogen reduction rate decreased due to the inhibiting properties of HRP on hydrogen recombination. In the presence of HRP, the SC crack initiation and propagation periods decreased probably due to the enhanced hydrogen permeation into the SC crack tip region. Incipient SC crack was initiated preferentially at the matrix_metallic inclusion To clarify the relation between PEE and SCC, PEE and SCC susceptibility was measured as a function of pre-exposure time, ageing state or applied potential by slow strain rate method. PEE and SCC fracture surfaces were observed with SEM. Sequence of SCC susceptibility with ageing state did not vary with applied potential. This imply that SCC mechanism does not change with applied potential. There was a similarity in ageing state dependence of PEE and SCC susceptibility. Reheat treatment, i.e., resolution treatment in vacuum and ageing in dry air, of the specimen pre-exposed to water vapour saturated air, resulted in pronounced increase in fracture energy relative to that of preexposed specimen, This suggests that PEE is caused by hydrogen embrittlement. When pre-exposure time was large enough (]5day), susceptibility and fracture surface of PEE was very similar to those of SCC. From the above experimental results, it is suggested that SC crack initiation and propagation is caused by hydrogen embrittlement.
In the second, the sCC behaviour of AA-7075 has been investigated by measuring the relative fracture energy under constant strain rate as a function of microstructural features such as precipitate free zone(PFZ) width, grain boundary precipitate size and matrix precipitate size. The SCC specimen were so aged that a given variation in one feature could be produced nearly without a concomitant change in the other variables. The respectively obtained change in microstructrual variable was identified by TEM. Matrix precipitate were found to be of primary importance and grain boundary being of secondary importance. Increasing tendency toward planar slip mode showed increased SCC susceptibility believed to be due to enhanced hydrogen transport by dislocation and higher stress concentration as SC crack tip.
In the third, kinetic of SC crack propagation behaviour of AA-7039 have been studied as a function of temperature, stress intensity factor, applied potential and NaCl concentration using a DCB specimen. Apparent activation energy for the SC crack propagation process in region I was found to be about 97-99 kJ/mole in 2.0-3.0 wt.% NaCl solution, and in region II about 32-38 kJ/mole in distilled water and about 54-65 kJ/mole in 2.0-3.5 wt.% NaCl solution. SC crack propagation processes were discussed in terms of electrode reaction, ion transport between bulk solution and SC crack tip, and hydrogen diffusion ahead of SC crack tip. SC crack propagation rate in NaCl solution was higher than that in distilled water due to enhanced hydrogen ion discharge rate in sC crack resulting from higher hydrogen ion concentration in NaCl solution compared with that in distilled water. The rate controlling step in region II was thought to be hydrogen discharge ($H^+$+e → H_ad$) in distilled water, and hydrogen diffusion ahead of SC crack tip in NaCl solution.
In the fourth, kinetics of corrosion-fatigue(CF) crack propagation in Al-Zn-Mg alloy have been studied as a function of applied potential, stress intensity factor range(ΔK) and temperature in 3.5 wt.% NaCl solution using a center notched specimen. This study especially concerned the SC crack propagation associated with CF crack propagation. CF crack propagation rate increased with increasing applied potential. CF and SC cracks propagated in a brittle intergranular manner. CF crack propagation was thought to be enhanced phenomena of SC crack propagation caused by dynamic straining in CF. By modifying the simple superposition model, synergistic interaction between pure fatigue and environmental effects was taken into consideration in evaluating the CF crack propagation rate. The present experimental data also led to the conclusion that the CF crack propagation rate is proportional to $(ΔK)^2.0_2.2$, and Bolbbbtzmann's factor. The apparent activation energy was found to be about 52 kJ/mole at the corrosion potential. It is suggested from the experimental results that CF crack propagation is controlled by hydrogen embrittlement.
