Abstract
The fundamental parameters of importance in determining the sub-critical crack propagation rate during corrosion fatigue and stress corrosion are reviewed for low- and medium-strength alloys in aqueous solutions. The shortcomings of current analytical methods of distinguishing between slipdissolution and hydrogen-embrittlement mechanisms of crack advancement are exposed. An alternative analytical technique is introduced which compares the maximum observed crack propagation rate with the maximum theoretical rates for both advancement mechanisms; these latter rates are calculated on the basis that they will be dependent on the electrode reaction rates occurring on a region of a crack tip which is continuously maintained in a bare condition. The analysis is applied to the specific case of a medium-strength aluminium 7% magnesium alloy in chloride and sulphate solutions. It is concluded that for this alloy, the environment-controlled component in corrosion fatigue is due to the slip-dissolution mechanism, rather than one based on hydrogen-gas rupture, and that the same basic mechanism applies in stress-corrosion cracking.
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