Kinetics of corrosion fatigue (CF) crack propagation behaviour of Al-Zn-Mg alloy have been studied as a function of applied potential and temperature in 3.5 wt% NaCl solution using a center notched specimen. CF tests were carried out over a range of temperature from 303 to 348 K and over a range of applied potential -840 to -640 $mV_{SHE}$ with a loading frequency of 3 cpm. Stress-corrosion cracking (SCC) tests were also carried out in 3.5 wt% NaCl solution using a center notched specimen to validate superposition model. A continuously recording electrical potential system was used for monitering crack propagation.
CF crack as well as stress-corrosion (SC) crack propagated in intergranular mode, irrespective of temperature and applied potential. This is due to low loading frequency. Since there were similarities in applied potential dependence of CF and SC crack propagation and in CF and SCC fracture surfaces, CF crack propagation was thought to be enhanced by SC crack propagation caused due to dynamic straining at a crack tip during CF. CF crack propagation rate, $(da/dN)_{CF}$, is the sum of three components - the fatigue crack propagation rate in a inert environment, $(da/dN)_{F}$, which represents the contribution of pure fatigue, a cycle-dependent component, $(da/dN)_{cf}$, that requires the synergistic interaction of fatigue and environmental attack, and the contribution by SCC, $(da/dN)_{scc}$.
Apparent activation energy for CF crack propagation process was found to be about 52 kJ/mole at corrosion potential and independent of stress intensity factor range. CF crack propagation rate was proportional to the square of stress intensity factor range and was expressed as follows
$(da/dN)_{CF} = A (\Delta K)^2 exp(-Q/RT)$
A equation for crack propagation rate was equivalent to the equation for hydrogen embrittlement crack propagation rate derived by H.W. Liu. It is suggested on the basis of CF and SCC fracture surface observation and dependence of CF crack propagation rate on the stress intensity factor range that CF crack propagation is caused by hydrogen embrittlement.
