Investigation on corrosion fatigue crack growth rate in 7075 aluminum alloy

The corrosion fatigue crack growth behaviors of 7075 alloy in seawater were investigated. The results showed that corrosion solution concentration, pH value, temperature as well as load frequency have great influences on the corrosion fatigue crack growth rate. The corrosion fatigue crack growth rate in 7075 alloy accelerates with increasing the corrosion solution concentration, raising the temperature, or lowering the pH value. The above three factors can be simply expressed by the surface current intensity, Icor. On the other hand, the corrosion fatigue crack growth rate increases with decreasing the load frequency, due to the longer exposure of the new-generated specimen surface to the corrosive environment. The fracture surface indicated that the corrosion pits extending along the direction of the crack growth are responsible for the acceleration of the crack growth rates, since the corrosion pits may act as physical defects and stress concentrations around the crack-tip. Furthermore, the electrochemical reaction occurring between the new-generated surface and the corrosion solution makes the crack closure difficult, which also facilitates the crack growth. In addition, a modified Paris model has been proposed on the basis of the experimental results, consisting of the surface current intensity Icor and the load frequency f.