Effect of corrosion on fatigue behaviour of aluminum alloys

Abstract

Corrosion has been and still is a worldwide problem today. Not only has it cost governments and companies millions of dollars, many have lost their lives due to the effects of corrosion on various man-man structures. These structures like bridges and aircraft experience fatigue stresses that causes the periodic propagation of cracks, leading to eventual failure. It has been well documented that corrosion acts as an existing defect, expediting fatigue failure by reducing the number of stress cycles it can withstand at a given stress level. Due to its superior strength to weight ratio, Aluminum is increasingly used in many such structures especially aircraft. 2 commonly used alloys AA2524 and AA7075 were thus analyzed to characterize pitting corrosion.

A confocal imaging microscope was used to analyze the surface of test specimens which varied in both material and corrosion time. The percentage area affected, depth and diameter of several pits were compared and discussed. The rate of increase of corrosion and its effects were also delved into. In depth analysis was done to be able to relate the pit dimensions and rate of corrosion with the composition of each alloy. The effect of corrosion on the fatigue behavior was also analyzed and compared.

The results show that although AA7075 is initially stronger than AA2524, due to its high susceptibility to corrosion, it experiences a more drastic drop in fatigue strength. This causes the fatigue strength of pre corroded AA2524 to be larger than that of AA2524. The results from the Image J processor show that the final depth, diameter and percentage area affected is all larger for AA7075. Although at varying rates, it also shows a general increase in these dimensions with time.

It has been concluded in this report that AA7075 is more prone to corrosion and is weaker than AA2524, due to the extensive amount of corrosion it faces.