Corrosion Fatigue of AM50 and AE44 Alloys at Various Temperature and R-Ratio

Reported in this paper are the experimental results obtained from corrosion fatigue tests of high pressure die cast (HPDC) AM50 and AE44 magnesium specimens immersed in NaCl and GM9540P solutions at various temperature, low and high R-ratio. Test specimens were loaded under cyclic loading at temperature of −5°, 25°, and 80°C. It is shown that fatigue life was reduced significantly in these corrosive environments. Although the fatigue life (in air) is longer for specimens tested in −5°C in comparison with the ambient temperature data, the relative reduction of fatigue life in corrosive environments is much significant than that at 25°C.

An investigation of the effect of stress level on AM50 fatigue life shows that, for R=0.1, there is more loss in fatigue life, due to corrosion, when the applied stress is low; when the applied maximum stress is above the yield point, the reduction in fatigue life is relatively less significant. This trend is quite different from the oxidation effects on AM50 fatigue behavior at room temperature. This phenomenon is explained on the basis that pit evolution produced in the corrosive solution is the most dominant factor for fatigue crack initiation, and at lower stresses, there is more time allowed for corrosion to take place. Cyclic testing under very high stress (130% YS) and high R ratio (R=0.9) conditions showed a significant loss of fatigue life by the corrosion environment. Use of a two-layer conversion coating system on AE44 alloys improved considerably the fatigue life, however cracks can form when there is a breakdown in the coating. Further increase in fatigue life required a three-layer coating system.