The Effect of Aluminum Cold Spray Coating on Corrosion Protection and Corrosion Fatigue Life Enhancement of Magnesium Alloy, AZ31B
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Magnesium alloys are of a recent interest for the transportation industry due to their excellent properties such as high strength and low density which will save energy and reduce gas emission and it will also improve the vehicle performance. However due to their high chemical activity, magnesium and magnesium alloys have unsatisfactory corrosion resistance and high tendency to corrode in humid and aqueous environments. Although alloying elements provides some improvement in magnesium against corrosion, further protection for these alloys is needed against the corrosion in different corrosive environments. Aluminum powder cold spray is a new coating technology with very promising results in corrosion protection of magnesium alloys. The effect of applying pure aluminum cold spray coating on wrought AZ31B Mg alloy from the corrosion and corrosion fatigue point of view is studied. This research comprises two parts. The first part is studying the corrosion behavior of AZ31B cold spray coated and uncoated coupons by performing an accelerated corrosion testing in a corrosion chamber. The results for both types of coupons have been compared to each other. The second part of this research is to study the fatigue strength by rotating bending machine of stress relieved and stress relieved/coated specimens in salt water environment. S-N curves for the two groups of specimens were plotted. After the analysis and comparison of all the testing results, it was revealed that pure Al cold spray coating considerably improved the corrosion resistance of Mg alloy AZ31B in 5% NaCl fog environment. The microstructural analysis revealed the presence of some secondary phases which act as a cathode and accelerate the corrosion of the anode, Mg alloy. Comparison of corrosion-fatigue S-N curves of stress relieved and stress relieved/coated specimens show fatigue life reduction after cold spray coating to a maximum percentage of 87.6%. The reason for that is the fact that the pure Al powder has much lower ultimate tensile strength than Mg alloy AZ31B. This fact will lead to an earlier crack on the Al coating surface during the fatigue cycles, from which the electrolyte will penetrate to the Mg substrate and cause a localize corrosion and failure.