Effect of phosphate conversion film on fatigue and corrosion fatigue behavior of an as-rolled Mg–3.08Zn–0.83Al (in wt.%) alloy

Through investigating and comparing the fatigue behavior of an as-rolled Mg–3.08Zn–0.83Al (in wt.%) alloy performing surface phosphate conversion film treatment, it revealed that the determined fatigue strength of surface treated samples at 10 6 cycles in air was 65 MPa, whereas the fatigue strength was only 35 MPa when tested in 3.5 wt.% NaCl solution. Failure analysis demonstrated that in air, the fatigue crack initiation was mainly dominated by the interaction between the retarding effect of phosphate conversion film on cyclic slips occurring in the underneath substrate. When the matrix cannot endure the accumulated stress concentration due to the irreversibility of cyclic slips, the fatigue crack will preferentially initiate at sample subsurface. Since the phosphate conversion film cracked easily under the cyclic loading and lost its protectiveness on the substrate in 3.5 wt.% NaCl solution, fatigue cracks were preferentially nucleated at the localized corrosion pits.