Effect of Mg concentration on interfacial strength and corrosion fatigue behavior of thermal-sprayed Al-Mg coating layers

This study aims at observing the effect of Mg concentration on the interfacial strength and corrosion fatigue behavior of Al-Mg coating layers on structural steel (SS400) substrates. Al-Mg coating has been applied to components of bridges as sacrifice coating layers. Although Al or Al-Mg coating layers are typically applied to the components located in severely corrosive environment, a mechanism behind the effective protection provided by increasing the concentration of Mg has not yet been clarified. The interfacial strength test using four-point bending revealed that a Al-Mg coating layer of higher Mg concentration showed a higher interfacial strength only before immersion in 3.5-wt% NaCl aq.. After immersion in 3.5-wt% NaCl aq. for 30 days, such the difference in interfacial strength was lost due to rapid dissolution of Mg in the coating layers. As regards the fatigue crack growth behavior, the Al-Mg coating with higher Mg concentration exhibited a lower resistance to vertical crack propagation and interfacial delamination. A fracture mechanics model, which includes both effects of corrosion and delamination/cracking was proposed. Numerical simulation based on the fracture mechanics model successfully predicted exposure lives of substrates due to fatigue failure of the Al-Mg coating layers. These results could provide a selection policy for Al-Mg coating layers in which a loading level and the severity of corrosive environment were considered.