Corrosion fatigue behavior of conversion coated and painted AZ61 magnesium alloy

The present study has been carried out in order to study the fatigue behavior of conversion coated and painted AZ61 magnesium alloy under different corrosive environments: (a) low humidity environment, (b) high humidity environment (80% relative humidity) and (c) 5% NaCl environment. It was found that under low humidity and high humidity environments, the coated and painted specimen showed the same level of fatigue limit, which was almost equal to that of bulk material under low humidity environment. In contrast, under NaCl environment, the coated and painted specimen showed about 11% of reduction in fatigue limit, while that of bulk specimen was about 85% of reduction. Fracture surface observations of the coated and painted specimens under high humidity and NaCl environments showed no existence of environmental attack in the crack nucleation region, where fatigue cracks were nucleated from the surface of coating and painting layer, which then propagated towards the substrate. The absence of environmental attack in the crack nucleation region suggests that the coating and painting layer can perfectly protect the substrate material from the attack of corrosive environments.

► Fatigue strength under high humidity and NaCl was effectively improved by coating.
► No effect of environmental attacks such as corrosion pit was observed.
► Fatigue process was dominated by a single crack nucleated from the surface.
► The coating can protect the substrate from the attack of corrosive environments.