In vitro corrosion-assisted cracking of AZ31B Mg alloy with a hybrid PEO+MWCNTs/PCL coating

The effects of multi-walled carbon nanotubes (MWCNTs) incorporation and polycaprolactone (PCL) posttreatment on the environmental-assisted cracking behaviour of a plasma electrolytic oxidation (PEO) coated AZ31B Mg alloy were elucidated in this study. Slow strain rate tensile (SSRT) experiments were carried out in simulated body fluid (SBF) for the bare material and different coating systems with and without MWCNTs and PCL overlay. Electrochemical impedance spectroscopy (EIS) and microscopic examinations (SEM and optical) were also conducted to reveal the role of corrosion on the mechanical response. In spite of the significant positive influence of the PEO coatings (with and without MWCNTs) on the bio-electrochemical behaviour of the AZ31B alloy, the environmental-assisted cracking performance was only marginally improved. Furthermore, PEO+MWCNTs/PCL coating system increased the fracture strain of the specimens by 7% compared to the uncoated specimens. Based on the SEM and optical micrographs, hydrogen embrittlement was suggested as the main cause of failure of the coated specimens under the in vitro slow strain rate test conditions.