The effects of multi-walled carbon nanotubes (MWCNTs) incorporation and polycaprolactone (PCL) post-treatment 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.

In this study, a phosphate-based conversion coating (PCC) was applied as a precursor before forming silicate-fluoride (SiF) and silicate-phosphate-fluoride (SiPF) based flash-plasma electrolytic oxidation (Flash-PEO) coatings on AZ31B magnesium alloy. The main novelty is the successful incorporation of calcium, zinc, manganese and phosphate species into the Flash-PEO coatings via a precursor layer rather than using the electrolyte. The precursor also led to longer lasting and more intense discharges during the PEO process, resulting in increased pore size. Corrosion studies revealed similar short-term performance for all coatings, with impedance modulus at low frequencies above 107 Ωcm2, and slightly better performance for the SiPF-based coating. Nonetheless, the enlarged pores in the PEO coatings functionalized with the PCC precursor compromised the effectiveness of self-healing mechanisms by creating diffusion pathways for corrosive species, leading to earlier failure. These phenomena were effectively monitored by recording the open circuit potential during immersion in 0.5 wt.% NaCl solution. In summary, this study demonstrates that conversion coatings are a viable option for the functionalization of PEO coatings on magnesium alloys, as they allow for the incorporation of cationic and other species. However, it is crucial to maintain a small pore size to facilitate effective blockage through self-healing mechanisms.