Protection properties of conversion coatings based on nitrates for resorbable Mg alloy

In this work, conversion coatings based on nitrates Ca(NO₃)₂ and Zn(NO₃)₂ were produced on the surface of MgZn49Ca4 to protect against corrosion. The main aim of this study was to prepare dense and uniform coatings using a conversion method (based on nitrates Ca(NO₃)₂ and Zn(NO₃)₂) for resorbable Mg alloys. The scientific goal of the work was to determine the pathway and main degradation mechanisms of samples with nitrate-based coatings as compared with an uncoated substrate. Determining the effect of the coatings produced on the Mg alloy was required to assess the protective properties of Mg alloy-coating systems. For this purpose, the morphology and chemical composition of coated samples, post corrosion tests and structural tests of the substrate were performed (optical microscopy, SEM/EDS). Immersion and electrochemical tests of samples were also carried out in Ringer’s solution at 37°C. The results of immersion and electrochemical tests indicated lower corrosion resistance of the substrate as compared with coated samples. The hydrogen evolution rate of the substrate increased with the immersion time. For coated samples, the hydrogen evolution rate was more stable. The ZnN coating (based on Zn(NO₃)₂) provides better corrosion protection because the corrosion product layer was uniform, while the sample with a CaN coating (based on Ca(NO₃)₂) displayed clusters of corrosion products. It was found that pitting corrosion on the substrate led to the complete disintegration and non-uniform corrosion of the coated samples, especially the CaN sample, due to the unevenly-distributed products on its surface.