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A protective novel (MgO) oxide layer (with 91.92% corrosion protection efficiency) was successfully applied on Mg–0.4%Ca alloy using protective oxidation at 390°C (without any pre-treatment). Oxide layer coated Mg alloy was then coated with pure epoxy coating (as top coat). This oxide inner barrier layer protected Mg substrate from 3.5wt % NaCl solution when it passes through from epoxy top layer. Lowest anodic current density and highest charge transfer resistance (1118.124 kΩ cm2) and impedance modulus (│Z│) at low frequency substantiated that the existence of thick epoxy top layer which is integrated (via physical anchoring) with the oxide inner barrier layer could considerably reduce the corrosion rate of magnesium substrate in the chloride containing solutions. Moreover, corrosion mechanism for coated Mg–0.4%Ca alloys was then suggested.

Single NbSi2 and duplex NbSi2/Nb5Si3 coatings were successfully applied on Nb substrates using halide activated pack cementation (HAPC) method at 1100 and 1300°C, respectively. Duplex NbSi2/Nb5Si3 coating with Nb5Si3 inner barrier layer, higher thickness, and lesser micro-cracks considerably decreased the infiltration of simulated body fluid (SBF) solution. This deduction was pertained to the high corrosion potential and lowest corrosion current density for the NbSi2/Nb5Si3 coated Nb alloy in corrosive solution. The Nb5Si3 inner barrier layer can protect the Nb substrate from corrosive solution when it passes through from NbSi2 top layer. Moreover, higher charge transfer resistance (Rct), impedance modulus at a low-frequency ( $$\left| Z \right|$$ ) and phase angle were also observed for the duplex NbSi2/Nb5Si3 coated Nb alloy. On the other hand, in vitro bioactivity test indicated higher capability of apatite formation on the silicide coatings compared to uncoated Nb substrate in SBF solution. In fact, modification of chemical composition and surface morphology of Nb substrate surface can provide suitable nucleation sites for apatite formation and growth in SBF solution.

Plasma electrolytic oxidation (PEO) coatings in the aluminate-silicate-based mixture electrolyte solution with different duty cycles were successfully applied on Mg alloy. The corrosion behavior of the samples was evaluated by water contact angle test, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and immersion tests. Hydrophobic PEO coating could be obtained by adjusting the duty cycle of the applied electric signal. This coating considerably diminished the Mg dissolution and could enhance the impedance values of Mg alloy in 3.5 wt % NaCl solution. However, the surface of other PEO coated samples showed more hydrophilic properties compared to that of the uncoated sample. Dense structure of the modified PEO multiphase (including Mg2SiO4, MgO and MgAl2O4 phases) coating and also its appropriate thickness provided an effective barrier to remarkably delay corrosive solution penetration into the PEO coating. This phenomenon led to major decrease in anodic current density of alloy in chloride solution.