Corrosion Resistance of Superhydrophobic Mg(OH)2/Calcium Myristate Composite Coating on Magnesium Alloy AZ31

Hydrophobic Mg(OH)(2)/calcium myristate (Ca[CH3(CH2)(12)COO](2), CaMS) and Mg(OH)(2)/magnesium myristate (Mg[CH3(CH2)(12)COO](2), MgMS) composite coatings were prepared on the alkali-treated AZ31 substrates via both electrodeposition and dipping methods. The morphologies, compositions, and constitutes of the coatings were investigated by using field-emission scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometry (FTIR) together with X-ray diffractometer (XRD). Furthermore, electrochemical (polarization and impedance spectroscopy) and hydrogen evolution tests were applied to evaluate the corrosion resistance. The wettability and adhesive strength of the composite coatings were characterized through water static contact angle (CA), sliding angle (SA), and nano-scratch tests. Results indicated that the better super-hydrophobicity, corrosion resistance, and adhesion were achieved via an electrodeposited process. The corrosion current density (i(corr)) and hydrogen evolution rate (HER) of the electrodeposited coating were three and one orders of magnitude smaller than the substrate, implying a significantly improved corrosion resistance. This scenario was ascribed to the super-hydrophobicity of electrodeposited composite coating with a contact angle (CA) and slide angle (SA) of 159.2 degrees +/- 0.8 degrees and 5.2 degrees +/- 0.8 degrees, respectively. However, the dipped composite coating was adverse to the improvement of corrosion resistance and adhesion due to the dissolution of the underlying Mg(OH)(2) layer and smooth surface with less organic fatty acid salt (MgMS)