One-Step Fabrication and Localized Electrochemical Characterization of Continuous Al-Alloyed Intermetallic Surface Layer on Magnesium Alloy

Improvement of AZ91 Alloy Corrosion Properties by Duplex NI-P Coating Deposition

The corrosion behavior of duplex Ni-P coatings deposited on AZ91 magnesium alloy was studied. The electroless deposition process of duplex Ni-P coating consisted in the preparation of low-phosphorus Ni-P coating (5.7 wt.% of P), which served as a bond coating and high-phosphorus Ni-P coating (11.5 wt.% of P) deposited on it. The duplex Ni-P coatings with the thickness of 25, 50, 75 and 100 µm were deposited on AZ91 magnesium alloy. The electrochemical corrosion behavior of coated AZ91 magnesium alloy was investigated by electrochemical impedance spectroscopy and potentiodynamic polarization method in 0.1 M NaCl. Obtained results showed a significant improvement in the corrosion resistance of coated specimens when compared to uncoated AZ91 magnesium alloy. From the results of the immersion tests in 3.5 wt.% NaCl, 10% solution of HCl and NaOH and 5% neutral salt spray, a noticeable increase in the corrosion resistance with the increasing thickness of the Ni-P coating was observed.

A Zinc-Rich Coating Fabricated on a Magnesium Alloy by Oxide Reduction

The corrosion resistance of magnesium alloys could be enhanced by covering metallic coatings on the surface. The zinc-rich coating is one of these metallic coatings. To fabricate a zinc-rich coating on magnesium alloys, the substrate should be pretreated carefully, and a protective atmosphere is usually required. In this research, a zinc-rich coating was successfully fabricated on the AZ91D magnesium alloy in air by a diffusion alloying method, with zinc oxide as the zinc source. At the same time, the pretreatment of the magnesium alloy matrix was greatly simplified. The as-diffusion-alloyed zinc-rich intermetallic layer was investigated, utilizing SEM, EDS, and XRD, respectively. It is inferred that zinc oxide was reduced into Zn atoms by the active Mg atoms, and the Mg atoms were coming from the magnesium alloy matrix. Then the Zn atoms passed through the oxide film and formed an intermetallic layer on the magnesium alloy surface. Thus, taking advantage of the activity of Mg atoms, magnesium alloys could be surface alloyed with oxides.

Corrosion Behavior of Fe/Zr Composite Coating on ZK60 Mg Alloy by Ion Implantation and Deposition

The Fe/Zr composite coating was prepared by duplex Fe/Zr ion implantation and deposition to modify the microstructure and corrosion behavior of Mg-5.5 Zn-0.6 Zr (in wt.%, ZK60) alloy. The surface and interface characteristics were investigated using X-ray diffraction (XRD), atomic force microscope (AFM) and scanning electron microscopy (SEM). The results showed that the Fe/Zr composite coating exhibited a bi-layer microstructure of outer Fe-rich layer and inner Zr-rich layer. Multi-phases of α-Fe, ZrO0.35 and Zr6Fe3O were formed on the modified surface. The electrochemical measurements and immersion tests revealed an improvement of corrosion behavior for the surface-modified sample due to the protective effect of Fe/Zr composite coating.