Micro-arc oxidation of magnesium alloys: a review

In this work, PEO process was carried out on SLM samples of AlSi10Mg, characterized by different grades of porosity, in direct-current mode using high current densities and short time in a basic silicate electrolyte. For comparison, the PEO process was also performed on samples of conventional cast AlSi10Mg alloy. The microstructure and the composition of the coatings was evaluated with SEM and XPS, whereas the phase analysis was performed with XRD. The corrosion resistance was analyzed by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests. The parameters used for PEO process allowed to obtain a continuous coating on all SLM samples, but its morphology resulted strongly influenced by the initial microstructure of the substrate. The coatings more homogeneous and less porous were produced on the samples with initial lower porosity. The corrosion performances of all SLM samples improved after PEO treatment and the PEO coatings with lower porosity resulted the more corrosion resistant.

Effect of microstructure and porosity of AlSi10Mg alloy produced by selective laser melting on the corrosion properties of plasma electrolytic oxidation coatings

Magnesium-lithium (Mg-Li) alloys have become promising metal engineering materials due to the outstanding weight reduction performance. However, the vulnerability to corrosion is a major obstacle for development. A variety of surface treatment methods have been applied to protect Mg-Li alloys against corrosion deterioration, among which micro-arc oxidation (MAO) is an effective and economic method. Additionally, MAO coatings also present high hardness, excellent wear resistance, and strong bonding to the substrate. This article reviews advances in MAO coatings on Mg-Li alloys, which includes summarizations for the MAO parameters and factors affecting the properties of the coatings. The main focus is given to characteristics of the MAO coatings on Mg-Li alloys, as well as the functionalization performance in a wide application. 

Advances in micro-arc oxidation coatings on Mg-Li alloys

Difference in formation of plasma electrolytic oxidation coatings on MgLi alloy in comparison with pure Mg

Formation of plasma electrolytic oxidation coatings on pure niobium in different electrolytes

Influence of secondary phases of AlSi9Cu3 alloy on the plasma electrolytic oxidation coating formation process

In this work, the formation behavior of PEO coatings on polymorph Ti6Al4V alloy with two different lattice structures (hexagonal close packed α-Ti and cubic body centered β-Ti phases) was studied, using two galvanostatically controlled processes. The phase formation as a function of treatment time was studied ex-situ locally on the two Ti phases by interrupting the treatment after certain time intervals and in-situ continuously in an integral manner by recording diffraction patterns from the changing surface during the PEO process. The initial discharges were preferred to start on β-Ti phases localized at the grain boundaries and then extends to the surrounding α-Ti grains. The presence of α- and β-Ti phases was also responsible for the different local coating surface morphology. A highly sintered morphology and larger-sized micro-pores developed firstly on the layer on β-Ti phases, with higher concentration of vanadium. These findings highlight the importance of microstructure of Ti6Al4V alloy on the formation of PEO coatings. Under the selected conditions of PEO processing, the higher current density promotes the growing of the coating and reduces the surface roughness of the resultant coating. The ex-situ characterization of the coatings after certain periods shows that the final coatings mainly consist of anatase, rutile and an amorphous phase containing phosphorous. The formation of rutile, transformed from anatase, is influenced by the effective temperature of the discharges, thus the ratio of anatase to rutile is decreasing with treatment time. The in-situ characterization of phase evolution showed the formation of an amorphous phase in parallel with an expanded α-Ti phase, which is a result of oxygen incorporation into solid solution into the titanium lattice. Interestingly, the main formation of crystalline phases occur only when the HV is turned off and the coating obviously cools down.

Ting Wu

Papers

Influence of secondary phases of AlSi9Cu3 alloy on the plasma electrolytic oxidation coating formation process

Difference in formation of plasma electrolytic oxidation coatings on MgLi alloy in comparison with pure Mg

Formation of plasma electrolytic oxidation coatings on pure niobium in different electrolytes

Role of polymorph microstructure of Ti6Al4V alloy on PEO coating formation in phosphate electrolyte