Comparison of plasma electrolytic oxidation coatings on Mg–Li alloy formed in molybdate/silicate and aluminate/silicate composite electrolytes

Abstract

The need of corrosion protection for metals in aggressive environments is an issue of prime importance for widespread applications. In this article, we demonstrate the properties of oxide coatings prepared in molybdate/silicate and aluminate/silicate composite electrolytes via plasma electrolytic oxidation (PEO) on Mg-Li alloy. To understand the nature of the two coatings, the surface and cross-sectional morphologies, phase composition, and chemical composition of the coatings are studied by scanning electron microscopy (SEM), electron dispersion X-ray spectroscopy (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The corrosion resistances of oxide films are evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution. In contrast to Mg-Li alloy substrate, the anti-corrosion properties of the two coated alloys are both remarkably improved. Furthermore, molybdate instead of aluminate as an additive in the electrolyte brings forth a denser and compact coating, which can provide longer corrosion protection for Mg-Li alloys. Thereby a surface protection method based on PEO in molybdate/silicate electrolyte is a promising means for producing a dark brown ceramic coating on the surface of Mg-Li alloys with the capability to corrosion protection.

Effect of Molybdate on Corrosion Performance of Oxide Coating Produced on 7075 Al Alloy Using PEO

“…Molybdate also acts as an oxidant, such as chromate, in the chemical conversion processes, and its adsorbed products can inhibit the penetration of Cl − ions to protect the substrate [10,11]. The inhibitive effect of molybdate on the corrosion of Al alloys has been the subject of several papers [11][12][13]. Wiggle et al [7] studied the effect of molybdate on the pitting corrosion of two Al alloys, AA3003 and AA7072, and found that the corrosion potential becomes nobler, increasing monotonically with the concentration of molybdate ions.…”

Optimization of AZ31B Magnesium Alloy Anodizing Process in NaOH-Na2SiO3-Na2B4O7 Environmental-Friendly Electrolyte

“…Recently, with the enhancement of environmental protection awareness, investigations on phosphorus-free, fluorine-free and chromium-free PEO electrolytes have become an important and urgent research area. Moreover, numerous works have been undertaken to identify environmentally friendly electrolytes, mainly focusing on the silicates system and aluminates system [27][28][29][30][31]. Thus far, most research has focused on the additions of the electrolytes and the electrochemical behavior of the anodic oxidation process [32][33][34][35][36].…”

Tribological Behavior of Al2O3-MoO2-SiO2 Composite Ceramic Coating on Al-Zn-Mg-Cu Alloy

“…As the concentration of Na 2 WO 3 in the electrolyte increases, the roughness decreases from 2.00 to 1.67 µm, the hardness increases from 1025 to 1465 HV and the wear performance and corrosion resistance of the coating improve. Li et al [26] report that the MAO coating formed in the silicate-based electrolyte containing Na 2 MoO 4 can enhance the wear properties of the Mg-Li alloy due to the addition of molybdenum oxide, compensating the structural defects of the MAO coating. Kaseem et al [27]…”