Preparation and properties of conversion phosphate-containing coatings on magnesium alloys doped with rare earth elements

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The purpose of our study was to synthesize and analyze the structure, qualitative and quantitative composition, and protective properties of phosphate-containing conversion coatings on WE43, ZRE1, and QE22 magnesium alloys doped with rare earth elements in the Hank’s Balanced Salt Solution. Scanning electron microscopy, energy dispersive X-ray analysis, and X-ray phase analysis methods were used to study the morphology, microstructure, the elemental and phase compositions of QE22, ZRE1, and WE43 magnesium alloys doped with rare earth elements, as well as conversion coatings formed on their surface during phosphating. Linear voltammetry and electrochemical impedance spectroscopy were used to study the kinetic properties of corrosion of the analyzed samples in the Hank’s Balanced Salt Solution (рН = 7.4) imitating the human body environment before and after phosphating. The study showed that the phosphating of magnesium alloys doped with rare earth elements results in the formation of low-soluble fine-grained coatings with a pronounced crystal structure and a thickness from 16 to 21 μm. The obtained conversion coatings are characterized by the following elemental composition: Са ≈ 40 wt.%; Р ≈ 15 wt.%; and О ≈ 35 wt.%. The crystal structure of phosphate-containing coatings is presented by the brushite phase (CaHPO4·2H2O). The electrochemical studies of the corrosion behavior of magnesium alloys in the model Hank’s Balanced Salt Solution (рН = 7.4) demonstrated that the corrosion current density decreases in the sequence QE22, ZRE1, WE43 and is icorr, A/cm2: 5.2·10–5; 2.5·10–5; 2.0·10–5. The obtained conversion coatings based on brushite reduce the corrosion rate of QE22, ZRE1,and WE43 magnesium alloys by 15.2, 7.8, and 6.3 times, respectivel