Surface Engineering

Oxide based ceramic conversion layers containing Ca and P were prepared on AZ91D Mg alloy by plasma electrolytic oxidation technique in Na₂SiO₃ and NaOH systems respectively. The conversion layers prepared in both systems were porous and grey white. The doping of Ca decreased the porosity of the conversion layers in Na₂SiO₃ system, while increasing the porosity of and the microcracks of the conversion layers in NaOH system. The conversion layer prepared in NaOH system was composed of MgO, and the one prepared in Na₂SiO₃ system was composed of Mg₂SiO₄ and MgO; P and Ca only existed in the form of amorphous state in the conversion layer. The doping of Ca decreased the relative content of P while increasing the relative content of Ca in both conversion layers. The ceramic conversion layers in both systems improved the corrosion resistance of the AZ91D Mg alloy in 0·9%NaCl solution by one or two orders of magnitude.

AM50 magnesium alloy is treated by PEO process using four different electrolytes which are combination of K₃PO₄ and Na₃PO₄ plus two different hydroxides, i.e. KOH and NaOH. Basic properties of the electrolyte such as pH, conductivity and breakdown voltage were measured. Moreover, coatings were characterised by scanning electron microscopy for the surface morphology and cross sectional study, X-ray diffraction for determination of phase composition and electrochemical impedance spectroscopy (EIS) for evaluation of corrosion resistance. The results indicate that KOH solutions need higher breakdown voltage and result in higher thickness of coatings. The values of the latter parameters are in the range of 150–160 V and 14–17 μm respectively. While those of NaOH solutions are in the range of 106–116 V and 5–6 μm thick. The final results show that presence of KOH in the solutions leads to a better corrosion performance of the coating compared to NaOH.

A composite double zinc phosphate conversion coating was obtained via forming a hopeite crystal layer on an amorphous phosphate layer on the AZ91D magnesium alloy. The surface morphology, structure and phase composition of the composite coating were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). As a comparison, the single zinc phosphate coating and the bare substrate were also discussed. The phosphate crystal clusters can form on the amorphous layer densely and fine. According to the open circuit potential (OCP) and potentiodynamic polarisation curve tests, the composite phosphate conversion coating on the AZ91D magnesium alloy yielded the best protective effect compared with the single amorphous layer and crystal layer.

An environmentally friendly anodising electrolyte for the corrosion protection of magnesium alloy AZ31 was optimised using orthogonal experimental design of four factors with three levels. Electrochemical tests were conducted to determine the corrosion resistance of anodic film. The microstructure and phase composition of the optimised film were characterised by scanning electron microscope and X-ray diffraction respectively. The results show that the optimised electrolyte is composed of 60 g L⁻¹ potassium hydroxide, 20 g L⁻¹ sodium silicate, 30 g L⁻¹ sodium tetraborate and 10 g L⁻¹ sodium phosphate. The significance of electrolyte factors affecting the corrosion resistance of anodic film can be ranked as sodium phosphate>sodium silicate>sodium tetraborate>potassium hydroxide. The optimised film is porous like the basic film, but it is more compact and therefore more corrosion resistant than the basic film. The optimised film is mainly composed of MgO, Mg₂SiO₄ and MgP₄O₁₁.

A cast AM50 magnesium alloy was plasma electrolytic oxidation treated in a silicate based electrolyte using a direct current power source. Plasma electrolytic oxidation coatings produced at four different visible discharge conditions were characterised for their microstructural features, composition, tribological behaviour and corrosion resistance. The chemical composition, thickness and the roughness of the coating were found to be influenced by the processing voltage and thick coatings produced at relatively higher voltage levels provided a better wear and corrosion resistance.

An attempt has been made to enhance the tribological properties of Mg–11Y–2·5Zn alloy by laser surface melting with a 6 kW continuous wave CO₂ laser processing system. The microstructure and microhardness of the surface layer on Mg–11Y–2·5Zn alloy were characterised using X-ray diffractometer, laser microscopy and Vickers hardness indentation. The laser surface melted zone consisted of fine dendrites and coarse dendrites growing epitaxially from the liquid/solid interface. Microhardness was improved from 69–70 HV for the substrate to 77–83 HV for the fine dendritic microstructure. The friction and wear characteristics were investigated using a pin on disc apparatus. The coefficient of friction curve of the laser surface melted specimen was similar to that of the untreated specimen. Laser surface melted Mg–11Y–2·5Zn alloy exhibited good wear resistance, which has been explained by refinement of microstructure in the melted zone. Four wear mechanisms including abrasion, delamination, thermal softening and melting, have been observed.

The corrosion behaviour of an AZ31 magnesium alloy coated with plasma electrolytic oxidation (PEO), polymer and a combination of them was assessed by electrochemical impedance spectroscopy (EIS) in 0·1M NaCl solution. The polymer coating was applied on a just cleaned surface (without any special preparation) and also on PEO treated magnesium substrate. While the polymer coated and the mere PEO treated magnesium alloy specimens lasted less than 50 h in the EIS tests, the specimens with the duplex coating (PEO+polymer) successfully resisted 1000 h in the EIS tests without showing any degradation. The improved adhesion of the polymer coating in presence of the PEO layer and the effective sealing of the pores in the PEO coating with the polymer were responsible for the enhanced corrosion resistance. The synergistic beneficial effect of the duplex coatings on the corrosion behaviour was also witnessed in the salt spray tests.

A novel galvanic conversion coating was studied on magnesium alloy AZ31B in an electrolyte system consisting of ammonium molybdate and magnesium chloride. The conditions of the conversion coating process and the characteristic of the coatings are studied in detail. The coatings were characterised by energy dispersive X-ray, infrared spectral and scanning electron microscopy studies. The thermoanalytical investigations have been carried out using thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). The space worthiness of the coating was evaluated by environmental tests, namely, humidity, thermal cycling, thermovacuum performance and thermal stability tests. Optical properties (solar absorptance and infrared emittance) were measured before and after each environmental test to ascertain its stability. The coating provides higher solar absorptance and infrared emittance in the order of ∼0·80. The developed procedure is simple, ecofriendly and economically viable.

Plasma electrolytic oxidation of AM50 magnesium alloy was performed in alkaline phosphate electrolyte with and without the addition of titania sol. The coatings produced in the phosphate electrolyte were constituted with MgO and Mg₃(PO₄)₂. The coatings obtained in the phosphate electrolyte with the addition of titania sol were blue in colour and contained additionally the TiO₂ and Mg₂TiO₄ phases. The phosphate PEO coating provided a two order of magnitude improvement in corrosion resistance to the AM50 magnesium alloy as was shown by the potentiodynamic polarisation measurements. With differences in the physical appearance, microstructural morphology and phase composition, the coatings produced in titania sol containing electrolytes offered an improved corrosion resistance compared to the mere phosphate PEO coating.

A phosphate–silicate composite coating was investigated for AM60 magnesium alloy. The coating was achieved in a phosphate–permanganate solution which involved sealing in sodium silicate solution. The morphology, composition and corrosion resistance on the coatings were analysed. The phosphate–permanganate conversion coating was shown to exhibit network feature and plenty of cracks. The coating containing increased Mn element was produced using acetic acid to adjust the pH of the phosphating bath. The corrosion resistance of the coating was improved. The phosphate–silicate composite coating became more compact. The cracks on the phosphate–permanganate conversion coating were filled in by silica gel through sealing. The main elemental compositions of the phosphate–silicate composite coating were Mg, O, K, P, Mn and Si. The electrochemical polarisation curves demonstrated that the phosphate–silicate composite coating provided more effective protection against corrosion than that of phosphate–permanganate conversion coating and traditional chromate conversion coating.

Using potentiodynamic polarisation, polarisation resistance and electrochemical impedance spectroscopy measurements, the effect of amide treatment to magnesium metal matrix composite in 0·6 N sodium chloride medium has been studied. The corrosion potential E_corr, corrosion current I_corr, polarisation resistance R_p and charge transfer resistance R_ct were determined from the corresponding plots. E_corr shifted towards more negative values while I_corr decreased with increasing treatment time. Surface analysis with SEM and AFM showed an improvement in surface due to formation of a film. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) studies confirmed the film formation and the coordination of magnesium to the oxygen in the amide compound.

Plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy was investigated in an alkaline electrolyte containing 100 g L⁻¹ sodium hydroxide, 60 g L⁻¹ sodium silicate, 20 g L⁻¹ sodium tetraborate and 50 g L⁻¹ sodium citrate. Different oxide coatings were prepared on AZ31 substrate by changing the oxidation time or current density. The corrosion resistance of PEO coatings was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarisation techniques. The microstructures and composition of the PEO coatings were examined by scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction. The results indicate that the coating prepared at 20 mA cm⁻² for 40 min has good corrosion resistance, and its average thickness is 48·1 μm. Similar to other PEO coatings, the coating is porous and mainly composed of MgO and Mg₂SiO₄. The elements of O, Mg and Si are distributed uniformly across the coating.