| Magnesium and its alloy offers some advantages, such as low density, higherspecific strength and rigidity, good shock absorption, cutting performance, conductivityand heat conductibility, and magnesium and its alloy as an environmental-friendlymaterial is easy to be casted, it has known as the “21stCentury Green EngineeringMaterial”. Micro-arc Oxidation is a new surface treatment technique, developed fromanodic oxidation, it uses micro-arc discharge in the metal oxide ceramic membrancesurface can increase the corrosion resistance of magnesium alloy. In this paper, aprocess of forming composite coating on magnesium alloy AZ91D under micro-arcoxidation and combining chemical conversion was developed. Oxide films have beenproduced with and without addition of powders. The surface and cross-sectionmorphology of samples were analyzed by Scanning Electron Microscopy(SEM)coupled with Energy Dispersive Spectroscopy(EDS), X-ray diffraction(XRD) and thegalvano-chemistry tests.Composite coatings with addition of nanoparticles Al2O3have been produced onAZ91D magnesium alloy by micro-arc oxidation. Researched the effect of nanoparticlesAl2O3and ultrasonic waves on film. Results are showed that after adding Al2O3powderspores in ceramic coatings decreased and the loose layer became more compact, newphase of Al2O3appeared; the Al element of the ceramic membrane elements increased.Ultrasonic waves promote the content of Al2O3in the coating. Electrochemicalcorrosion experiments in3.5%NaCl solution showed that corrosion resistance ofceramic coatings added Al2O3powders was also improved greatly.Composite coatings with additon of nanoparticles TiO2have been produced onAZ91D magnesium alloy by micro-arc oxidation. Researched the effect of nanoparticlesTiO2on film.Results are showed that after adding nanoparticles TiO2pores in ceramiccoatings decreased and the loose layer became more compact, new phase of titaniumoxide appeared. Electrochemical corrosion experiments in3.5%NaCl solution showedthat corrosion resistance of ceramic coating added TiO2powders was also improved.And the ceramic coating had photocatalysis which realized the muti-function on the coating.Oxide coatings have been procuced on AZ91D magnesium alloy by micro-arcoxidation with addition of metal particles(Ni, Fe, Ti, Co). The corrosion resistance ofceramic coatings of process optimization was analyzed by the salt water immersion testsand the galvano-chemistry tests. The cossosion resistance of ceramic coating ofmicro-arc oxidation with differernt metal particles were compared and Ti, Ni compositecoatings were particularly significant. The corrosion morphology of the matrix alloy iscomplete corrosion, while that of the anodized film is pitting, the lightest pitting is thepowder composite ceramic coating, indicating the superior in corrosion resistance of thepowder composite ceramic coatings. With the addition of metal particles, the surface ofceramic coating was relatively smooth and dense, but still existed some smallmicro-porous. Vertical cross-section of ceramic membrane showed it was composed ofan outer loosed layer and an inner compact layer. Owing to the metallurgical bonding,the inner layer had good interfacial connection with the substrate, the outer layer hadmicro-porous. Electrolyte powder went into the micro-arc oxidation coating and existedin the form of oxides in the coating. The phase composition of the coating can beadjusted by changing the parameters of powder concentration, which was conductive tothe further modification of the coating.Combined the chemical conversion and micro-arc oxidation new processed to formblack composite ceramic coating. Researched the effect of transformation solutioncomposition on micro-arc oxidation film. Used the L9(34) orthogonal test optimized thechemical conversion treatment fluid best technological conditions: K2MnO4+Na2MoO48g/L, Ni(CH3COO)2·4H2O8g/L, Time:10min, pH=3. Results showed Mn,Mo, Ni elements were into the ceramic coating through the chemical conversionpretreatment. The composite coating contained NiO, Mn5O8, MoO3. The surface ofcoating was smooth and compact.72hours salt water immersion tests and thegalvano-chemistry tests showed the coaing corrosion resistance was improved. |
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