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Conversion coatings on cast AZ91D magnesium alloy were prepared in the manganese dihydro phosphate baths. The corrosion behavior of the coated and uncoated alloys has been investigated by polarization curve methods. The morphology and composition of coated surfaces were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX ) as well as X-ray photoelectron spectroscopy (XPS) techniques. It is found that the conversion coating surface characterized by crystal particle and was composed of Mn3(PO4)2. Electrochemical investigation results indicated that the phosphate conversion coating could enhance the corrosion potential in great extent of about 1300mV compared with Mg alloy substrate.

Phosphate conversion coatings was investigated on the surface of AZ91D magnesium alloy in a manganese dihydro phosphate baths. The influence of Ca2+ on coating microstructure was evaluate by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques, and the corrosion resistance of the coating was tested by polarization curve methods in 3.5%NaCl. Surface observation results show that the conversion coating presents in crystal feature, and the coating containning Ca was uneven with large grains and small grains. The adding of Ca2+ compound in the bath was found to be important for the attainment of good combination between the coating and Mg alloy substrate. Polarization curve results indicate that the phosphate conversion coating containing Ca could enhance the corrosion potential and improve corrosion resistance.

A non-chromate conversion coating was prepared from molybdate based bath on the surface of Zn-5%Al hot dip galvanized steel. The influence of the preparation conditions such as temperature, bath pH and treating time on the corrosion resistance of the obtained conversion coating was investigated. The corrosion performance of the film was researched using electrochemical technique in 3.5%NaCl. The results indicate that the anodic branch of the polarization curves present in passivation character. Electrochemical impedance spectroscopy (EIS) illustrate that the radius of the capacity impedance for conversion coating varies distinctly according to the preparation condition. The surface morphology was observed using scanning electron microscopy (SEM), and the results show that the surface of Zn-5%Al hot dip galvanized steel present in eutectic feature after conversion coating treatment. The chemical composition of the coating was detected by energy dispersive X-ray (EDX), and the results illustrate that the conversion coating is composed of Zn, Al , O, P and Mo.

A conversion coating was obtained in the molybdate based bath on hot dip galvanized steel. The influence of the bath service life and bath component variation on the corrosion resistance of the conversion coating were investigated. Corrosion resistance of the coating was evaluated by electrochemical technique in 3.5%NaCl. It is found that the anodic branch of polarization curves presented in passivation character. Electrochemical impedance spectroscopy (EIS) results confirm that the radius of the capacity impedance for molybdate film decreased with treating area, which reveals that the bath life and bath component variation affected anti-corrosion performance of the coating remarkablely. The bath constituent was analysed by inductively coupled plasma atomic emission spectrometry (ICP), and the results show that the bath will lose the passivation ability after treating 1.16m2sample area for new prepared treating solution of 500ml. The surface morphologies of the coatings were observed using scanning electron microscopy (SEM), and the element composition of the coatings was detected using energy dispersive X-ray (EDX). The investigation results indicate that uniform and compact film is formed on the surface of the galvanized steel, and the film is composed of zinc,oxygen, phosphor and molybdenum.

Molybdate-phosphate conversion coating was prepared by adding addition agent towards molybdenum-phosphorus compounds based bath on comercial hot dip galvanized steel. The anti-corrosion performance of the coating was evaluated by electrochemical technique in 3.5%NaCl. It is found that the anodic branch of polarization curves presented in passivation character. Electrochemical impedance spectroscopy (EIS) results indicate that the radius of the capacity impedance for molybdate-phosphate film increased distinctly with pH decrease, which validate the excellent anti-corrosion performance of the film. The surface morphologies were observed using scanning electron microscopy (SEM), and the results show that the surface of hot dip galvanized steel is covered by a spiculate and floccule shape conversion coating. The chemical composition of the surface coating was investigated by energy dispersive X-ray (EDX), which illustrate that the conversion coating is composed of zinc,oxygen, phosphor and molybdenum.

Manganese phosphate conversion coating was prepared by adding addition agent towards phosphate bath. The structure was investigated using SEM/EDX, and the anti-corrosion performance was evaluated by electrochemical technique. It was found that the film presented in crystal structure, and the crystalloid grain transformed from large grain toward small particles upon the addition. The corrosion resistance was studied by polarization curves and EIS in 3.5%NaCl. Polarization curves showed that the corrosion potential of the coating shifted remarkably towards positive direction comparing with that of AZ91 substrate. EIS results indicated that the radius of the capacity impedance for phosphate film increased distinctly, which validated the excellent anti-corrosion performance of the film.

Phosphate conversion coating, which is considered as all alternative to chromium conversion coating for improving the corrosion resistance of die-cast AZ91D magnesium alloy is studied. The structure and formation mechanism of the coating was investigated in details using ESEM/EDX, XRD, EPMA, ICP and electrochemical method. It was found that the conversion coating was composed of complex phosphate containing magnesium and aluminum and showed amorphous structure. The thickness of the coating was about 10 mu m. A possible formation mechanism for phosphate conversion coating was proposed. Local pH rise due to the evolution of hydrogen in the vicinity of micro-cathode attributed to the movement of hydrolysis reaction balance for manganese dihydro phosphate in treating solution and resulted in the phosphate coating precipitated oil metal surface. Initial depositing position was related to the microstructure of substrate. Galvanic effect between alpha phase and beta phase caused the flower-like phosphate nuclei preferential depositing oil beta phase. Ball-like phosphate nuclei deposited mainly in alpha phase interior due to the micro-galvanic effect within alpha phase. The primary alpha phase was removed from the metal matrix due to the corrosion dissolution, which suggested the anodic dissolution mechanism during phosphate conversion coating growth. The inner stress in dried film resulted in random distribution of the network micro-cracks oil film surface, the crack site did not correspond to beta phase. (C) 2007 Elsevier Ltd. All rights reserved.

A phosphate conversion coating was deposited on diecast AZ91D magnesium alloys, the film was a complex phosphate containing Mg and Al,which showed amorphous structure. Corrosion resistance of conversion coating in 3.5%NaCl was investigated by polarization curve. It was showed that the anodic branch of polarization curve for the phosphate conversion coating presented typical passivation characteristic. Immersion test results showed that the corrosion rate of phosphate conversion coating was less than that of chromate Dow7 film.