| Magnesium alloys have a number of desirable features,including low density,high strength-to-weight ratio,good conducts electricity and heat capability,excellent cast capability,good mach inability,damping characteristics and reusing,and its use in a variety of applications,particularly in aerospace,automobile,mechanical,electronics components,military industry.Magnesium alloys are recognized as the green materials in 21st century.However,Magnesium is very active and is susceptible to corrosion when exposed in practical application environment because of the oxidation coating is very loosen.The limited use of magnesium alloys in engineering applications results mainly from its poor corrosion resistance.In consideration of the environment friendly technology,zinc phosphate coating should be one promising method for magnesium alloys anticorrosion and pretreatment process before paint,tends to replace chromate treatment where the environment polluted hexad chromium was involved.Zinc phosphate coatings have been successfully used to underneath the paint of steel and aluminum for many years because they can enhance the adhesion of the paint and substrates.Phosphatization of magnesium alloy is rather difficult in comparison with the traditional phosphatization of steel and aluminum because of high electrochemically activity of magnesium.In the acid phosphating bath resolving rate of magnesium must be restrained in order to obtain dense and high quality phosphate coating.Although the growing interests in magnesium alloys,there are only limited studies about phosphate treatments on the surface of magnesium alloy, and this process is now far from completeness.Therefore,zinc phosphate conversion on the surface of AZ91D magnesium alloy was studied in this dissertation and obtained the phosphate conversion films with well corrosion resistance based on the corrosion mechanism and anticorrosion treatment research of the papers in the recent years.Orthogonal design experiment was used to acquire the basic phosphate solutions and processing parameters.The effects to the surface morphology,chemical composition,phase status and corrosion resistance of zinc phosphate conversion coatings with different additives on magnesium alloy AZ91D were studied.The properties of the coatings were investigated by scanning electron microscopy(SEM), X-ray diffraction(XRD),DSC-TGA electrochemical impedance spectroscopy(EIS), potentiodynamic polarization curves and so on.The formation mechanism of phosphate conversion coatings was also studied.The main works include:(1) The influence of different additives on zinc phosphate coating of AZ91D magnesium alloyPhase components of coatings are different by adding different additives.Results indicated that the phosphate coatings consist mainly of Zn3(PO4)2·4H2O(hopeite) and Zn,when N,N-dimethylformamide,dodecyl sulfate sodium and triethanolamine as additive were added in the phosphate bath.Diethanolamine or monoethanolamine (MEA) as additive phase components of phosphate coatings was Zn3(PO4)2·4H2O phosphate crystals without Zn.An amorphous phosphate layer was obtained by adding ethylenediamine in the phosphate bath.The existence of zinc is not conducive to the corrosion resistance of phosphate coating.And the performance of monoethanolamine was better than the other additives.(2) Design of composite phosphate coatings on AZ91D magnesium alloy by secondary zinc phosphating processA composite double zinc phosphate conversion coating was obtained via forming a hopeite crystal layer on an amorphous phosphate layer on AZ91D magnesium alloy. Amorphous film can supply a equal potential surface on magnesium alloy AZ91D The phosphate crystal clusters can form on the amorphous layer densely and fine and the composite phosphate conversion coating can yield the better protective effect compared with the single layer.(3) The effects to the structure and electrochemical behavior of zinc phosphate conversion coatings with monoethanolamine on magnesium alloy to the AZ91D magnesium alloyThe addition of MEA can be propitious to form new crystal nucleation and make grain refinement.The existence of the MEA altered the preferential orientation of Zn3(PO4)2·4H2O crystal and got a more uniform structure,so that the corrosion resistance of zinc phosphate coating on AZ91D magnesium alloy was promoted effectively.The phosphate coating formed in the bath containing 1.2 compact outer crystal layer just with the least flaws.Its shows the highest the polarization resistance promoted 10 times than that of the bare substrate in 3.5 wt.%NaCl solution and 160 times in the borate buffer solution(4) The micro structure of zinc phosphate coating on AZ91D magnesium alloy and its effect on corrosion resistance.The phosphate conversion coatings formed a duplex structure:a crystal outer layer and an amorphous inner layer.The inner layer was fiat with non-penetrating micro cracks to the substrate.The outer layer was made up by the crystal clusters hopeite (Zn3(PO4)2·4H2O).The outer crystal layer has more effect on impedance resistance of phosphate coatings.According with the size of defect,in order of from small to big as follows:the surface of shallow porosity＜micro flaws between the cracks or micro-cracks＜open defects.(5) Zinc phosphate coating on AZ91D magnesium alloy degradation mechanismThe degradation process of zinc phosphate coating on AZ91D magnesium alloy in borate buffer is divided into three stages:ⅰ) the initial immersion stage,the electrolyte infiltrate the crystal outer layer slowly,the structure of outer layer was unspoiled.ⅱ) the medium-term,defects of the crystal outer layer became expanding for the detrimental effect of corrosion products,anti-corrosion performance of the phosphate coating decline rapidly,ⅲ) late immersion,the corrosion reaction on the inner layer achieved balance,the electrochemical parameters such as resistance of coating is not changed.(6) The influence of Aging time on zinc phosphate coating of AZ91D magnesium alloyThe new prepared work electrode surface unstable with a 'diffusion effect' and its FTIR spectra was with some miscellaneous peaks.After 72 h aging,"diffusion effect" and miscellaneous peaks were both disappeared.Namely,the film needs to go through after the aging 72 h to reach stability.(7) Function mechanism of ethanolamine in the phosphating processFunction mechanism of ethanolamine in the phosphating process is the combined effect of amino group and hydroxyl group in its molecule.Amino group is able to stabilize the complex phosphating bath;N-terminal amines can form salts and the adsorption of the amine salts and hydroxyl on the electrode surface can inhibit the surface dissolution of magnesium alloyα-phase,increased the cathodic polarization, so that increase the grain nucleatio,the grain has been refined.In short,complex role of the amino group and hydroxyl group result in the change of magnesium alloy surface state,and improve the performance the zinc phosphate coating on AZ91D magnesium alloy effectively.(8) Formation mechanism of zinc phosphate coating on AZ91D magnesium alloy Zinc phosphate coating on AZ91D magnesium alloy forming in the phosphating bath process is divided into five stages:the initial nucleation stage,the rapid dissolution stage,the rapid growth stage,steady-state growth stage and the deposition-dissolution equilibrium stage.The hopeite nucleation formation on AZ91D magnesium alloy in the zinc phosphating bath is not in the same time a form,but formedin batches.The first batch of hopeite nucleus grow into grain gradually, continuous nucleation of new generation occurred simultaneity.The coverage of phosphate grain increased gradually the alloy surface,until all the surface completely covered and the boundary of grain growing closed,the phosphating process is end.The formation of nuclei does not occur in the metal matrix of the grain boundary with priority.It is not very different between the internal grain and grain boundary, with the growth of nucleation and its epitaxial the phosphate coating formed of on AZ91D magnesium alloy.
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