Papers by Author: Da Yong Shan

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Authors: Yong Wei Song, Da Yong Shan, En Hou Han
Abstract: Magnesium alloys are considered as “difficult to plate” metal. The pretreatment processes play a key role for plating. The pretreatments of alkaline cleaning, chromic acid etching and hydrofluoric acid activating were carried out and the microstructures investigated by SEM and EDAX in detail. The hydrofluoric acid activating time of 8 minutes was determined by Potential-Time curve. The experimental results showed that the alkaline cleaning moved the grease and oils off the substrate surface, but there was no apparent surface morphology change. The chromic acid etching was a very critical step, and the optimum etching time of 1 minute was chosen; the eutectic α was severely etched, and the chromic compounds were mainly formed on the surface. During the hydrofluoric acid activating, the white activating product of magnesium fluoride was preferentially generated on the β–phase, and obvious morphology change has been observed on the alloy substrate surface.
Authors: Rong Fa Zhang, Da Yong Shan, Rong Shi Chen, En Hou Han
Abstract: Micro arc oxidation is an effective method to improve corrosion resistance of magnesium alloys and the selected electric parameters have great effects on coating properties. In this paper, the effects of the unipolar pulse and the bipolar pulse on surface morphology, chemical compositions and corrosion resistance were studied. When working voltage was higher than breakdown voltage, sparks appeared on sample surface whether the unipolar pulse or the bipolar pulse was used. However, under the same positive working voltage, the spark size and lifetime obtained by the bipolar pulse were evidently larger and longer than those by the unipolar pulse, which resulted in rougher anodic coatings and worse corrosion resistance by the former than the latter.
Authors: En Hou Han, Wan Qiu Zhou, Da Yong Shan, Wei Ke
Authors: Zhuo Qun Li, Da Yong Shan, Wei Ke, En Hou Han
Abstract: Microstructural examinations of T6-treated WE54 magnesium alloy were conducted after different aging conditions, and the influence of aging on electrochemical behavior of this alloy was investigated. For three batches of samples, with increasing aging time, the amount of precipitate phases was greatly promoted, and they formed in a strengthening continuous way. Electrochemical study showed that the value of corrosion potential followed the tendency to decrease when the aging condition transformed from under-aged to peak-aged. However, after peak-aging, the corrosion potential was raised to higher values. Explanations for the observed phenomena were based on the relationship between microstructure, especially precipitate phases, and electrochemical behavior. At under-aged condition, matrix served as galvanic cathode and precipitates acted as galvanic anode. Because the amount of precipitate would rise with prolonging aging time, it is natural that corrosion potential decreased. For over-aged alloys, metastable precipitate phases might in situ change to equilibrium phases, rendering increase of corrosion potential possible.
Authors: Da Yong Shan, Rong Fa Zhang, En Hou Han
Abstract: Al2O3 thin film was formed on pure magnesium and its anodic coating by sol-gel method. SEM observation and EDX analysis showed that Al2O3 film could not totally cover the surface of base metal, but they blocked small pores when they were formed on anodic coating. Potentiodynamic polarization test indicated that Al2O3 films improved corrosion resistance of both anodizing coating and base metal, and the effect on anodizing coating was better than on base metal.
Authors: Wan Qiu Zhou, Wei Tang, Da Yong Shan, En Hou Han, Wei Ke
Abstract: Initial corrosion behavior of AZ91 magnesium alloy in simulating acid rain containing Cl- is investigated by SEM/EDX and electrochemical method. The results show that corrosion of magnesium alloy in diluted NaCl solution was localized corrosion, and the attack only occurred around the scattered AlMn particle which induced the departure of AlMn from metal matrix. HSO3- in acid rain induced the dissolution of corrosion in α phase, β phase was not be corroded and presented in passive state. The corrosion of magnesium alloy in simulating acid rain containing Cl- was mainly induced by HSO3-, and the existence of Cl- accelerated the corrosion process.
Authors: Wan Qiu Zhou, Da Yong Shan, En Hou Han, Wei Ke
Abstract: 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.
Authors: Rong Fa Zhang, Da Yong Shan, En Hou Han
Abstract: Substrates have major effects on coating properties. In this paper, pure magnesium, AZ91HP and Mg-Li were separately anodized in a neutral anodizing solution containing hydrofluoric acid, phosphate acid and borate acid. The coating properties such as surface morphology, thickness and compositions were separately observed and detected. The behavior of voltage with time indicated that reaching the same final voltage under the same electric parameters, anodizing time for AZ91HP was the shortest and Mg-Li the longest. The uniformity of anodic coatings formed on AZ91HP was the best and Mg-Li the worst. Analysis by EDX, the phosphorus content of anodic coatings formed on Mg-Li was the highest and that on AZ91HP the lowest. Alloying elements of aluminum and lithium slightly decreased and considerably increased the coating thickness, respectively. The effects of lithium and aluminum on coating properties can be explained by different moving speeds of Mg2+, Al3+ and Li+ as well as Pilling-Bedweorth ratios (PBRs) of MgO/Mg, Al2O3/Al and Li2O/Li.
Authors: Rong Fa Zhang, Da Yong Shan, En Hou Han
Abstract: Among four AZ91 samples, two were once anodized and the others were twice anodized in two electrolytic baths. After twice anodization, two coatings coexist on the base metal. Surface morphology showed that for one of twice anodic coatings (sample 2), the second coating only existed on some places and sealed some pores of the first coating after short time on the second anodizing. However, for another twice anodic coating (namely sample 4), the second anodization spent very long time and the coating was thick and loose. Salt spray testing showed that sample 2 had the most excellent corrosion resistance and the reason is discussed.
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