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Jiang J.,Hohai University | Jiang J.,Changzhou Hohai Technology Co. | Ma A.,Hohai University | Ma A.,Changzhou Hohai Technology Co. | And 9 more authors.
Journal of Materials Science | Year: 2012

Ultrafine-grained (UFG) Al-26 wt% Si alloy was obtained through multipass equal-channel angular pressing (EACP) procedure and subsequently tested in 3.5 wt% NaCl solution for the evaluation of electrochemical corrosion. The results show that the ECAPed alloy with increased number of pressing passes obtain lower mass-loss ratios, nobler E corr and E pit, lower Icorr values, and higher anode polarization. The improved corrosion resistance of the ECAPed alloy results from the homogeneous UFG structure with the breakage of brittle large primary silicon crystals, which contributes to a higher pitting resistance. The oxidation product with improved adhesion force and protection efficacy can be formed with greater ease on UFG alloys. It implies that grain refinement through severe-plastic-deformation can enhance anticorrosion behavior of hypereutectic Al-Si alloys, besides the well-known strengthening and toughening effects. © 2012 Springer Science+Business Media, LLC.

Jiang J.,Hohai University | Jiang J.,Changzhou Hohai Technology Co. | Zhou Q.,Hohai University | Yu J.,Baoyou Electrical Equipment Manufacturing Co. | And 8 more authors.
Surface and Coatings Technology | Year: 2013

By micro-arc oxidation (MAO) process in a silicate electrolyte, the oxide coatings were successfully formed on a coarse-grained casting AZ91D Mg alloy and an ultra-fine grained (UFG) one fabricated by equal-channel-angular-pressing (ECAP). The effect of substrate microstructure on morphologies and corrosion resistance of the MAO coatings on AZ91D alloys were investigated by scanning electron microscopy observation, immersion tests and electrochemical impedance spectrum (EIS) measurements. In comparison, the MAO coating on the UFG Mg alloy has lower corrosion rate and larger Rf value. Three factors enhance the corrosion resistance. The first is the MAO coating with uniform size and distribution of micro-pores due to homogenous micro-arc discharge. Secondly, the number of open holes decreases due to the finer β-phase after ECAP process. The most important one is the higher compactness and thickness of the coating on the UFG Mg alloy. It implies that the MAO coatings have a great potential in surface protection of UFG Mg alloys. © 2012 Elsevier B.V.

Zhou Q.,Hohai University | Ma A.-B.,Hohai University | Ma A.-B.,Changzhou Hohai Technology Co. | Jiang J.-H.,Hohai University | And 4 more authors.
Corrosion and Protection | Year: 2013

The micro-arc oxidation (MAO) coatings were successfully formed on a bulk ultra-fine grained (UFG) AZ91D alloy in a silicate electrolyte, and then the MAO process parameters were optimized via an orthogonal test to improve the surface protection of the UFG Mg substrate. The optimum MAO parameters are 400 V of maximum peak forward voltage, -15 V of peak negative anode voltage, 600 Hz of operating frequency, 30% of duty cycle ratio and 20 min of oxidation time. The MAO coating formed under the above conditions had a lower porosity (8.3%), greater even thickness (19.23 μm), higher pitting potential (-0.199 V), larger Rt value (7 765 kΩ · cm2) and longer proctection time (1.29 times).

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