Nickel coatings with different microstructures were synthesized by pulse jet electrodeposition technique.Their morphology and microstructure were investigated by scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The corrosion property of the coatings was studied by using polarization,electrochemical impedance spectroscopy（EIS）,potential of zero free charge（PZFC） measurements and Mott-Schottky（M-S） relationship.The results showed that the coating with grain size of 50 nm possessed higher corrosion resistance than that with grain size of 10 nm.This abnormal behavior may be related to the existence of nanoscale twins in the coatings and the lower concentration of acceptor in the passive films.
Guozhe MengYang LiYawei ShaoTao ZhangYanqiu WangFuhui WangXuequn ChengChaofang DongXiaogang Li
Selective growth of oxidation coating was observed with Al18B4O33 crystal whisker as reinforced phase） on Mg matrix composite Al18B4O33w/AZ91 （a composite when this composite was treated by microarc oxidation （MAO） technique, and then the role of Al18B4O33 whisker in the process of MAO was analyzed. The protective properties of MAO coating also were investigated. Scanning electron microscopy （SEM） was used to characterize the existing state of Al18B4O33 whisker in MAO process and the microstructure of MAO coating. Corrosion resistance of the bare and coated composite was evaluated by immersion corrosion test and potentiodynamic polarizing test. Wear resistance of MAO coating was investigated by a ball-on-disc friction and wear tester. The results showed that sparking discharge did not occur on Al18B4O33 whisker and the whisker existed in the coating as a heterogeneous phase when MAO coating grew on the composite; then the whisker would be covered gradually with growing thick of the coating. Corrosion current density of the coated composite was decreased by 4 orders of magnitude compared with that of the uncoated composite; excellent corrosion resistance was closely related to the compact whisker-coating interface since Al18B4O33 whisker did not induce structural defects. The coating also exhibited high wear resistance and a slight adhesive wear tendency with bearing steel as its counterpart material.
A novel approach was successfully developed to fabricate bulk carbon nanotube-reinforced Mg matrix com-posites with uniform carbon nanotubes(CNTs).The approach consists of pre-dispersion and ultrasonic vibration.Homoge-neous and single CNTs on flake Zn powder can be achieved simply by slurry blending.The pre-dispersed CNTs were added to Mg melt,and then,the melt was ultrasonically pro-cessed.After ultrasonic vibration,the CNTs/Mg-6Zn melt was cast into a metal mold.Most CNTs distribute homoge-neously and singly in the bulk composites.Moreover,good interfacial bonding is achieved,and Raman spectroscopy analysis shows that the damage to CNTs is insignificant.Meanwhile,CNTs evidently improve the ultimate tensile strength,yield strength and elongation.The Kelly-Tyson formula agrees well with the experimental tensile value,and the load-transfer efficiency is nearly equal to 1.
Selective growth of oxidation coating was observed on Mg alloy AZ91 D when this alloy was treated by microarc oxidation （MAO） technique, and then the role of intermetallic phase Mg17AI12 （β phase） during MAO was investigated. Corrosion resistance and anti-corrosion mechanism of the MAO coating were also studied. Optical microscopy and scanning electron microscopy （SEM） were used to characterize β phase and coating microstructure. Corrosion properties of the coated alloy were studied by potentiodynamic polarization test and electrochemical impedance spectroscopy （EIS） in 3.5% NaCI solution. Results showed that sparking discharge preferentially occurred on ^-Mg phase rather than on β phase at the early stage of MAO; however selective growth of the coating disappeared gradually with the increasing oxidation time and β phase would not further inhibit coating growth at the prolonged stage of MAO. MglTAI12 phase ultimately was unable to destroy the integrity and continuity of MAO coating. The MAO coating could restrain charge transfer process and then greatly enhance corrosion resistance of AZ91D alloy. Sealing treatment of MAO coating by stearic acid could further improve the corrosion resistance of AZ91 D alloy.
SiC particles reinforced AZ91 Mg matrix composites （SiCp/AZ91） with SiC volume fractions of 5%, 10% and 15% were fabricated by stir casting. After T4 treatment, these composites were extruded at 350 °C with an extrusion ratio of 12：1. In the as-cast composite, particles segregated at a microscopic scale within the intergranular regions. Hot extrusion almost eliminated this particle aggregation and improved the particle distribution of the composites. In addition, extrusion refined the grains of matrix. The results show that hot extrusion significantly improves the mechanical properties of the composites. In the as-extruded composite, with the increase of SiCp contents, the grain size of the extruded composites decreases, the strength and elastic modulus increase but the elongation decreases.
Nanocrystalline nickel coatings with grain size of 50 nm were annealed in vacuum at 200 ℃ and 400 ℃ for 10 min, Their microstructures were investigated by transmission electron microscopy （TEM）. And their corrosion behaviors were studied by means of polarization and electrochemical impedance spectroscopy （EIS）, The results showed that their grain size grew up to about 60 nm （200 ℃） and 500 nm （400 ℃）, respectively, The specimen annealed at 200 ℃ possessed higher density of twins in compared with the counterparts of as-deposited and annealed at 400 ℃, The normal grain size effect on the corrosion behavior was not observed, However, it was found that the corrosion resistance of the coating linearly changed with the density of twins.
Guozhe MengYang LiYawei ShaoTao ZhangYanqiu WangFuhui WangXuequn ChengChaofang DongXiaogang Li
