Research On Several Corrosion Issues Of Magnesium And Its Alloys

In this work, three corrosion issues were chosen from the preparation, connection and application of magnesium alloys, and the focus of our research was mainly put on the ultrasonic vibration treatment during casting, welding joint and the influence of magnetic field on the corrosion behaviour of magnesium and its alloys. Efforts had been made to investigate the relativity of the corrosion properties and the microstructure.With ultrasonic vibration treatment applied in the process of casting, the corrosion behaviour of AZ80 alloy was studied by electrochemical noise (EN), potentiodynamic polarization curves and local electrochemical impedance spectrum. It was shown that the grain size andβphase were clearly refined after the ultrasonic treatment, and nano scale (3 phase appeared which was much easier to fall off the specimen surface. Since P phase was the cathode in the micro-gavanic corrosion, the fallenβphase would result in the decrease of the amount of cathode, which could magnificently restrain the micro-gavanic corrosion. By the reccurence quantification analysis (RQA) of EN data, it was indicated that the pitting initiation process and pitting growth process were both restrained after ultrasonic treatment.The corrosion behaviour of AZ31 at the welding joint was investigated by wire beam electrodes technology, optical microscope (OM) and scanning electron microscope (SEM). In contrast with AZ31 bulk whose grain size ranged from 20μm to 50μm, the grain size in the heat affected zone (HAZ) was mainly in the range of 50μm to 100μm, which indicated that the quantity of grain boundaris decreased. Since the grain boundary had a serious corrosion tendency, the corrosion susceptibility of HAZ decreased as well, which gave HAZ a relatively high potetial and made HAZ protected in the gavanic corrosion. The grain size is symmetrical in the fusion zone (FZ), ranging from 40μm to 50μm. Precipitation particles and oxide particals were observed in FZ, which were the cathode in the micro-gavanic corrosion and could accelerate the anodic dissolution around them. So generally, gavanic corrosion would occur on the surface of AZ31 around FZ. HAZ had a high corrosion resistance and played the role of cathode which would be only corroded slightly; FZ and bulk showed a lower corrosion resistance and played the role of anode in the gavanic corrosion which would suffer serious corrosion.The effect of magnetic field on the corrosion susceptibility of pure magnesium was investigated by electrochemical measurements and SEM observation. In the presence of magnetic field, the corrosion process of magnesium showed two distinguishing stages, and magnetic field played contrary roles on the corrosion susceptibility during two stages. During the passivation stage, pure magnesium in the presence of magnetic field showed a passive behavior, which ascribed to the acceleration of oxygen diffusion due to magnetohydrodynamic (MHD) effect. During the pitting corrosion stage, both the pitting initiation rate and pitting growth probability were accelerated by magnetic field, which indicated the significant increase of corrosion susceptibility.