Effect Of High-pressure Torsion On Structure And Corrosion Behavior Of GW83K、Mg-Zn-Ca And AZ91 Mg Alloys

Magnesium and its alloys have rich reserves and wide application prospects, but their low strength and poor corrosion resistance are the main limitation for their development. Torsion high-pressure (HPT) is an effective method to improve mechanical properties and corrosion resistance of magnesium alloys, because of its remarkable effect on grain refinement. It can expand the application of magnesium alloys. Herein, Mg-8Gd-3Y-0.4Zr (GW83K), Mg-1Zn-0.13Ca and AZ91 magnesium alloys were used to study the effect of HPT on the microstructure and corrosion resistance of magnesium alloys, by microstructure observation, X-ray diffraction spectroscopy analysis, microhardness test, open-circuit potential(OCP) test, potentio-dynamic polarization (PDP) test and electrochemical impedance spectroscopy (EIS),combining scanning Kelvin probe test (SKP) and local electrochemical impedance spectroscopy (LEIS).The results show that the grain size and phase constitution of the GW83K alloy after 5 HPT turns do not change distinctly in compared with the unprocessed specimen, but the microhardness on the edge is much higher than that of the center because of plenty of twins and dislocation formed in the fine-grains. After HPT, the GW83K alloy has a lower interfacial voltage in air. In 3.5% NaCl solution, the open circuit potential (Eoc) of the HPT-processed specimen comes to be steady quickly(about one hour),but is more negative than the unprocessed specimen. It has a larger corrosion current density(icorr) and a smaller electrochemical impedance (Rp) which decreases with the immersion time. In 0.1M NaOH solution, the Eoc value and the electrochemical impedance of the two kinds of specimens do not have obviously difference in the beginning. However after 6 hours, the electrochemical impedance of the unprocessed specimen is significantly greater than the HPT-processed specimen. The polarization curves show the HPT-processed specimen has a smaller stable passivation range, a larger current density and more negative pitting voltage in alkaline solution.Mg-1%Zn-0.13%Ca samples were analyzed after 5 cycles of HPT plus annealing at 200℃ and 350℃, respectively. The results showed that the grain size of the HPTed sample was refined obviously, and the annealed samples have lower hardness, the larger grain size and higher interfacial voltage. After immersion in the Hank’s solution for 0.5h at 37℃, the Eoc value of the HPTed sample is slightly positive, the electrochemical impedance is higher and icorr is smaller. It indicates that the corrosion resistance of the HPTed sample is better than that of the annealed ones. With the extension of immersion time, the electrochemical impedances of the three kinds of samples decrease and then increase. The surface potential difference became larger and the interfacial voltage shifted positive, because of the increase of calcium phosphate on the surface and the cover of the substrate.The microstructures of AZ91 magnesium alloy with and without HPT are very different. The HPTed sample has higher micro-hardness, finer grain size, lower surface potential and the P-phase network structure is damaged. At initial immersion stage(0.5h) in 3.5wt% NaCl solution, the HPT-treated AZ91 alloy exhibited better corrosion resistance than the as-cast one. The HPT-treated alloy has more positive Eoc value spending shorter time and larger Rp value. Its polarization curves show that it has more positive corrosion potential and smaller icorr. During 24 hours immersion time, the electrochemical impedance and the local electrochemical impedance of the as-cast sample increases after the initial decrease and then decreases. The interfacial voltage of the sample distributed unevenly and the potential difference becomes larger with a long immersion. LEIS results show that the cast sample experienced from uniform corrosion into localized corrosion, and the HPTed one undergoes localized corrosion into serious uniform corrosion. The corrosion susceptibility of the HPTed one attacked by Cl reduced at initial stage. The corrosion morphology of the HPTed sample is obviously different in the sample center and the edge region, which indicated local corrosion. It was obvious that the cast one appeared uniform corrosion.The effect of HPT processing on corrosion behavior of magnesium alloys depends on two respects. HPT has contributed to the rapid formation of the passive film so that corrosion susceptibility of material decreased, resulting in better corrosion resistance at initial immersion stage. However, the protective film still can not protect the substrate effectively. High-energy crystal defects in the grain of HPTed materials lead to the magnesium matrix more active and dissolve more quickly,which results in the worse corrosion resistance. If the former is dominant, the corrosion resistance of materials increase (and vice versa). On the other hand, the uneven structure caused by HPT leads to the occurrence of micro-galvanic corrosion and accelerates partial damage. It is another risk using HPT processing.