| Based on in-situ corrosion observation, the corrosion behaviors of Mg-Gd-Y-Nd-Zr and MB8magnesium alloys in3.5%NaCl solution, as well as the effect of various second phase particles on localized corrosion of both alloys, were investigated by optical microscope (OM), scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS) and saline immersion test. Furthermore, surface modification of these two magnesium alloys was conducted by plasma immersion ion (Ti, N) implantation. Then, the chemical composition of alloys surface after ion implantation was characterized by electron probe micro-analysis (EPMA), and the influence of Ti, N implantation on the surface performance of both alloys was analyzed by means of dynamic potential polarization, hydrogen evolution technology and micro-hardness testing.As for Mg-Gd-Y-Nd-Zr alloy, pitting is the typical feature at the initial stage of corrosion. Gd-rich and Y-rich particles promoted corrosion of a-Mg as cathode of corrosion galvanic cell, while both Mg in Zr-rich particle and a-Mg close to Zr-rich phase corroded preferentially. Moreover, the exact location of corrosion was concerned with the orientation between second phase particle and the matrix. In the process of localized corrosion, the second phase with higher content of RE elements or Zr showed better corrosion resistant. Furthermore, the area distributed with intensive second phase particles exhibited better corrosion resistance, which was ascribed to the barrier effect derived from the interaction between particles.As for MB8magnesium alloy, filiform corrosion and pitting corrosion were the main corrosion forms. After an induction period, filiform corrosion occurred while there are a large number of hydrogen bubbles generated at the front end of the corrosion filament. Filiform corrosion grew in all directions to around in the cross section, while the corrosion filament extended as strips like the distribution of second phases in the vertical section. However, in the region without filiform corrosion, pitting occurred due to the galvanic effect of Mg matrix coupled with second phase particle.The surface morphology and microstructure of both magnesium alloys remained relatively unchanged after plasma ion implantation (PSII). For Mg-Gd-Y-Nd-Zr alloy after Ti, N plasma implantation respectively, the corrosion potential of surface shifted towards the anodic direction, and lower corrosion rate was obtained. Moreover, the N-implanted samples showed superior corrosion resistance than the Ti-implanted samples. The micro-hardness of modified layer of Ti-implanted and N-implanted Mg-Gd-Y-Nd-Zr alloy was enhanced by36.8%and28.5%, respectively. And with regard to MB8magnesium alloy, filiform corrosion still occurred in3.5%NaCl solution after Ti, N plasma implantation. But the differences were that the induction period was longer and the resistance of chloride ion attacks was advanced evidently. In addition, the growth rate of filiform corrosion was reduced after ion implantation, and a lower corrosion rate was obtained by N ion implantation. The micro-hardness of modified layer of Ti-implanted and N-implanted MB8alloy was also increased by26.0%and57.4%, respectively. |
