| Energy crisis and climate change have become two major problems in the present world, and then improving energy efficiency and reducing carbon emissions are important means for solving these problems. Automotive industry is the strategic pillar of national economy, reducing the body weight of the car could effectively promote energy efficiency and reduce carbon emissions which are very helpful for the realization of energy conservation and environmental protection.For weight reduction, magnesium alloy is an ideal material to replace the traditional materials, owing to these remarkable advantages, such as, high specific strength, low density, good mechanical processing properties and fine cast performance. Thus, magnesium alloy receives widespread attention.However, bad corrosion resistance plus low strength has become the"bottleneck problem"which significantly suppresses the development of magnesium alloy; thus magnesium alloy could hardly perform as load-bearing parts in use. Still, security risk exists as the bad performance of magnesium alloy in corrosion and strength.In order to address security problems and improve product safety, cast AZ91D magnesium alloy was adopted as the target alloy for investigating the dynamic strength and establishing evaluation Criteria based on the systematical research on the electrochemical corrosion and the microbiologically influenced corrosion. Simultaneously, rare earth Ce and Y were introduced to ameliorate the dynamic-mechanical property of magnesium alloy and suppress the decay rate of the corrosion residual strength of magnesium alloy.As a result, the corrosion residual strength of AZ91D magnesium alloy follows negatively exponential decay within 372h immersion in 3.5% NaCl aqueous solution. For magnesium alloy, the nucleation and propagation of corrosion pits should be directly responsible for the decay of corrosion residual strength in corrosive conditions. On the one hand, corrosion pits lead to the reduction of the effective loading area; on the other hand, corrosion pits are the positions where stress concentration happens when external load acts on the magnesium alloy parts.Consulting the international practice, the extreme depth of corrosion pit was introduced to evaluate the local corrosion of magnesium alloy, and the statistical results shows that the extreme depth of corrosion pit follows power exponent function, and thus it increases fast at the very beginning and then slowly in the following time range. Further analysis indicates that the corrosion residual strength of AZ91D magnesium alloy is linearly dependent on the extreme depth of corrosion pit, which lays a solid foundation for the evaluation of corrosion residual strength for AZ91D magnesium alloy.Previous studies indicated that rare earth elements Ce and Y could effectively improve the corrosion resistance and mechanical properties of AZ91D magnesium alloy. The results from the experiments shows that the rare earth Ce/Y modified AZ91D magnesium alloy exhibits a similar regulation in dynamic strength in corrosive conditions as compared with the unmodified AZ91D magnesium alloy. However, the addition of rare earth significantly improves the mechanical properties of magnesium alloy by reducing the effect of stress concentration at corrosion pits, and therefore remarkably suppressing the decay of corrosion residual strength. Electrochemical potentiodynamic polarization curves analysis indicates that rare earth could accelerate the refinement of the microstructure of AZ91D magnesium alloy, which suppresses the micro-galvanic effect in corrosion process and promotes the overall macro-corrosion resistance of AZ91D magnesium alloy. Simultaneously, the addition of rare earth could suppress the nucleation of corrosion pits judging from the corrosion morphology; therefore the extreme depth of corrosion pit to the rare earth modified AZ91D magnesium alloy is lower in contrast to that of the unmodified AZ91D magnesium alloy. With the development of corrosion, the refinement of theβphase lead resulted in the weakening of its resistance effect on the propagation of corrosion pits; in this case, the extreme depth of corrosion pit to the rare earth modified AZ91D magnesium alloy is higher than the unmodified AZ91D magnesium alloy. In general, rare earth modification could effectively suppress the decay of the corrosion residual strength of AZ91D magnesium alloy, which is an ideal mean for the protection of the dynamic-mechanical properties magnesium alloy.Rare earth Ce and Y exhibit significant difference in modification, i.e., rare earth Y shows better solid solution capability than Ce, while rare earth Ce could remarkable promote the precipitation of compounds. The mechanical properties of AZ91D magnesium alloy is further improved by the appropriate adjustment of the ratio between rare earth Ce and Y. In contrast to the unmodified AZ91D magnesium alloy, the tensile strength of the rare earth Ce and Y complex modified AZ91D magnesium alloy is increased by 22%. Pitting corrosion is further restricted by further adjusting the ratio of rare earth Ce and Y, and simultaneously the corrosion residual strength of the complex modified AZ91D magnesium alloy after 108h immersion is improved obviously. For the 1.0Y modified AZ91D magnesium alloy, 0.2%Ce is the optimal concentration, the mechanical property and the corrosion resistance are the best. For the 1.0Ce modified AZ91D magnesium alloy, appropriately increasing the concentration of Y could promote its mechanical property and corrosion resistance effectively. In general, the corrosion residual strength of AZ91D magnesium alloy is remarkably improved by combining the advantages of the two rare earth elements, which calls for much more research and exhibits higher research value.Common corrosion tests indicate that corrosion pits emerge on the surface of AZ91D magnesium alloy when corrosion happens, which results in the fast dropping corrosion residual strength. However, except common electrochemical, microbial metabolism is also an important reason for the corrosion of magnesium alloy. In this paper, the microbiologically influenced corrosion of magnesium alloy was investigated by a specially designed in-situ corrosion method, and the results indicated that the metabolism of bacteria could accelerate the degradation of corrosion film which is in favor of pitting corrosion. The degradation of corrosion film by bacteria is the direct reason for the drop of corrosion residual strength. The large-scale finite element analysis was adopted to calculate the stress field and strain field, the results revealed that stress concentration mainly emerges in the corrosion pit. Further more, the stress and strain field in the corrosion pit is also uneven, in particular stress concentration mainly happens in a long and narrow area under the axial tension state. The emergence of stress concentration is a root cause for the failure of magnesium alloy in low stress condition. In fact, this result is closely consistent with the experimental results.In order to systematically analyze the mechanism for pitting corrosion, a polycrystalline multi-electrode couple (PMC) model was adopted for the thermodynamics calculation based on the experimental results, which is also used for the explanation of pitting corrosion mechanism.Evaluation criteria, , for the dynamic-strength of magnesium alloy was founded on the basis of the relationship between the corrosion residual strength and the extreme depth of corrosion pit, i.e., , base on a large number of experiments. When corrosion happens, the EDCP value is larger than the allowed value [EDCP], the magnesium alloy components will be failed, and S F could be a value between 1.0 and 2.0 according to the engineering requiments. The foundation of the dynamic evaluation criteria could provide the safety evaluation for magnesium alloy products. It could also accelerate the application of magnesium alloy, which shows long social efficiency and remarkable economic value.
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