Age-hardening Al-Mg-Si alloy is widely used in automobile industry because of its good formability, high strength and excellent corrosion resistance. The aging hardening effect comes from the hindrance of dislocation movement by nano-precipitates formed during aging. In the alloy, magnesium-silicon particles always play the role of local anode in acidic solution. It is usually suggested that the size of precipitates should be controlled below the critical value, so that there are fewer interface defects between precipitates and al matrix, that is, the pitting corrosion resistance of alloys is improved at the expense of lower strength. However, intergranular corrosion (IGC) of Al-Mg-Si alloy is the most serious under-aging. The metastable pitting corrosion rate of Al-Mg-Si alloy is the highest in the peak aging state, which means that it is possible to achieve a good balance between pitting corrosion resistance and IGC resistance at the expense of partial strength in the over-aging state. At present, the influence of corrosion behavior and the type and size of MgSi nano-precipitate on the excellent pitting corrosion resistance and IGC resistance of overaged Al-Mg-Si alloy has not been clarified.
Recently, the team of Professor Du Yong and Associate Professor Li Kai from the State Key Laboratory of Powder Metallurgy of Central South University broke through the limitations of traditional second-phase corrosion behavior analysis methods and analyzed the corrosion behavior of MgSi precipitates. The related results were published in the top journal of China Academy of Sciences, Applied Surface Science, titled "Formation of an Amorphous Precipitate in Corrosion Overaged Al-Mg-Sialloy". Based on the principle of accelerated corrosion of anode samples in electrolyte, the corrosion behavior of nano-precipitated phase in nitric acid-methanol acidic electrolyte during electrolytic double spray sampling was studied by transmission electron microscope.
Paper link: https://doi.org/10.1016/j.apsusc.2021.149329.
Due to the dissolution of magnesium and the formation of amorphous structure of silica, coarse β? Nano-precipitate was corroded as anode, and the proportion of amorphous precipitate increased with the aging time of the sample, even reaching 74.6% when it was seriously over-aged. Energy dispersive X-ray (EDX) analysis confirmed that the amorphous precipitated phase was SiO2 _ 2, and compared with the sample prepared by ion thinning, it was confirmed that amorphization occurred during the preparation of electrolytic double spray sample.
The larger volume nano-precipitate with higher MgSi is preferentially corroded by the acidic solution used in the electrolysis process. The coarser the precipitated phase of MgSi, the higher the Mg/Si ratio, and the easier it is to be corroded. Besides, β? The interface between precipitates and Al matrix is corroded uniformly by acidic solution, and the interface is more stable than the matrix.
This discovery is the first time to explain the reason why the sensitivity of pitting corrosion and intergranular corrosion of over-aged Al-Mg-Si alloy decreases with the extension of aging time from the perspective of nano-precipitates in grains.
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