Corrosion Behavior Of Pure Magnesium,AZ91D And ZE41 Magnesium Alloy Under Thin Electrolyte Layer Based On In Situ And Nondestructive Electrochemical Techniques

Magnesium and its alloys are the lightest metal structural materials in the world, which are widely used in aerospace and automobile industry and also show good development prospects in the field of 3C electronic industry and medical material. Despite the advantages of the magnesium alloy, the current primary problem restricting the development of magnesium and its alloys is the poor corrosion resistance of them and the corrosion of magnesium and its alloy exposed to atmospheric environment accounts for more than half of the total corrosion. So far the most widely used methods to investigate the atmospheric corrosion behavior are outdoor exposure experiments, which can provide some macro ex-situ corrosion information mainly by weightless method and physical characterizations of the corrosion products and surface images. As well known, the nature of the atmospheric corrosion of metals is the electrochemical behavior under thin electrolyte layer (TEL), thus in-situ nondestructive electrochemical researches on the corrosion mechanism of atmospheric corrosion of magnesium and its alloys is necessary and important.In the current research, we studied the corrosion performance of magnesium, magnesium alloy AZ91D and ZE41 under thin electrolyte layers (TELs) by employing in-situ nondestructive electrochemical methods and some physical characterizations. The main results are as follows:The electrochemical technologies for instance Cathodic Polarization Curves, in situ nondestructive Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise (EN) combined with SEM/EDS, XRD and Scanning Kelvin Probe Force Microscopy (SKPFM) were employed to study the corrosion behavior of magnesium, magnesium alloy AZ91D and ZE41 under TELs with the existence of 3.5 wt.% NaCl solutions.The research results indicate that the corrosion behavior of magnesium and AZ91D under TELs is different from that in bulk solution. Cathodic polarization curves show that the corrosion rates of magnesium and AZ91D under TELs decrease with the thinning of TELs, which can be characterized by the fact that the cathode and anode reactions are both suppressed with the cathode current density reducing and the corrosion potential rising. EIS results show the corrosion rates of magnesium and AZ91D under TELs are lower than in bulk solution and continuously decreasing with the TEL thinning. The energy distribution plots (EDPs) corresponding to the electrochemical potential noise of AZ91D bring some information with respect to the corrosion form. In bulk solution the energy of D7+D8 dominates indicating that the corrosion electrode is under uniform corrosion. Under TELs the relative energy concentrates in the region of D1, D2 and D3 and the ratio of the energy increases with the TEL thinning, which manifests that the local corrosion behavior occurs and the ratio of it increases during the thinning of the TEL. SEM/EDS and SKPFM results reveal that for AZ91D the surface Volta potential of the β phase is 92 mV higher than that of a phase and 202 mV higher than the adjacent matrix, and the matrix adjacent to the β phase is the most susceptible to corrosion. Furthermore, magnesium is composed by a single matrix phase with some tiny defect holes. And the measured Volta potential fluctuates slightly indicating that there is no obvious Volta potential difference.The corrosion behavior of ZE41 under TELs with the existence of 3.5 wt.% NaCl solutions is similar to that of Mg and AZ91D i.e. under TELs both the cathode and anode behaviors of ZE41 are restrained and the corrosion resistance increasing with the TEL thinning. What’s more, according to the changing of Rsurf+Rt value, the corrosion of ZE41 can be divided into three different types. First, in bulk solution, with the immersion time extending, Rsurf+Rt value decrease continuously. Under the TELs of 407,301 and 200 μm, Rsurf+Rt value increase and then decrease with time prolonging. Rsurf+Rt value shows increase from 0-3 h and 0-6 h corresponding to the 105 and 50 μm respectively and keep increase along with severe fluctuations with the time. The electrochemical potential noise signal indicate that at the initial stage of corrosion in bulk solutions an uniform corrosion is observed, with time increasing localized corrosion behavior appears, in addition, under TELs the proportion of localized corrosion behavior such as metastable pitting is higher than that in bulk solutions and increase with the decrease of the electrolyte layers and the exposure time. The results of the average noise resistance show a little differences from the EIS conclusions, which may result from the fact that the EIS technology mainly reflect the overall information of corrosion while the EN pay more attention to localized corrosion. EDS and XRD results illustrate that at the initial stage of corrosion, the corrosion products contain the elements of Mg, O, Al and Zr. Comparing to the initial stage, as corrosion time extends to 24 h, the content of corrosion products increase and the amorphous state compounds containing the element of C arise at the thin electrolyte layers of 304,200,105 and 50 μm.