The Corrosion Behavior Of Pure Magnesium

Magnesium and its alloy is a kind of low density, high strength ratio metal, resulting in wide concern. However, the high corrosion susceptibility limits the magnesium alloy’s application in actual life. During the magnesium’s corrosion, the anodic reaction is Mgâ†'Mg2++2e-and the main cathodic reaction is2H2O+2e-â†'H2+2OH-. Because the main cathodic product is hydrogen, corrosion rate of magnesium alloy can be estimated by collecting the hydrogen when the magnesium is under open circuit potential. One more characteristic of hydrogen evolution during magnesium’s corrosion process is the Negative Difference Effect (NDE), which means the hydrogen evolution increases with the anodic polarization increasing. There have been some theories to explain the NDE, but none of them is perfect until now. So Nowadays, the NDE is still one hotspot of magnesium’s research now. In order to investigate the NDE of magnesium, it’s important to collect the hydrogen accurately. One shortcoming of classical hydrogen collection is that the measuring data are average value of entire electrode surface during certain immersion period.Here, the thesis reported a study on hydrogen evolution of localized sites (point, line and surface) in various concentration of NaCl, Na2SO4and different polarization potential by Scanning Electrochemical Microscopy (SECM) with substrate generation-tip collection (SG-TC) mode. The hydrogen evolution increased with NaCl concentration increasing due to the Cl-ion destructive effect on passive film of magnesium, while hydrogen evolution decreased with Na2SO4concentration increasing due to the combination of SO42-and H+. The higher SO42-concentration, the lower H+concentration, resulting in lower hydrogen evolution. The hydrogen evolution was more serious in Na2SO4solution than that in NaCl solution when the electrolyte concentration was0.1and0.3M, but the trend was opposite in0.5and1M solution. Because of short immersion time (10min), there was not enough attack to the passive film by lower concentration Cl-. The hydrogen evolution and distribution increased with more higher polarization voltage. The NDE can be also found by SECM improved that SECM technology was a reliable technique to detect the hydrogen evolution. The obtaining of hydrogen distribution on magnesium was one advantage of SECM compared to the classical hydrogen collection method.Magnesium surface can be oxidized to form a thin MgO/Mg(OH)2film in natural condition, which is easy to breakdown when the magnesium was exposed in aqueous solution containing aggressive ions, or under anodic polarization condition, since the film is thin and uncompact. Here, we represent the effects of immersion time, NaCl concentration, polarization potential and pH on the breaking down of passivation film by feedback mode of SECM combined with polarization curve and EIS measurement. The results indicate the bare magnesium active sites could be generated during immersion time, and these active sites can be repassivated along with immersion time; The higher NaCl concentration, the more active sites due to the Cl-,aggressivity to the passive film, which is similar to the hydrogen collection experiment; The anodic polarization was in favor of breaking down the passivation film, while the cathodic polarization was opposite. The low solution pH accelerates the breakdown of passivation film, but the high solution pH was opposite, since the passive film MgO/Mg(OH)2was not stable and easy to react with H+.