| The corrosion behaviour of AZ91D magnesium alloy were studied in pure air , SO2-pollutedair and simulated acid rain environment by means of optical microscopy(OM), scanning electronmicroscopy(SEM), energy depressive spectroscopy(EDS), X-ray diffraction(XRD). Theelectrochemical measurement device was used to investigate electrochemical behaviour ofmagnesium alloy under thin electrolyte layer. Meanwhile,the kinetic models were built on the basisof experiment.In pure air of 20℃,30℃and 40℃with relative humidity 95%, the corrosion weight gainfollowed exponential growth law. The increase of temperature had an accerlerated effect oncorrosion of magnesium alloy.The cellular corrosion products mainly grew on the surface ofαphase. The corrosion products consisted of MgO and Mg(OH)2.The content of Mg(OH)2 increasedwith the rise of temperature.In SO2-polluted air, the corrosion rate of magnesium alloy increased with the increase of SO2content. The corrosion weight gain followed exponential growth law in air containing 0.4×10-6 ,1×10-6 and 2×10-6volume fraction of SO2.The corrosion weight gain can be divided into two stagesin air containing 4×10-6volume fraction of SO2. The first stage accorded with linear increase law,and the second stage accorded with exponential growth law. The continuous corrosion layer formed.Meanwhile, large granular corrosion products appeared in the cracks of corrosion layer. Thecorrosion products mainly consisted of MgO, Mg(OH)2, MgSO3·6H2O and MgSO4·6H2O. Thequantity of MgO and Mg(OH)2 decreased, while the quantity of MgSO3·6H2O and MgSO4·6H2Oincreased.In simulated acid rain environment, the morphology of magnesium alloy included weakcorrosion area and strong corrosion area. Pits were observed on the surface of AZ91D magnesiumalloys at the initial period, which was caused by the dissolution of magnesium alloy. As thecorrosion going on, the surface was covered by the acicular corrosion products. Occluded areaformed inside pits, which resulted in autocatalytic process.The corrosion morphology and corrosionmechanism of air containing SO2 were different from that in simulated acid rain environment, andthe difference resulted from containing aggressive anions and high acidity in simulated acidrain.The cathode reaction of magnesium alloy was oxygenation reaction in SO2-polluted air, andThe cathode reaction of magnesium alloy was hydrogen evolution reaction in simulated acid rain environment.With the thickness of electrolyte layers thinning, the corrosion potential firstly decreased andthen increased, both corrosion current and limited diffusioncurrent density firstly increased and thendecreased in pure air and air containing SO2.The Nyquist figure consisted of high frequencycapacity reactance arc and low frequency diffusion impedance, and showed two time constants.With corrosion going on,1/Rct increased. The order of corrosion tendency under different thicknesselectrolyte layer containing SO2 was as follows: 50μm>20μm>200μm>bulk.Based on the data got from indoor simulated experiment,the three parts were gas dissolution,gas hydrolysis in thin layer and formation of corrosion products.The amount of corrosion productsformed in different periods were accumulated with integration formula. The mass gain ofmagnesium alloy was calculated in pure air and air containing SO2. Compared with experimentdata,the model was verified.
|
PDF Full Text Request