| At present,the building formwork on the market can be divided into steel formwork,wood formwork,plastic formwork and aluminium alloy formwork according to the raw materials.But the steel formwork is too heavy,the operation of which is inconvenient;the durability of wood formwork is poor,and the turnover times of wood formwork are less;the plastic formwork is difficult to recycle;the aluminium alloy is not alkali-resistant.The magnesium alloy has low density,high ratio strength,specific stiffness and good alkali-resistance.Therefore magnesium alloy is an important choice for building template.So far,no one has studied the corrosion resistance of magnesium alloys in alkaline cement environment,so it is of great significance to study the corrosion resistance of magnesium alloys in the environment of building formwork.In this thesis,the corrosion resistances of AZ80 magnesium alloy in 3.5wt.%NaCl solution,alkaline simulated solution and cement were compared by alloying different elements and different contents of element.In order to better study the effect of precipitation on the corrosion resistance of alloys in specific environments,the(3-Mg17Al12 phase was decomposed by heat treatment to study the effect of different content of alloying elements on corrosion resistance.Then the uniform and dispersive ?-Mg17Al12 phase was precipitated by heat treatment,and the comprehensive effect of all precipitation on the corrosion resistance of alloys was studied.The full text draws the following conclusions:(1)Adding trace Nd or La elements to AZ80 magnesium alloy can effectively reduce its corrosion rate and improve its corrosion resistance,and the effect of Nd is better.The essence is that Nd or La precipitates decompose and form protective film during the corrosion process to hinder the corrosion process;Adding trace As elements to AZ80 magnesium alloy can reduce the corrosion resistance of the alloy.Mg3As2 phase will be formed in the alloy with arsenic.The hydrolysis reaction of Mg3As2 phase in magnesium alloy can form Mg(OH)2 which will accelerate the corrosion process.(2)With the increasing Nd content in AZ80 magnesium alloy,the corrosion resistance of AZ80 alloy first increased and then decreased,and the corrosion resistance of AZ80-1.2wt.%Nd alloy was the best.The phase containing Nd in AZ80 magnesium alloy were Al2Nd phase and AlMnNd phase,and the potential of Al2Nd is positive than that of magnesium matrix.Though Al2Nd phase had the effect of accelerating matrix corrosion.On the other hand,AlMnNd phase decomposed to form Nd-containing oxide layer to protect the alloy in the corrosion process;when the Nd content was low,the amount of Al2Nd phase in the alloy was more than that of AlMnNd,and the accelerating effect of Al2Nd played a dominant role;when the Nd content increased gradually,AlMnNd phase formed Nd-containing product on the surface to protect the alloy.The protective effect of Nd-containing phase was dominant at this time;when the content of Nd reached the highest level,the acceleration of Nd-containing phase was dominant again,because a large number of Nd-containing phase aggregated and formed serious pitting corrosion.(3)Decomposition of ?-Mg17Al12 phase by heat treatment reduced the corrosion resistance of the alloy,because a large number Al atoms dissolved into the matrix maked the alloy in metastable state.The chemical potential of the alloy was high,which was easy to spontaneously react;when heat treatment causes the precipitation of Al atoms in the matrix to form granular?-Mg17Al12 phase,the alloy is in the metastable state,and the chemical activity of the matrix decreases. |
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