Corrosion Behavior Of Az91d Magnesium Alloy In Solution

In this thesis, the effects of types, compositions and pH values of corrosive solutions on corrosion behavior of AZ91Dmagnesium alloy were studied. The effects of ion, and gases in the solutions, composition and microstructure of alloys, and microstructure of oxide film on the alloys were also discussed in details. The corrosion behavior of AZ91D magnesium alloy was described through analyzing the corrosion rate, morphology of the corroded surface, corrosion mechanisms and nature of surface film. The results indicated:Low hydrogen electrode potential of magnesium (-2.37, almost lower than those of any other elements in the alloy) was the thermodynamics factor which made the corrosion run, and loose and poriferous surface film, impurities in the alloy, and active negative ion in the solutions were the kinetics factors.In sodium chloride solution, the main corrosion mechanism was the selective pitting because of the special characteristics of chloride ion (high solubility, high conductivity, high adsorption and small apparent size), resulting in the final alveolate morpholopy; the corrosion rate increased with the increasing of the chloride ion concentration. In acid, alkali or neutal solution, uniform corrosion occurred and a morphology of open pit formed. The thermodynamics of corrosion increased in acid solution because of the increasing of H+concentration, which led to the increase of the equilibrim hydrogen voltage. But in alkali solution, it was just opposite to that in the acid solution and the corrosion was impressed.The corrosion of AZ91D magnesium alloy was a hydrogen evolution process in quiescent solution. The effect of dissolved O2 was light in the solution. Only in the neutral solution, the dissolved CO2 had relative large influence on corrosion.A model of double electric layer and a model of corrosion behaviour were established according to the corrosion characteristics of AZ91D magnesium alloy in the used solutions.