Az91d Magnesium Alloy Corrosion In The Coolant And The Atmospheric Environment Research

Magnesium alloys were recognized as the green engineering materials in 21st century because of their low density, high strength/weight ratio, perfect damping capacity and shock energy absorption capacity, easy processing, as well as a good recycling capacity and compatibility with environment, and the resource of the magnesium is very abundant in the nature. The application of them in automobile,aerospace components,communication parts,electronic equipments,mechanical equipments and so on, has been increasing . However, magnesium alloys usually have poor corrosion resistance, which is actually one of main obstacles that impedes the application of magnesium alloys in practical environments. Therefore, the study on the corrosion mechanism of magnesium alloy is a major issue for research and development of magnesium alloys.In this paper, the corrosion of AZ91D magnesium alloy in automotive engine coolant and the atmosphere are studied. Corrosion rates of AZ91D magnesium alloy in engine coolant and the atmosphere are investigated by weight-loss method. The composition of corrosion products,micrograph,chemical composition have been tested and analyzed by metallogrophy,scanning electron microscopy(SEM),energy dispersive spectrometer (EDS),X-ray diffraction (XRD). The main research work and conclusions are presented as follows:1,Corrosion products of AZ91D magnesium alloy are composed of Al2O₃, MgCO₃ in automotive engine coolant. With the increase of aluminum content, corrosion rate of magnesium alloy in engine coolant decrease. The corrosion resistance of AZ91D magnesium alloy is 2.2 times that of AZ31 magnesium in commercial engine coolant .The corrosion resistance of AZ91Dmagnesium alloy is 3.7 times in 50% ethylene glycol solution . The corrosion resistance of AZ91D magnesium alloy is 34 times in 85% ethylene glycol solution .2,Corrosion rate of AZ91D magnesium alloy is 0.01065mm/a.Corrosion products of AZ91D magnesium alloy are composed of Al2O₃, MgCO₃,MgSO₃·6H₂O and MgSO4O in Linfen industrial atmosphere. CO₂ in the atmosphere speed up the solution of the metal surface oxide film. Dissolved Mg²⁺ and CO₃^2- generate MgCO₃, which block the corrosion in the latter part. SO₂ in the atmosphere is absorbed by the metal surface to form H₂SO₃, and H₂SO₃ is resolved into H+ and SO₃^2-. Due to the existence of O₂ ,at the same time part of the SO₃^2- is oxidize SO₄^2-. MgO and Mg(OH)2 are translated into the soluble MgSO₃·6H₂O and MgSO4·6H₂O, which cause increased corrosion of magnesium alloy. But Al2O₃, MgCO₃ provides a protective effezct to AZ91D magnesium alloy.3,Corrosion rate of AZ91D magnesium alloy is 0.12mm/a.Corrosion products of AZ91D magnesium alloy are composed of Al₂O₃ and MgCO₃ in Qingdao sea atmosphere . In addition, there is some dust falling attached to the surface of magnesium alloy. CO₂ is absorbed and dissolved in the surface thin liquid film of magnesium alloy, so that the thin liquid film is acidulated and pH value lower. At the same time it promote the cathode reaction. The Micro-melting corrosion products MgCO₃ block further corrosion a certain extent, and play a role in the protection.