Study On Corrosion Behavior Of Magnesium Alloy In Simulated Acid Rain

Magnesium alloy is the lightest structural materials, and it is characteristic of excellent specific strength, good damping capacity, castability and machinability, especially easy to collect and reuse. Magnesium alloy is found using widely in aeronautical, automotive and electronic industry. However, its poor corrosion resistance has hindered its widespread use in many applications. The corrosion behavior of metals could be influenced apparently by acid rain. It had been proved that the acid rain can accelerate atmospheric corrosion rate of metals. In this dissertation, the study was focused on the corrosion behavior of AZ91D and AM60B alloys in simulated acid rain. The electrochemical impedance spectroscopy was employed to study the corrosion behavior of the two alloys in Chongqing city atmosphere. The goal of this dissertation was to supply fundamental experiment data for clearing mechanism that the effects of acid rain on the corrosion of magnesium alloy.With polarization curve, EIS, SEM and XRD, the influence of pH value and corrosive ions of solution and immersion time to corrosion behavior of magnesium alloy in simulated acid rain solution was investigated. The results showed that: when the pH was increased, the corrosion potential of AZ91D and AM60B magnesium alloy were gradually shifted in positive direction, and current density was decreased. Their anodic polarization current was decreased evidently at same potential. Integrated film was formed on the AZ91D alloy surface and partly protected parent metal. However, effective protective film on the surface of AM60B magnesium was destroyed; NO₃^-, Cl^- and SO₄^2- were made the polarization resistance of magnesium alloys decreased, so increased the corrosion current density; In simulated acid rain solution, the open-circuit potential(OCP) and impedance of AZ91D alloy was increased at first, afterwards decreased. When the immersion time was 2h, inductive loop appeared. This meant that pitting occurred. EIS contained two capacitive loops when the immersion time reached 72h which indicated that comparative film was formed on the electrode surface. To AM60B alloy, the OCP and impedance was increased with immersion time. When the immersion time was 2h, the inductive loop in EIS translated into a capacitive loop. The film of corrosion products began to take shape; The whole surface of magnesium alloys were eroded after immersed in simulated acid rain, and deep pits were formed on the surface. The main corrosion type was pitting. Corrosion products were layered and susceptible to exfoliation. A mass of Al was dissolved. The corrosion products were mainly composed of MgO and MgAl₂(SO₄)₄·2H₂O.Atmospheric corrosion testing results indicated that the corrosion progress of magnesium alloy attributed to the relative humidity (RH) and temperature. The higher relative humidity was, the faster corrosion rate was on the magnesium alloy surface; Atmospheric corrosion was an alternate process between dry and wet condition. Furthermore, it also can be seen that the atmospheric corrosion was characteristic of unstabilization and complication process from Rp.