| Magnesium alloy is one of the lightest metal materials in current applications, which is considered as"the green engineering material of the 21st century". In this thesis, advances in corrosion and protection of magnesium alloy as well as current status in the research of corrosion of magnesium alloy in engine cooling system are summarized. Engine coolant is the inevitably liquid medium for ensuring the normal operation of engine. The chief problem faced by the magnesium alloy used in automobile engine is the corrosion of magnesium alloy in cooling system. By simulating the corrosive environment of engine cooling system, the influences of concentration of the main components in glycol type engine coolant, service duration and temperature on the corrosion behavior of AZ91D magnesium alloy are investigated using immersion test and electrochemical corrosion test; the corrosion morphology as well as the composition and structure of corrosion products of the magnesium alloy is analyzed using scanning electron microscope (SEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and energy depressive spectroscopy ( EDS), and the corrosion mechanism and damage process are also studied; Based on the corrosion mechanism, a novel rare-earth chemical conversion coating with low cost, high performance, green environmental protection is prepared on the surface of magnesium alloy, which is adapted to preventing the magnesium alloy from corrosion damage. The conclusions are as listed as below:1. The influences of the concentration of glycol and service duration on the corrosion behavior of AZ91D in the engine cooling system are investigated. Results show that AZ91D in the pure glycol and glycol - water system exhibits completely different corrosion mechanism and damage process. In glycol-water cooling system, the corrosion rate decreased and the service duration increased with the increase of the concentration of glycol. The average corrosion rate of the AZ91D in the coolant of glycol decreased with immersion time, which can be attributed to the formation of protective MgO/Mg(OH)2 surface film and consequently inhibited the corrosion of the alloy. 2. The influences of temperature (room temperature to 120℃) on the corrosion process and mechanism of AZ91D in the glycol type coolant. Results show that the temperature accelerates the corrosion of magnesium alloy in coolant, the corrosion rate increases considerably with the increase of temperature. The relatively stable MgO/Mg(OH)2 protective film could not be formed on the surface of AZ91D under high-temperature corrosion conditions.3. Different action mechanisms of several inorganic salts on the corrosion behaviors of AZ91D in the glycol type coolant are analyzed using electrochemical corrosion method. Consequently, a novel rare earth chemical conversion coating with high performance and green environmental protection is prepared. Such conversion coating with compact and uniform structure has good adhesion strength with the substrate, which effectively improves the corrosion resistance of magnesium alloy in the coolant. The electrochemical corrosion test shows that the corrosion potential of the prepared conversion coating increased about 400mv～600mv, and the corrosion current density decreases nearly two magnitude order when compared with the untreated AZ91D magnesium alloy.
|
PDF Full Text Request