Corrosion Behavior And Mechanism Of Magnesium In Haze And Marine Atmospheric Environment

Magnesium(Mg)alloys have been widely used in aerospace and automotive due to their low density and high strength to weight ratio.However,their poor corrosion resistance limits the applications.The corrosion behavior and mechanism of pure Mg in haze and seawater were studied by immersion test and electrochemical method.The field exposure test of Mg in marine atmosphere was also complemented.The main achievements can be summarized as the following:(1)The concentrations of the haze constituents were determined.The corrosion behavior of Mg in a Mg(OH)2-saturated solution containing each individual constituents of PM2.5 was studied.The results indicated that the corrosivity of these constituents to pure Mg decreased in the following order:(NH4)2SO4>haze-contaminated-solution>NH4NO3>NaCl?KCl>Na2SO4?MgCl2>CaSO4>NH4Cl>Mg(OH)2(basic solution)>Ca(N03)2.(2)The acceleration effect of NH4+ on Mg in alkaline condition was attributed to the dissolution of the inner MgO product film,providing a path for solution to reach the matrix and accelerating the corrosion of Mg.(3)The corrosion rate of Mg in(NH4)2SO4 was higher than that in NH4Cl at the same normal concentration,which was mainly related to the higher concentration of the free NH4+in(NH4)2SO4.This difference was caused by the different hydrolysis of NH4+ in(NH4)2SO4 and NH4Cl solutions when the pHs were adjusted to 10 using NaOH.(4)The corrosion rate of Mg in Ca(NO3)2 containing solution was the lowest,which is related to the protectiveness of Ca(OH)2 film formed on pure Mg surface and the passivation of surface film enhanced by NO3-.(5)The corrosion rate of Mg immersed in artificial seawater was significantly lower than that in 3.5 wt.%NaCl.The MgCl2 and Na2SO4 were found to be the main reasons.The presence of MgCl2 might be beneficial to the formation of somewhat protective Mg(OH)2 film,and the competitive absorption of SO42-against the Cl-on the Mg surface might be responsible for the reduced corrosion damage of Mg in the artificial seawater.(6)The corrosion rate of pure Mg in artificial seawater(pH=8.2)was about 0.4 mm/y,and that in haze solution(pH=10)was about 0.5 mm/y,It is because that the corrosivity of NH4 is much more serious than that of Cl-.(7)The corrosion rate of pure Mg exposed in ocean and the site closed to ocean were 6.4 ?m/y and 5.5 pm/y,respectively,which is mainly attributed to the high relative humidity and sea salt concentration of the ocean.