The Research Of Organic Magnesiumalloys Coating Corrosion Resistance At Salt Spray Environment

Aluminum alloy is widely used in electric power, transportation, aerospace,military equipment and other industries for its light specific gravity, high specificstrength, high specific stiffness, excellent shock absorption and other excellentproperties: in particular, with the development of military equipment, more and morecorrosion resistant aluminum alloy used in ships and weaponry area, which requiresgreat corrosion resistance under high temperatures and salty environment.The property of Mg-rich coating is excellent and proved to be an effective meansto overcome the above disadvantages, furthermore, it’s environmental and avirulent tosome extent. However, pure magnesium particle is applied as filler to the coating, whichis chemically active and has great tendency of autolysis, leading to short service life ofthe coating protection for the short time protection of the cathode.Mg-rich coating is vulnerable and inclined to lose effectiveness under salt-sprayenvironment, for the sake of enhancing the corrosion resistance of magnesium alloy insuch an environment, this subject was taking AZ91D magnesium alloy as filler andorganic rich AZ91D coating was prepared so as to investigate the corrosion resistanceunder the salt-spray environment.The corrosion resistance of the coating is closely related to the content of thepigment, the pigment of epoxy-coated AZ91D with PVC volume concentration of35%,50%and65%respectively are introduced to conduct corrosion resistance test under thesalt spray environment, as the results show: apparent foam occurs on the surface of thePVC volume concentration of35%and65%coating under the salt spray environmentfor960h, while the coating with PVC volume concentration of50%has no significantchange, which shows great corrosion resistance, the EIS test indicates that the coatingwith the PVC volume concentration of35%and65%forms plenty of micro hole on thesurface when the salty time is960h. The impedance value of the low frequency end of Coating with the PVC volume concentration of35%and65%(0.01Hz) drops to lessthan107·cm2and fail, and50%coating remains relatively high impedance value underthe salt spray environment for960h, which indicates that PVC50%coating has greatcorrosion resistance. Surface morphology illustrates: plenty of micro-hole occurs on thesurface of the coating with PVC volume concentration of35%and65%, and that is themain reason leading to rapid coating failure, however, the50%coating is better in termsof the surface micro hole.The comparison of corrosion resistance of Mg-rich coating and AZ91D epoxycoating in a salt spray environment is conducted, the results indicates: the foam of Mg-rich coating occurs after360h salt spray, however, the epoxy AZ91D rich coating hasno significant changes on the surface. The contrast of the electrochemical impedance isalso conducted, the impedance value of the Mg-rich coating in the low-frequency end inthe salt spray360h decreases to less than107·cm2, which shows the failure of thecoating, conversely, the impedance modulus of the epoxy AZ91D rich coating stillmaintain a high value. From the morphology we can observe that the surface of the Mg-rich coating shows large numbers of micro hole and consumption of magnesium-richcoating is relatively faster, the corrosion resistance is worse than that of AZ91D epoxycoating.The corrosion resistance mechanism of the epoxy AZ91D rich coating is studiedtoo, from which we find that the epoxy AZ91D rich coating failure process can bedivided into four stages: the first stage, the coating shield stage before corrosion, thecoating shows effect of shielding; The second stage is the coating activation phase,during which the corrosion electrolyte enter into the coating and react with the AZ91Dparticles in the coating, then the impedance modulus decreases; the third stage, thesubstrate of the coating act as cathodic protection material, the corrosion products startto accumulate and diffuse, in this stage, the Nyquist diagram low-frequency end showsapparent diffusion tail; the last stage is all about failure, the double arc in Nyqusit appears, the coating particles of AZ91D exhausts.