Study On The Corrosion Of Anodizxed AZ91D Magnesium Alloy

Magnesium and its alloys are increasingly implemented into a number of components such as automobile parts,mobile personal computers,cellular phones, etc.,because of their specific advantages such as the lowest density and very excellent specific strength.Due to the superior properties of strength,cast, anti-corrosion,etc.,AZ91D magnesium alloy has been drawn more attention.In the last several decades,researchers have done a lot of studies about the corrosion behavior of the AZ91D magnesium alloy and obtained a number of achievements. Moreover,the methods to enhance anti-corrosion performance of magnesium alloy have been developed unceasingly,which will expand the application scope of the alloy.And anodizing has been thought to be one of the most effective methods. However,at present commonly used methods for research about the corrosion behavior of AZ91D magnesium alloy are seemed to be slightly frail.If the richer test/analysis methods can be used,we will gain more comprehensive and thorough understanding about the corrosion process of the alloy.In addition fewer attentions have been paid towards the formation mechanism of anodic film.In view of the situation about AZ91D magnesium alloy's research,the present paper has carded out the research from two major parts to it:The first part has focused on the study of the corrosion behavior of the AZ91D magnesium alloy.Besides of salt spray test,polarization curve,electrochemical impedance spectroscopy(EIS),SEM&EDX,the technique of electrochemical noise (EN) which could be measured in-situ and non-disturbance to the electrode system was utilized in the research about the corrosion behavior of the anodized or not AZ91D magnesium alloy in neutral aqueous NaCl solution.Fractal theory was first used into the data analysis.The conclusions obtained are shown as following:Such as salt spray tests,polarization curve,EIS,EN and etc.measurements results showed the corrosion behavior of AZ91D presented characteristic changes due to anodization.For example,the unsealed anodic film could be sustained in 5.0 %NaCl salt spray beyond 200 hours,E_corr of anodized magnesium alloy shift up and the inductive period for pitting corrosion was obvious prolonged,the impedance modules in high frequency of anodized alloy were several thousand times as those of unanodized alloy.All these proved that anodization process made the alloy gain excellent property to anti-corrosion.SPD obtained by EN showed pitting corrosion was the main corrosion type of the anodic film and base alloy in 1.0%NaCl solution.Results of EIS and EN indicated that the corrosion process of anodized or not alloy could be divided into three stages:occurrence,development/repair of pitting corrosion in the initial immersion time;passivation of the active reaction points on the electrode surface due to basification in the medium immersion time;occurrence and propagation of pitting corrosion in the last immersion time.EDP showed in the initial and last immersion time,energy were mainly accumulated in the small timescale(D₂-D₃) and large timescale(D₇-D₈).The former arised from the fast reaction of pitting corrosion occurred in the anodic film of base alloy,and the latter was related with the slow diffusion process of aggressive ions or hydrogen gas bubble near the active points.In the medium time,energy distributed in the medium crystals was increased, which was corresponding to the development/passivation process of pitting.The evolution of fractal dimension(D_f) in the corrosion process of AZ91D magnesium alloy was shown as slightly increased in the initial,gradually speeded in the medium and slowed down in the last.D_f increased slightly in the initial indicated that the corrosion process might be baffled to some extent.And with the immersion time increasing,pitting corrosion's development and repair/die made the state of electrode unstable and roughness increase,which led to D_f speeded gradually in the medium.At last the D_f was more larger than that of in initial,which demonstrated the electrode surface had been damaged badly by corrosion.However the evolution of D_f in the corrosion process of anodized AZ91D magnesium alloy was different from that of unanodized and shown as increased rapidly in the initial and remained comparatively at a stable value.Rapid increasing might be related with the pitting corrosion on the anodic film and remained at a value may be due to the development of stable pits.SEM images showed that with the increasing immersion time,surface of unanodized AZ91D magnesium alloy had been suffered from only one pit to some pits connected with each other,and corroded severely in the initial and last time but relaxed in the medium.EDX analysis indicated magnesium oxide might exist in the corroded region in the medium time,which supported the conclusion of passivation.The second part has focused on the initial formation mechanism of anodic film of AZ91D magnesium alloy.We utilized several electrochemical measurements such as EIS,chronoamperometry,etc.,combined with SEM,to study the initial formation mechanism and kinetics of anodic film of AZ91D magnesium alloy in optimized anodizing electrolyte.Conclusions are shown as following:The results of cyclic voltammogram and EIS indicated that the initial anodizing process of AZ91D magnesium alloy in optimized electrolyte was controlled by diffusion and followed the 3D nucleation and subsequent grain growth mechanism.Nucleation/growth of anodic film should start preferentially on the active points without or only with thin oxide film and high potential could speed anodizing process.The same conclusion could be also drawn from SEM images.Results of chronoamperometry showed with the increasing potential, nucleation type changed from progressive nucleation to mainly instantaneous nucleation,partly progressive nucleation.The results tallied tightly with those obtained using EIS and SEM techniques,i.e.,in the case of low applied potential range,formation of anodic film should first occur at the sites uncovered by original oxide film and result in progressive nucleation.While in the case of high anodizing potential range,all sites of the surface were activated rapidly and almost simultaneously,and result in instantaneous nucleation.