Investigation Of Microstructure And Corrosion Behavior Of Mg-Al Series Magnesium Alloy With/without Rare Earth Element (Ce,La) Addition

Called as green engineering material, magnesium and magnesium alloy has been used in many applications and become the most important and potential lightweight material in21Century due to its special properties, such as high specific strength, good relative mechanical properties, good damping property and castability, perfect thermal conductivity and vibration reduction, easy to recycle. However, as construction material, one of its main drawbacks is high cost. Moreover, their poor corrosion resistance in comparison with other metals has greatly limited their further utilizing. It is well known that that addition of rare earth (RE) elements is an effective way to improve the corrosion resistance of magnesium alloy. The microstructure and corrosion product of magnesium alloy is improved after rare earth addition, then the physical changes has an influence on the corrosion behavior of magnesium alloy. As most used inducstrical Mg-Al alloy, its corrosion behavior and corrosion protect has attracted more and more attentions. In this dissertation, the corrosion behavior of Mg-Al magnesium alloy (AM60and AZ91) in bulk solution and under thin electrolyte layer (TEL) successfully monitored via electrochemical methods with the aid of home-made equipment. The main contents are listed as follows:The influence of chloride ion concentration on the corrosion behavior of AM60magnesium alloy in bulk solution has been investigated by hydrogen evolution and electrochemical test, such as open circuit potential (OCP) curves, polarization curves, electrochemical impedence spectroscopy (EIS) and so on. The results indicate that corrosion resistance decreases with increasing Cl-concentration. The more concentrated NaCl solution promotes initiation of localized corrosion and the breakdown of surface film as shown in corrosion morphology and corrosion product, proving that diffusion tail at low frequencies in EIS plots was transforned to inductive arc with Cl-concentration increasing. A corrosion model is proposed to explain the influence of chloride ion concentration on the corrosion behavior of magnesium alloy, implying that more and more chloride ions adsorb on the surface of magnesium alloy in high concentrated NaCl solution and react with Mg(OH)2to form soluble MgCl2, corrosion is promoted in the corrosion zone resulting in higher hydration of the surface components. Meanwhile, the diffusion of CO2to the electrolyte is limited and the amount of insoluble corrosion product containning CO32-is relativly low and corrosion of magneisum becomes more and more serious.New magneiusm alloys AMRE were prepared in a crucible furnace under the shelter of CO2and SF6with cerium (Ce) and lanthanum (La) in a water cooled metallic model. Their corrosion behavior was evaluated by hydrogen evolution and electrochemical methods. The microstructure of magnesium alloy is optimized after rare earth element addition with new intermetallic compounds y forms and the fraction of (3phase decreases. The results of SKPFM indicate that y phase has more active potential as cathodic phase in the microgavanic couple. The potential difference between y phase and a phase is less than that between β phase and a phase. Moreover, the potential distribution of AMRE1is more even compared with AM60, indicating less micro-galvanic corrosion. Corrosion product film which is enriched in RE and Al element and more compact and thin, on the RE-containning alloy surface is another key factor to the inhibition of further corrosion. The corrosion resistance of AM60was improved by RE addition. However, the content of rare earth element has a significant effect on the corrosion behavior of magnesium alloy, the excess addition of RE deteriorates corrosion resistance due to more y phases formed and the number of microgavanic couple between y and a phases was increased.The nature of atmospheric corrosion, which occurs under thin electrolyte film or even adsorbed layers, is electrochemical reaction between micro-anodes and micro-cathodes. Thus, investigation on corrosion behavior under thin electrolyte layers (TEL) plays an improtant role in understanding atmospheric corrosion of magnesium alloy. The corrosion behavior of magnesium alloy under thin electrolyte layers is different with that in the bulk solution.The results of cathodic polarizaiton curves indicate cathodic current density is depressed and cathodic reaction is inhibited with TEL decreasing. The EIS measurement domenstrates that corrosion resistance is enhanced by TEL thinning. Whereas, the corrosion resistance in the vicinity of100μm is closed to or even lower than that in bulk solution. As RE addition as concerned, there is the second kinetic region in the cathodic polarization curves of AMRE1alloy when cathodic potential shifed300mV with respect to open circuit potential (OCP); and there are capactive loops and inductive arcs altnatively appeared at LFs under200-300μm thickness, which results from the competition between reaction of Mg+on the local anodic area located where film broke down and reaction of H+on the cathodic surface of intermetallic compound (y phase). However, an inductive loop always showes up at LFs for AM60no matter how thick the electrolyte layer is.The composition and structure of corrosion product is very improtant in corrosion behavior of magnesium alloys. The corrosion product and corrosion morphology was respectively investigated by XRD, EDS. FTIR and SEM as complement to the corrosion behavior of AMRE1under TEL compared with AM60. The results indicate that the composition of corrosion products is not affected by TEL for the same type of magnesium alloy. The corrosion morphology of AMRE1magnesium alloy under TEL has two characteristics excpect at the vicinity of100μm: one is the severely localized corroded section, the other one is " unaffected area". The corrosion product are Mg(OH)2and compounds containing CO32-in seriously corroded section and only latter one is present in "unaffected area". In the condition of TEL, the corrosion product of AMRE1is enriched in RE and Al element, forms a skeleton structure to inhibit the corrison progress. Moreover, the corrosion morphology of AMRE1discloses the localized corrosion under bulk solution, different with AM60in uniform corrosion form. The pit initiation is inhibited under TEL and the pit growth is accelerated.Corrosion of magnesium is related to the destruction and repassivation of its surface film (or corrosion production film). Corrosion of magnesium is initiated from free-film region where the pit corrosion is the main corrosion form. The pregnance and growth of acitive points in free-film region is affected by the concentration of Cl-and the composition of surface film. These small active regions or points can be detected by Scanning Electrochemistry Microscopy (SECM). In the initial of immersion, the number and electrochemical activity of active points on AZ91and AM60magnesium alloy increase with Cl-concentration. While, high acitve points on magnesium alloy AMCel disappear in the immersion due to its dense surface film. Moreover, the Cl-concentration has a weak impact on the electrochemical activity of active points on AMCel magnesium alloy. The SECM images mainly reflect the surface morphology of magnesium alloy. According to the result of line scan across the scratch on AM60magnesium and corrosion morphology, dissolution of surface film is accelerated with Cl-concentration increasing. Compared with AM60, the line scan curves of scratch on AMCel magnesium slightly change with immersion time. Meanwhile, the strength and width of current peak across the scratch is slightly influenced by Cl-concentration. The result of corrosion morphology of scracth on AMCel magnesium alloy indicate that surface film of AMCel is more dense and provides strong protection for substrate and weakly affected by Cl-concentration.