| With increasing requirements of vehicles lightweight and environmentprotection, magnesium alloys have been used widely in automobile industry.However, the corrosion issue of magnesium alloys has not yet been understoodcorrectly. In consequence, it is essential for promoting magnesium alloysapplication, enhancing development of our country magnesium industry andaccelerating the proceeding of vehicles lightweight by research on the corrosionbehaviour of magnesium alloys, discussion for corrosion mechanisms ofmagnesium alloys and searching protective methods of magnesium alloys fromattack. The paper is investigated on the corrosion behaviours and corrosionmechanisms of die cast magnesium alloy AZ91D and as-cast AZ91D/TiC_p incorrosive media, and evaluated improving corrosion resistance for die cast AZ91Dmagnesium alloy with protections on die cast AZ91D by chromate-free conversionfilm treatments, electroless nickel plating and polymer coating. It will providepreparatory theoretical preparedness for further application of magnesium alloys inpractice.It is different for corrosion behaviour of die cast AZ91D in distilled water and3.5% NaCl solution. Die cast AZ91D was corroded very slowly in distilled water.The open circuit potential decreased with temperature increase either in distilledwater or in 3.5% NaCl solution. Corrosion resistance of magnesium alloysdecreased at higher temperature in the same medium. At the same temperature, thecorrosion potential of die cast AZ91D moved negatively and corrosion resistancebecame poor when it was exposed in the solution containing aggressive chlorideCl-ions. Pitting is major corrosion type. Pits occurred on the surface of magnesiumalloys at random and propagated on the surface and deeply penetrated withimmersion time increase. The corrosion rate decreases with immersion timeextending. Corrosion mechanism is accorded with surface film mechanism onmagnesium alloys. When the surface film of magnesium alloy is nearlycomplete or cathodic polarisation is applying, the anodic and cathodic reactionrates can be given as Ia～Iapexp[(E-Eae)/bap],Ic～Icpexp[-(E-Ece)/bcp];when thesurface film begins to break down as the applied potential is increased above acritical value, the anodic and cathodic reaction rates can be given as Ia =θ(E,pH,Y)Ia0exp[(E-Eae)/ba0];Ic=θ(E,pH,Y)Ic0exp[-(E-Ece)/bc0].The microstructure of as-cast AZ91D/TiCp consists of primary α phase withAl dissolved , Mg-Al intermetallic compound Mg17Al12 (The amount of β phasewas less and coarse) and TiC particles TiCp. TiC particles were distributednon-uniformly and presented spherical shape with average diameter 1-2μm.Corrosion rate of AZ91D/TiCp in 3.5%NaCl solution increased remarkably. It wasabout three times faster than die cast AZ91D. Attack began from pits. No passivefilm formed on AZ91D/TiCp , which led to corrosion resistance of AZ91D/TiCpwas not been improved, in contrast, reduced. At the same conditions, corrosionresistance of AZ91D/TiCp was far poor than die cast AZ91D.The corrosion resistance of coatings, which were gotten by dacromet,dacromet+organic lacquer, micro-arc anodizing and organic lacquer to protect diecast magnesium alloy from attack, was investigated through salt spray. The resultshowed that the corrosion resistance of coatings for magnesium alloy by organiclacquer and micro-arc oxidization were better than that by dacromet anddacromet + organic lacquer. Corrosion resistance of dacromet + organic lacquercoating was the worst in these four methods and poorer than naked die castmagnesium alloys.A Chrome-free chemical conversion film on die cast magnesium alloyAZ91D was formed by acetic salt-permanganate solution. The film was yellowand squama-like and composed of Mg and Mn oxide and Mg, Mn hydroxid.Corrosion potential shifted about 30mV than die cast magnesium alloy AZ91D in3.5% NaCl solution. The conversion film was almost non-corroded in 3.5% NaClsolution for 24h. The corrosion mechanism is the same as die cast magnesiumalloys and also described by surface film mechanism. When the electrode potentialis lower than its corrosion potential, the conversion film is not corroded;when theelectrode potential is higher than its corrosion potential, the conversion film beginsto break down and magnesium alloy surface is corroded. This chrome-freeconversion film has better protective effect for magnesium alloy.The phosphation treatment included ultrasonic degreasing, alkaline cleaningand phosphatation, omitting pickling and activacation. Phospation films on diecast magnesium alloy AZ91D presented different morphologies in phosphationbaths. In the bath only containing the phosphoric acid, the phosphate ions and theaccelerator agents NaNO3/NaNO2, no crystalline structure coatings were formedon magnesium alloy suface and visible cracks in the chemical conversion filmwere observed, which was conmosed of amorphous Mg3(PO4)2 and Mg17Al12;Thefilm formed in the bath containing Zn2+ ions was lamelated and consists of Znand hopeite;the crystallized film was flower-like and the nuclei and growth werefaster than the others when the bath containing Zn2+ and F-ions and the film wasmade up of hopeite. Phosphatation treatment is a kind of electrochemical process.On the microanodic sites, magnesium is dissolved;and on the microcathodic sites,hydrogen evolution and insoluble phosphate precipitation occurred. Magnesiumreacts with hydrogen ions in the solution. Hydrogen ions are consumed, whichaccelerates free phosphoric acid ions PO43-formation, the phosphoric acid ionscombining with zinc ions to form an insoluble zinc phosphate(hopeite). Zincphosphate precipitates on the microcathodic sites when it is exceeded its solubilityin the solution. It was showed that these phospatation films enabled to improvecorrosion resistance of mangensium alloys to some extent by polarisation curves in3.5% NaCl solution. The film with zinc and hopeite had better corrosion resistancethan others due to its corrosion potential was highest among them. Immersion testshowed that protection of phospatation films for magnesium alloys was not good.Electroless nickel-plating on die cast AZ91D magnesium alloy wasinvestigated by nickel acetate as main salt. The process concluded ultrasonicdegreasing, alkaline cleaning, acid pickling, fluoride activating andelectroless-nickel plate. The alkaline cleaning did not etch the surface. Afterchrome-nitric pickling and fluoride activation, the sample surface was etched andcrevices were observed. The autocatalytic reaction for nickel deposition is initiatedby catalytic dehydrogenation of the reducing agent with release of hydride ions,which then supplies electrons for the reduction of nickel ions. Hypophosphite ionsacting with H2O form hydrogen ions H+, hydrogen atoms H and phosphite ions;Nickle ions reduced by hydrogen atoms H become nickel;at the same time,hypophosphite ions acting with hydrogen atoms form H2O, OH-and P;hypophosphite ions and hydrogen atoms react by catalytic action to form H+,HPO32-and H2. These actions circulating time by time lead to the deposition ofnickel and phosphorus on the magnesium alloy. Nickle has been preferentiallynucleated on the crevices and it is formed on primary α-phase after the crevicesare covered. The coating was consist of nickle dissolved phosphorus which wasabout 4% in the deposition. The microstructure of the coating was compact andaverage diameter of most Ni particles was 2-4 μm. The adhesion was evaluatedby a heat-quench test and scratch test. The result suggested the excellent adhesionof the plating coating to the die cast AZ91D alloy substrate. The open circuitpotential of the Ni–P coating with time immersed in 3.5% NaCl solutionapproached to -0.36V(SCE). The corrosion potential shifted 1000mV bypolarization in 3.5% NaCl solution. This indicated the significantly improvedcorrosion resistance of the plating coatings compared with the bare magnesiumalloy. The coating was not corroded immersed in 3.5% NaCl solution for 24h. Alsothe coating covered with an organic coating was not attacked immersed in 3.5%NaCl solution for 240h. The results showed that Ni-P coating had good corrosionresistance for magnsiusm alloys. Eletroless nickel coating and the conversion filmby acetic salt-permanganate solution provided good protection for magnesiumalloys among the conversion film by acetic salt-permanganate solution,phospatation films, eletroless nickel plating coating. It was believed that eletrolessnickel plating was a better measure for protection of magnesium alloys because ofthe higher corrosion potential of Ni-P coating.Since late 1970's a high polymer was reported first, conductive high polymershave been new materials merged into metals and polymers. One of the mostimportant applications is used as a coating to protect metals from corrosion. Undercertain electrochemical conditions, conductive polymers act with metals to achieveprotection. A polypyrrole(Ppy) film was synthesized by electropolymerization onthe electroless nickel planting coating, which was deposited on die cast magnesiumalloy AZ91D, in an aqueous solution with pyrrole monomer and tartrate. Thepolypyrrole film was like cauliflowers, unconpact and weak adhesion to thesubstrate. The composition of the polypyrrole film was simple compared withpyrrole monomer. IR spectrum of Ppy film showed that there were N-H strongbroad band flexible vibration absorbed peak near 3431cm-1, C=C flexible vibrationabsorbed peak near 1618cm-1, CH3 bend vibration absorbed peak near 1396 cm-1and 1384 cm-1,and C-N flexible vibration absorbed peak on the ring near 1089cm-1. The mechanism of Ppy film is that pyrrole monomer is oxidated to positiveradicel by loosing a hydrogen ion;positive radicels react with each other, 2polymerization, 3 polymerization … until the chain molecule is formed with npolymer degree. Counter anions are doped in constant proportion between positiveradicels. Synthesis process of polypyrrole at room temperature and charateristics ofpolypyrrole film are required to be investigated further and it will be a promisingmethod for corrosion protection of magnesium alloys.
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