| Al-Si foundry alloys are widely used in industry because of their favorableproperties such as low density, high strength-to-weigh and so on. How to improve theproperties of AI-Si foundry alloys has become an important part of work to researchersall the time. At the present time, the studies on Al-Si alloys focus on improvement ofmechanical properties, while there are few reports on the effects of the elements andheat treatment on corrosion resistance of Al-Si-X(X=Zn, Cu, Mg, Mn, Sn, Sb, Ti and Zr)alloys. The corrosion behaviors of AI-7Si alloys and ZL114A alloys in 3.5％NaCl(wt％)solution were studied by electrochemical and immersion corrosion tests. At the sametime, the effect of heat-treat on electrochemical corrosion parameters of ZL114A alloyswas investigated, and the morphologies of tested alloys before and after immersioncorrosion were observed and analyzed by optical microscope, scanning electronmicroscope, EDS spectrum et al. The results show that:(1) The effects of alloying elements on the microstructure of AI-7Si foundry alloys:the addition of Zn and Sn result in segregation in microstructure, the element Zn issolutionized in matrix, and the network Sn distribute in grain boundary and the"peritectic-type" structure of Si surrounded with Sn is formed in microstructure; Siphase is slender with the addition of Sb but is spotted state or globularity and coarseningwith the addition of Mg, because it decreases rate of coagulation; Cu, Mn, Zr and Tihave the grain refinement effect on alloys. (2) The effects of alloying elements on the electrochemical corrosion parameters ofAl-7Si foundry alloys: corrosion potential changes to negative potential and corrosioncurrent density increases with the addition of elements; however, Cu and Zn are themost important, Sn is less, the others elements aren't important.(3) The effects of alloying elements on the corrosion behavior of Al-7Si foundry alloys:Al-7Si foundry alloys appear mainly pitting in the paripheral zone of Si. Obviouscorrosion is detected in the enrichment zone of Zn and the paripheral zone of Si with Znadded, and obvious inter-granular corrosion is detected on the alloys with Cu added.The others elements aren't important.(4) The analysis of immersion corrosion: the corrosion results from theelectrochemistry effect of cathode phase, activity effect of Cl-, and the segregation of thesecond phase. That is the mechanism of immersion corrosion. The mechanism ofelectrochemistry effect of cathode phase: The matrix around the cathode phase has thepriority to dissolving, then the etch pit appear. If the chains of precipitation distributesin the grain boundary, the point etch evolutes inter-granular corrosion. The mechanismof activity effect of Cl-: Cl- can penetrate corrosion layer of alloy which appears etch pitbecause of the minor radius of CI. The mechanism of the segregation of the secondphase: The binding energy of grain-boundary will be decreased and the potentialdifference between inter-granular and intracrystalline will be increased if alloys appeargrain-boundary segregation, however, the tensile stress of corrosion layer of alloy willappear and the potential difference among all micro-area will be increased if alloysappear coring segregation.(5) The analysis of the effect of age treatment on corrosion: only pitting corrosioncan be found in all nonageing conditions and the corrosion resistance of the alloy insolution treatment is better than as-cast metal. Corrosion resistance of alloys isdeteriorative due to ageing treatment, and peak-ageing is sensitive to inter-granularcorrosion, while the underageing or overageing is no visible inter-granular corrosion butpitting corrosion in underageing and overageing.(6) The age-hardening characteristics of ZLl14A alloys have been investigated in thispaper. The results prove that the characteristics of age-hardening curves are related to the precipitation sequences of tested alloys. The peak of age-hardening curve occursdepending on the transition form G.P zones toβ" metastable phase in peak-ageing state.The hardness increases because of the occurrence of G. P zones in underageing, while thehardness decreases because of the transformation from theβ" metastable phase toMg2Si stable phase in overageing.(7) The analysis of the effects of age treatment on electrochemical corrosionparameters: there is maximum of corrosion potential and minimun of corrosion currentdensity in solution treatment, but there is minimun of corrosion potential and maximumof corrosion current density in peak-ageing.(8) In this paper the ageing precipitation behavior is reflected by measuringelectrochemical corrosion parameters. When the maximum corrosion potential and theminimun corrosion current density occurs, the alloys stay at the solid solution state atthis time, the hardness is the least, and the microstructure of alloy is composed ofsupersaturated a solid. When the minimun corrosion potential and the maximumcorrosion current density occurs, the alloys stay at the peak-ageing state at this time, thehardness is the least, and the microstructure of alloy is composed of needle-like orrod-likeβ" metastable phase.
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