Microstructure And Corrosion Behavior Of As-cast AZ91-3Ca Magnesium Alloy

In this paper,a variety of experimental methods such as optical microscope?OM?,scanning electron microscopy?SEM?,X-ray diffraction?XRD?and electrochemical test system were used to study the microstructures and corrosion behavior of AZ91-3Ca magnesium alloy after being homogenized at 400?and500?for different time were studied.At the same time,the protective effect of the chemical conversion coating formed by the treatment of AZ91-3Ca magnesium alloy on alkaline solution,alkaline solution and acid solution on the surface of the alloy was also studied.The experimental conclusion is as follows:?1?As-cast AZ91-3Ca is mainly composed of?-Mg phase,?-Mg₁₇Al₁₂phase and?Mg,Al?₂Ca.The content of Al in the?-Mg matrix near the phase boundary is higher than that in the frontier of the phase boundary Al content.?2?The microstructure of?-Mg₁₇Al₁₂phase and?Mg,Al?₂Ca phase did not dissolve obviously when treated at 400?.After 400?homogenization,the concentration gradient of Al near the phase boundary was obvious reduce.?3?After homogenizing the AZ91-3Ca magnesium alloy at 500?for 2.0 h,the?-Mg₁₇Al₁₂phase completely disappeared to form a quaternary phase containing four alloying elements of Mg,Al,Zn and Ca Was honeycomb.After homogenization at500?for 8.0 h,the Al element in the matrix has been uniformly distributed,and the alloy burns at 500?for 24.0 h.?4?The corrosion resistance of AZ91-3Ca magnesium alloy after being annealed at 500?for 16.0 h in NaCl aqueous solution is the best.The corrosion potential of AZ91-3Ca magnesium alloy is-1330 mV and the current density is 1.222×10^-4A/cm².The corrosion rate after soaking in 3.5%NaCl solution for 32.0 h was 0.11 mg/cm²h.The reason is that the content of Al element in the matrix increases and the self-corrosion potential of the alloy increases.?5?The chemical conversion coating treated with NaOH aqueous solution was flocculent and consisted of Mg?OH?₂ and CaO.The conversion coating on the surface of the alloy was more uniform in all phases.The flocculation membrane of?-Mg matrix was more uniform and compact,Part of the surface film of?-Mg₁₇Al₁₂phase was broken feather,?Mg,Al?₂Ca phase film was dark flocculent.The corrosion rate of AZ91-3Ca magnesium alloy after NaOH aqueous solution slowed down.?6?The chemical conversion coating treated by NaF solution is composed of granular MgF₂.?-Mg₁₇Al₁₂is more likely to produce chemical conversion coating than?Mg,Al?₂Ca phase,resulting in uneven distribution of the coating.As the immersion time prolongs,the granular layer gradually,fter growing for 96.0 h,the film thickness is about 5?m.With the sample in the treatment of NaF aqueous solution time extension,corrosion resistance gradually increased.?7?The surface of AZ91-3Ca magnesium alloy treated with phosphate-potassium permanganate showed a typical "dry river",and the chemical conversion coating formed in the solution with pH=4.5 had better surface protection effect on the alloy,With the pH decreased,the resulting chemical conversion film to reduce the protective effect on the alloy.