Effect of aging treatment on microstructure and corrosion behavior of Al-Zn-Mg aluminum alloy in aqueous solutions with different aggressive ions

The microstructure and corrosion behavior of Al-Zn-Mg alloy(namely 7×××)after natural aging treatment(NAT)and artificial aging treatment(AAT)in aqueous NaCl solutions containing different aggressive ions have been investigated in current work.Results of microstructure characterization demonstrate that the aging treatment has a great influence on the grain size and precipitates.The grain size is relatively sizeable and no evident precipitates are observed in alloy after NAT comparable with that after AAT.The electrochemical corrosion behavior of alloy was studied by polarization curve and electrochemical impedance spectroscopy(EIS).The corrosion potential(E_(corr))of the aluminum alloy is more negative in 3.5 wt.%NaCl containing 0.052 wt.%NaHSO_(3)solution than that in 3.5 wt.%NaCl solutions with or without 0.907 wt.%NaHCO_(3).Charge transfer resistance(Rct)results reveal that alloy after AAT has an enhancement of corrosion resistance compare with that after NAT.With the immersion time increasing,mostly pitting spreads over the surface of the alloy only in NaCl solution,whereas exfoliation corrosion mainly occurs in NaCl solutions containing NaHSO_(3)or NaHCO_(3).

Insight into the triggering effect of(Al,Mg,Ca,Mn)-oxy-sulfide inclusions on localized corrosion of weathering steel

The surface properties of weathering steel(WS)is very important for its service performance and safety,and the localized corrosion induced by inclusions is closely related to the surface properties of WS and its application.In the current work,a common spherical(Al,Mg,Ca,Mn)-oxy-sulfide inclusion was selected to investigate the corrosion evolution of complex inclusion and its effect on localized corrosion on WS surface.The results indicate the inclusion in WS consists of(Ca,Mn)sulfides part and(Ca,Al,Mg)oxides part with complex core-shell structure.Locally preferential dissolution occurs in(Ca,Mn)sulfides part as well as metal matrix around the inclusions.Furthermore,both parts of the inclusions with poor conductivity and high-density dislocation at metal matrix around the inclusions was found,which suggests that traditional micro-galvanic corrosion cell may not be the cause of inclusion-induced localized corrosion on WS surface at initial stage of corrosion.The variation in maximum and average depth around the inclusion or selected region with immersion time indicates that localized corrosion induced by inclusions is overwhelmed by uniform corrosion of WS in the latter stage of immersion,then the rust formed on WS surface consists of two layers.

Corrosion behavior of pure zinc and its alloy under thin electrolyte layer

The corrosion behavior of zinc and its alloy under thin electrolyte layers （TEL） in 0.1M NaC1 solution was investigated by cathodic polarization curves and EIS. There was only one phase in pure zinc while zinc alloy consisted of eutectic phase and primary phase. Corrosion rate of zinc alloy was faster than that of pure zinc due to the effect of the micro-galvanic couples between the primary phase and the eutectic phase. The results indicated that corrosion rate of zinc alloy was greatly enhanced under TEL than that in bulk solution. Pure zinc exhibited minimum corrosion resistance as TEL decreased to 198 #m. Zinc and its alloy exhibited localized corrosion under TEL while it was more uniform in bulk solution. There were two capacitive loops in high frequency （HF） and middle frequency （MF） respectively, with finite length diffusion in low frequency （LF） presented in EIS. For pure zinc under TEL below 300 pm an additional inductive loop presented in MF-LF. The corrosion products and morphology were respectively examined by X-ray diffraction （XRD）, FTIR （Fourier Transform Infra Red） and SEM-EDS. FTIR micro spectroscopy results indicated that the component of the corrosion products was similar at different section of the specimen surface but different in content.