Electrochemical behaviour of cast Al-Zn alloys in natural chloride solution were investigated by potentiodynamic measurements and electrochemical impedance spectroscopy (EIS). Results from open circuit potential (OCP) measurement against immersion time showed a stable free corrosion process was achieved after 15 kiloseconds and alloys with more Sn contents had shifted OCP value towards more negative direction. Potentiodynamic polarisation curves showed that the corrosion potential (Ecorr) of aluminium alloys with Sn addition were much active compared to alloy without Sn. The anodic curves were characterized by no sign of passive current due active dissolution on the surface of the Al-Zn-Sn alloy. SEM images show that the presence of Sn in Al-Zn alloys produces better and uniform dissolution morphology. EIS results confirm that the presence of Sn is beneficial in improving anodic dissolution of Al-Zn alloys by reducing resistance to polarization (Rp). The presence of 0.1%wt. Sn in Al-Zn alloy has been found to be useful in activating electrochemical reaction at alloy-solution interface based on inductive loop in EIS diagram.
In this paper, a study on the electrochemical behaviour and electrical properties of Al-Zn and Al-Zn-Sn alloys in tropical seawater using open circuit potential (OCP) measurement and electrochemical impedance spectroscopy (EIS) are reported. The results from both the OCP and EIS tests show that surface activation was observed in the Al-Zn alloy with the addition of 1.34 wt.% Sn which can be manifested by the shift of OCP values towards more electronegative direction. The EIS spectra of Al-Zn alloy exhibits a semicircle loop, while the EIS spectra for the Al-Zn-Sn alloy exhibits a semicircle with a semicircle inductive loop. The change in EIS spectra for Al-Zn-Sn alloy is correlated to the increase of surface activation resulting in a less stable passive layer. Equivalent circuits models were proposed to fit the impedance spectra and the corresponding electrical parameters with optimum values were deduced. The modulus impedance in the low frequency region or polarization resistance,Rpolobtained for the Al-Zn-Sn alloy,Rpol= 2.76 kΩ cm2) is slightly decreased compared to the corresponding value of the Al-Zn alloy,Rpol= 3.97 kΩ cm2), indicating a considerable reduction in the protective capability of the oxide layer on the Al-Zn-Sn alloy. It appears that the heterogeneous oxide film and pores formed on the Al-Zn-Sn alloy play a key role in reducing total resistance to the flow of electron at the alloy-electrolyte interface.
In natural seawater, pure aluminum develops oxide layer which forms a barrier, protecting against corrosion. Alloying with other elements prevents the development of oxide layer by introducing localized galvanic cells. Different aluminum alloys exhibit different electrochemical behavior. The scope of the project was to study the electrochemical behavior of different chemical composition of aluminum alloys in natural seawater. The significance of understanding the corrosion behavior of aluminum alloys is essential in the fabrication of sacrificial anodes to be used in cathodic protection in corrosion control. Aluminum alloys were fabricated using alloying elements Zn, Sn, Mg, Cu, Fe, and Si. Divided into two groups, samples of aluminum alloys in Group 1 contain same weight percent of Zn, and different weight percent of Sn. Aluminum alloy samples in Group 2 contains same weight percent of Mg, Cu, Fe and Si with different weight percent of Sn. The samples were then subjected to corrosion behavior experimentation which includes Tafel plot, corrosion potential and potentiodynamic scan. It was found that alloying with Mg, Cu, Fe, and Si instead of alloying only with Zn and Sn further increases the negative potential, the density of particle distribution and further reduces the corrosion rate of aluminum alloys. The activeness of aluminum alloys also increases when alloyed with Zn, Sn, Mg, Cu, Fe, and Si.
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