Research On Electrodeposition And Corrosion Resistance Of Al-Mn Alloy On Steel Surface

Study on protection of steel from corrosion has never stopped. Considering the excellent corrosion resistance of the Al-Mn alloy, electrodeposition of Al-Mn alloy on steel plate in low temperature molten salt was carried out as well as studying the corrosion behavior of the coated alloy.The ternary molten salt system AlCl3-NaCl-KCl is adopted with the mole ratio of0.66:0.17:0.17which appears as a kind of Lewis acid. The ternary phase diagram is calculated and CV results show there are two steps respectively presents the reduction of Al2Cl7-and AlCl4-and the reduction potential is respectively-0.46V and-0.60V. Crystalline Al can be reduced on surface of the steel cathode. It is calculated that aluminium and manganese can deposited together from the chloride molten salt system since the deposition potential is-0.14V vs. Al/Al3+, which is also proved by CV analysis. CV results show that the deposition of Al-Mn can be divided into two steps and this process is controlled by diffusion process.Amorphous Al-Mn alloy deposit was formed on steel surface when MnCl2was added into the molten salt. The effects of current density, temperature, MnCl2content, deposition time on coating quality of the Al-Mn alloy were studied. It is discovered that MnCl2concentration and temperature both play predominent impact on the quality of the Al-Mn alloy layer. Current density ranging from26.7mA·cm-2to48.0mA·cm-2, electrodeposition temperature ranging from200℃to220℃and MnCl2concentration ranging from0.5wt.%to2.0wt%are benefit for producing deposition layer with good quality. The coulombic efficiency decreased with depositing time because the efficiency became lower for the thick deposits.The addiction of CeCl3can improve the coating quality of the Al-Mn alloy.The corrosion resistance is studied with the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in1.0mol L-1NaCl solution.According to the polarization curves and EIS analysis, amorphous Al-Mn alloy showed higher corrosion resistance than steel and pure aluminium deposits. The deopsit obtained at current density of48.0mA·cm-2showed the samllest corrosion current density of9.114×10-7A·cm-2. The corrosion resistance of amorphous coating with Mn content of21.6～39.7wt.%is better than the crystallined Al-Mn alloy formed at a higher Mn content. The addition of CeCl3improves the corrosion resistance of Al-Mn alloy.