Low-temperature Molten Salt Anodized Aluminum And Aluminum-manganese Alloy

Aluminum and aluminum alloys are well known for their comprehensive applications in steel protection. But aluminum and its alloys can only be electrodeposited in a non-aqueous system for the standard potential of aluminum is much lower than that of hydrogen. In this paper, a mixture of 0.66 : 0.17 : 0.17(mole ratio) AlCl₃-NaCl-KCl was chosen as the electrolyte for the deopsition of Al and Al-Mn alloys. Al coatings were obtained on iron substrate from AlCl3-NaCl-KCl molten salt by D.C.-plating. Surface activeness tetramethylammonium chloride was added into the molten salt in order to get better electrodeposits. A controlled content of anhydrous MnCl₂ was added into the eutectic chloride molten salt and a series of Al-Mn alloys coatings were obtained. Dual-pulses plating was applied to enhance the performance of the obtained Al and Al-Mn alloy coatings.Scanning electron microscope, metalloscopy, X-ray diffraction, energy dispersive spectroscopy and polarization curves were used to analyse the morphogy, microstructure, composition and corrosion resistance of the obtained coatings. Deposition mechanism of Al and Al-Mn alloys on iron substrate in the molten salt was investigated by cyclic voltammogram.The results showed that surface morphogy of the obtained aluminum coatings was determined by current density. When cathode current density is 25mA/cm², acicular aluminum coating is reliable to be formed. When cathode current density is 60mA/cm², the obtained aluminum crystalline grain is a mixture of acicular and spherical. When current density is 100mA/cm², spherical aluminum coating is reliable to be formed. XRD results reveals that (200) preferred orientation are featured in AlCl3-NaCl-KCl molten salt, and that preferred orientation was weakened by the increase of cathode current density. The results of cyclic voltammogram showed that Al deposition obtained owing to reduction of Al₂Cl₇^- which has a peak value of-0.23V and the oxidation peak at 0.15V is attributed to the dissolution of the deposited Al.Aluminum dendrite was effectively controlled and crystalline grain was fined by adding in TMA. Also, (200) preferred orientation was destroyed. A cross loop appeared in cyclic voltammogram for the nucleation process appeared in the deposition.The surface morphogy and phase structure of the obtained Al-Mn alloys determined by Mn content in the deposits. Several structure steps were transitted along with the increase of Mn content in deposits, there are face-centered aluminium, manganese in solid aluminium melt, single amorphous phase, mixture of Al₈Mn₅ and amorphous phase. A single amorphous phase alloy coating was obtained when Mn content is between 20.84% and 29.74%, while a mixture of amorphous and crystal structure was obtained beyond that. The deposits of Al and Al-Mn alloys obtained by dual-pulses plating is much denser and smoother. A cross loop appeared in cyclic voltammogram by adding in anhydrous MnCl2, also irreversibility of reaction was enhanced.The pitting potential of the obtained aluminium coating from AlCl₃-NaCl-KCl molten salt is -0.68V, the pitting potential of Al from 1.0%TMA contained molten salt is -0.56V, while that obtained by dual-pulses plating is -0.53V. The pitting potential of the obtained pure aluminum coating was enhanced for 180-360mV by the addition of manganese. Corrosion resistance of single amorphous phase alloys is better than that of the duel-phase structure deposits, while corrosion resistance of the coatings that obtained by dual-pulses plating is even better.