9Magnetron-sputtered aluminum (Al) and aluminum-manganese (Al-Mn) films with structures 10 ranging from nanocrystalline to amorphous were obtained by tuning the Mn% up to 20.5 at.%.
11Corrosion behavior of the films was investigated in 0.6 M and 0.01 M NaCl aqueous solutions by 12 potentiodynamic polarization and electrochemical impedance spectroscopy. Pitting corrosion was 13 found to be strongly affected by alloy composition. The amorphous Al-20.5 at.% Mn exhibited 14 the best pitting resistance during short term exposure. However, over longer immersion in 0.01 M 15 NaCl up to 108 hrs, nanocrystalline Al-5.2 at.% Mn showed the highest corrosion resistance. The 16 dual-phase Al-11.5 at % Mn alloy was found to have higher nominal corrosion rate compared to 17 its nanocrystalline or amorphous counterparts. 18 19 1Aluminum (Al) films are frequently applied as a corrosion protection coating for steel [1, 2], 2 magnesium alloys [3, 4], and NdFeB magnets [5] etc., replacing the toxic cadmium coatings [6].3 Al coatings can be deposited using various methods including electroplating [1], physical and 4 chemical vapor deposition [3, 7], and hot-dipping [2]. An amorphous semiconducting passive 5 film forms on the surface of Al in neutral (pH 4-9) solutions to protect the metal underneath from 6 corrosion [8,9]. However, this passive film is still vulnerable to local attack from different halide 7 ions in saline environments, which eventually leads to pitting and crevice corrosion of Al. In 8 addition to its pitting susceptibility, the low mechanical strength of Al coating also leads to poor 9 wear resistance, which greatly limits its potential usage in industrial applications where 10 mechanical contact and corrosive environment coexist [10].
12Alloying has been found to be a highly effective method to strengthen Al by forming precipitates 13 or secondary particles [11]. However, the presence of precipitation and secondary particles 14 enhances corrosion by catalyzing oxygen reduction, increasing the alloy corrosion potential, and 15 localizing the electrochemical activity due to chemical inhomogeneity from the Al matrix [10]. 16 Recent studies show that alloying can increase the pitting potential (E pit ) of Al provided that the 17 alloying elements are retained in solid solution [12][13][14][15]. For example, age-hardened 2000 series 18 Al alloys (with Cu as the major alloying element) exhibit poor corrosion resistance [10], but Kim 19 et al. observed an ennoblement of E pit with increasing Cu content in Al-Cu solid solutions [12, 20 13]. Alloying Al with appropriate transition metals (TMs) such as Mo, Mn, W, Nb, Cr, Ta, V, 21and Zr in metastable solid solutions offers the possibility to significantly enhance its pitting 22 resistance in chloride solutions [16]. These TMs improve corrosion resistance and decrease 23 pitting susceptibility of Al by increasing the overpotential for anodic dissolution and decreasing 24 metastable pit initiation and growth rates [12]. In addition, alloying is an effective way to 1 minimi...
