Passivation and localised corrosion susceptibility of new Al–Cr–Fe complex metallic alloys in acidic NaCl electrolytes

Al-based complex metallic alloys (CMAs) are crystalline intermetallic phases characterised by very complex crystal lattices, showing intriguing surface properties such as low wetting, low friction and hardness which include, for the Al–Cr–Fe system, high corrosion resistance. However, their passivation mechanisms (nm-thick oxide layer formation) still need to be understood in relation with localised corrosion susceptibility. New single phases were therefore characterised by electrochemical Microcell technique. The chromium cations in the oxide proved to play a significant role in stabilising the passive layer at low pH. Corrosion resistance of single- and polycrystalline samples was first compared by means of inductively coupled plasma mass spectrometer (ICPMS) analysis during immersion. Higher dissolution rates have been measured for the low Cr containing (12.5 at%) polycrystals compared to the single crystals. The resistance to localised corrosion attack of the single crystals in an aggressive 0.01 M HCl + 1 M NaCl solution at pH 2 was high, with an anisotropic behaviour of the anodic breakdown potential related to the grain orientation. The orthorhombic (0 0 1) termination with higher Cr/Al atoms ratio formed a passive layer with increased resistance against pitting. Ageing of the samples and the formation of surface Al oxy-hydroxide appeared to decrease the pitting corrosion resistance.