X-ray photoelectron spectroscopy study of the interaction of ultra-thin alumina films on NiAl alloys with NaCl solutions

We report a XPS study of the reactivity in 0.5 M NaCl aqueous solution of NiAl(100) surfaces covered by ultra-thin aluminium oxide films grown by thermal oxidation of the alloy at 900 degrees C. Detailed analysis of the film thickness, the oxide stoichiometry and the alloy composition below the oxide was performed to characterize the drastic modifications induced by chloride exposure. Chloride entry in the oxide was observed by AR-XPS that revealed the existence of two chemical states of chlorides, one associated to aluminium oxide, the other one to aluminium hydroxide. The film modifications were initiated in the inner part and characterized by the formation of chloride-containing hydroxide species. The growth of these species spread later to the entire film, accompanied by a marked thickening of the modified alumina layer by selective oxidation of the alloy. Longer immersion times (>20 h) led to the formation of mixed nickel-aluminium chloride-containing hydroxide species, indicating the loss of corrosion protection. Copyright (C) 2010 John Wiley & Sons, Ltd.