Influence of the type of alkali metal cation on the interaction of soluble silicates with porous anodic alumina

Abstract

Aqueous sodium silicate solutions are commonly used in pretreatments of aluminium alloys as cleaners and corrosion inhibitors. Silicate films are a promising alternative for corrosion protection and are able to confer high hydrophilicity to the metal surface. An interaction mechanism of soluble sodium silicates with anodic alumina was proposed in previous publications, where the sodium cation acts as 'bridging' element between the silicate anions and the anodic oxide. In this work, this model of interaction is extended by considering the effect of different types of alkali cations. Porous anodized aluminum oxides were silicate treated by dipping in water-based silicate solutions of different monovalent cations, namely lithium, sodium and potassium. The surface modifications occurring on the oxide layer during immersion in solution are studied as a function of the type of cation at ratio SiO 2 /M 2 O = 1 (M = Li, Na, K) at two different temperatures of the silicate solution (30 C and 70 C). The morphological changes occurring on the oxide surface during immersion are observed by field emission scanning electron microscopy, while the reactions happening at the anodic alumina surface are studied by monitoring the open circuit potential as a function of time. The attack of the oxide layer at low silicon-to-cation ratio is quantified by means of wavelength dispersive X-ray fluorescence.