Mechanism of formation of silicate thin films on porous anodic alumina

Sodium silicate solutions, commonly known as “water-glass”, are water-based solutions containing dissolved glass. These aqueous silicates are used for a wide variety of applications. In particular, they are commonly used in pretreatments of aluminium alloys as cleaners and corrosion inhibitors. Silicate films are known to be a promising alternative for corrosion protection and to confer high hydrophilicity to the metal surface. One of the main issues in the study of the formation of silicate films on metal oxides is the complexity of the anionic speciation in aqueous solution. The mechanism of silicate deposition on alumina is not clear yet. One of the main reasons is that it is not known how the silicate anions adsorb on the alumina surface and what type of deposition is formed on the surface. In this work, different aluminium oxides were silicate-treated by dipping in a water-based sodium silicate solution; the effect of the oxide morphology on the deposition was studied. The anionic speciation in the silicate solution was studied by means of 29Si NMR and DLS. The morphological changes occurring on the oxide surface upon adsorption were observed by FE-SEM. WDXRF measurements allowed quantifying the deposited silicate on the porous oxide layer. The presence of silicate in the pores of the anodic layer was studied by means of GDOES. A mechanism of the deposition was proposed.