Electrochemical Study of Modified Cerium-Silane BI-Layer on Al Alloy 2024-T3

Chromate conversion coating (CCC) is the most successful and effective pre-treatment for corrosion protection of high-strength Al alloys (AA); however, they present high toxicity both for the human health and environment, and are subject of severe regulations. Therefore, there is a need of effective and environmentally friendly alternatives. This work studies modified Ce-silane bi-layers for replacement of CCC in the protection of AA 2024-T3. The treatment consists of an inner layer of cerium oxides/hydroxides and an outer silane layer. The silane treatment was further modified by the addition of silica nanoparticles and/or cerium (IV) ammonium nitrate (CAN). The corrosion behaviour of this system was studied by EIS and potentiodynamic polarisation. The morphological characterisation was performed via SEM and AFM and the chemical composition studied by XPS and FTIR. The electrochemical results showed that, when added up to a certain limit, the modifiers contribute for improved anticorrosion properties of the bi-layer. The morphological characterization indicated that, when added above a certain amount, the silica nanoparticles tend to agglomerate, introducing defects in the silane structure and contributing for earlier deterioration of the bi-layer. On the other hand, the XPS and FT-IR analyses showed better BTSE reticulation due to the addition of CAN.