The Effect of Pre-Treatment on Corrosion Properties, Surface Chemistry and Interfacial Properties of Sol-Gel Coating on Aluminium and Aluminium Alloys
Authors: Tiringer Ursa , Abrahami Shoshan , van Dam Joost , Terryn Herman , Milosev Ingrid , Mol Johannes M. C. 
Published: May 2020

Hybrid sol-gel coatings are amongst the most promising environmentally friendly replacements for chromate conversion coatings for corrosion protection of aluminium alloys (1). However, there is still a lack of understanding about the importance of the pre-treatment of the substrate prior to sol-gel coating deposition. The goal of this study is to improve the understanding of the effect of different pre-treatments on corrosion, surface chemistry and interfacial properties of sol-gel coating on aluminium (Al) and aluminium alloys AA7075 and AA2024.

The hybrid sol-gel coating used in the present study is based on alkoxysilane precursors (3-glycidyloxypropyl)trimethoxysilane (GPTMS) and tetraethyl orthosilicate (TEOS) (2). A wide variety of Ce-based chemistries are found to be effective corrosion inhibitors for aluminium alloys and can be incorporated into sol-gel coatings to achieve active corrosion protection in case of a coating defect (1). The corrosion inhibitor studied in the present work is cerium nitrate Ce(NO3)3. Due to oxygen reduction at cathodic sites during initial stages of corrosion, OH-ions are formed increasing the local pH, establishing the condition for local precipitation of cerium oxide/hydroxide reducing the cathodic and hence corrosion kinetics of the substrate (3).

It is known that adhesion increases with the hydroxyl fraction at the surface of the substrate, resulting in stronger bonding between aluminium and the silicone from hybrid sol-gel coatings, resulting in better corrosion protection properties of coatings (1). For that purpose, alkaline (KOH), acidic (HNO3) and pseudoboehmite (DI boiling water) pre-treatments were used to control the formation of oxide layers with different hydroxyl fractions, prior to hybrid sol-gel coating deposition.

General observations, morphology and composition, of differently pre-treated samples were obtained by scanning electron microscopy equipped with electron dispersive spectroscopy (SEM/EDS). SEM analysis confirmed the significant effect of pre-treatment on the morphology of Al, AA7075 and AA2024. Surface compositional maps, carried out with EDS, were recorded to obtain the elemental distribution after different pre-treatments. EDS showed that AA7075 and AA2024 are enriched with copper after alkaline pre-treatment because of the galvanic coupling between copper and aluminium matrix resulting in preferential and accelerated corrosion of the aluminium matrix during the pre-treatment process (4).

Surface oxide chemistry of differently pre-treated samples was studied by X-ray photoelectron spectroscopy (XPS). The hydroxyl fraction of differently pre-treated Al, AA7075 and AA2024 were calculated from O1s and C1s peaks fitting. A relatively high hydroxyl fraction is observed after pseudoboehmite pre-treatment (DI boiling water) for all substrates (Figure 1), which is in accordance with earlier studies (5).

Electrochemical impedance spectroscopy (EIS) of different pre-treated substrates Al, AA2024 and AA7075, with the sol-gel coating on the top, is measured to study the effect of pre-treatment on corrosion protection properties of the deposited sol-gel coatings. The results show the highest low-frequency impedance modulus values after pseudoboehmite pre-treatment, for all substrates (Fig. 2).

Finally, we determined the effect of differently pre-treated sol-gel coatings on the dry and wet adhesion of overlaying epoxy-based adhesive Araldite®, by using pull off tests. Furthermore, the delamination kinetics of this adhesive on all pre-treated substrates coated with sol-gel is measured by using scanning Kelvin probe (SKP). Results show that a high hydroxyl fraction resulting from the pseudoboehmite pre-treatment, provides the best corrosion protection properties and the highest adhesion after sol-gel coating application for all studied substrates. Roughness measurements also confirmed that surface chemistry has a dominant effect on adhesion compared to surface roughness.

The financial support was provided by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 707404.

References:

1. R. B. Figueira, C. J. R. Silva, E. V. Pereira, Journal of coating technology and research
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2. U. Tiringer, A. Durán, Y. Castro, I. Milošev, Journal of the electrochemical society
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3. A. J. Davenport, H. S. Isaacs, M. W. Kendig, Corrosion Science 32 (1991) 653.

4. O. Gharbi, N. Birbilis and K. Ogle, Journal of The Electrochemical Society
163 (2016) C240-C251.

5. J. Cerezo, P. Taheri, I. Vandendael, R. posner, K. Lill, J. H. W. de Wit, J. M. C. Mol, H. Terryn, Surface and coatings technology
254(2014) 277-283.

Figure 1