Understanding the role of thin film pre-treatments on steel surfaces

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The interface between a metal substrate and a protective organic coating plays a vital role in determining the long-term anti-corrosion performance of painted metal products. Prior to the application of an organic coating system to protect against corrosion, the metal surface is subjected to a cleaning regime, typically involving exposure to an alkaline environment, followed by a pre-treatment which deposits a thin inorganic film. This in turn provides an inert surface which improves adhesion with organic-based primer coatings, and in certain circumstances, also offers additional active corrosion protection at the metal-organic coating interface. This project forms part of a larger partnership involving BASF Coatings Ltd, in collaboration with Swansea University and Imperial College London, where the main theme will involve furthering the understanding of corrosion-induced failure of protective organic coatings when applied to metal surfaces. The principal focus of this project will be directed towards a fundamental understanding of the influence of the metal surface prior to the application of a corrosion protective organic coating system on durability. The research will concentrate on two specific areas of interest:(i) the chemical nature of the cleaned metal surface prior to pre-treatment and how this affects the formation and composition of current state-of-the-art inorganic conversion coatings.(ii) the influence of the topography and chemical composition of the pre-treated surface on the durability of a corrosion protective organic coating, initially concentrating on a simple model paint system.The investigation will be carried out using comprehensive in-situ and ex-situ electrochemical characterization by means of Scanning Kelvin Probe (SKP), Scanning Vibrating Electrode Technique (SVET), alongside potentiodynamic and electrochemical impedance spectroscopy methods in the laboratories of the Swansea University corrosion research group. Surface chemical and structural characterization will be carried using a suite of instrumentation including X-ray-photoelectron spectroscopy (XPS), glancing angle X-ray diffraction (XRD), and field emission gun scanning electron microscopy (FEG-SEM).Project Tasks: - Investigate the influence of various industry standard chemical cleaning technologies on the surface chemistry of cold-rolled galvanised steel and zinc-magnesium-aluminium (ZMA) alloy coated steel substrates.- Study the effect of cleaning regimes on the efficiency of thin inorganic film formation using current pre-treatment technologies such as phosphate, zirconate and titanate-based types.- Investigate the influence of both the cleaned and pre-treated surface on resistance to corrosion-driven failure when overcoated with an organic layer. For steel and galvanised steel surfaces, this will include an evaluation of the organic coating failure via cathodic disbondment and anodic-driven organic coating failure.