Tailoring of oxide chemistry on aluminum by means of low-pressure water and argon plasma surface modification was performed to influence the kinetics of the self-assembly process of octadecylphosphonic acid monolayers. The plasma-induced surface chemistry was studied by in situ FTIR reflection-absorption spectroscopy (IRRAS). Ex situ IRRAS and X-ray photoelectron spectroscopy were applied for the analysis of the adsorbed self-assembled monolayers. The plasma-induced variation of the hydroxide to oxide ratio led to different adsorption kinetics of the phosphonic acid from dilute ethanol solutions as measured by means of a quartz crystal microbalance. Water plasma treatment caused a significant increase in the density of surface hydroxyl groups in comparison to that of the argon-plasma-treated surface. The hydroxyl-rich surface led to significantly accelerated adsorption kinetics of the phosphonic acid with a time of monolayer formation of less than 1 min. On the contrary, decreasing the surface hydroxyl density slowed the adsorption kinetics.

Please cite this article as: T. Lostak, S. Krebs, A. Maljusch, T. Gothe, M. Giza, M. Kimpel, J. Flock, S. Schulz, Formation and characterization of Fe 3+ -/Cu 2+ -modified zirconium oxide conversion layers on zinc alloy coated steel sheets, Electrochimica Acta (2013), http://dx. AbstractZirconium oxide conversion layers are considered as environmentally friendly alternatives replacing chromate-based passivation layers in the coil-coating industry. Based on excellent electronic barrier properties they provide an effective corrosion protection of the metallic substrate. In this work, thin layers were grown on HDG-steel-substrates by increasing the local pH at the surface and were characterized using potentiodynamic polarization technique.The influence of Cu(NO 3 ) 2 3H 2 O or Fe(NO 3 ) 3 9H 2 O on morphology and thickness of deposited protective layers were investigated by XPS, ToF-SIMS and FE-SEM. A significant film thickness increase was found by adding Cu 2+ or Fe 3+ ions to the conversion solution. In addition, growth kinetics were studied by in-situ measurements of corrosion potential using potentiodynamic polarization technique.

Barrier properties of self-assembled octadecylphosphonic acid (ODPA) monolayers on plasma-modified oxyhydroxide-covered aluminum surfaces were analyzed by means of in situ photoelastic modulated infrared reflection absorption spectroscopy (PM-IRRAS). The surface hydroxyl density prior to ODPA adsorption was increased by means of a low-temperature H(2)O-plasma treatment. Adsorption isotherms of H(2)O on ODPA self-assembled monolayer (SAM) modified surfaces in comparison to bare oxide covered aluminum surfaces showed that the ODPA SAM leads to a strongly reduced amount of adsorbed water based on the inability of water to form hydrogen bonds to the low-energy aliphatic surface. However, the ODPA SAM covered surfaces did not show a significant inhibition of the H(2)O/D(2)O isotope exchange reaction between the D(2)O gas phase and the hydroxyl groups of the aluminum oxyhydroxide film, as the interfacial layer between the ODPA SAM and the metal substrate, while the interfacial phosphonate group as well as the orientation of the SAM is not affected by the adsorption of water. It can be followed that the strong adhesion promoting and high corrosion resistances of organophosphonate monolayers on oxyhydroxide-covered aluminum is a result of the strong acid-base interaction of the phosphonate headgroup with the Al ions in the oxyhydroxide film, even in the presence of high interfacial water activity and the molecular interactions of the aliphatic chains. However, the barrier effect of such monolayers on the transport of water is negligible.

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