In this work, the influence of the iron oxide acid−base properties on the adsorption of model epoxy compounds was examined. To study this, iron oxide layers with a different surface hydroxyl fractions were prepared in controllable and reproducible conditions. The surfaces were characterized by X-ray photoelectron spectroscopy (XPS). An expression for the hydroxyl fraction of the oxide films was deduced, starting from the measured XPS intensities. Two model epoxy compounds characteristic of an epoxy/amide system were adsorbed on the oxides:  N,N‘-dimethylsuccinamide and N-methyldiethanolamine. Additionally, an amine molecule without alcohol groups, N,N‘-diethylmethylamine, was adsorbed to investigate the role of alcohol functionalities on the amine adsorption mechanism. The interaction between the oxide layers and the nitrogenous model compounds were studied by examination of the O 1s and N 1s XPS photopeaks. The data showed that the amine and amide nitrogen adsorbed via two different bonding modes:  via Lewis-like acid−base interactions and via Bronsted-like interactions or protonation. A direct correlation was found between the protonation level of the adsorbed nitrogenous molecules and the hydroxyl fraction in the outer oxide layer. Additionally, it was noted that the protonation level depended on the mobility and flexibility of the adsorbed molecules. It was also observed that the presence of alcohol groups in the amine molecular chain had a beneficial effect on the number of adsorbed amine molecules.

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Steel is used for many applications, yet little research has centred on the optical properties of steel and its oxides compared to low-density materials such as aluminium and titanium.In this paper, the optical properties of iron oxides on interstitial free steel substrates are studied. The oxide layers were thermally grown under an oxygen atmosphere at 250°C. Two iron oxide types were present, i.e. a-Fe 2 O 3 (hematite) and Fe 3 O 4 (magnetite). The optical constants of the oxide layer are determined by visual spectroscopic ellipsometry (VISSE). VISSE allows a fast and non-destructive determination of both layer thickness and optical constants. An optical model was constructed to allow for the extraction of the layer thickness from the ellipsometric data. From this model, the total reflection, and therefore also the colour of the sample were calculated. The model is a good predictor of the system's colour, which proves that the colour originates from both interference and absorption effects.

In this article, the interface dipole formed after adsorption of model compounds on thin iron oxide films is studied by scanning Kelvin probe (SKP). Amine and amide molecules with the same organic functionalities as an epoxy/amide coating system and a carboxylic acid molecule representing a maleic anhydride grafted or copolymerized polyolefin polymer were used as adsorbants. First, the effect of the functionality type on the Volta potential of a thermally formed iron oxide was investigated. It was shown that after carboxylic acid adsorption, the surface potential shifted in positive direction, whereas the amine and amide molecules induced a negative potential shift. Second, the Volta potential shift after adsorption of the amine and amide molecules on oxides with different amounts of surface hydroxyls was studied. A changing overall dipole moment as a function of the hydroxyl amounts was observed. Lewis acid-base interactions and protonation took place between the amine or amide functionality and the oxide surface. It was found that the Volta potential shift was mainly affected by the number of protonated amine or amide molecules. The individual interface dipole moments of the two binding types could be deduced from the Volta potential shift, assuming that the individual dipole moments of the two binding types contributed linearly to the overall dipole moment. For the protonation reaction, an interface dipole of -1.28 and -1.54 D was observed for the respective amine and amide molecule, whereas for the Lewis acid-base interaction, a dipole of +0.10 and +0.14 D was found.

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aThe scanning Kelvin probe (SKP) was used as a tool to receive information about the interfacial interaction strength between coatings and oxide surfaces. Potential shifts induced by the application of two polymer films (polyurethane and epoxy-amide) on differently treated steel substrates were measured as a function of the physicochemical condition of the oxide surface (hydroxyl fraction and surface energy). The results showed that, in the case of strong interfacial acid-base interactions, a direct correlation exists between the Volta potential drop and the adhesion strength of the coating. This phenomenon indicated that the interface dipoles formed by oxide-coating acid-base interactions were reflected in the SKP potential shifts and that these dipoles could be detected, even when they were covered by a thick coating.