Interaction between S-organic compounds and iron surfaces

Abstract

Detaching of organic polymer coatings as a consequence of a corrosive attack of the substrate material means as serious problem for corrosion protection [1]. Because of the coating thickness and the complex nature of practical corrosion protection systems, direct studies of the phase boundary metal/coating are impossible up to now. Therefore, the mechanism of deadhesion is largely unknown, The aim of this work now is to produce a strong chemical bond between the metal surface to be protected and selected organic monomers (concept of chemically modified metal surfaces [2]). This bond should be stable against aggressive media like strong acid or basic electrolytes. The thickness of the adsorbed organic film is only that of several molecular layers. Therefore, the binding conditions at the phase boundary can be investigated with electron spectroscopic (AES, XPS) and electrochemical (cyclovoltammetry, rotating ringdisk-electrode) methods. Up to now these studies are restricted to the interaction of n-decanethiol (n-DMc) with polycrystalline iron surfaces of different conditions. At room temperature n-DMc is a water like liquid. A first preparation technique means to drop the organic liquid without any further solvent onto the air formed oxide on top of the iron surface covered by a contamination layer. Within a second method the organic liquid is used as a second phase on top of an aqueous acid electrolyte (1 tool/1 HC10,). The sample is removed from the electrolyte at conditions that stabilize metallic iron and is passed through the n-DMc layer. At the phase boundary organic liquid/electrolyte the thiol molecules are oriented in such a manner that the polar (SH)-groups look into the electrolyte. By this a coupling of the sulphur to the metallic substrate occurs when the iron substrate passes the phase boundary. The same procedure is also performed under anodic conditions, so that a surface oxide is stabilized. The results of the surface analytical studies can be summarized as follows: For the oxide free surface the intensity ratios of the Auger peaks of carbon (CKLL) and of sulphur (SLMM) in the freshly prepared state and during Ar+-ion sputtering prove that the preferred orientation of the organic molecules at the phase boundary organic liquid/electrolyte is transferred to the chemisorbed molecules. The thiol binds to the metallic iron surface by the mercapto group (-SH). The aliphatic chains are oriented more or less perpendicular with respect to the surface. For preoxidized and contaminated iron surfaces the dropping technique results in a statistical arrangement within the thiol film. By modifying the iron substrate under anodic conditions only traces of n-DMc could be adsorbed on the oxidized surface and no further investigations were performed for these samples. The sensitivity of adsorbed thiol against oxidation depends on the orientation of the organic molecules and the chemical state of the iron surface. High resolution XP-spectra of the S-2p-niveau are discussed and interpreted in comparison to results reported by Holm [3] and to own reference measurements. At oxide free surfaces the main signal results from the chemical interaction between metallic iron and the mercapto function, the binding energy for the S-2p-electrons being comparable to values obtained for FeS and segregated sulphur atoms by Panzner et al. [4]. On the preoxidized surface only oxidation products of the thiol could be detected as for example sulfonate ions (R-SO~) and species exhibiting an electron density at the sulphur atom similar to organic disulfides (R-S-S-R). Additionally performed electrochemical investigations lead to the following conclusions: The n-DMc film bound to the metal under cathodic potential control is extremly stable in an Ar-purged borate buffer (pH 8.5) at potentials between -1 .1 VSHE and +0.0 VsnE. In the corresponding cyclovoltammograms no signals of the clean iron could be observed. The application of more anodic potentials leads to the formation of one monolayer Fe3+-ions without any damage of the n-DNc layer. An oxidative desorption of the thiol molecules occurs only then, if the sample is polarized to -1 .1 VSnE in an O2-purged borate buffer for several minutes.