Investigations of the electroreduction process of anodic films grown on tin in carbonate-bicarbonate solutions

Anodic passivation of tin by alkanethiol self-assembled monolayers examined by cyclic voltammetry and coulometry

The self-assembly of medium chain length alkanethiol monolayers on polycrystalline Sn electrodes has been investigated by cyclic voltammetry and coulometry. These studies have been performed in order to ascertain the conditions under which their oxidative deposition can be achieved directly on the oxide-free Sn surface, and the extent to which these electrochemically prepared self-assembled monolayers (SAMs) act as barriers to surface oxide growth. This work has shown that the potentials for their oxidative deposition are more cathodic (by 100-200 mV) than those for Sn surface oxidation and that the passivating abilities of these SAMs improve with increasing film thickness (or chain length). Oxidative desorption potentials for these films have been observed to shift more positively, and in a highly linear fashion, with increasing film thickness (~75 mV/CH2). Although reductive desorption potentials for the SAMs are in close proximity to those for reduction of the surface oxide (SnOx), little or no SnOx formation occurs unless the potential is made sufficiently anodic that the monolayers start to be removed oxidatively. Our coulometric data indicate that the charge involved with alkanethiol reductive desorption or oxidative deposition is consistent with the formation of a close-packed monolayer, given uncertainties attributable to surface roughness and heterogeneity phenomena. These experiments also reveal that the quantity of charge passed during oxidative desorption is significantly larger than what would be predicted for simple alkylsulfinate or alkylsulfonate formation, suggesting that oxidative removal involves a more complex oxidation mechanism. Analogous chronocoulometric experiments for short-chain alkanethiols on polycrystalline Au electrodes have evidenced similar oxidative charge densities. This implies that the mechanism for oxidative desorption on both surfaces may be very similar, despite the significant differences in the inherent dissolution characteristics of the two materials at the anodic potentials employed.

Early Stages of Film Formation and Surface Growth on Tin Electrodes at Bicarbonate Medium: a Modelistic Approach

A modelistic approach of the early stages of film formation and surface growth on tin electrodes at bicarbonate medium under dynamic conditions is presented. The modelistic treatment was applied to both anodic and cathodic processes. In order to organise the paper, the potential interval of interest was divided into two potential windows, each one involving one of the anodic peaks and their corresponding cathodic counterparts. The Müller–Calandra model (also known as the layer-pore resistance model) was considered suitable to interpret the film growth processes occurring during the positive scan. A derivation of the former, developed by Devilliers et al., which take into account the conductive electrical nature of the surface compound, was applied to explain the reduction of the film formed within the first potential window. On the other hand, it was verified that the use of a simple model to interpret the cathodic process occurring at the second potential window was not possible due to the complex nature of the process. In this case, a mixed control kinetics was proposed and the current was considered as the sum of two contributions. In order to complement the description and specially to determine the role of diffusion and mass transport in film formation, rotating disk measurements were also carried out.

Corrosion et passivation de l'étain dans un milieu simulé à une eau industrielle

The corrosion behaviour of tin in a synthetic medium similar to an industrial water has been studied by using electrochemical methods (plotted potentiodynamic curves and polarization resistance measurement). The effect of pH variation, temperature and concentration of anions in solution has been investigated. The results obtained show that pH variation (around the value 8.2 of the synthetic medium) has no effect on the pitting sensitivity of tin. However, the increasing of temperature in the range of 20–70°C accelerates the corrosion process. The presence of anions Cl– and SO42– leads to pitting corrosion of tin. Cl– ions are more aggressive than SO42– ions, whereas HCO3– ions acted as an inhibitor for pitting corrosion of tin in the studied medium.Key words: tin, corrosion, pitting, industrial water.