Electrooxidation and localized attack of iron in slightly alkaline solutions
Vatankhah Mohammadabadi, Gholamreza
Many electric components in Québec in operation are in underground vaults in contact with free waters. These components (such as transformers, circuit breakers, disconnecting switches, etc.) are protected by paintings which make it possible to delay corrosion. However this mild steel part of these equipments is subject to delamination of painting with time. Nevertheless, the micro-defects initially present in painting, those which are induced at the time of the installation and, the scratches make that the material of these equipments can be in contact with free waters. This research project was developed by simulating the conditions which can be found in the underground vaults, i.e. in solutions containing bicarbonates, phosphates, sulfate and chlorides. Iron was selected as a material to study the effects of these ions on it. It will be useful as a reference material in a more exhaustive study carried out in the laboratory. In order to understand the electrochemical behavior of iron in these media, several electroanalytical methods were used like the cyclic voltamperometry, the chronoamperometry, the chronopotentiometry, the rotating disk electrode disc, etc. The electrochemical techniques were supplemented by measurements of scanning electron microscopy, the Fourier transform infra-red spectroscopy, x-ray fluorescence, and inductively coupled plasma. The addition of the sulfate ions on the electrochemical behavior of iron electrodes in the presence of bicarbonate ions was examined. In solutions which contain only sulfate ions, iron shows a uniform corrosion. The addition of sulfate ions in the presence of bicarbonate ions cause pitting corrosion. At the time of the localized attack, there is formation of a passivating product of corrosion which covers the site. The various electrochemical behaviors were compared and explained by the analysis of the formation of the probable products and their physicochemical properties during electrochemical polarization. The electrochemical behavior of iron in aqueous solutions of phosphate ions was examined in the presence of chloride or of sulfate ions. It was shown that the phosphate ions behave like very effective inhibiting ions against a localized corrosion. In the presence of sulfates ions, there is a similarity with the phosphate ions in the active-passive dissolution region. The electrochemical behavior of iron electrode with respect to the localized attack induced by the presence of chloride and/or sulfate tons in aqueous bicarbonate solutions has been investigated by using full factorial design and pitting potential as an object function. It has been shown that both chloride and sulfate ions in the presence of bicarbonate ions are aggressive, but interactively with bicarbonate ions, chloride ions are aggressive and sulfate ions are inhibitive toward iron attack when these anions are present together in the solution. These observations had been linked to the presence of particular corrosion products (green rusts), and to their properties and reactions.
- Sciences – Thèses