The use of microelectrodes in the study of localized corrosion of aluminum 6111-like alloys

by 1972- Abdullah, Aboubakr M.

This thesis is classified into four chapters. While chapter 1 is only an introductory chapter that summarizes the two mechanisms that are trying to explain localized corrosion of metals and alloys, Chapter 4 is the conclusions and future work chapter that presents the new achievements that have been added to the field of corrosion science by this work besides the proposed work to be done in the future. Chapters 2 and 3 are the core of this thesis and each of them has its own abstract, objective, and literature review, experimental, results and discussion, conclusions, and references list. In chapter 2, Crevice corrosion of an experimental Al 6111-like alloy with 0.68% Cu has been studied using potentiodynamic and potentiostatic techniques in different concentrations of sodium chloride and in 0.1M nitric acid solutions. Potentiodynamic experiments show that the alloy has an active/passive transition in a saturated sodium chloride solution at elevated temperatures (90 and 70 oC). Also, a steep electrode potential gradient, E(x), within the crevice has been measured using a microprobe connected to a SCE and controlled by a 3-D manipulator, and the observed locations of corrosion on the crevice wall are similar to the locations seen previously for systems in which it was clear that the shift of E(x) into the active peak region of the polarization curve on the crevice wall stabilized the crevice corrosion process. The same active peak E(x) relationship was observed for the Al 6111-like alloy in 0.1 M nitric acid solutions at room temperature. iv In chapter 3, focuses on the corrosion of a peak aged aluminum 6111-like alloy of 1.47% copper using the artificial pit configuration (pencil electrode) with relatively larger specimens in the form of plates rather than wires. The sample which was flush with the surface of the epoxy resin mount was anodically polarized in neutral NaCl solutions of different concentrations and pH 6.9. The alloy dissolved uniformly at a certain rate to form a local cell with epoxy resin walls and the alloy surface as its bottom. Intergranular corrosion (IGC) also occurred and was recorded every 900 seconds using a stereo-microscope (through the transparent epoxy) connected to a PC uploaded with time-lapse software. IGC rate was in the order of 0.5 mm/day. The potential drop profile within the cavity was recorded on a daily basis. It showed that the total potential drop is in the order of 50 mV. The pH in the local cell was measured using a commercial mono pH microelectrode coupled with a homemade Pyrex (glassy) microprobe connected to a SCE and controlled by a 3-D manipulator. pH measurements showed the medium within the cavity is alkaline with a pH ranges from 9-12. Also, the chloride ion concentration within the local cell was measured using a mono Ag/AgCl microelectrode coupled with the same glassy microprobe, which is connected to a SCE. It showed that the chloride concentration within the cavity is at least one order of magnitude higher than that in the bulk solution. The study of the effect of the chloride ion concentration on the IGC rate showed that the increase in the chloride ion concentration increases only the initial rate of the IGC but the overall rate remained almost the same. v