Electrochemical Oscillations, Surface Morphology and Corrosion of Selected Thermal Sprayed Alloys.

Abstract

ABSTRACTThe effects of the surface morphology on the electrochemical behavior of various metal substrates and coated materials have been studied by analyzing impedance spectra, and electrochemical current oscillations in acidic solutions. The objective of this study is the search for a correlation between the surface complexity of the metallic samples and the impedance spectra, the structure of the current chaotic oscillations, the corrosion dynamics. Electrochemical current oscillations were studied for pure metals such as copper (Cu), iron (Fe) and titanium (Ti) in form of wire-electrodes. From the current time series the dynamics was reconstructed by using time delay and embedding techniques. The results showed, in agreement with other studies, the presence of a deterministic dynamics on chaotic, quasi periodic and periodic attractors in the reconstructed phase space. Furthermore, for the metal wire-electrodes the fractal dimension indicated the presence of low dimensional chaos. We then studied bigger pure metallic samples with a different electrochemical cell configuration. The results showed again deterministic behavior with the development of chaotic dynamics. The fractal dimensions in this case, however, were considerably larger than in the previous experiments with the microcell. In both cases the electrochemical oscillations changed structure as the surface undergoes a metamorphosis due to corrosion processes, formations of salt films or because on purpose we changed the roughness of the surface. Bifurcations from one time behavior of the current to another were observed. To better analyze the influence of the surface morphology on the current oscillations and in general on the electrochemical behavior of the sample we coupled our chronoamperometric studies with an analysis of the impedance spectra. We studied three pure iron samples mechanically at three grain sizes (600,400, 240). The dimension of the chaotic attractor decreased with the roughness. The constant phase angle (CPA) calculated from the impedance spectra increased with the roughness. The corrosion rate, as measured by the polarization resistance technique, increased with the roughness. Using the previous results as a “standard”, with the same techniques, we then studied steel samples coated with various “flavors” of Tungsten Carbide coatings, provided by Cooper Oil Tools Co. The roughness of the samples was measured using a Scanning Tunneling Microscope (STM). Atomic Force Microscope (AFM). We measured the CPA from the impedance spectra and compared with the roughness data. The results obtained indicate no trivial correlation and the need for new characterization of surface complexity more connected to the electrochemical sensitivity of the surface morphology. This sensitivity has its fingerprints in the structure of the electrochemical oscillations and the CPA. While this study is presently in progress, the results of this investigation showed a new avenue to study the electrochemical behavior and corrosion resistance of thermal spray coated materials and their substrates.