| Localized corrosion, i.e., corrosion confined to local regions, is difficult to predict and control. In general, it appears as a result of heterogeneities, either in the metal or in the corrosive environment. One of the phenomenological features of localised corrosion is the development of specific environments close to active sites. In practice, the micro-environment near the metallic surface is different from that of the bulk medium and varies with time and conditions of the system under study. Localized electrochemical techniques based on the use of microelectrodes are well suited for in situ sensing the distribution of potential, current and chemical species in active zones, pores or defects. Most of the published work dealing with microelectrodes in corrosion is mainly exploratory and their use in a regular basis is still rare in corrosion research. This work presents the development and application of microelectrodes as experimental tools capable to assess local electrochemical reactions on metallic substrates immersed in aggressive solutions. Traditional electrochemical techniques used in corrosion research give the average response of the global activity of the whole surface not discriminating local effects and local chemistry. Therefore they are unable per si to properly characterize localized phenomena and the corresponding corrosion, inhibitive mechanisms and, sometimes, even to select the best protection methods. It is important to know local parameters such as pH, micro-galvanic effects, concentration of oxidizing (like O2) and aggressive (like Cl-) species, and formation of surface films. The Scanning Vibrating Electrode Technique (SVET) was used for local measurements of ionic currents in solution. The technique detects the potential distribution in solution associated to ionic currents in solution. For the analysis of the chemical species involved in the corrosion process, electrochemical microsensors were used in SIET (Scanning Ion Selective Electrode Technique) mode. Microelectrodes sensitive to pH, dissolved oxygen and metal cations (namely, Mg2+, Zn2+ and Cu2+) were developed and characterized. Work was also done with microeloectrodes sensitive to Al3+, but with less success. Then, they were used for investigating the reactivity on defects and corrosion inhibition on coated aluminium and magnesium alloys, detecting the micro-distribution of chemical species in solution close to the corroding surface of Zn, Cu and a Zn- Fe galvanic couple specimens. ix Limitations and difficulties exist for the use of these techniques in corrosion research due to the inherent reactivity of corroding metals, with the formation of corrosion products and sharp changes of pH, O2 and ionic strength along the samples surface. In spite of the difficulties, the results presented here demonstrate that the SVET/SIET mapping gives useful information for the quantification of electrochemical processes at the micro-level. The data are of prime importance for the modelling and simulation of corrosion mechanisms, selection of new corrosion inhibitors and development of ''smart" coatings that suppress the corrosion processes. |
