ABSTRACT
Microscopy ................................................................................. 177 3.2.5 Scanning Reference Electrode Technique (SRET) and
Scanning Vibrating Electrode Technique (SVET) .......................178 3.2.6 Local Electrochemical Impedance Spectroscopy and
Mapping (LEIS/LEIM) ............................................................... 179 3.2.7 Other Scanning Probe Techniques .............................................. 179
3.3 Basics of Corrosion .................................................................................. 180 3.3.1 Mixed Potential Theory ................................................................181 3.3.2 Passive Metals and Localized Corrosion ..................................... 184 3.3.3 Methods of Corrosion Control ..................................................... 186
3.4 Applications of SECM to Corrosion Studies ........................................... 187 3.4.1 Iron and Its Alloys ....................................................................... 188 3.4.2 Aluminum and Its Alloys ............................................................ 204 3.4.3 Other Metals ................................................................................ 209
3.5 AC-SECM for the Study of Corrosion ......................................................216 3.5.1 An Early Approach .......................................................................216 3.5.2 More Recent Work ........................................................................216 3.5.3 Constant Distance Imaging ..........................................................219 3.5.4 Optimizing Contrast .................................................................... 221
3.6 SECM for Probing Coating Properties .................................................... 223 3.7 SECM for the Study of Electron Transfer at Active Metals .................... 233
3.7.1 Titanium ....................................................................................... 233 3.7.2 Tantalum ...................................................................................... 234 3.7.3 Iron ............................................................................................... 236
The corrosion of a material may be de†ned as the irreversible reaction of the material with its environment, usually resulting in degradation of the material and/or its properties. For pure metals and metal alloys, the reactions are redox reactions involving the oxidation of one or more of the metals comprising the alloy, driven by reduction of one or more oxidants, such as dioxygen, hydrogen ion, and/or water. These oxidants are typical of those encountered in aqueous (or environmental) corrosion under near-ambient conditions, the most common corrosion environment, and the principal subject of this chapter. Corrosion of metals has also been called “extractive metallurgy in reverse” [1], re«ecting the fact that considerable energy must be expended to extract and purify a metal such as Al or Fe from its ore (i.e., its oxide forms). Thus, corrosion is the thermodynamically favored process by which the metal reverts back to its oxide form(s). As a result, it is virtually impossible to completely stop such a thermodynamically favorable process. Therefore, corrosion control strategies are designed to slow the corrosion rate by retarding the rate of metal oxidation, the rate of oxidant reduction, the rate of ion movement between sites of oxidation and reduction (such ion movement is required for maintaining overall charge balance), or some combination of these approaches.
