Applications of Electrochemical Scanning Tunneling Microscopy in Corrosion Research

Scanning tunneling microscopy is a technique that is used to image atomic resolution conducting surfaces, find the density of states of the sample surfaces and is used for atomic manipulation of species on the sample surfaces. Electrochemical scanning tunneling microscopy is a specialized scanning tunneling microscopy where the tip and the sample are in in an electrolyte environment. Electrochemical scanning tunneling does not only have the tunneling current but also Faraday current and capacitative current. Special care is given to keep the faradic current and capacitative current at the lowest so mostly tunneling current is achieved to image the surface. This is achieved by coating the scanning wire with insulating material and keep the tip of the wire exposed so the tunneling current is through the tip of the wire only. Electrochemical scanning tunneling microscopy is based on the continuous oxidation and reduction of the electrolyte between the tip and the sample which creates the tunneling current. Corrosion is a natural process of converting pure metal to a more stable, compound state thus releasing energy in the process. The underlying reason for corrosion is the thermodynamic instability of chemically active sites against water, air, or other chemicals. Since the corrosion reaction is mostly electrochemical, corrosion behavior of metals can be evaluated with electrochemical techniques. Application of Electrochemical scanning tunneling microscopy in the field of corrosion has elevated the research to another dimension and has advanced the understanding of the corrosion nucleation sites, pitting initiation, crevices, surface disintegration, passive coating formation, and helped in developing corrosion inhibitors. In this chapter, different examples of electrochemical scanning tunneling microscopy and spectroscopy study of surfaces like nickel, copper and gold in different chemical environments are given to understand the importance of this technique in study corrosions in metals.