Pitting corrosion resolution

Williams, D.E., Mohiuddin, T.F and Zhu, Y.Y. (1998) Elucidation of a trigger mechanism for pitting corrosion of stainless steels using submicron resolution scanning electrochemical and photoelectrochemical microscopy. Journal of the Electrochemical Society, 145, 2664— 2672. 

Corrosion processes can be very complex and, as the above examples show, surface analytical techniques can often provide unique information important for the understanding of these processes and to the solution of corrosion problems. By their basic nature, surface sensitive methods excel at examining thin layers at surfaces and interfaces that are difficult to detect and analyze by other methods but which can have a large influence in corrosion. The higher spatial resolution surface techniques are particularly useful for analysis of small area corrosion problems such as pitting and corrosion of electronic components and integrated circuits. 

Many models exist to predict the conditions within these sites (e.g., 34,35). However, if the primary need is to determine the extent of corrosion damage (e.g., the depth of corrosion penetration), these models are not sufficient. Generally, electrochemical techniques contain no spatial information, since the current measured is the sum of currents from all individual corrosion sites. In the case of pitting, this limitation is being slowly erased as scanning techniques capable of spatial resolution are being developed. However, the ability to resolve local corrosion sites within fixed occluded areas such as cracks and crevices remains minimal. 

The three examples shown In this paper Indicate that mlcrovoltam-metrlc electrodes are useful tools to probe chemical heterogeneities In solution, and furthermore to characterize these phenomena under dynamic conditions. To achieve greater spatial resolution, smaller electrodes will need to be employed. Automation of this type of measurement would be desirable and can readily be accomplished with piezoelectric mlcroposItloners and other such devices which can be remotely controlled. Such developments will lead to a form of dynamic chemical microscopy which would be useful to measure such events as secretion from single cells, corrosion processes In pits and cracks, or further studies of solution flow. 

The shear force constant distance mode SECM was later mounted onto an inverted optical microscope, in a so-called Bio-SECM configuration, in order to study individual living cells. The Bio-SECM instrument was notably used to detect nitric oxide released from single cells [78]. In parallel, the shear force setup was modified by replacing the optical detection system, used to monitor the tip vibrating motion, by a piezoelectric element [79]. Electrical detection of the tip vibration was shown to be much easier and more convenient than optical detection, partly because the delicate laser alignment on the tip was made unnecessary. Further developments to shear force SECM have seen the implementation of high-resolution constant distance mode AC-SECM, which was used for the visualization of corrosion pits on stainless steel samples [80,81]. 

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