20 Years of Corrosion Sensing and Microvisualization of Corrosion Processes

Localized corrosion processes continue to be a major cause for degradation failure in damaging fluids.  Initiation stages of response remain difficult to characterize, arising in part because of the difficulty in being able to predict the location and onset of the attack.  Propagation stages are seldom constant but change with time due to competing processes of dissolution and product deposition at the corrosion sites.  The progress of the attack changes the local environmental concentrations which are especially important at buried interfaces that are isolated from bulk conditions.  The present paper will describe some of the recent advances in optical, electrochemical, and microscopy that are directed at providing local information on “precursor sites” and vulnerable areas on metal and covering oxide surfaces.  Previous studies of coupled reactions on reactive surfaces provide a general basis for the work to be described here.

The objectives of the experimental techniques are: (1) to measure the local corrosion rate; (2) to locate breakdown sites, and; (3) to study the fundamental properties of passive films and how they are related to the stability of the films and damaging environments.  The latter is of particular interest since it will allow one to predict why a particular precursor site is more vulnerable to breakdown as compared to neighboring areas.

In locating precursor sites, entire regions of the corroding surface can be imaged concurrently or point-by-point using scanned probe or beam.  Both classes will be discussed with their capabilities.  The techniques used in our laboratory are: (1) the Quartz Crystal Microbalance (QCM); (2) Optical Fiber Micromirrors (FOP); (3) Confocal Laser Scanning Microscopy (CLSM); (4) Photoelectrochemical Microscopy (PEM); (5) Scanning Electrochemical Microscopy (SECM); (6) Scanning Photoelectrochemical and Electrochemical Microscopy (SPECM); (7) Near Field Scanning Optical and Spectroscopic Microscopy (NSOSM).  The experimental setup for each and the capability help to reveal the advantages that are offered by them.

Optical sensing by the QCM and optical fiber micromirrors have been used to measure corrosion rates, with sensitivity of fractions of an atomic layer.  Similar to them but with the capability to image a larger reactive surface is the CLSM to determine local attack rates at various surface sites.  The NSOSM is valuable to obtain both vertical and lateral super resolution images of reacting surfaces of about 60 nanometers. The techniques support the development of miniature corrosion sensors devoted to monitoring reactive surfaces in remote and occluded regions, coupled with wireless transmission.