IN-SITU MONITORING OF UNDERCOATING CORROSION DAMAGE ACCUMULATION BY DIRECT OPTICAL INTERROGATION (DOI)

We have developed a simple in situ method for direct observation of corrosion damage accumulation underneath organic coating systems. In this method, thin metallic films several nanometers in thickness are deposited on to optically transparent glass substrates and then coated with a pigmented or unpigmented organic resin. These surfaces are then exposed to an aggressive environment, which stimulates undercoating corrosion through natural or artificial coating defects. The thinness of the metallic film results in rapid penetration to the glass substrate by corrosion followed by lateral spread of the corrosion resulting in a change in optical contrast when the sample is viewed through the transparent substrate while it is illuminated from its opposite side. Corrosion occurring underneath the coating can be tracked using low-power optical microscopy and time-lapse video capture. Using this approach, uniform and localized corrosion modes can be distinguished, and direct estimates of undercoating corrosion site growth kinetics can be obtained. Results show that localized corrosion growth is episodic with short bursts of growth occurring between long periods of passivity. Site growth rates during active dissolution range from 1 to 4 A/cm², which are consistent with growth rates of pits reported for bare aluminum surfaces. Examples illustrating how in DOI carried out simultaneously with electrochemical impedance spectroscopy (EIS) enables a more insightful interpretation of the EIS response will also be given.