Smart Coatings for Corrosion Detection in Steel Alloys

When steel alloys are used, they may require a coating.  It would also be advantageous to provide an early warning system for corrosion detection.  One method is to use coatings that will fluoresce or lose fluorescence based on corrosion activity.  Corrosion in steel alloys can be detected based on coatings that incorporate pH sensitive indicators that possess such properties.  The authors have used positive indicators for steel; they are initially non-fluorescent, but glow over areas of corrosion.

Many coating systems consist of a primer layer and a topcoat layer with the indicator in the primer layer.  Therefore, most of the specimens in this study were prepared in this way, with a nominal coating thickness of 30 microns each for the primer and topcoat layer.  However, specimens were also prepared with different coating layer thicknesses, as well as different coating layers, eg primer or topcoat layer only.

The specimens were exposed to a 3.5 wt% NaCl solution and examined as a function of time using a UV lamp in combination with optical microscopy.  They were also examined using electrochemical impedance spectroscopy (EIS) during exposure.

The influence of the various layers will be described, as well as the effect of coating layer thickness.