Craig Melton - Dye Containing Coatings for Corrosion Sensing Applications

Presentation - pdf

Craig Melton¹, Dr. Simon Gill¹ & Dr. Shiladitya Paul^2
1 University of Leicester; ² TWI

Coating systems are used to protect metallic structures against corrosion in aggressive environments. However, over their design life insufficient maintenance of these systems can result in concealing significant metal loss beneath the coating. The incorporation of pH sensitive dyes in to coating systems aims to highlight these areas of underlying corrosion for inspection engineers to observe.

This research presents the compatibility and functionality of two pH sensitive dyes (phenolphthalein and thymol blue) in combination with three coating formulations; liquid epoxy, powder epoxy, and powder polyester. These dyes and coatings were administered to three different substrates; glass slides, S355 steel and thermally sprayed zinc/aluminium (TSZA) (85%/15%) on S355 steel. The glass slides were then exposed to 0.1M NaOH solution to investigate dye chromophore activation. The steel and TSZA samples were exposed to synthetic seawater for 35 days to investigate dye chromophore activation as a result of iron or zinc/aluminium oxidation.

The results demonstrate that the liquid epoxy coating was not compatible with the dyes, however successful corrosion sensing behaviour, via dye chromophore activation, was demonstrated for the epoxy and polyester powder systems on all three substrates, with the thymol blue containing systems being the easiest to observe.

Q&A

From  Stuart Lyon : I believe phenolphthalein goes colourless again at pH > 12.5
From  Philip Gill : pH > 10 it turns colourless again
From  Peter Collins : From my A level practical days (48yrs ago) I seem to recall that PP indicator can be tricky because atmospheric CO2 absorption leads to acidity which turns it back colourless, so you can easily overshoot the end point. I guess that the pink colouration might fade with time for the same reason.
Answer: These are fair comments. Phenolphthalein was our first choice because we thought we thought that pink is bright and would contrast. It is not really an engineering colour so it would be easy to detect. Phenolphthalein has more problems, like the ones I showed. It can function, but not as well as it would be needed for inspection engineers to use. In terms of real engineering is not the most feasible candidate. We looked at it because we thought that there were some practicalities.

From  Bob Luigjes : What is the UV stability of these organic dye’s? Will the dye stay in its current form when exposed in outdoor weathering conditions?
Answer: I’m not sure. I have no thought about that, but that would be a useful bit of information to test about. We have some of these coating systems installed offshore in the coastline in Spain and taking regular photographs and we can look at this.