Development and Evaluation of Novel Corrosion Sensing & Protective Coatings based on Polymeric Powders for Marine Environments

Research regarding the development of corrosion-sensing polymeric coatings has taken the form of microcapsule or micro-particle incorporation. This is based on the need to shield the corrosion-sensing components from the curing agents of the polymer. This research has avoided this by making novel use of powder based coatings to design two corrosion-sensing coatings (epoxy based and polyester based respectively) without the need for microcapsules or micro-particles. This document defines the design requirements and evaluations taken in their development. This included, defining the UV/Vis absorbance of the corrosion-sensing components (pH sensitive dye phenolphthalein - phph and thymol blue - TB) as well as their thermal behaviour up to the curing temperature of the epoxy powder. In addition to this attempt to determine sensitivity of the dyes to corrosion of steel demonstrated that TB will sense after 35.6 mg and phph will sense after 64.0 mg. Combinations of the dyes and polymers were shown to sense hydroxide when immersed in hydroxide containing solutions. Coating samples were then developed on steel substrates and thermally sprayed zinc/aluminium (TSZA) coated substrates. These samples were exposed to four corrosive environments: laboratory immersion in synthetic seawater, neutral salt spray, a real C5 coastal environment, and a real CX offshore environments. Monitoring of their corrosion-sensing functionality showed that TB in combination with epoxy or polyester showed the most promising corrosion-sensing with functionality in all environments. phph proved difficult to observe in combination with phenolphthalein due to grey-scaling of the pink dye and green pigmentation of the epoxy. Further analysis showed that dye-containing solutions can lose their colour change as a result of UV exposure in only a few hours (1hr 40 mins for phenolphthalein and 4 hours for thymol blue).

A rust assessment of the exposed samples reported no significant consequence to corrosion protection performance to either polymer coating as a result of substrate dyeing. In the “harsher” real environments steel samples dyed with phph coated in epoxy samples reported Ri 4 25% and 60% in CX and C5 environments respectively. The TB samples fared slightly better with Ri 4 30% and 60% respectively. Polyester equivalent samples reported performed poorer in the harsh environments reporting between Ri 4 70% (phph C5) and Ri 5 100% (phph CX).

The TSZA samples revealed very little corrosion (Ri 2 5% at worst) this is likely due to the TSZA not the polymer coatings, which is highly corrosion resistant. Additional analysis came in the form of pull-off testing, defect propensity and electrochemical impedance spectroscopy (EIS). The pull off testing showed the polymer coatings to be stronger than the TSZA. The polyester coated steel samples (dyed and undyed) reported similar pull of strengths (4.04 Mpa to 5.59 Mpa), all of which were within standard specifications. The epoxy coated steel samples

showed that TB dying reduced the pull of strength failing with a 20 mm dolly at 12.0 Mpa, while the phph and undyed sample exceeded the capacity of the meter.

Defect propensity analysis (holiday detection) was not possible on TSZA coated samples due to the high roughness and porosity of the coating. The use of sealants could overcome this but are negative to corrosion sensing. On the roughened steel samples phph epoxy was shown to have the least defects (average of 5.97) on a 100 mm × 100 mm sample. Where as the epoxy without any dye had the most (average 15.6). Dye beneath the polyester powder was shown to have little effect on defect propensity.

EIS of 70 mm diameter samples was found to be quite challenging due to the sample roughness and porosity (TSZA). However, demonstrated dyeing the substrates to have only marginal effect on the samples impedance to a 10 mV excitation over 1 hour and 24 hour scans.