Testing protective coatings for metal conservation: the influence of the application method

Abstract

The lack of a standardised methodology for the application of coatings on heritage metals can lead to non-comparable results. A careful and meaningful study requires considering the different application variables involved, especially in the preparation of the coupons. In this work, the effect that these application variables can have on the protective capacity of the coatings used has been studied. The influence of the thickness, number of layers and drying time (between layers and final) of Paraloid B-72, C80 microcrystalline wax and Incralac has been evaluated. Coatings have been applied on bare steel coupons, thickness measured with an induction probe, and subject to artificial ageing on UV light/water condensation cycles. The performance of the coatings has been studied by visual inspection and electrochemical impedance spectroscopy (EIS). Morphology of the layers has been characterised by optical microscopy and the composition of the corrosion products by Raman spectroscopy. Results show that the number of layers is not always proportional to the final thickness of the coating, and that drying time is a critical factor affecting the thickness and the protective properties of the coatings. After accelerated ageing, some coatings that had been left to dry for 14 days have a much better protective capacity than those that were left to dry for less time. Without taking into account these factors, performance measured can be wrong and comparison between materials misleading.