Low-frequency EIS interpretation with the potential to predict the durability of protective coatings

Abstract

Obtaining impedance response with predictive value for coating durability is central to meeting today's protective coatings end-user requirements. Here we present a novel approach to interpreting the low-frequency impedance of industrial coatings that takes into account the relevance of the coating properties that determine their barrier and adhesion durability. Modern, high adhesion, low VOC polymer coatings utilize cross-linking chemistry with polar or hydrogen bonding materials, leading to the apparent paradox that coatings exhibit corrosion creep values that are very low and inversely related to the barrier effect derived from the EIS response. In practice, once the corrosion creep rate is negligible, the ability of a coating to act as a long-term effective barrier, especially at edges and welds, becomes a major concern, and the improvement in durability relies on the improvement of the barrier. In the present study, we investigated the barrier effect of conventional partial and full offshore coating systems with epoxy and epoxy mastic layers applied without and with zinc-rich or zinc phosphate primers and polyurethane topcoats. The panels were aged for two years in an atmospheric offshore-like conditions and subsequently exhibited defects due to inadequate coating thickness, number of layers, surface preparation and inadequate application practice. EIS measurements were performed on the panels in the laboratory in dry and wet state. EIS provided a quantitative rating of the barrier effect that can serve as a basis for informed upgrading of coating solutions, application practices or workmanship quality, particularly in structures with limited access where high durability is an imperative.