This paper will discuss methods to evaluate the corrosion of substrates and breakdown of protective coatings subject to wet-dry cycling in natural and accelerated environments. Principally the discussion concerns the ability to monitor on-going corrosion deterioration to predict coating failure in a real-world environment without having to accelerate (i.e., artificially harshen) the environment.
The author previously introduced a method to evaluate protective coatings using a novel segmented cell approach. (1) Briefly, the technique intended to monitor natural current exchange between isolated segments, coated or uncoated, to supplement visual rankings of coating performance. The goal was to predict failure earlier than might otherwise be visible or to distinguish between coatings that had a similar visual appearance at the end of the proscribed test period. The experimental design also allowed for the instrumented segments, which act as sensors, to be prepared and coated as intended for a real-world industrial exposure, i.e., the metallic surfaces could be abrasive blasted and painted unlike thin-film, foil-like sensors also explored for similar purposes.
The initial work focused on the type of data that might be obtained and demonstrating the utility of the approach. This testing included longer-term data, wherein the segmented cell approach was used to study coatings intended for immersion service. Testing for immersed conditions allowed the inclusion of a continuous water film across the coating surface, simplifying the testing and data analysis. Testing under atmospheric conditions was limited at the time. The present work expands on these earlier presentations.
Figure 1 provides a plan-view photograph of a segmented-cell design. The design includes seven (7) elements. Each is nominally 0.6-cm thick. The center segment is nominally 14-cm long and 2.5-cm wide. (For some testing, the center segment was 0.6-cm wide; this difference related to fabrication issues and was not deemed significant in the data generated.) The outlying six (6) segments are nominally 4.4-cm long and 2.5-cm wide.