A Comparative Study of Two Coating Systems Exposed for 2 Years in the Field and in Accelerated Atmospheric Corrosion Chamber Environments
Authors: Hansen Douglas C. , Petry Leanne 
Published: February 2020

Analysis of a magnesium rich and a full chromate coating system on AA2024-T3 panels exposed at two field sites, Pt. Judith, RI and a University Oceanographic Laboratory Ship (UNOLS) based out Seattle, WA for two years were compared to identical samples exposed in a B117 test chamber (2000 hours), and a modified salt fog chamber with ultra-violet radiation and ozone gas (2000 hours). Both coating systems utilized a topcoat of polyurethane. The analysis was performed utilizing FT-IR ATR on multiple locations for each panel as well as scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). When comparing the IR spectra for the UV/ozone chamber and field exposure samples to the baseline data for one formulation of topcoat in the magnesium rich coating system, it was observed that a decrease in the % reflectance peaks for various components of the polyurethane had occurred suggesting the degradation of the urethane component of the topcoat. The change in the FT-IR spectra indicating topcoat degradation on the field and modified chamber exposures is in contrast to the spectra from the samples exposed to the B117 chamber, which looks more like that of the baseline data. Therefore, the FT-IR data indicates that degradation of the topcoat components of the magnesium rich coating system exposed in the modified chamber after 2000 hours was much more similar to the identical coating exposed for 2 years at the Pt. Judith and the Seattle-based UNOLS ship, whereas the coating exposed to the B117 chamber was much more similar to the baseline sample that had not been exposed. When considering the IR spectra for the full chromate coating system, there were some slight differences in the spectra, this presumably being attributed to different formulations between the topcoat for the Mg rich system and the topcoat for the full chromate system. In terms of observable differences in the spectra between the exposures and the baseline samples, all major peaks were diminished under the UV/ozone exposure after 2000 hours. In terms of the B117 exposure, all of the major peaks are larger in magnitude than the baseline or field and UV/ozone chamber exposures; Therefore, the FT-IR data for the full chromate coating system formulation tested in the UV/ozone indicates that it is more aggressive in causing the degradation of the coating system components than the B117 chamber test. For the SEM-EDS results, the coated test panels that were subjected to the modified chamber exposure with UV/ozone/sodium chloride solution had higher relative percentages of oxidation in the scribe over the duration of exposure when compared to the B117 exposure chamber. It was observed for the coating surfaces analyzed for the fully chromated and Mg rich coating systems that longer exposures in the modified chamber contribute to not only increased oxidation of the scribe but also of the coating as compared to the baseline analyses of the full chromate or Mg rich coatings. Furthermore, higher percentages of aluminum, titanium, and especially silicon are noted with time in the modified chamber. These results are significant because the UV/ozone conditions in the modified chamber are at or higher than the cumulative values observed in the field and, in this instance, are an indication of degradation of the advanced performance coatings in the modified chamber that are normally very resistant to weathering, chalking, and color changes in the field. It also must be noted that the modified and B117 chamber exposures were in constant spray conditions, whereas the field sites experienced intermittent wet and dry exposure cycles. These results suggest, therefore, that the coating resin is becoming more enriched in pigment particles at the coating surface and are associated with coating system degradation over time, and that it may be possible to tailor the chamber exposure conditions to yield coating degradation specific to an exposure site in the field.