Scribe Creep and Underpaint Corrosion on Ultra-High Molecular Weight Epoxy Resin-Coated 1018 Steel Part 1: Comparison of Field Exposures to Standard Laboratory Accelerated Life Tests

The corrosion and scribe creep of ultra-high molecular weight epoxy resin (poly[bisphenol A-co-epichlorohydrin] glycidyl end-capped [C₁₈H₂₂O₃]n·C₂₂H₂₆O₄)-coated AISI 1018 steel (UNS G10180; 0.15% C, 0.7% Mn, balance Fe) was investigated. Selected standard laboratory-accelerated life tests were compared to field exposures and full immersion tests to elucidate similarities and differences between scribe creep under the paint and corrosion of the steel. To accurately discern these phenomena, comparisons between laboratory-accelerated life tests, full immersion tests, and field exposures were made using many methods to interrogate both coating and substrate degradation. The methods included electrochemical impedance spectroscopy, x-ray diffraction, 3D optical microscopy, Raman microscopy, and Fourier transform infrared spectroscopy. The results demonstrated that LALT without ultraviolet radiation and wet/dry cycling did not correlate well with field exposure in terms of coating and corrosion degradation as compared to tests that incorporated these environmental factors. This points to the need for ultraviolet radiation to damage the polymer coating and wet/dry cycling to mimic the natural diurnal cycles found at field sites. Part 1 of this study finds that standard laboratory tests that have certain environmental severity factors enable scribe creep that is similar to field tests. Part 2 constructs a model that yields scribe creep length as a function of these selected environmental severity factors and time.