The Impact of Selected Environmental Severity Factors on Paint Scribe Creep, Polymer Degradation and Corrosion of Epoxy Coated 1018 Steel

by Merrill Tayler

Institution: University of Virginia
Degree: PhD
Year: 2014
Record ID: 2041844
Full text PDF: http://libra.virginia.edu/catalog/libra-oa:7029


The discrepancy between lab accelerated life tests (LALTs) and field exposures of organically coated aerospace alloys subjected to corrosion is a well-known problem. For example, some Mg-based chromate-free primers for AA2024-T351 perform well in field corrosion testing but perform poorly in LALTs such as the ASTM B-117. Conversely, some primer coatings on metals have been found to perform well from a corrosion standpoint in LALTs but poorly in the field. Currently, it is not well understood whether various differences in environmental severity factors (ESFs), such as chloride and UV, cause such discrepancies. A lack of understanding of how ESFs affect the coating/substrate system is a consequence of previous studies’ reliance on low fidelity interrogation methods, such as visual inspection and coating gloss measurements, which do not adequately capture the entire range of interactions between ESFs, the coating and the substrate. Subsequent LALTs are developed by trial and error. This research seeks to take the first steps to understand the influence of selected specific ESFs on metal corrosion and polymer degradation in scribe creep. To begin to understand the influence of environmental severity factors on underpaint corrosion, we compare the similarities and differences in corrosion and scribe creep results from standard LALTs, field sites and lab full immersion tests (FIT). Ultra-high molecular weight epoxy resin (Poly(Bisphenol A-co-epichlorohydrin) glycidyl end-capped (C18H22O3)n•C22H26O4; CAS No. 25036-25-3,trade name Eponol) coated AISI 1018 steel samples (UNS# G10180; 0.15% C, 0.7% Mn, Fe; wt. %), with controlled scribes to expose the bare metal, were used in all tasks. In this initial investigation, comparisons between standard LALTs, FITs and field exposures were made using a suite of high-level surveillance methods: electrochemical impedance spectroscopy (EIS), Fourier Transform Infrared Spectroscopy (FTIR), X-Ray diffraction (XRD), and Raman spectroscopy. These methods maximize the ability to detect corrosive degradation both to the bare substrate at the scribe and to the substrate under the coating, as well as improve detection of degradation of the coating itself. By elucidating corrosive changes at a high level that could be missed with low fidelity surveillance techniques, these methods also improve our ability to make comparisons between LALT, FIT and field tests, and demonstrated a methodological improvement upon previous studies. Tests were conducted according to the LALT standards and were interrogated at predetermined time points (i.e., 0, 1, 3, 5, 10 and 15 days) during a total exposure time of 15 days. Briefly, results from this work demonstrate that there is a positive correlation between mass loss on bare 1018 steel samples and scribe creep length on coated steel for both lab- and field- exposed samples. Additionally, there is an inverse correlation between scribe creep length and the low frequency electrochemical impedance of the coating for coated steel near the scribe for lab and field…