The objective of this effort was to develop a comprehensive test protocol to accurately predict all aspects of the performance lifetime of Department of Defense (DoD) coatings and alloys. This test protocol was to be comprised of a test methodology which would include the development of a test chamber, modified to include the synergistic effects of UV and ozone and the exposure of bare and coated samples to yield an accelerated corrosion test. This test would result in not only accelerated corrosion rates for the bare metals, but also similar corrosion chemistries on the surface of the exposure test coupons as were found on the field exposed samples. If this could be replicated, then the test chamber environment would be applied to coated samples as well. Indeed, DoD service environments are variable in nature (e.g., beachfront vs. desert) and therefore the intent of the test protocol was to be either specific to a particular service environment or dynamically “tunable” to match the particular service environment in which the coating or alloy substrate is intended to be used in service. Finally, the test protocol was to allow a reasonable prediction of performance lifetime based upon a relatively short timeframe accelerated test.
Coupon panels of bare aluminum alloys AA2024-T3, AA6061-T6, AA7075-T6, and 1010 steel, pure silver and pure copper were exposed to eight atmospheric test site environments and within two laboratory corrosion test chambers. One of the laboratory corrosion test chambers operated in 5% NaCl salt fog in accordance with ASTM B117, and the other was modified to allow concurrent exposure of ultraviolet (UV) light, ozone, and 5% NaCl salt fog in accordance with ASTM B117. Various analyses (mass loss determinations, corrosion morphology and elemental analysis by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and electrochemical determinations of corrosion product thickness) were performed in order to investigate a possible correlation between corrosion behavior of the panels exposed at the outdoor sites and those in the laboratory tests. The atmospheric test site environments were: Wright-Patterson AFB, Kirtland AFB, Tyndall AFB, Hickam AFB, Pt. Judith, RI and Daytona, FL. There were two additional sites located on University National Oceanographic Laboratory Ships. A second set of panels consisting of AA2024-T3 and 1010 steel substrates coated with various organic coating systems (both chromate-containing and chromate-free), and an AA2024-T3 lap joint ensemble with Cd-plated steel fasteners, were exposed at the eight atmospheric sites and the two laboratory corrosion test chambers. The coated panels were mounted on racks adjacent to the bare metal samples at each exposure location. Qualitative coating system (performance rankings) and quantitative coating system determinations (SEM-EDS, FTIR) on the coated substrates and lap joint samples were made and compared after the one and two year exposure cycles. Bare sample coupons were retrieved at three month intervals over two years and the coated panels were retrieved at one year intervals. Weather data over the two year exposure period was either recorded on deployed weather monitoring systems at each exposure site (temperature, relative humidity, total UV irradiation and ozone levels) or downloaded from nearby EPA monitoring sites. The cumulative frequencies of the four weathering parameters (UV, ozone, temperature and RH) measured were determined for the exposure sites, which allowed comparison to mass loss data and provided input to the cumulative damage model. Analysis of the agreement between deployed weather monitoring systems and EPA monitoring sites was performed, where applicable. Additionally, a proof of concept model for predicting atmospheric corrosion rates of 1010 steel was developed, using a cumulative damage non-linear modeling and simulation approach. The model used inputs from weather data including time dependent temperature, relative humidity (%RH) and atmospheric contaminant (SO2 and ozone) levels and chloride deposition rate (mass per unit volume of rainwater).
Coupon panels exposed at atmospheric sites and in the laboratory chambers were analyzed and several important trends were discovered. It was determined that the amount of corrosion experienced by the coated panels in outdoor environments, including lap joint specimens, correlated more strongly to elevated temperature and %RH than other parameters measured. In addition, comprehensive analysis suggests that the cumulative amount of time that a coated sample was exposed to damaging environments (as measured by temperature, %RH, ozone, UV, Cl, and SO2) was a dominant factor in determining the severity of corrosion that occurred. It was found that even short exposures to “elevated” UV and ozone levels under constant salt fog in the laboratory study resulted in an accelerated corrosion phenomenon in the scribe and that was more severe than similar exposure time to salt fog alone, or after two years of exposure at the most aggressive sites in the field. Other observations: degradation of the coating system was also evident in the FT-IR analysis; degradation of the components of the high performance polyurethane coatings exposed in the UV/ozone chamber were more pronounced than when exposed in the B117 chamber; and the degradation of the Mg-rich coating system in the UV/ozone chamber was more like the degradation seen at two exposure sites after two years. For the full chromate coating system, the degradation of the coating in the B117 chamber was similar in appearance to two of the outdoor test sites but did not resemble the appearance of coated panels exposed in the UV/ozone chamber. These results suggest that it may be possible to tailor the chamber conditions to yield coating component degradation to replicate field exposures.
In examining the data from the laboratory tests and the outdoor environments for the bare metal coupons, elevated levels of UV and ozone significantly increased corrosion of the three aluminum alloys and pure copper in the laboratory, but increased corrosion was not observed in the coupons exposed in the outdoor environments with high UV and ozone. This discrepancy was likely due to the fact that the modified ASTM B117 test performed in this evaluation was much more aggressive than natural outdoor environments and does not contain all factors of influence- for example, other atmospheric contaminants such as SO2, wet/dry cycles with dilute electrolytes, temperature and humidity cycling, mechanical stress, etc. Elevated levels of UV and ozone in the modified salt fog test resulted in lower corrosion rates for 1010 steel than those observed for the low UV/low ozone levels. Of the three aluminum alloys studied, AA2024-T3 exhibited the greatest corrosion rate when subjected to the high UV/high ozone conditions, which was consistent with the observed increase in corrosion rate that the high UV/high ozone condition had on pure copper, since AA2024-T3 alloy has the highest weight percent of copper in its composition of the three aluminum alloys. Accelerated formation of AgCl films was demonstrated, with the film formation rate greater in the B117/UV/ozone chamber than in the B117 chamber over time. Correlation of the AgCl film thickness with hours of exposure time in the B117/UV/ozone chamber to similar thicknesses in the field exposures was achieved, indicating that it may be possible to replicate the parameters required for the formation of the AgCl film thicknesses seen in the field at various exposure sites.
The cumulative damage corrosion model was developed to predict mass loss on 1010 steel due to atmospheric corrosion using inputs of temperature, relative humidity, ozone and SO2 concentration, and deposition rate (mass per unit volume of rainwater) of chloride. The R2 values for the calibration and validation data sets, comprised of the eight atmospheric test sites in the study, were 0.96 and 0.86 respectively. The accuracy of this model exceeded any atmospheric corrosion rate prediction models published in the literature to date.
This project and other similar efforts have laid the groundwork for research program investments in multiple DoD laboratories (e.g. AFRL, NAVAIR) that are developing and implementing new accelerated test methodologies for management of weapon systems. DoD laboratory activities in accelerated test methodology and other technology development are now coordinated via the science and technology working integrated product team (S&T WIPT) which meet three times a year as part of the DoD Corrosion Forum which are sponsored by the Office of Secretary of Defense (OSD) Office of Corrosion Policy and Oversight (CPO).