A Comparative Study of Two Coating Systems Exposed for Two Years in the Field and in Accelerated Atmospheric Corrosion Chamber Environments

Over the last few decades the analytical characterization of bare and coated surfaces undergoing atmospheric corrosion has improved, resulting in a more complete understanding and consideration of environmental parameters, corrosion layers, and degradation of polymeric coatings. However, the corrosion processes involved and the role that the environmental parameters play in what is a multiphase system is rather complex, involving chemical reactions and equilibria, ionic transport phenomena, and gaseous, aqueous, and solid phases.¹  Various corrosion products, specific to the metallic (or polymeric) substrates in the system, and the corrosive species present (anions, cations, acidic and basic salts, particulates, etc.) which interact with each other all vary in amounts and residence time. For the purposes of atmospheric corrosion, the electrochemical nature of the corrosion process requires the presence of an electrolyte, provided by the atmospheric precipitation or adsorption of water molecules on the surface of the metal or polymeric coating. Adsorbed water layers, which can range from 15 to 90 atomic layers thick depending upon the relative humidity (RH), play a central role in supporting the electrochemical process.² 

Indeed, the presence of corrosive species and corrosion products, which are hygroscopic, can attract water vapor above a critical relative humidity and solubilize, further catalyzing the corrosion process. A study by Vernon in 1935 established that for iron and steel, oxidation of the metal surface did not commence until an RH of 70% was attained.³  However, other researchers have reported on the role of atmospheric contaminants and the critical %RH required for atmospheric corrosion to occur on several metal substrates.⁴  This critical relative humidity therefore depends upon the type of metal surface, the type of corrosion products or species, and particulates/pollutants that are present in the system. Also, atmospheric constituents found on metal surfaces are a function of the atmosphere itself, with sulfates, nitrates, nitrites, chlorides, carbonates, hydrogen ions, ammonium, metal ions, particulates, and organic compounds commonly found in electrolyte chemistries or corrosion layers. Each of these have an effect on the corrosion processes on exposed surfaces.⁵  These result either directly from the deposition processes or from aqueous phase reactions of the deposited atmospheric constituents, whereas sulfates, nitrates, and nitrites originate either directly from the wet deposition process, from particle deposition, or from reactions of the gaseous air pollutants in the aqueous phase of the adsorbed electrolyte.¹  Atmospheric oxidizing agents, such as ozone and hydrogen peroxide, and hydroxyl radicals are also important reactants on the surface, facilitating the formation of numerous organic compounds.

Many investigations have been performed in the last 6 y to clarify the role of environmental and climatic factors in the atmospheric corrosion of commonly used structural metals and coatings, as well as to simulate in the laboratory their observed corrosion behavior in the field.^6-10  Nearly every coating’s test and qualification organization has documented instances where the corrosion performance of a series of coating systems in accelerated laboratory tests^11-15  do not correlate with the rank order of performance in an outdoor exposure environment.^16-18  This disparity has become more pronounced with the introduction of non-chromate-based corrosion inhibitors,^19-22  as the ASTM standard protocols such as G85,¹¹  D5894,¹²  D4587,¹³  B117,¹⁴  and G154¹⁵  were originally developed around quality control of chromated systems.¹⁴  These same works,^7-8,19  in addition to others,^23-24  have documented cases in which uncoated metal substrates do not show the same corrosion products in laboratory tests compared to outdoor exposure. Clearly, there are fundamental chemical and thermodynamic differences between these two environments.

The first step toward developing better accelerated test methods is to analyze and accurately reproduce these environments in a laboratory setting. The second step in this process is to accelerate the kinetics of the simulated environment to enable performance evaluation in a reasonably short period of time to predict long-term outdoor performance. Properly accelerating the kinetics involves knowledge of both the interaction of corrosive species, as well as the effects within the polymeric coating system. These effects are specifically related to the diffusion rate of electrolyte into primer, which contains inhibitor species, dissolution rate of the inhibitor, and transport phenomena and kinetics of inhibitor ions to suppress corrosion in damage/scribe sites. The method used to accelerate total corrosion performance must equally accelerate all of these factors, otherwise both false positives and false negatives are possible. A successful accelerated test method, therefore, would be environmentally tunable and provide accurate, predictable results for any substrate with any type of protective barrier layer or coating present.

In the present work, a matrix of coated specimens for baseline coating performance evaluation was exposed at various geographic locations in order to understand the role of atmospheric contaminants and environmental conditions on the degradation of coated metals. The database generated from these field exposures was used in the development of an accelerated corrosion test protocol on identical coated specimens using a modified laboratory atmospheric exposure chamber. Analyses of both field- and chamber-exposed specimens were compared to determine if similar degradation behavior could be obtained under “field-like” accelerated exposure conditions.^25-27 

Samples of 10 separate coating systems on aluminum alloy (AA) 2024-T3 substrates were processed as described in Table 1. The coated panels (Figure 1) were deployed (in triplicate, except for shipboard exposures) as two sample sets to be retrieved at 12 month intervals over 2 y. Analysis of the coated samples from the field exposures, as well as samples exposed in the accelerated chamber, was performed in the 4,000 cm⁻¹ to 600 cm⁻¹ spectral range using a Nicolet iS50^† Fourier transform infrared (FTIR) attenuated total reflectance (ATR) spectrometer outfitted with a HeNe laser, tungsten halogen white light source, diamond ATR crystal, and ZnSe beamsplitter.

Weather monitoring stations capable of measuring total ultraviolet (UV), RH, ozone, and temperature (°F) were deployed at six of the eight locations. These six locations were: The University Oceanographic Laboratory Ship (UNOLS) R/V H.R. Sharp, based at the University of Delaware, Lewes, DE (designated as the East Coast Ship); Wright-Patterson Air Force Base (AFB); Kirtland AFB; Tyndall AFB; Pt. Judith; and the UNOLS R/V T.G. Thompson, based at the University of Washington, Seattle, WA (designated as the West Coast Ship).

Coated panels were deployed to eight sites as noted in Table 2. Of these deployments, there were six land-based and two ship-based sites. Weather data were collected using an external temperature/RH data logger and a remote UV sensor. The data were recorded on an hourly basis and downloaded approximately every 3 months using a 4-channel external data logger. An image of a deployed weather monitoring instrumentation station is presented in Figure 2. Collection of weather data was designed to be coordinated with the retrieval of exposed cards and panels. A weather monitoring station was not deployed at the Daytona Beach Battelle site because they already have a monitoring station there; a system was also not deployed at the Hickam AFB, HI site because there was not a reliable power supply in close proximity to the exposure site.

In order to investigate the role of environmental and climatic factors in the field exposure sites, a standard corrosion test chamber capable of conducting ASTM G85¹¹  and ASTM B117¹⁴  exposure tests was modified with the introduction of both ultraviolet A (UVA) illumination and ozone gas. The UVA lamps were installed on the exterior of the chamber lid and illuminated the coupons within the chamber through quartz windows. A commercial ozone generator, ozone monitor, and a proportional-integral-derivative (PID) controller with microprocessor were put in line and plumbed into the exposure chamber to provide and control ozone levels. The ozone level within the chamber as a function of time and under continuous salt spray conditions was monitored during the exposures: the exhaust effluent gas from the chamber was passed through a graham condenser to remove the majority of the salt from the flow stream and the effluent gas was measured for ozone concentration; the ozone monitor was wired in a signal feedback loop to the PID controller microprocessor, which actuated a control valve regulating the flow of ozone into the chamber. The UVA irradiance levels as a function of distance from the light fixtures and location within the chamber were measured and mapped. Placement of the replicate coated panels was randomly distributed within the chamber to ensure that UV irradiance levels could be accurately correlated with each sample. Therefore, using this modified exposure test chamber, the effect of UV and ozone on the corrosion behavior of coated specimens, which were identical to the samples deployed at the eight exposure sites, was investigated. Two of the three coating systems exposed in the chamber were designated as either fully chromated or magnesium-rich primer (MgRP). The third coating system, designated the rare earth conversion coating (RECC), was a replacement coating system for one that was no longer commercially available for testing when these individual panels were made for the accelerated chamber testing. For these chamber exposures, a total of 10 panels were prepared per coating system. Five panels were exposed in B117, while the remaining five panels per coating system were exposed in UV/ozone. One panel per 400 h time interval was removed for analysis, with the total length of exposure being 2,000 h.

An image of the modified exposure chamber system is shown in Figure 3. The map diagram of UVA intensities as a function of location within the chamber is presented in Figure 4. The maximum and minimum irradiance levels for the illumination fixtures were measured to be 46.80 W/m² and 5.44 W/m², respectively. These values represent the total irradiance level immediately adjacent to the lamps for the entire wavelength range of 315 nm to 400 nm. The maximum total UVA intensity measured is comparable to the maximum UVA intensity of 50 W/m² reported by Wan, et al.,⁷  in their modified MB117 test at a peak emission of 340 nm.

Based upon the maximum and minimum levels of total irradiance of the UVA light fixtures and the levels of ozone that the system was capable of controlling with precision, the exposure chamber test levels for the coated samples were much lower for the sample locations within the chamber. The measured ozone and irradiance levels as a function of sample location within the chamber were:

• •

  Low Ozone (100 ppb) / Low UVA (3.87 W/m²)

• •

  Low Ozone (100 ppb) / High UVA (27.93 W/m²)

• •

  High Ozone (800 ppb) / Low UVA (3.87 W/m²)

• •

  High Ozone (800 ppb) / High UVA (27.93 W/m²)

The coated panels were exposed under high ozone/low UVA conditions for 1,000 h, immediately followed by low ozone/high UVA conditions for an additional 1,000 h, for a total of 2,000 h exposure testing. The coated panels were removed at 400 h intervals and analyzed and compared with identical coated samples exposed for 1 y and 2 y in the field, as well as identical samples exposed for 2,000 h using the standard ASTM B117¹⁴  test protocol.

The weather data monitored at the field exposure sites were UV, ozone, temperature, and %RH. Ozone levels monitored at the land-based exposure sites were found to be in good agreement with the local Environmental Protection Agency (EPA) monitoring sites. Ozone concentration, temperature, and %RH at Daytona Beach were utilized from Battelle’s weather monitoring system. The weather monitoring station was not deployed at Hickam AFB because of a lack of a reliable and continuous power supply. The average values for the four weather parameters monitored at the field sites are presented in Table 3.

Three coating systems were exposed in both the modified chamber and the B117 chamber. Table 4 is a summary review of the coating systems tested. Coating systems A and H are the same systems as deployed in the field (Table 1), only having been re-designated with a different alphabet code in the coating processing to minimize bias in the analysis of results. For the accelerated laboratory exposure chamber tests, coating system H is the fully chromated coating system (designated as coating system A when deployed in the field). Coating system A is the MgRP coating system (designated as coating system G when deployed in the field). Coating system K (designated as coating system C in the chamber exposures) is a replacement for coating system F and is not shown in Table 1. This system was under testing and evaluation under another Department of Defense (DoD) programmatic effort; thus, it was decided to include this coating in the chamber studies.

Coated panels of each coating system in Table 4 were exposed under high ozone/low UVA conditions for 1,000 h, immediately followed by low ozone/high UVA conditions for an additional 1,000 h, for a total of 2,000 h exposure testing. The coated panels were removed at 400 h intervals and analyzed and compared with identical coated samples exposed for 1 y and 2 y in the field, as well as identical samples exposed for 2,000 h in the B117 test chamber. Table 5 is a summary of the resulting rankings of the coating system performance (degree of blistering) as per ASTM D714 from the chamber exposures.²⁶  It is important to note that the rankings were assigned to the sample in comparison to the other samples within the same sample set (in this case the B117 and UV/ozone chamber exposures); the numerical rankings assigned to chamber samples should not be compared to numerical rankings of the coated samples retrieved from the field. From the visual inspections of these coated samples, it was determined that the fully chromated system’s performance in both chamber exposures was worse than the 2 y outdoor exposures, even at the shortest time intervals, when the corrosion present in the scribe was compared to the East Coast Ship, West Coast Ship, and Pt. Judith exposure sites, which were determined to be the most aggressive outdoor exposure sites for the full chromate coating system (see Figure 5). The MgRP coating system performed better in the field exposures (see Figure 6) than the chamber exposures. These results are representative of the performance of the full chromate and magnesium-rich coating systems, where their performance was determined to be worse in the chambers than in the field exposures (see Figure 7).

Analysis of the MgRP and full chromate coating system samples from the Pt. Judith and the West Coast Ship field exposure sites (2 y), the B117 test chamber (2,000 h), modified chamber (2,000 h), and the baseline coating system panels were also done by FTIR ATR on multiple locations for each panel. A summary of the identified functional group frequency (peak) assignments for the baseline coating systems is presented in Table 6. Plots of the FTIR ATR spectra for each coating system and overlays for the exposures at the field sites and chambers are presented in Figures 8 through 11. The % reflectance (or transmittance) is the opposite of absorbance for the energy bands.

For both of the ship-based exposure sites, the East Coast Ship and West Coast Ship weather data, good agreement was observed between coastal EPA sites (when available) and the monitoring system contained on the ships. For the coated samples, the most aggressive field exposure sites were determined to be Pt. Judith and the West Coast Ship locations. This was the case even though these two sites had markedly different average annual values for UV and ozone levels, but very similar temperature and %RH average values (Table 3). These results suggest that it may not be the annual average value that is important, but rather the cumulative amount of time that a coated sample may be exposed to a certain environmental parameter or combination of parameters.

Therefore, an analysis of the cumulative frequency distribution for the weather parameters was performed²⁸  and compared to the average values presented in Table 3. The frequency distribution enabled the determination of how often a specific environmental parameter condition (UV/ozone/RH/temperature) existed at each exposure site. This comparison was more informative than using just averages for the purpose of tuning the modified chamber conditions for site specific degradation replication. The cumulative frequency distribution of the ozone levels ranged from 50 ppb to 80 ppb 99% of the time; for the UV levels, there was a range of 55 W/m² to 85 W/m² for all locations at a cumulative frequency of 99%. In terms of temperature, there was a fairly close range of 82°F to 98°F (27.8°C to 36.7°C) for the 99% cumulative frequency, and in terms of RH, for 99% of the cumulative frequency, all of the exposure sites exhibited 100% RH except for the Kirtland AFB, which was much lower at approximately 90% RH. Thus, the low ozone level of 100 ppb was the chosen condition in the modified chamber because it represented the minimum concentration that could be regulated and it was near the upper range of the distribution 99% of the time in the field. The 800 ppb ozone concentration was chosen to be an accelerated condition. In terms of UV irradiance, the average minimum and maximum settings represented the average lower and upper limits of the illumination fixtures. Even though the average maximum UV level was lower in the modified chamber than the cumulative frequency of 55 W/m² to 85 W/m² noted in the field, the degradation noted for the coated samples was more similar than those specimens exposed in B117.

The performance of the MgRP and RECC coating systems fared much worse in the UV/ozone chamber than in the B117 test. These results suggested that regardless of coating stack-up, the coated test panels that were subjected to UV/ozone in addition to the sodium chloride electrolyte had higher relative percentages of oxidation in the scribe over the duration of exposure when compared to the B117 exposure. It was observed for the coating surfaces analyzed for the fully chromated, MgRP, and RECC coating systems that longer exposures in the modified chamber contributed to increased oxidation of the coating as compared to the baseline analyses of the full chromate or MgRP coatings. 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 are significant because the UV/ozone conditions in the modified chamber are at or lower 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.

Additionally, the topcoats for these coating systems can be composed of urethanes, esters, acrylates, ethers, hydroxyls, aromatics, and fluoropolymers. Musto, et al.,²⁸  note that natural weathering in an atmospheric environment with prolonged exposure to direct sunlight contributes to extensive photo-degradation and photo-oxidation of specialized coatings used in civilian and military applications. Diepens and Gijsman²⁹  corroborate this chemical degradation process for simulated weathering conditions. When comparing the spectra for the UV/ozone chamber and field exposure samples to the baseline data for Topcoat B in the MgRP coating system (Figures 8 and 9), it can be seen that a decrease in the % reflectance peaks at 2,933 cm⁻¹ and 2,858 cm⁻¹ has occurred, indicating either a breakdown of the primary and secondary amines, or possibly more likely in the case of the West Coast Ship at 3,525 cm⁻¹, 3,447 cm⁻¹, and 3,375 cm⁻¹, the N-H bonds and C-H bonds (see Figure 9 and Table 6).^9-10,28-29  Diminution of the 1,728 cm⁻¹ and 1,682 cm⁻¹ peaks is attributed to the changes in aldehyde/ketone carbonyl and amide carbonyls, respectively, suggesting possible degradation of the polyurea and polyurethane groups.²⁸ 

The decrease in the peaks at 1,462 cm⁻¹ and 1,242 cm⁻¹ suggest reduction of C-H and C-O resulting from corresponding vibrational scissoring and stretching actions, which indicate the possible formation of carbamic acid, which is unstable. The absence of the 1,242 cm⁻¹ peak in the Pt. Judith data indicates degradation of urethane from C-O scission, and the decrease in peak 1,526 cm⁻¹, which is indicative of the presence of amides (a component of urethanes), also suggests the degradation of the urethane component of the topcoat. The change in the FTIR 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 look more like the baseline data: peaks 1,728 cm⁻¹ and 1,682 cm⁻¹ are similar in magnitude for both B117 exposed and the baseline for Topcoat B (carbonyl stretching and amide formation from polyurea); the magnitude of peak 1,526 cm⁻¹ is similar between the B117 exposure and the baseline (for amides, a component of urethane); and peak 1,462 cm⁻¹ is similar in magnitude between the B117 exposure and the baseline for Topcoat B (urethane component, scissor vibration of C-H). Therefore, the FTIR data indicate that degradation of the topcoat components of the MgRP coating system exposed in the modified chamber after 2,000 h was much more like the same coating exposed for 2 y at Pt. Judith and the West Coast Ship, whereas the coating exposed to the B117 chamber was much more similar to the baseline sample that had not been exposed. Collectively, these spectroscopic results for the specific functional groups identified in Table 6 point to degradation of the coatings as a result of molecular breakdown of the chemical bonds upon exposure to a photo-oxidative process.^9-10,28-29 

When considering the IR spectra for the full chromate coating system (Figures 10 and 11), there are some slight differences in the spectra. These differences can be attributed to different formulations between Topcoat B (for the MgRP system) and Topcoat A: for the baseline spectra, the peaks 3,618 cm⁻¹, 3,525 cm⁻¹, 3,447 cm⁻¹, and 1,242 cm⁻¹ are absent, and peak 1,728 cm⁻¹ is present as a shoulder on the adjacent 1,682 peak, with these being representative of amides and urethane components. In terms of observable differences in the spectra between the exposures and the baseline samples, all major peaks are diminished under the UV/ozone exposure after 2,000 h. In addition, the 2,925 cm⁻¹ and 2,855 cm⁻¹ peaks (either amines, components of polyurethane, or aliphatic hydrocarbons) under Pt. Judith are diminished with their magnitude between the baseline and 2,000 h UV/ozone exposure. 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 except for the 1,065 cm⁻¹ peak, which is indicative of the ester component (C-O stretching), which could indicate the initial breakdown occurring on the samples exposed at Pt. Judith and the West Coast Ship. Therefore, the FTIR data for the full chromate coating system formulation tested in the UV/ozone indicate that this environment is again more aggressive in causing the degradation of the coating system components than the B117 chamber test.

These results indicate that the coated samples exposed in the modified chamber with the addition of UV and ozone experienced more degradation than those exposed in the B117 chamber. This coating degradation experienced in the modified chamber was more reminiscent of that observed in the field exposures. Furthermore, different coating formulations experienced different amounts of degradation in the chambers as well. These results also suggest that an accelerated laboratory test that incorporates environmental factors such as UV and ozone in addition to salt spray, temperature, and humidity has a profound impact on the mechanism of coating degradation as compared to current standard test protocols that do not combine these factors. Thus, it may be possible to tailor the chamber exposure conditions to yield coating degradation specific to an exposure site location in the field. Current efforts are underway in testing an evaluation of next generation exposure chambers that incorporate these additional environmental factors.

• 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; the degradation of the MgRP coating system in the UV/ozone chamber was more like the degradation seen on the same coating system exposed at Pt. Judith and the West Coast Ship after 2 y. For the full chromate coating system, the degradation of the coating in the B117 chamber was more like that of the samples from Pt. Judith and the West Coast Ship than the UV/ozone chamber. These differences indicate that depending upon the coating formulation, it is possible to tailor the chamber conditions to yield coating component degradation to replicate field exposures.

This work was sponsored by the SERDP-ESTCP Program under contract FA8601-06-D-0013, Dr. Bruce Sartwell, Program Manager, Weapons Systems and Platforms. Special thanks to Mr. James Postel (Univ. Washington, R/V Thomas G. Thompson), Capt. William Byam (Univ. of Delaware, R/V Hugh R. Sharp), Lt. Col. P. Legendre (Kirtland AFB, NM), Mr. John Puu (Hickam AFB, HI), Mr. P. V. Mitchell (AFRL, Tyndall AFB, FL), and Dr. J. Moran (ALCOA) for outdoor exposure placement and retrieval of the sample coupons and weather monitoring systems; the AF Weather Agency, Det 3, 16 WS and Maj. John Crane, Mr. Kirk Lehneis, and Mr. Paul Gehred for their assistance with weather monitoring instrumentation and coordination with weather monitoring sites and database mining.