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Effect of Inhibitor Leaching from Several Aircraft Primers on the Corrosion Protection of AA7075-T651

Jesse C. Kelly, Luna Innovations, Inc, [email protected]

Adam Goff, Luna Innovations, Inc

Jeier Yang, Luna Innovations, Inc

Charles Sprinkle, Luna Innovations, Inc

Sarah E. Galyon Dorman, SAFE Inc

Keywords: Corrosion Protection, Accelerated Testing, Inhibitors, Leaching, Chromate, Chromate-Free

ABSTRACT

Controlling corrosion is critical in reducing military sustainment costs and increasing mission readiness, and primers are a principal means of protecting equipment and infrastructure. When a primer is breached and the substrate is exposed, protection is dependent on inhibitor leaching and/or galvanic coupling with active metal pigments. Coatings containing hexavalent chromium inhibitors (Cr6+) are un-surpassed in providing long-term corrosion inhibition. Heightened awareness of their toxicity, however, has led to increased regulation and petitions for their replacement with new primers based on rare earth or metal-rich pigments. Accelerated corrosion and outdoor testing have traditionally been used to screen and qualify new primer coatings. Unfortunately, these methods do not provide insight into the leaching behavior of corrosion inhibitors or active metal pigments, which can be used to optimize coat-ing chemistries and filler packages. Luna Innovations has developed an approach to monitor leaching of soluble inhibitors in chromate, rare earth, and aluminum/magnesium-rich primers. The two-step ap-proach evaluates inhibitor leaching from coatings immersed in solution as well as coated, inert coupons exposed to accelerated cyclic corrosion testing. The two-step approach enables correlation between leaching and accelerated testing, and may help guide the development of non-chrome primers that pro-vide equivalent or greater protection compared to chromated systems.

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2019 Department of De-fense – Allied Nations

Technical Corrosion Con-Paper No. 2019-XXXX

INTRODUCTION

The key responsibility of military coatings systems has been to protect equipment and infrastructure from environmental damage. One of the biggest threats to operational readiness is corrosion and there is a recognized need to improve corrosion prevention and control to reduce costs, maintenance labor hours, and days of equipment non-availability. Primers containing hexavalent chromium inhibitors (Cr6+) are unsurpassed in providing corrosion inhibition.1 Heightened awareness of their toxicity, however, has led to increased regulation of Cr6+ and to petitions for their complete replacement with non-chrome primers.2 By eliminating Cr6+, safety improves and waste handling and disposal costs from coating re-moval can be reduced. Superior corrosion protection is sought without the use of these inhibitors, and modern primers may hold the promise of improved corrosion protection without the environmental haz-ards of chrome.

Primer coating systems protect the integrity of underlying metal substrates through a combination of corrosion prevention and corrosion mitigation. In preventing corrosion, the primer acts to provide adhe-sion with the substrate to prevent defects, limit introduction of water or corrosive contaminants, and pre-vent formation of a corrosion cell. In the event that a corrosion cell is formed, corrosion inhibitors in the primer protect the substrate by either diffusing to the active corrosion site (traditional primers) or form-ing a sacrificial galvanic couple (metal-rich primers). The barrier properties of primer coatings have a significant effect on the diffusion of inhibitors to active corrosion sites. Migration within the primer is in-versely proportional to the barrier properties of the coating. High barrier properties, as a result of in-creased coating thickness or crosslink density and primer composition, limit the ingress of water and other corrosive contaminants. Low barrier properties increase the primer’s ability to absorb moisture and provide a pathway for inhibitors to diffuse to active corrosion sites. A compromise between high and low barrier properties is needed in new non-chrome primers for protection equivalent to chromated systems.

Inhibitor leaching from a polymer matrix is determined by the resin type, crosslink density, primary cor-rosion inhibitor technology, and barrier properties. A fundamental understanding and prediction methodology of leaching is needed to optimize new primers. Several studies of chromate leaching from a primer matrix have been conducted utilizing a variety of analytical techniques.3-6 Of these techniques, a combination of salt fog testing and electrochemical impedance spectroscopy (EIS) has, historically, provided information on chromate ion presence in the primer and its mobility to a defect site. These techniques, however, do not enable correlation of corrosion resistance to inhibitor leach rates. Luna In-novations, in partnership with SAFE Inc., has developed a new approach to monitor leaching of chro-mate and non-chrome corrosion inhibitors in traditional primers and active aluminum/magnesium pig-ments in common metal-rich primers. The method consists of a leach rate study where primer coated coupons are immersed in an electrolyte solution, in parallel with leaching analysis on inert coupons ex-posed to cyclic corrosion test conditions. Solution based leaching, which provides the rate of diffusion within the film over time, was compared to inhibitor or metal pigment remaining in coated inert coupons during corrosion cycling. The two-step approach enables correlation between leaching and atmospheric

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Technical Corrosion Con-

corrosion testing, and may help in the development of new polymer matrices and non-chrome corrosion inhibitors for aerospace and naval applications.

EXPERIMENTAL PROCEDURE

Primers and SubstratesThree primer coatings were evaluated in this study. Table 1 shows the characteristics of the three coat-ing systems, their coating designation, and the inhibitors or active metal pigments contained in each. The primary inhibitors of interest were strontium chromate (SrCrO4) in the Cr-Primer, dipraseodymium trioxide (Pr2O3) in the NC-Primer, and elemental aluminum (Al) and magnesium (Mg) in the MR-Primer. Metal substrates were aluminum alloy AA7075-T651 (UNS A97075). Aluminum coupons used for solu-tion based leaching and accelerated corrosion testing were cleaned with Bonderite C-IC 33 Aero Acid Aluminum Deoxidizer (Henkel) and pretreated with the SurTec 650V tri-chrome conversion coating prior to primer application. Polyester substrates (Leneta, P300-7C) used for atmospheric leaching were metal-free and were used as received.

Table 1. Description of the applied primer coating systems

Primer Specification Description Designation Inhibitor or Metal Pigment

MIL-PRF-23377Type 1, Class C2 Primer

Chromated,solventborne epoxy Cr-Primer Strontium Chromate (SrCrO4)

Barium Chromate (BaCrO4)

MIL-PRF-23377Type 1, Class N Primer

Non-chrome,solventborne epoxy NC-Primer Dipraseodymium Trioxide (Pr2O3)

Gypsum (CaSO4)

Metal-Rich Primer Aluminum alloy pig-

mented, solventborne epoxy

MR-Primer Aluminum (Al)Magnesium (Mg)

Primer Inhibitor ConcentrationAn acid digestion procedure was used to determine the total inhibitor or active metal pigment concen-tration in the selected primers (Table 1). The primary basis for acid digestion was EPA Method 3031, which uses multiple acids in series (nitric, sulfuric, and hydrochloric) to digest organic/inorganic compo-nents in coatings for spectroscopic analysis.7 In this study, digestion involved the dissolution of cured primer samples into solution, by adding strong acids/oxidizers and heating, until the following occurred: 1) the complete decomposition of the polymer matrix, 2) the release and dissolution of the analyte(s) of interest, and 3) the decomposition of larger inorganic materials (e.g. metal oxides) into smaller compo-nents that could be safely analyzed via spectroscopic methods. Digestates (samples) were diluted and analyzed in triplicate using inductively coupled plasma (ICP) spectroscopy. Samples were evaluated against calibrated solutions of chrome (Cr), praseodymium (Pr), aluminum (Al) and magnesium (Mg), and results from ICP analysis were used to back calculate the average weight percent of inhibitor or ac-tive metal pigment in fully cured coatings (Table 2).

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Table 2. Corresponding concentrations of corrosion inhibitors and active metals in selected primer coatings following ICP analysis on primer digestates

Primer Inhibitor/Active Metal Concentration in Cured Primer (Wt. %)Cr-Primer Elemental Chrome (Cr3+

and Cr6+) 4.30

Chromate (CrO42-) 9.58

Strontium Chromate (SrCrO4) 16.82

NC-Primer Elemental Praseodymium (Pr3+) 1.74

Dipraseodymium Trioxide (Pr2O3) 2.04

MR-Primer Elemental Aluminum (Al) 45.3

Elemental Magnesium (Mg) 18.9

GMW14872General Motors Cyclic Corrosion Laboratory Test standard, GMW14872, was used to assess atmo-spheric leaching of inhibitors/active metals and the accelerated cyclic corrosion performance of the se-lected primers. GMW14872 involves a combination of cyclic conditions to accelerate metallic corrosion and evaluate a variety of corrosion mechanisms. A cycle of the GMW14872 test method spans a 24-hour period and consists of an ambient stage (23 °C, 45% RH, periodic salt spray), a humid stage (23 °C , 100% RH, no spray) and a dry stage (60 °C, <30% RH, no spray). The electrolyte used for the salt spray is a standard 0.9 wt. % sodium chloride (NaCl), 0.1 wt. % calcium chloride (CaCl2), and 0.075% wt. % sodium bicarbonate (NaHCO3) solution. The GMW14872 schedule is shown in Table 3 and Fig-ure 1.

Table 3. Standard GMW14872 Schedule for 1 CycleStep Duration Temperature, °C Salt Spray Humidity Notes

1 15 sec 23 Yes 45% Salt spray

2 90 min 23 No 45% Ambient hold







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9 480 min 49 No 100% Humid stage

10 480 min 60 No <30% Dry stage

Figure 1. Standard test conditions for 1 cycle (24 hours) of GMW14872 accelerated testing8

Corrosion assessment was conducted on primer coated AA7075-T651 (UNS A97075) panels. Prior to placing panels in the chamber and initiating testing, substrates were sealed on the backsides and edges with wax and scribed with an X pattern using a Gardco carbide scribe tool. Performance of coated coupons subjected to GMW14872 environmental testing was evaluated through visual inspec-tion of both scribed and unscribed areas. For evaluation of the scribed areas, coupons were rinsed im-mediately upon removal from the environmental chamber with DI water at 45°C. The Air Blow-Off method was determined to be the most appropriate evaluation method for the primer coated coupons. Using an air gun that met specifications in ASTM D1654, the nozzle was angled at 45° and air was blown along the scribe, disturbing the area adjacent to the scribe to ensure an opening for the air. Scribes were evaluated and rated according to Table 1 in ASTM D1654. Subsequently, the unscribed areas of the coupons were evaluated for degree of blistering by comparison with Figures 1-4 in ASTM D714.9 The main characteristics of blistering focused on the size distribution and the frequency of oc-currence within the primer. For further evaluation of undercutting or corrosion spots on the metal sub-strate, primer coated coupons that underwent 120 cycles were stripped. The coupons were soaked in El Dorado PR-3500 paint remover for 10 min and thoroughly cleaned with deionized water and iso-propyl alcohol. The scribed and unscribed areas of the coupons were then evaluated under a stereo-scope.

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Leaching Test MethodsInhibitor leaching was first monitored on primer coated AA7075-T651 (UNS A97075) coupons im-mersed in salt solutions identical to those used in GMW14872 testing. Test cells were designed for in-hibitor sampling after primer soaking over extended periods of time. A7075-T651 coupons (UNS A97075) were used as the substrate. Coupons were cleaned with Bonderite C-IC 33 and pretreated with SurTec 650V prior to masking (Figure 2). The goal of the masking process was to 1) control the area and thickness of the exposed coatings when immersed in solution for accurate comparison be-tween trials and coatings, 2) prevent the exposure of bare aluminum or defects in the coating, 3) limit active corrosion development at the underlying or exposed aluminum, and 4) create a water tight seal around the test cell to prevent evaporation or leaking of the GMW14872 solution or analytes of interest. Two layers of a waterproof adhesive mask (Gamry, PTC1 Porthole Electrochemical Sample Masks) were applied to the coupon at three locations, creating three 1 cm2 exposed areas of the coupon. Primers were then spray applied onto the coupons according to the manufacturer’s specifications and the top masking layer was removed within 5 minutes of application while the coatings were wet. After removal of the top mask, coated primer coupons were allowed to cure for 24 hours.

Figure 2. Masking and coating process for solution based leaching assessmentA section of the remaining mask was removed after 24 hours to leave behind a 2.54 cm diameter por-tion of the PTC1 film (Figure 3). Nine masked and coated substrates were made, with each of the primers being applied to three separate AA7075-T651 (UNS A97075) coupons. After the masking process was complete, a 2.54 cm OD polycarbonate (PC) tube was placed on top of each masked area and securely fastened using an industrial grade marine adhesive that prevented evaporation or leaking.

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Figure 3. Masking and coating process for the NC-PrimerOnce a leaching analysis was initiated and the GMW14872 solution was added to the test cells (50 mL, time = 0), a rubber septum was placed over the top and was securely fastened to create an airtight seal (Figure 4). Each primer system was evaluated at the three temperatures experienced during GMW14872 cycling (25 °C, 49 °C, and 60 °C). Leaching was performed over a 90-day period and all sampling was performed using the pull and replace method using a precision syringe with a long Luer Lock needle. Samples were removed in 5 mL quantities and stored for analysis, with an equivalent amount of fresh GMW14872 solution being returned to the test cell (constant solution volume over the course of 90 days). The use of the pull and replace method has been shown to be effective in the anal-ysis of chromate leaching and allowed for an accurate determination of inhibitor/active metal pigment concentration in solution over time. As demonstrated by Lau et al., the diffusion of primer inhibitors does not depend on the amount of solution in the cell or the amount of inhibitor/pigment already dis-solved as long as the solution volume is significantly higher than the surface area of the coating.4,5

Figure 4. Schematic of the fully assembled leaching test cells (left) and leaching test cells for the Cr-Primer

Sampling times followed the recommended guidelines in the literature.4,5,10 Table 4 shows the primers, temperatures, and sampling times employed for the solution based leach rate study. ICP spectroscopy

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was used to determine inhibitor/active metal concentrations in samples pulled. Samples were evaluated against calibrated solutions of chrome (Cr), praseodymium (Pr), aluminum (Al), and magnesium (Mg) to determine the concentration of inhibitor at each sampling time (μg/L). Concentrations were then con-verted to the total amount of inhibitor/active metal that leached per area of primer coating (μg/cm2), ac-counting for the volume pulled and replaced. Analysis provided the rate of diffusion from the film over time (g/cm2 of coating), however, this method could not differentiate between the various oxidation states of dissolved species such as trivalent (Cr3+) and hexavalent (Cr6+) chrome.

Table 4. Primers, temperatures and sampling times for solution based leaching

Solution Based Leach Rate Analysis

Primers Cr-Primer NC-Primer MR-Primer

Inhibitor/Pigment Analyzed Total Chrome (Cr3+, Cr6+)

Pr2O3

(Pr3+)

Aluminum & Magnesium (Al3+/Mg2+)

Temperatures 25 °C, 49 °C, 60 °C

Sampling Day 1 5, 10, 20, 40, and 60 minutes

2, 4, 6, and 12 hoursSampling Days 2 – 90 2, 5, 15, 30, 45, 60, 75, and 90 days

Inert polyester substrates were used for accelerated atmospheric testing. Substrates were cut into small 2” x 1” pieces and coated with selected primers using a similar masking process previously de-scribed. Coatings were applied to two areas of each coupon according to the manufacturer’s specifica-tions. A small hole was then punched into the center of each coupon and used to attach three coupons to an inert fiberglass panel with fiberglass fasteners (6 samples per panel). Coupons were arranged on the fiberglass panels to reduce cross contamination (samples at 45°) as water collects and drips down the substrates. The entire sample holder/system, including the polyester substrate, fiberglass fasteners and fiberglass panel, were metal free. A schematic and representative images of the inert coupon pan-els are shown in Figure 5. Twenty coupons were coated using each of the primer systems, resulting in 40 sample areas for each primer system (total of 120 samples). Panels were then exposed to GMW14872 accelerated atmospheric testing. In 10-day intervals, three samples of each primer system were removed for analysis with the final samples being removed at 120 cycles. Four samples of each primer were used for baseline testing (t=0).

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Figure 5. Schematic and representative images of inert leaching panels for atmospheric based leaching assessment

Coupons were submerged in 20 mL of the GMW14872 solution as they were pulled from the GMW14872 chamber at 10-day intervals. The submersion process lasted for 15 days. ICP analysis was used to determine the concentration of leached inhibitors/active metal remaining in the coatings after soaking was complete (µg/cm2). All ICP analysis was referenced back to the original, unexposed base-line samples that underwent similar submersion for 15 days.

RESULTS

Solution Based LeachingICP results for the Cr-Primer are shown in Figure 6. Based on the manufacturer’s dry film density and results from acid digestion, coatings were determined to contain 174 µg/cm2 of total chrome (Cr3+/Cr6+). At all three temperatures, Cr leached rapidly into solution in the first hour with reduced leaching after 12 hours, and solutions at 60 °C showed a lower Cr concentration compared to those exposed to 25 °C and 49 °C. The reduced concentration may have resulted from a decrease in chromate (CrO4

2-) solubil-ity at high temperatures, or AA7075-T651 (UNS A97075) corrosion and CrO4

2- deposition at the under-lying substrate. For the former, higher temperatures can lead to a drop in solution pH and lead to higher rates of CrO4

2- leaching; however, elevated temperatures and a pH in the 4-7 range (such as the GMW14872 solution used here) have been shown to decrease the total concentration of leachable Cr.4,5,10 For instance, Klomjit and Buchheit have shown that chromate leaching is highest in the 30-40 °C range, followed by the 20-30 °C and 50-60 °C ranges, respectively.10 For the latter, elevated corro-sion on the underlying aluminum substrates at 60 °C may increase the amount of chromate that miti-gates active corrosion sites, therefore decreasing the amount of leachable inhibitor remaining in solu-tion.

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Figure 6. Cr released (µg/cm2) from the Cr-Primer after 48 hours (left) and 90 days (right). Coat-

ings were 1 cm2 with an average thickness of 24 µm.The amount of Cr in solution continued to increase with immersion time then slowly decreased after 30 hours at all three temperatures (Figure 6). Solutions at 25 °C demonstrated a high Cr concentration up to 30 days before decreasing, and those exposed to 49 °C stabilized and began to slowly decrease af-ter 12 hours. At 60 °C, the amount of Cr stabilized after 12 hours and showed almost no change in con-centration up to 90 days. The drop in Cr concentration, or stabilization, may be another indication that corrosion is occurring at the underlying substrate at elevated temperatures or long periods of exposure, and soluble Cr is depositing at active corrosion sites. At all three temperatures, the amount of leached chrome never exceed 10% of the total chrome available in the primer coating (174 µg/cm2), indicating that only a small fraction of the inhibitor is available for corrosion mitigation. While this fraction is low, the primer’s optimized properties (crosslink density, composition, filler package) and controlled release characteristics may help provide insight into how chromated primers slowly manage corrosion over long periods.

Results from ICP analysis on the NC-Primer are shown in Figure 7. The NC-Primer contains two corro-sion inhibitors, including dipraseodymium trioxide (Pr2O3) and a gypsum compound consisting primarily of hydrated calcium sulfate (CaSO4·2 H2O). While both components help suppress corrosion, only the Pr2O3 inhibitor has been shown to slowly dissolve/leach in the presence of water and re-precipitate at active corrosion sites over long periods of time.10 CaSO4·2 H2O rapidly dissolves and is typically con-sumed in very short periods. Since the GMW14872 solution contains small amounts of CaCl2, it was dif-ficult to accurately determine where calcium originated and only the concentration of Pr was analyzed. Based on the manufacturer’s dry film density and results from acid digestion, coatings were determined to contain 83 µg/cm2 of total praseodymium (Pr3+). For all three temperatures, the Pr concentration in-creased immediately after submersion (5 minutes). The initial spike in concentration was likely the re-sult of rapid dissolution of the Pr2O3 inhibitor or rapid reaction of the Pr2O3 with components in the GMW14872 solution. After the first hour, solutions at each temperature reached a steady-state value. Only 10% of the leachable Pr was released into solution and the concentration remained the same up to 90 days. Results indicate that the non-chrome corrosion inhibitor, Pr2O3, can rapidly leach from the

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epoxy binder system when wet and only a very small quantity of the Pr inhibitor is able to leach from the primer. While this type of leaching mechanism is useful for mitigating short term corrosion, there may be opportunities to modify the primer’s cross-link density and increase the inhibitor concentration to allow for corrosion protection comparable to the chromated primer in Figure 6.

Figure 7. Pr released (µg/cm2) from the NC-Primer after 48 hours (left) and 90 days (right). Coat-

ings were 1 cm2 with an average thickness of 29 µm.ICP results from solution based leaching of the MR-Primer are shown in Figure 8 and Figure 9. The se-lected MR-Primer was believed to contain both elemental Al and Mg and their oxides. Early work in metal-rich primers suggested that coatings provided galvanic protection to aluminum substrates, and aluminum was unable to leach from the primer due to its insolubility at neutral pH conditions (pH 4.5-7.5). Recent research into metal-rich primers has determined that the presence of Mg may contribute to corrosion inhibition on aluminum substrates due to its solubility under neutral pH conditions. For this reason, both Al and Mg leaching from the selected MR-Primer were evaluated. Based on the manufac-turer’s dry film density and results from acid digestion, coatings were determined to contain 4770 µg/cm2 of total aluminum (Al3+) and 1985 µg/cm2 of total magnesium (Mg2+). At all temperatures, the MR-Primer showed a small increase in Al leaching within the first 20 minutes followed by a decrease in con-centration and stabilization within 12 hours (Figure 8). The initial Al concentration spike may have re-sulted from unbound aluminum alloy pigment migrating into solution followed by rapid precipitation. Fol-lowing the initial spike, the Al concentration remained steady for up to 1000 hours.

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Figure 8. Al released (µg/cm2) from the MR-Primer after 48 hours (left) and 90 days (right). Coat-

ings were 1 cm2 with an average thickness of 70 µm.The concentration of Mg increased with immersion time at all three temperatures with higher tempera-tures leading to more rapid dissolution of Mg (Figure 9). Higher temperatures lead to a drop in solution pH, however, Mg is less soluble at low pH conditions and the rapid dissolution is likely the result of a fa-vorable Mg solubility constant at high temperatures. Less than 0.1 wt. % of the available Al leached in the first 1000 hours, but increased after 1000 hours at 49 °C and 60 °C. Conversely, high levels of Mg in the MR-Primer leached within the first 10 hours, and rapidly increased to 11%, 16%, and 30% of the total magnesium after 1000 hours at 25 °C, 49°C and 60 °C, respectively, before decreasing. This con-current change in leached Al/Mg indicated that Mg initially protects the aluminum alloy pigment, and rapid dissolution leads to corrosion and leaching of aluminum, as well as possible precipitation of Mg, in solution. This dual approach to corrosion protection in the MR-Primer, through a combination of Mg shielding of the aluminum alloy pigment, then protection by aluminum alone, may provide insight into the capabilities and performance of metal-rich primers utilizing magnesium/aluminum pigments. The leaching of Mg provides a mechanism outside of galvanic coupling to protect underlying substrates and the discrete Al particles present in the primer, however, future work should assess metal-rich primers that do not contain high levels of magnesium for comparison.

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Figure 9. Mg released (µg/cm2) from the MR-Primer after 48 hours (left) and 90 days (right). Coat-

ings were 1 cm2 with an average thickness of 70 µm.

Atmospheric Based LeachingThe area of coupons for atmospheric leaching was 3.63 cm2 and coatings were 55.9 µm, 38.1 µm, and 48.7 µm thick on average for the Cr-Primer, NC-Primer, and MR-Primer, respectively. Based on the manufacturer’s dry film density and results from acid digestion, Cr-Primer coatings contained 408 µg/cm2 of Cr and NC-Primer coatings contained 104 µg/cm2 of Pr while the MR-Primer contained 3100 µg/cm2

of Al and 1290 µg/cm2 of Mg. Baseline primer coated, inert coupons not exposed to GMW14872 conditions were submerged for 15 days in the GMW18472 electrolyte and analyzed using ICP spec-troscopy. The concentration of inhibitor or metal pigment that leached from the baseline coatings (t=0, cycle 0) was determined to be 56.8 µg/cm2 of Cr for the Cr-Primer, 0.84 µg/cm2 of Pr for the NC-Primer, and 0.01 µg/cm2 of Al and 386.5 µg/cm2 of Mg for the MR-Primer. The total amount of leached inhibitor/active metal was small in comparison to the total concentration in the primer determined during acid digestion, but concentrations were similar to the values seen during solution based leaching. At each 10-day interval, coupons were pulled and submerged for 15 days in the GMW18472 electrolyte. The amount of inhibitor left in the coating was referenced back to the original amount of inhibitor leached from the baseline coupons not exposed to GMW14872 conditions (at t=0, day 0). In this man-ner, the amount of inhibitor that was still available to leach out of the coating could be tracked over a period of 120 days. While the timescale and conditions of the GMW14872 chamber (salt spray, humid-ity, drying, etc.) were drastically different compared to solution based leaching, the same general trends during were observed.

Figure 10 shows that magnesium in the MR-Primer slowly depleted over time by almost an order of magnitude, indicating that the active Mg is capable of leaching out of the coating and/or diffusing to ac-

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tive corrosion sites on aluminum particles over time. The decrease in Mg inhibitor during GMW14872 testing was equivalent to the rapid leaching seen in solution based leaching. Aluminum showed almost no leaching from the inert coupons subjected to GMW14872 cycling, with both baseline coupons and those exposed to 120 cycles having the same low concentration of leachable inhibitor. This behavior demonstrated that Al particles in the MR-Primer were highly stable during cycling as opposed to solu-tion based leaching, where the amount of Al leached increased after magnesium had been depleted. For the Cr-Primer, the Cr inhibitor showed a similar trend compared to the solution based leaching study. Chrome slowly diffused out of the coating over time, which presumably leads to the excellent corrosion mitigation performance of chromated primers over long periods of accelerated testing. For the NC-Primer, the Pr inhibitor showed a much lower rate of leaching from the coating when compared to solution based leaching study. The amount of Pr did not decline under GMW14872 conditions, but baseline amount available was still drastically lower than Cr in the Cr-Primer and Mg in the MR-Primer.

Figure 10. Leachable inhibitor/active metal remaining in the Cr-Primer, NC-Primer, and MR-Primer over 120 days of GMW14872 cycling. The total concentration of inhibitor/active metal

present in the coatings is available for reference. GMW14872 Corrosion AssessmentCr-Primer, NC-Primer, and MR-Primer coated coupons were subjected to GMW14872 environmental testing and evaluated through visual inspection. Scribed panels were exposed to 120 cycles (days) of accelerated testing with samples pulled every 30 days for assessment. Visual inspection of corrosion developed on the coated and scribed panels in accordance with ASTM D1654 and ASTM D714 is shown in Figure 11. Results from solution and atmospheric leaching correlated well with accelerated corrosion testing (GMW14872) on scribed panels, where the NC-Primer showed low rates of leaching over long periods of time and both the Cr-Primer and MR-Primer demonstrated prolonged levels of con-trolled release. The Cr-Primer showed the best performance during GMW14872 cycling with no blister-ing or undercutting and minimal discoloration along the scribes after 120 cycles. During both the solu-

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tion and atmospheric leaching assessments, CrO42- in the Cr-Primer leached slowly from the primer

over periods significantly longer than the NC-Primer, indicating long-term corrosion protection was pos-sible with chromated primers. The NC-Primer exhibited failure of the coating between 90 and 120 cy-cles, with small blisters evenly distributed throughout the panel. Scribes showed surface discoloration of the metal substrate with minimal development of white corrosion. Both the blistering and discol-oration indicate that the Pr inhibitor may have been exhausted (depleted), was unable to leach due to high barrier properties in the polymer matrix, and was unable to protect the underlying AA7075-T651 (UNS A97075) substrate long-term. The MR-Primer showed signs of white corrosion along the scribes within 30 cycles of testing and discoloration gradually progressed throughout the full testing period. Un-scribed areas showed no signs of blistering, however, discoloration in the scribed area suggested that Mg may have leached from the primer to a point where corrosion protection was governed only by the remaining aluminum pigment. Ratings for the primer coated coupons are shown in Table 4.

Figure 11. Evaluation of primer coated coupons exposed to GMW14872 conditions. Coated coupons were sampled every 30 cycles and immediately inspected for blistering and corrosion.

Table 5. Evaluation of coupons subjected to GMW 14872.

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Test Panel Cycles Scribe Rating(ASTM D1654)

Blister Rating (ASTM 714) Observations

Cr-Primer 30 10 10 Scribe in good condition with shiny metallic appearance

- 60 10 10 Scribe in good condition with shiny metallic appearance

- 90 10 10 Scribe in good condition with shiny metallic appearance

- 120 10 10 < 5% of scribes showed brown-black dis-coloration

NC-Primer 30 10 10 Scribe in good condition with shiny metallic appearance, some discoloration visible

- 60 10 8-10 Scribe in good condition with shiny metallic appearance, some discoloration visible

- 90 10 10 Scribe in good condition with shiny metallic appearance, some discoloration visible

- 120 9 8-10 >90 of scribes have darkened with < 5% of white rust; some discoloration

MR-Primer 30 9 10 ~50% of scribe is covered with white rust

- 60 9 10 ~25% of scribe is covered with white rust

- 90 9 10 ~25% of scribe is covered with white rust; chalky appearance

- 120 9 10 >90 of scribe have white rust; chalky ap-pearance

Stripped coupons (120 cycles) were inspected under a stereoscope and are shown in Figure 12. The unscribed areas of the NC-Primer exhibited corrosion spots approximately 0.5 mm in diameter and some undercutting along the edges of the scribe. Few corrosion spots were identified in the unscribed areas of the MR-Primer that were not visible before removal of the paint. The spots appeared less fre-quently and more randomly throughout the panel and elongated in shape and size. The undercutting corrosion along the scribe is more uniform whereas the non-chrome scribe demonstrates erratic corro-sion behavior.

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Figure 12. Stereoscopic images of stripped panels after 120 Cycles of GMW 14872 testing.

CONCLUSIONS

The leaching and release behavior of corrosion inhibitors from three commercial primers was character-ized using a two-step approach. Both a chromated primer and non-chrome primer were examined in addition to a metal-rich primer containing discrete aluminum and magnesium particles. Solution based leaching monitored inhibitor or active metal pigment release from primers coated on AA7075-T651 (UNS A97075) coupons immersed in a GMW14872 electrolyte. In parallel, atmospheric based leaching monitored the rate corrosion inhibitor or active metal pigment from primer coatings exposed to GMW14872 accelerated cyclic corrosion conditions. Results from solution and atmospheric leaching provided insight into the short- or long-term release behavior of inhibitors from each primer, and results were compared to corrosion performance of coated and scribed AA7075-T651 (UNS A97075) panels during GMW14872 cycling. The commercial chromated primer demonstrated long-term, controlled re-lease of inhibitors (Ba CrO4, SrCrO4) which correlated to excellent corrosion protection after 120 cycles of GMW14872. The non-chrome primer displayed rapid, short-term release of praseodymium inhibitors (Pr2O3), and coated and scribed panels showed blistering and corrosion development within 30 days. Finally, a metal-rich primer demonstrated continuous release of magnesium (Mg) without significant leaching of the primer’s aluminum (Al) pigment. Scribed and coated panels showed small amounts of white rust after 30 days of GMW14872 cycling that gradually increased up to 120 days. The combined solution and atmospheric leaching methods developed during this effort can be used to better under-stand the actual corrosion behavior of coupons when exposed to specific atmospheric conditions. Simi-lar methods can be employed in a similar manner based on expected coating service use on actual DoD assets, if a representative atmospheric exposure protocol is determined. Additionally, these meth-ods may support the optimization of existing primers (resin type, crosslink density, primary corrosion in-hibitor technology, and barrier properties) and can be used to develop new non-chrome primers to com-pete with chromated standards.

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ACKNOWLEDGEMENTS

Luna gratefully acknowledges that this material is based upon work supported by SAFE Inc. under a United States Air Force Academy (USAFA) Broad Agency Announcement (FA-7000-15-2-0012) by the Office of Naval Research Science and Technology. Any opinions, findings and conclusions or recom-mendations expressed in this material are those of the author(s) and do not necessarily reflect the views of, the US Air Force Academy or the US Government.

REFERENCES

1. Kendig, M.; Buchheit, R., Corrosion inhibition of aluminum and aluminum alloys by soluble chro-mates, chromate coatings, and chromate-free coatings. Corrosion 2003, 59 (5), 379-400.2. Department of Defense, Defense Acquisition Regulations System, Defense Federal Acquisition Reg-ulation Supplement; Minimizing the Use of Materials Containing Hexavalent Chromium. In DFARS Case 2009-D004, May 5, 2011; Vol. 76.3. Petry, L., et al. In Analysis of Isocyanate-Free Aircraft Topcoats by Electrochemical Impedance Spectroscopy, CORROSION 2007, NACE International: 2007.4. Scholes, F. H., et al., Chromate leaching from inhibited primers: Part I. Characterisation of leaching. Progress in Organic Coatings 2006, 56 (1), 23-32.5. Furman, S. A., et al., Chromate leaching from inhibited primers: Part II: Modelling of leaching. Progress in Organic Coatings 2006, 56 (1), 33-38.6. Sinko, J. In Pigment grade corrosion inhibitors: a review of chemistry and relevant concepts, DoD Corrosion Conference, 2009.7. EPA Method 3031 (SW-846): Acid Digestion of Oils for Metals Analysis by Atomic Absorption or ICP Spectrometry. 1996.8. Ascott Corrosion Testing Standards, GM9540P: A Standard for the Prediction of Cosmetic Corrosion on Cold Rolled Steel (Replaced by GMW14872). https://www.gm9540p.com/.9. ASTM D714 - 02: Standard Test Method for Evaluating Degree of Blistering of Paints. Reapproved 2017.10. Klomjit, P.; Buchheit, R. G., Characterization of inhibitor storage and release from commercial primers. Progress in Organic Coatings 2018, 114, 68-77.

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