Treatment of Mg powder with carbonic acid and the effect of treatment variables and treated Mg ratios on coating performance in salt spray tests.
Keywords Mg-rich primers, Corrosion performance, Salt fog tests
Magnesium (Mg) powder has been used successfully as an anticorrosive pigment on aluminum and aluminum alloy substrates due to its high electrochemical activity. (1-3) Although magnesium-rich primers (MgRPs) often perform very well in outdoor exposure, they can exhibit heavy blistering very early in accelerated salt fog testing (ASTM B117), which is still a key test in certifying coatings for use by the Department of Defense (DoD) in corrosion protection applications. In previous research, the origin of this unique performance duality has been related to the protective presence of magnesium carbonate formed by treating Mg particles with carbonic acid. (4) The protocol for outdoor testing and the rate of corrosion in these environments were shown to provide sufficient time for the Mg carbonate formation to occur naturally, resulting in an excellent protective influence. Contrarily, almost immediate failure in the ASTM R117 salt fog chamber testing typically occurs after material application before little, if any, Mg carbonate has formed. The Mg hydroxide that does form (due to the significant humidity, pH, salt concentration, and relatively reduced C[O.sub.2] concentration, Table 1) is poorly protective and partially soluble in water. Other studies investigating the corrosion of Mg in humid environments have also established that early corrosion rates are reduced in the presence of C[O.sub.2] in comparison with a C[O.sub.2]-free environment. (5), (6) In a recent cathodic corrosion experiment performed by Maier and Frankel, (7) it was shown that Al AA2024 panels coated with MgRPs showed a significant increase in pH and visible corrosion attacks and pits on the Al substrate when the panels were exposed to air or argon, while no changes or attacks were observed on samples exposed to a C[O.sub.2] atmosphere. The increased corrosion resistance in the presence of C[O.sub.2] has been attributed to the formation of a carbonate layer on the surface. (4), (8), (9) These studies suggest that the formation of a carbonate layer is both necessary and beneficial for increased corrosion protection.
Table 1: Concentration of C[O.sub.2] in salt spray chamber (ASTM B117) and at USM, Hattiesburg, MS (180 feet elevation, latitude 31[degrees]19'43"N, longitude 89[degrees]20'7"W) Location C[O.sub.2] level (in ppm) Min Max Ave USM (Roof), Hattiesburg temperature = 400 440 420 8[degrees]C, Humidity = 86% Salt spray chamber (ASTM B 117) 280 320 299 temperature: 35[degrees]C, Humidity = 100%
In a previous study, Mg powder was treated with aqueous carbonic acid and primers formulated with it did not exhibit early blistering while similar primers prepared with untreated Mg did blister in the salt fog test. (10) The carbonate compound formed during treatment was identified as nesquehonite (MgC[O.sub.3]*3[H.sub.2]O). In a subsequent paper, the Mg powder treatment process was analyzed to determine the conditions necessary to form nesquehonite. (11) In the final part of this study, the treatment process was analyzed further to understand the nature of the carbonation process, treatment variables that affect the corrosion resistance of Mg-rich primers, and the effect of treatment time and treated Mg ratios on corrosion performance of MgRPs. Coatings prepared with different ratios of treated and untreated Mg were evaluated via salt fog testing to determine the optimized ratio of treated and untreated pigments for maximum corrosion protection.
Materials and experimental method
Mg powder (325 mesh, 99.8% pure) was purchased from Alfa Aesar. Carbon dioxide gas cylinders were procured from Nordan Smith. [(MgC[O.sub.3]).sub.4]*Mg[(OH).sub.2]*5[H.sub.2]0 (magnesium carbonate hydroxide pentahydrate) and Mg[(OH).sub.2] (reagent grade, 95%) were purchased from Sigma-Aldrich. MgC[O.sub.3] (powder) was purchased from Fisher Scientific. Nesquehonite (monoclinic, divergent radiating, acicular, colorless) was procured from Select Minerals. Eponol[R] Resin 53-BH-35, an ultra-high molecular weight, diglycidyl ether of bisphenol-A-based epoxy resin in a blend of methyl ethyl ketone and propylene glycol methyl ether (3:1 by wt) with a reported specific gravity of 0.934 at 25[degrees]C and solids 35.5% by weight was obtained from Hexion Specialty Chemicals, Inc. All chemicals were used as received.
Treatment of Mg powder
A 1-L glass jar equipped with a C[O.sub.2] gas inlet and an outlet was employed to prepare carbonic acid (C[O.sub.2]-[H.sub.2]0) solution in situ by purging C[O.sub.2] via a flow meter at 20 mL/min through 400 mL of deionized water for about 30 min. Next, 20 g Mg powder was added to the glass jar while maintaining a continuous C[O.sub.2] purge at the same flow rate. Aliquots of the treated Mg powder were collected at various time intervals and dried at ambient conditions.
Preparation and application of Mg-rich primers
MgRPs formulated at 45% pigment volume concentration (PVC) were prepared by blending Mg powder (treated/untreated) in Eponol and a blend of methyl ethyl ketone and propylene glycol methyl ether (3:1 by weight). The samples were blended in a speed mixer (FlackTek, Inc.) for 2 min at 2000 rpm before being transferred to a ball mill to roll overnight to insure good dispersion.
Preparation of test samples
AA 2024-T3 panels (115 mm x 75 mm x 1 mm) were cleaned with acetone before use. The primers were applied using an automatic drawdown machine at six mils wet film thickness. The coated panels were placed in a dry box at ambient conditions for 7 days before testing. The average dry film thickness of the panels was 60 [+ or -] 10 pm.
Salt fog testing was performed by placing coated panels at an angle of 45[degrees] in a test cabinet designed and operated in accordance with ASTM B117. A 5% salt solution (pH 6.5-7.2) prepared by dissolving sodium chloride into water that met the requirements of ASTM D1193 specification for reagent water, Type IV, was supplied to the test chamber (preconditioned to the operating temperature of 35[degrees]C) via a continuous indirect spray that fell onto the specimens at a rate of 1-2 mL/80 [cm.sub.2]/h. (12)
Fourier transform infrared spectroscopy (FTIR)
Spectra were collected in reflectance mode using a Digilab FTS 2000 infrared spectrometer over a frequency range of 650-4000 [cm.sup.-1] via 32 scans at a resolution of 4 [cm.sup.-1].
X-ray diffraction (XRD)
X-ray diffraction (XRD) is a nondestructive analytical technique that reveals information about the crystallographic structure, chemical composition, and physical properties of materials and thin films. XRD studies were conducted on a Rigaku Ultima3 X-ray diffractometer with a fine structure air-insulated X-ray tube (type FK 60-04) with a copper anode (Cu K[alpha]1 5406 [Angstrom]), with a wavelength scintillation detector 0.027 nm < [lambda] < 0.05 nm at the following measurement conditions: aperture 0.025[degrees]; exposure 1 s; scan speed of 0.5[degrees]/min; and scan angles between 10[degrees] [less than or equal to] 5 2[theta] 5 [less than or equal to] 90[degrees]. To address the possibility that minor phase(s) could be missed because of the relatively high detection limit of XRD, the results were complemented with energy dispersive X-ray studies.
Scanning electron microscopy and energy dispersive Xray analysis
Changes occurring in the Mg powder and the surface morphology of the primers were examined via scanning electron microscopy (SEM) with an FEI Quanta 200 equipped with a Thermo System 7 energy dispersive X-ray (EDX) analyzer. SEM utilizes an electron beam for sample surface imaging at magnifications ranging from 25 to 50,000.
Optical micrographs were collected using a Keyence digital microscope (VHX-600).
Elemental analysis was accomplished through an outside analytical testing lab (Galbraith Laboratories, Inc.).
Results and discussion
Understanding the treatment process
When C[O.sub.2] is introduced into deionized water, carbonic acid formation causes the pH to drop and stabilize at while the temperature drops by ~15[degrees]C. (13)
C[O.sub.2] + [H.sub.2]O [??] [HC[O.sub.3].sup.-] + [H.sup.+], pKa = 6.65 (1)
[HC[O.sub.3].sup.-] [??] [C[O.sub.3].sup.2-] + [H.sup.+], pKa = 10.33 (2)
Upon addition of Mg, the pH immediately shifts to ~10. The interaction of Mg with water is expected to result in the formation of basic Mg[(OH).sub.2] (equation 3). (6) Subsequently, the presence of C[O.sub.2] results in the formation of magnesium carbonate in two steps that proceed nearly simultaneously (equations 4 and 5). (14-16)
Mg (s) + [H.sub.2]0 [right arrow] Mg[(OH).sub.2]+ [H.sub.2] (3)
Mg[(OH).sub.2] [??] M[g.sup.2+] + 20[H.sup.-] (4)
M[g.sup.2]+ C[O.sub.2](aq) + 2 O[H.sup.-] [right arrow] MgC[O.sub.3] [H.sub.2]O (5)
When Mg powder was added into deionized water (neutral pH), the pH immediately increased to 11.3 due to the formation of Mg[(OH).sub.2], as previously explained. Subsequent incorporation of C[O.sub.2] results in the formation of MgC[O.sub.3] (pH reported to be 1011). (17) When the pH of deionized water was reduced to 4.3 by addition of crotonic acid to simulate the initial acidic reaction conditions in the absence of C[O.sub.2], a similar increase in pH was observed upon introduction of Mg. Individually, a slurry of equal mass units of magnesium carbonate and magnesium hydroxide in deionized water exhibited a. pH of 10.4. These simple experiments both support and confirm that the pH increase observed in the initial stages of the treatment is due to the formation of Mg[(OH).sub.2] and MgC[O.sub.3] as shown in the equations above. Deionized water is a necessary medium for the reactions to take place. When Mg was treated with C[O.sub.2] in methyl ethyl ketone, no increase in pH was observed and no reaction with Mg occurred, suggesting that Mg will not react with C[O.sub.2] singularly in the absence of water.
The reaction between Mg and carbonic acid is exothermic and the temperature rose to 40[degrees]C within 10 min of Mg addition. After its initial rise to ~10, the pH decreased slightly and returned to ~10 as an equilibrium was established between the competing reactions, i.e., carbonic acid formation and its consumption by reaction with Mg (Fig. 1).
[FIGURE 1 OMITTED]
The FTIR spectra in Fig. 2 capture the changes occurring during first 7 min of the reaction. In the first minute, a peak at 3691 [cm.sup.-1] appears and is attributed to -OH stretching vibrations from Mg[(OH).sub.2] formation, (18) confirming the fact that Mg[(OH).sub.2] is the first product formed by the interaction of Mg with water.
[FIGURE 2 OMITTED]
The peak at 3691 [cm.sup.-1] disappears completely within 5 min and is attributed to the reaction of Mg[(OH).sub.2] with C[O.sub.2] to form magnesite (MgC[O.sub.3]), (5), (19) accompanied by a steady decrease in solution basicity.
It has been reported that at high relative humidities, magnesite forms a stable hydrated magnesium carbonate (nesquehonite) as shown in equation (6). (20), (21)
MgC[O.sub.3](s) + 3[H.sub.2]0 [right arrow] MgC[O.sub.3] * [H.sub.2]O (s) (6)
In Fig. 2, symmetric carbonate stretching appears at 7 min as a shoulder at 1510 [cm.sup.-1]and continues to be a distinct peak while the two absorption bands at 1407 and 851 [cm.sup.-1] relate to the asymmetric stretching and bending mode of carbonate moieties, respectively. (22-24)
The split between 1403 and 1510 [cm.sup.-1] and the shoulder appearance at 1470 [cm.sup.-1] is described as a characteristic representing nesquehonite. (10), (21) Similarly, the absorption band at 1099 [cm.sup.-1] illustrates the internal vibration mode of nesquehonite due to symmetric stretching. (22) The band around 1647 [cm.sup.-1] is ascribed to an -OH bending mode of [H.sub.2]O, and is consistent with the presence of small amounts of water on the nesquehonite crystal surface. (24-26) As seen in Fig. 3, the split at 1510 [cm.sup.-1], i.e., nesquehonite formation, starts around 7 min after adding Mg to the carbonic acid solution. As nesquehonite formation progresses, the split at 1510 and 1403 [cm.sup.-1] deepens, while the shoulder at 1470 [cm.sup.-1] becomes more noticeable, as shown in Fig. 4. Figure 4 compares the FTIR spectra of a sample treated for 15 and 360 min and validates the presence of nesquehonite early in the treatment process.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
XRD analysis of the reaction products isolated at various times as the reaction progressed confirmed that nesquehonite formation was initiated around 7 min of treatment (Fig. 5). The peaks that appear at 13[degrees] and 23[degrees] 2[theta] degrees were characterized as nesquehonite based on previous studies. (25,) (27), (28) The intensity of the peaks at 32[degrees], 34[degrees], and 36[degrees] 2[theta] degrees (attributed to Mg) diminishes as nesquehonite formation progresses, indicating a reduction in the amount of Mg as nesquehonite is formed.
[FIGURE 5 OMITTED]
Another important observation was that nesquehonite formation is represented through a color change during the reaction process. Figure 6 shows the color change of treated Mg (T-Mg) in the first 15 min after adding Mg. The color change is visibly evident within 5 min of adding Mg and coincides well with the time nesquehonite formation was confirmed in Figs. 2 and 4. As the treatment proceeded, the reaction medium progressively shifted from dark to light gray in color (Fig. 7) with increasing levels of nesquehonite. Nesquehonite is a colorless mineral in its pure form.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
To further understand the reaction dynamics, aliquots of treated and dried Mg particles were analyzed using SEM (Fig. 8). The effect of [H.sub.2]C[O.sub.3] on Mg particles was proven to be evident at 3 min (Fig. 8b). At 5 min, the granular boundaries became more visible (Fig. 8c) and at 7 min. crystal formation can be noticed on the Mg particle surface (Fig. 8d). The crystal forms in Fig. 8 correspond well with the nesquehonite Formation reported elsewhere. (4), (10) The SEM images are consistent with the FTIR, XRD results, and color changes reported earlier.
[FIGURE 8 OMITTED]
As the treatment continued, nesquehonite formation was further reinforced and spread over a larger percentage area of each sample; the crystals grew in length and were more visible with clear boundaries. Eventually the Mg was completely converted to the nesquehonite crystal form. Figure 9 captured the process using timed SEM pictures of treated and dried Mg aliquots after 15, 60, and 360 min of adding Mg to the carbonic acid solution.
[FIGURE 9 OMITTED]
Salt fog testing
Coating sample preparation at different conversion rates and ratios of treated Mg
Critical pigment volume concentrations (CPVC) of untreated Mg and treated Mg were measured to be 42 and 39.1%, respectively, using equation (7) below in combination with the oil absorption (OA, expressed in mL/mL) of the pigments (following ASTM D281).
CPVC = 1 + OAV / 1 (7)
Conventional cathodic protection requires the active materials to achieve a percolation threshold and, to ensure that these two materials exceeded the CPVC, a PVC of 45% was adopted as the loading level for coatings formulated with each untreated Mg and treated Mg pigments and Eponol Resin 53-BH-35. Coatings were prepared individually with the treated and untreated Mg pigments and blended at different ratios, as shown in Table 2, to prepare coatings containing both treated and untreated Mg pigments.
Table 2: Coating compositions Blend rations of Sample # Coating with Coating with Treatment time for Mg untreated treated (min) Mg (wt%) Mg (wt%) 1 100 0 N/A 2 0 100 15 3 80 20 15 4 60 40 15 5 40 60 15 6 20 80 15 7 0 100 60 8 80 20 60 9 60 40 60 10 40 60 60 11 20 80 60 12 0 100 360 13 80 20 360 14 60 40 360 15 40 60 360 16 20 80 360
Evaluation of salt fog test results vs pigment state and loading ratio
The coatings were applied on AA2024-T3 panels at six mils wet film thickness and dried for 1 week at ambient conditions before being evaluated in a salt fog cabinet following ASTM B117. MgRPs prepared with 100% untreated Mg (sample 1) were used as a control. The earliest change was clearly detectable as we observed blister formation within 5 h of accelerated testing for these control panels (denoted as samples la and lb in Fig. 10). Critically, all samples based upon any level of treated MgRPs were blister free.
[FIGURE 10 OMITTED]
After 100 h of salt fog exposure, the panels other than the controls were still free of blisters, although occasional corrosion spots were detectable on some panels (Fig. 11). Blisters are an immediate failure on appearance and while they are not a sign of immediate corrosion, they were considered to be a valid reason for rejecting the coating since blistering is indicative of delamination and porosity within the coating and any compromise in film integrity opens channels for corrosion. Upon visual inspection the poorest corrosion resistance resulted from the coatings that were based on Mg treated for 360 min, specifically, sample 12, which contained 100% treated Mg. Again, based upon visual inspection, the best corrosion resistance performance for samples treated for 360 min was represented in sample 13, which contained the highest amount of untreated Mg.
[FIGURE 11 OMITTED]
For further confirmation that a combined mechanism is necessary for corrosion control, the best and worst performing panels after 300 h of accelerated testing are summarized in Fig. 12. The coating that displayed the best corrosion performance within the first 300 h of salt fog exposure was sample 3, which contained 80% untreated Mg and 20% treated Mg (treatment time 15 min). The worst corrosion performance in the same timeframe was noted with sample 12, as discussed earlier. Although blisters were not seen, this panel possessed several corrosion spots. These two panels, which performed at the opposite ends of the data set, are shown in Fig. 12. The complete data set can be seen in Fig. 11.
[FIGURE 12 OMITTED]
The performance a sample 3 confirms that the short treatment time of 15 min, which enables only a limited percentage of nesquehonite formation, was sufficient to prevent blister formation on the samples and resist corrosion over the 300 min time frame. In combination, these experiments further validate that it is critical to have a significant amount of untreated Mg in order to provide the direct cathodic protection. Additionally, the data support that with the extended and longer treatment times, greater Mg is converted into nesquehonite, reducing the concentration of untreated Mg. This leaves insufficient amount of elemental Mg to function for the role of cathodic protectant, as supported through the presence of corrosion spots on sample 12.
Accelerated salt spray chamber testing again confirmed that control samples containing 100% untreated Mg as the starting point blistered almost immediately during salt fog chamber testing and the blisters were attributed to the release of hydrogen as a byproduct when Mg converts to Mg[(OH).sub.2] under these specific conditions, i.e., high humidity and rapid salt introduction. In contrast, when the carbonic acid-treated Mg ratio was at the highest levels, the films were absent of blisters but proved incapable of preventing corrosion from occurring, i.e., there was insufficient elemental Mg to provide cathodic protection. In the presence of a small percentage of treated Mg, the Mg carbonate acts to mediate as a sacrificial barrier, protecting the Mg in the coating, reducing the rate and concentration of Mg[(OH).sub.2] formation which has been consistently attributed as the cause for immediate blistering in MgRPs during accelerated testing. Based on these data, it was confirmed that the presence of both Mg and nesquehonite are required for optimal corrosion protection.
[c] American Coatings Association & Oil and Colour Chemists' Association 2012
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S. S. Pathak, S. K. Mendon. M. D. Blanton.
J. W. Rawlins
School of Polymers and High Performance Materials. The University of Southern Mississippi, Hattiesburg, MS 39406.
T. Turel ([??])
Youngstown State University. Youngstown. OH 44555.
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|Author:||Turel, Tacibaht; Pathak, Shashi S.; Mendon, Sharathkumar K.; Blanton, Michael D.; Rawlins, James W.|
|Date:||Jul 1, 2013|
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