Scientific Investigation of the Corrosion Performance of Magnesium and Magnesium Oxide Primers on Al Alloy 2024-T351 in Field Exposures Open Access

Protective chromate-based coatings function by releasing chromate ions, which inhibit the corrosion of AA2024-T351 (UNS A92024⁽¹⁾).^1-6  Concerns over the carcinogenicity of hexavalent chromium have motivated the search for reliable chromate alternatives like magnesium-rich primer (MgRP).^7-8  Recent investigations have assessed the corrosion inhibition performance of MgRP on aerospace aluminum alloys like AA2024-T351.^9-23  This primer can be applied over a variety of pretreatments^11-12  and is generally finished with an exterior topcoat for additional protection against environmental degradation^14-15  (Figure 1). MgRP coating has gained attention as a possible alternative to conventional chromate-based coatings widely used in the aerospace industry.^9-23 

This Mg-based coating can prevent corrosion damage with active corrosion protection mechanisms that suppress alloy substrate oxidation at remote defects or uncoated scratches where corrosion is at the greatest risk. MgRP protects AA2024-T351 primarily via sacrificial anode-based cathodic protection, whereby oxidative degradation of the substrate is drastically decreased by the sacrificial oxidation of Mg pigment instead.^{9-12,14-18,20-21,23}  Extensive analysis has been conducted on the effect of various aspects of the MgRP coating system toward the mediation of this galvanic couple such as Mg pigment volume concentration (PVC), pretreatment, topcoat, and coating defect size.^{9-12,14-15,18,20-21,24}  These factors were tested by exposing MgRP systems to full immersion in NaCl electrolyte, field exposure in rural inland and coastal marine environments, and laboratory accelerated lifecycle tests (LALT) such as cycle test and standard and/or modified ASTM B117.^9-23  Samples of MgRP were deployed with and without a topcoat and also with and without the presence of a machined scribe defect which exposed the underlying AA2024-T351 substrate to the environment. Electrochemical diagnostic testing was used to assess the barrier properties of the coating, the charge capacity of residual Mg pigment, and the galvanic couple potential of the MgRP-AA2024-T351 system in full immersion.^{9-10,12,14-15,18,20,23,25}  Nondestructive characterization was performed to assess the depletion of Mg pigment, the formation of Mg corrosion products, the corrosion damage in the scribe, and the repartitioning of Mg from the primer into the scribe.^{12,14-15,18,20}  When a MgRP was applied without a topcoat (MgRP), cathodic protection was immediately prevalent and was able to remotely protect the exposed scribe. When a topcoat was applied over the primer (MgRP/TC), cathodic protection was mediated by the high ohmic resistance of the polyurethane-based topcoat and remote protection was greatly attenuated by ohmic potential drop.^14-15  However, adding a topcoat limited Mg self-corrosion and greatly increased the barrier properties of the coating and improved the resistance of the primer to degradation in various exposures via electrolyte penetration and solar UV radiation. For this reason, in-service coatings are topcoated and generally provide satisfactory protection.²⁶  Similarly, nonresistive substrate pretreatments resulted in immediate coupling of the substrate to nontopcoated MgRP, whereas resistive pretreatments resulted in delayed or suppressed galvanic coupling depending on the magnitude of the resistance.^11-12,24  Notably, Mg²⁺ repartitioning from the primer into the scribe was observed in all cases, even where resistive pretreatment had mediated galvanic coupling and self-corrosion of Mg enabled Mg²⁺ production.^12,14-15  These studies support MgRP as a viable alternative to chromate-based primers based on its multi-functional protection.

Another alternative Mg-based coating has been developed to replace chromium in the aerospace industry, which is pigmented with MgO instead of Mg.^25,27  As with MgRP, magnesium oxide-rich primer (MgORP) can be applied over a variety of AA2024-T351 surface pretreatments and finished with a variety of exterior topcoats. This MgORP has shown promise as a corrosion preventing coating on AA2024-T351 by visual inspection of scratch protection in field exposure deployment.²⁶  Figure 2 provides scanning electron microscope (SEM) micrographs of coated and uncoated AA2024 substrates for coastal marine exposure. The damage for the bare, uncoated AA2024-T3 (Figure 2[b]) is much more extensive than for the AA2024-T351 substrate present within the scribe for both MgRP (Figure 2[c]) and MgORP (Figure 2[d]) coatings. Diagnostic electrochemical testing was conducted on unexposed MgORP to systematically assess its corrosion inhibiting performance at various stages of primer depletion.²⁵  This study made use of the cycle test; a cyclic regiment of diagnostic electrochemical tests.^(2),25  The performance of MgORP was compared to that of the well-studied and reported MgRP.

MgORP is a novel Mg-based coating in its own regard and can also serve as a proxy to probe how a field-exposed/converted MgRP is able to protect the coated AA2024-T351 substrate as well as bare substrate in scratches.⁽³⁾ This is especially relevant to long-term exposures in field environments for which the Mg pigment in MgRP has effectively all been converted to corrosion product. In the absence of galvanic protection afforded by Mg pigment, it is of significant interest to determine the chemical protection mechanism by which MgORP and exposure-converted MgRP are able to protect the AA2024-T351 substrate. Previous research hypothesized chemical protection by Mg²⁺ in solution and in corrosion product^25,27-28  similar to other known inhibitor ions.^{11-12,14-15,18,27,29-36}  Improved understanding of the long-term protection behavior of MgRP will assist in determining the realistic protection lifetime of this coating.

The aim of this investigation was to determine the mechanisms and extent of protection brought about by Mg-converted and MgO pigments toward artificial scratches in coatings on AA2024-T351. This report focuses on how the performance of a MgO primer compares to the performance of a field-exposed MgRP that has been sufficiently oxidized from Mg to Mg-corrosion products. Long-term field-exposed MgORP/TC coating was investigated to determine its performance after years of exposure. All stack-ups investigated here included nonresistive pretreatments (Table 1). The longest reported marine field exposure of MgRP/TC was conducted for 2 y.³⁷ 

Coated panels consisted of an epoxy-based primer with magnesium oxide or metallic magnesium pigment and polyurethane-based topcoat coated over pretreated AA2024-T351 substrate of 1.6 mm thickness (Figure 3). The AA2024-T351 surface was treated with a nonfilm-forming (NFF) pretreatment before being coated with either a 40 μm to 60 μm thick MgRP (45% PVC) or a 20 μm to 30 μm thick MgORP (approximately 2.5 to 5 PVC²⁵ ) and finished with a 55 μm to 100 μm thick polyurethane topcoat (Table 1). Figures 3 and 4 give a schematic and experimental profile-view of the scribe in the coating, respectively. Fully coated panels contained a machine-scribed defect in the coating which exposed the underlying substrate (Figure 1). The scribe width was approximately 0.7 mm and was in the shape of an “X” or a “V.” Table 1 summarizes the coating systems used in this investigation. Table 2 specifies component details.

Characteristics of Mg and MgO primer systems were explored after 0 y, 2.5 y, and 4.25 y of field exposure and the effect of Mg corrosion product on the corrosion protection was examined. Environmental exposure testing was conducted on topcoated MgRP and MgORP for up to 4.25 y in coastal marine conditions at the Kennedy Space Center (KSC) on the Atlantic coast of Florida. Samples were stationed 30 m off of high tide and fully exposed facing the Atlantic Ocean oriented 30° from horizontal as prescribed by ASTM G-4³⁸  and G-50.³⁹  Environmental parameters typical of this exposure are summarized in Table 3.

Full immersion studies were conducted to explore various aspects of the intact coating system after various levels of primer depletion. All diagnostic electrochemical testing reported was performed in 5 wt% NaCl solution in an electrochemical flat cell open to ambient lab air over the “intact” coating far from any scribes unless otherwise specified. A three-electrode cell was used for electrochemical experimentation as discussed elsewhere.¹¹  The open-circuit potential (OCP) was monitored to assess the extent of galvanic coupling of the primer to the substrate.⁽⁴⁾ Electrochemical impedance spectroscopy was used to assess the intact barrier properties and coating degradation of MgRP/TC and MgORP/TC systems as a function of exposure time. The OCP impedance was determined over a frequency of 10⁵ Hz to 10⁻² Hz with 10 points per decade at an amplitude of 80 mV to 90 mV root mean square (RMS).⁽⁵⁾ The capacity to discharge electrochemically active Mg pigment in the coating was probed via potentiostatic holds at −0.8 V_SCE for various lengths of time. The rationale for utilizing these methods for coating assessment in the periodic testing regimen known as the cycle test has been discussed elsewhere.^11,18,25 

X-ray diffraction (XRD) was used to monitor the depletion of the crystalline Mg and MgO pigments and formation of any crystalline corrosion products as a function of exposure time.^8-11  The technique can detect crystalline phases above 3% to 5% by volume and penetrate up to 120 μm into Al as a function of 2θ.^40-42  Spectra were normalized to the face-centered cubic (fcc) Al peak for the <200> system at 44.63° 2θ for comparison.

SEM and energy dispersive spectroscopy (EDS) were used to map the location of various elements in a scribe. Plane-view images were taken to examine the repartitioning of magnesium from the primer into the scribe over time. Diagonal cross-section analysis was used to map element locations throughout the coating. EDS was used in three ways: color-coded elemental maps gave the spatial distribution of elements in the electron image, line scans tracked the elemental intensity across the length of a scribe, and analysis quantified the weight percentages of elements present within the scribe. The analyze function considered the 10 constitutive alloying elements for AA2024-T351 to yield the weight percent of each present within the scribe.⁽⁶⁾ Incident beam penetration was approximately 3 μm for Al and 5 μm for MgRP.⁴³ 

The molecular identity of corrosion products was verified with a Renishaw Leica Raman Microscope^† calibrated to a Si standard reference. Identification was made with the RRUFF™^† Project Raman online database.⁴⁴  Identification of MgCO₃ must be carefully considered as these peaks also match the Raman signal of the epoxy used to mount the sample for analysis (i.e., C–O bonding). Operating parameters for characterization techniques are specified elsewhere.²⁵ 

Diagonally cross-sectioned profiles were taken of both MgRP/TC and MgORP/TC after 2.5 y of field exposure. Figure 4(a) shows an EDS elemental map of a segment of an unexposed MgRP/TC coating. Figure 4(b) shows the elemental map for unexposed MgORP/TC. Figures 4(c) and (d) show the same maps for 2.5 y exposed MgRP/TC and MgORP/TC, respectively. Elemental O and Mg EDS maps are shown for both coatings in Figure 5 both near and far away from the scribe. The Mg content within the primers far from the scribe differs, with a greater concentration of Mg remaining within MgRP/TC than MgORP/TC after 2.5 y based on Mg Kα signal intensity contrast with respect to the base alloy Mg content (Figure 5). A Mg-depleted zone is observed in the primer at the MgRP/scribe interface up to a clearly defined depth (approximately 125 μm).^14-15  Additionally, the O elemental map shows oxide penetration into the AA2024-T351 substrate beneath the MgORP/TC, suggestive of corrosion damage.

Mg repartitioned from the primer into the scribe exposing AA2024-T351 in previous studies.^11,14-15,18  Figure 6 shows the elemental Mg profiles for line scans across two locations in the MgRP/TC (a) and MgORP/TC (b) scribes. The MgRP/TC sample exhibited major repartitioning of Mg from the primer into the scribe for both locations indicated. The Mg intensity is larger within the scribe than over the coating due to the thick topcoat layer over the Mg-pigmented primer.⁽⁷⁾ Mg also repartitioned from the MgORP/TC into the scribe, but to a lesser degree. Figure 7 shows that, after 2.5 y of exposure, a greater amount of Mg corrosion product is accumulated within the scribe for MgRP/TC as compared to MgORP/TC, the formation of which may be a function of the exposure environment.⁴⁵  The amount of Mg repartitioning into the scribe for MgORP/TC is small but still measurable, as seen in Figures 6 and 7. Interestingly, the amount of Mg within the scribe increased throughout the full 4.25 y exposure for MgRP/TC.

Raman spectroscopy probed the molecular identity of corrosion products within the MgRP/TC coating. The spectra provided in Figure 8 indicate the presence of Mg(OH)₂ as a major corrosion product from peaks at 276 cm⁻¹ and 445 cm⁻¹. Another corrosion product detected was consistent with MgCO₃–derivative compounds (likely MgCO₃•3H₂O^22-23 ) as evidenced by peaks at 1,111 cm⁻¹ and 1,608 cm⁻¹. Additional peaks may correspond to other corrosion product or primer chemistries.

XRD results showed the depletion of Mg and MgO from their respective topcoated primer systems over the course of environmental exposure at the KSC. Figures 9 and 10 report Mg or MgO depletion for the MgRP/TC and MgORP/TC systems, respectively, after various lengths of exposure ranging from no exposure to 4.25 y exposure. Peaks were indexed for the AA2024-T351 substrate, Mg pigment, MgO pigment, and Mg(OH)₂ corrosion product, which are included in Figure 11. All other nonindexed peaks arise from other crystalline pigments used within the primer and topcoat polymer matrix.^(8),25  Peaks corresponding to Mg and MgO pigment steadily decreased over the course of the exposure to final intensities, which were scarcely above the baseline. A more accurate determination was made by integrating the area under a specific Mg, MgO, and Mg(OH)₂ peak to compare the amount of each species present over the course of exposure, as reported in Figure 12. These integrated intensities were normalized to the maximum intensity of the initial unexposed signal for Mg and MgO and the 4.25 y peak for Mg(OH)₂ to yield relative concentrations (Figure 12). The XRD results indicate that MgO is almost fully depleted from the primer after 2.5 y of exposure (Figure 12).⁽⁹⁾ Mg is also depleted from the primer over the course of the exposure, but not completely as Mg diffraction peaks are still present even after 4.25 y (Figure 12), indicating that metallic Mg remains in the primer.⁽¹⁰⁾ Mg was continually oxidized in place to form Mg(OH)₂ (Figures 9 and 12). After 4.25 y, the Mg(OH)₂ peaks rivaled those from Al in the Al alloy substrate. Detectable amounts of Mg remained in the primer after 4+ y of exposure, while the Mg(OH)₂ peaks increased. In contrast, there was no evidence of transformation of MgO to crystalline Mg(OH)₂ in the case of MgORP/TC, even though there was evidence of MgO depletion. The significance of this observation is discussed below.

Unexposed and 2.5 y exposed MgRP/TC and MgORP/TC were subjected to the diagnostic full immersion electrochemical cycle test.^12,18,25,27  The OCP measurements are shown in Figures 13 and 14. The global galvanic protection potential of an unexposed AA2024/MgRP/TC was initially very negative as the substrate was sufficiently well-connected to the Mg pigment (Figure 13[a]). Over the course of the cycle test, Mg was depleted and the global galvanic protection potential shifted to a more noble potential. The potential trend for MgRP/TC after a 2.5 y field exposure was almost the reverse (Figure 14[a]). At first, the global galvanic protection potential was relatively positive (near the OCP of bare AA2024-T351) for the 2.5 y field-exposed MgRP/TC. However, over the course of the test, the global galvanic protection potential became more negative, to the point where it was lowered below the OCP of the bare substrate (Figure 14[a]).

In contrast, the OCP cycle test result for the unexposed MgORP/TC was very stable and close to the value of the AA2024-T351 OCP throughout the cycle exposure (Figure 13[b]). The trend for the 2.5 y exposed MgORP/TC was similar (Figure 14[b]). After starting at more positive potentials, the OCP stabilized near the OCP for the AA2024-T351 substrate. The MgORP/TC OCP at the end of the cycle test was similar for unexposed condition as it was for field-exposed condition (Figures 13 and 14). These findings are consistent with the fact that MgO is not electrochemically active.

The charge was monitored in Figures 15 through 18 for unexposed and 2.5 y exposed MgRP/TC and MgORP/TC during potentiostatic holds of the intact coating in the diagnostic cycle test. Figures 15(a) and (b) show that for the unexposed MgRP/TC, anodic charge was measured at the hold potential as Mg⁰ was oxidized to Mg²⁺. However, as more cycles elapsed the potential of the intact coating shifted to more positive values. After about 30 h, the OCP of the substrate was more positive than the global hold potential and the charge became negative, or cathodic. This implies no galvanic couple protection by Mg. Figures 16(a) and (b) show the charge evolved during the potentiostatic holds for unexposed MgORP/TC, which was negative (cathodic) for all cycles. This indicates that the cycle test hold potential is more cathodic than the OCP of the topcoated MgORP system and that there is no sacrificial anode-based protection afforded by MgO pigment.

The charge for field-exposed MgRP/TC was negative during the initial stages of the cycle test but then switched to anodic values after approximately 30 h of immersion (Figure 17). Figure 18 shows cathodic charge readings for 2.5 y exposed MgORP/TC for the full exposure cycle, indicating that the potentiostatic hold produced net cathodic reactions on the AA2024-T351 substrate. The OCP remained close to the potential expected for uncoupled AA2024-T351 and the cycle hold potential of −0.8 V_SCE produced cathodic current, likely from ORR.

Impedance spectroscopy assessed the coating barrier resistance and the ingress of electrolyte to the substrate. The total impedance at low frequencies is of particular interest, as it reflects the total contribution of all resistances in the system. High impedances at low frequencies suggest high coating and polarization resistances. The low-frequency cycle test impedance response for unexposed MgRP/TC in Figure 19(a) is higher than that of the 2.5 y exposure in Figure 20(a), which was just below 10⁸ Ω·cm². The impedance for the unexposed MgORP/TC in Figure 21(a) was also just below 10⁸ Ω·cm². However, after 2.5 y of exposure the impedance behavior of the MgORP/TC decreased by an order of magnitude, with a notable decrease in the coating capacitance (Figure 22[a]).

Impedance spectroscopy also characterized the initial as-received condition (separate from the cycle test), comparing the unexposed, 2.5 y, and 4.25 y exposure aging times. Bode plots (Figure 23) report the Bode magnitude impedance and phase shift of the MgRP/TC ([a] and [c]) and MgORP/TC ([b] and [d]) at various exposure times. The impedance magnitude of both unexposed primers was initially high at zero exposure time, though MgRP/TC had a higher impedance than MgORP/TC. The impedance for both coatings decreased after 2.5 y of exposure, though by a greater extent for the MgORP/TC. Overall, the impedance of the MgRP/TC remained relatively steady and high throughout the 4.25 y of exposure (Figure 23[a]). The impedance of the MgORP/TC decreased more appreciably with exposure time and was lower than MgRP overall.

An important and well-established protection mechanism afforded by MgRP is cathodic protection of the substrate via galvanic coupling to the Mg pigmented primer.^{9-10,14-15,18,20-21,23}  When the AA2024-T351 substrate is connected to the Mg pigment, it is cathodically polarized toward the galvanic couple potential between Mg and AA2024-T351 (as Mg is oxidized to Mg²⁺) as regulated by ohmic resistance and described by mixed potential theory. Galvanic coupling and cathodic protection is exhibited immediately in the case of the unexposed MgRP/TC, which provides global galvanic protection early on (Figures 13 and 15). However, as the cycle test progressed, most of the initially well-connected Mg pigment was oxidized and depleted, and as the remaining Mg pigment was unavailable for galvanic coupling, cathodic protection diminished (Figures 13 and 15). This is analogous to environmental exposure where readily accessible Mg pigment (pigment that has easier access to the substrate) is depleted. As the readily connected Mg pigment is oxidized and spent, MgRP loses capacity for sacrificial anode-based cathodic protection.

Initially, the 2.5 y exposed MgRP/TC did not exhibit galvanic coupling, due either to depletion of the Mg pigment or insufficient connection of the pigment to the substrate. However, despite the general depletion of Mg through either galvanic couple corrosion or through self-corrosion, XRD results indicate that there was still some metallic Mg⁰ present within the MgRP/TC after 2.5 y and after 4.25 y of field exposure at KSC as seen in Figures 9 and 12. Potentiostatic cycle testing (Figure 17) indicates that after enough cycles, this small amount of preserved Mg pigment in the primer is sufficient to form an active galvanic couple with the AA2024-T351 substrate even after 2.5 y of exposure aging. One argument for this resumed coupling after an apparent incubation time is that the −0.8 V_SCE hold potential which initially promoted cathodic reactions at the sample interface caused cathodic disbondment and coating degradation which exposed preserved Mg pigment⁽¹¹⁾ to a low resistance connection with the substrate¹⁸  (Figures 17[a] and [b]). This indicates that not only is there residual metallic Mg left in the MgRP/TC after 2.5 y of exposure (as confirmed by XRD data in Figure 9) but that these remaining pigment particles can enable galvanic coupling, which resumes cathodic protection of the substrate. This resumed galvanic coupling after 2.5 y of exposure contributes to an explanation of how MgRP/TC protects AA2024-T351 substrate for long exposure times. While galvanic coupling in an intact MgRP/TC coating was observed after 2.5 y following a sufficient number of cycles in the cycle test, it is doubtful that galvanic coupling persisted continuously throughout the 4+ y exposure for every wetting event. Alternatively, the switch from cathodic to anodic charge may be due to a drop in the substrate OCP below −0.8 V_SCE due to increased alkalinity caused by the promoted cathodic reactions of the potentiostatic hold.^25,46  However, this switch has not been observed for any other system, including a 0% PVC MgRP.¹⁸ 

EDS analysis indicated that Mg segregates from the primer and into the scribe over the course of the exposure with segregation being more pronounced for the MgRP/TC than for the MgORP/TC (Figures 6 and 7). Mg repartitioning from the MgRP/TC into the scribe continued over the 4.25 y field exposure in a coastal marine environment (Figure 7). The mechanism for repartitioning is attributed to the cathodic polarization of the scribe by the galvanic couple with MgRP. As the scribe becomes a galvanic cathode it is negatively polarized and cathodic reactions are promoted. The combination of the negatively polarized scribe and pH increasing cathodic reactions promotes electromigration of Mg²⁺ ions produced from Mg pigment oxidation to the scribe where they precipitate corrosion products such as Mg(OH)₂. This mechanism is operative as long as there is sufficient galvanic coupling of the scribe to the MgRP to both promote Mg²⁺ production at the primer and drive diffusion and migration of Mg²⁺ and corrosion product formation in the scribe.

As the coating barrier properties of MgORP/TC degrade more than those of MgRP/TC (from Z_{0.01 Hz} ∼10^7.6 to 10^6.7 for MgORP versus ∼10^8.3 to 10^7.9 for MgRP in Figure 23) and galvanic protection is nonoperative (as the Mg²⁺ is already in a discharged state), there must be an alternative mechanism for MgORP to provide corrosion protection to the AA2024-T351 substrate. MgORP provides a chemical protection mechanism in which corrosion is suppressed by Mg²⁺ and its corrosion products.²⁵  Despite the inability for MgO pigment to galvanically couple to the substrate, the OCP of a nontopcoated MgORP on AA2024-T351 has been observed to be drastically lower than the OCP expected for bare AA2024-T351 in this same solution (Figure 13).²⁵  It was concluded that the chemical dissolution of MgO pigment into solution changed the solution chemistry proximate to the substrate interface (by increasing [Mg²⁺] and pH⁴⁵ ) such that the natural OCP of AA2024-T351 was reduced to more negative values than that expected for bulk solution chemistry.^25,46  This phenomena and its effects will be discussed in subsequent investigations. The dissolving MgO pigment affected the electrochemical kinetics of the substrate in a chemical manner instead of in the direct electrochemical manner Mg pigment exhibits via galvanic coupling. When the MgORP was topcoated, the chemical effect of MgO on the OCP was not observed and the OCP was near that expected for bare AA2024-T351.⁽¹²⁾ Both OCP values of the unexposed and the 2.5 y field-exposed topcoated MgORP were near that expected for bare AA2024-T351.

Pigment depletion may explain the differences in impedance character of the two coatings after 2.5 y of exposure. Diffraction results indicated that Mg globally converts to Mg(OH)₂ in MgRP, preventing pore development with Mg(OH)₂ formation at the site of the pigment. Mg(OH)₂ has a larger molar volume than Mg, expanding by approximately 77% upon oxidation (based on the density and molecular weight of Mg and Mg(OH)₂; 1.7 g/cm³ and 2.3 g/cm³, and 24.3 g/mol and 58.3 g/mol, respectively), leading to the development of compressive stresses in the coating around Mg(OH)₂ during a pore filling process. This conversion to Mg(OH)₂ may prevent defect formation in regions of compressive stress proximate to this product, providing an explanation for the retained, high impedance of the MgRP. As MgORP/TC exhibited severe depletion of pigment to chemical dissolution into solution and no formation of crystalline product (Figure 10), it is likely that pores developed in place of MgO. This pore formation would explain the notable decrease in impedance magnitude character of the MgORP/TC after exposure (Figure 22). Pore formation is further evidenced in Figure 23 by the increase in the breakpoint frequency of the MgORP/TC (200 Hz) over the MgRP/TC (10 Hz) after 2.5 y of exposure. The breakpoint frequency (the frequency at which the phase angle becomes −45°) is a common metric for defect formation.^14-15,18,47 

Electrochemical dissolution kinetics may explain why Mg converts to Mg(OH)₂ in MgRP. In contrast, MgO is largely chemically depleted with no conversion in MgORP. As both Mg-based pigments dissolve, they produce Mg²⁺ and OH⁻ (Equations [1a] and [1b]). When the equilibrium [Mg²⁺] and pH is reached, Mg(OH)₂ will form (Figure 24, Equation [2]). It may be that the dissolution kinetics of Mg is fast enough that the critical chemistry is established at the pigment surface for Mg(OH)₂ formation, whereas the dissolution of MgO is slow enough that diffusion of Mg²⁺ and OH⁻ into solution prevents the interfacial buildup of Mg²⁺ and OH⁻ ions needed to form Mg(OH)₂ at the site of dissolution⁴⁸  (k₁ ≫ k₂). Thus, the interplay between interfacial reaction kinetics and diffusion kinetics is crucial. This hypothesis was tested by monitoring the pH transient of dissolving bare Mg and MgO interfaces. Commercially pure Mg was able to reach its equilibrium solution chemistry before cold-pressed MgO pellets, as monitored by pH change upon dissolution into a stirred 5 wt% NaCl solution with a pH probe. Mg was able to establish chemical equilibrium within 2.2 h, whereas MgO established equilibrium within 5.1 h (Figure 25).

EDS and XRD analysis indicated there was more total Mg (elemental/metallic/compound) in the MgRP than in the MgORP (Figures 5 and 12). It was previously determined that the capacity of Mg within MgORP is much less than that of MgRP (by a factor of 25).²⁵  It is not surprising then, that after 2.5 y of field exposure, the amount of retained Mg is far less for MgORP than for MgRP.²⁵  This will have implications for the relative performance of MgRP vs. MgORP.

In the case of long exposure times where Mg is either mostly depleted or physically isolated from the substrate, the case of pretreatment mediated galvanic coupling, and the case of MgO pigmented primer where galvanic coupling is absent, Mg repartitioning to the scribe is not driven by the cathodic polarization of the scribe via galvanic coupling. For these three cases, repartitioning is likely due to the development of localized chemistry on the AA2024-T351 substrate exposed in the scribe, which is favorable for precipitation and deposition of Mg-based products. In the absence of cathodic protection, the chemistry over the AA2024-T351 will progress under natural conditions at open circuit, such that cathodic and alkaline reactive sites can develop, which are favorable for Mg-based corrosion product formation. Mg, MgO, or Mg(OH)₂ dissolution can all provide Mg²⁺, which can operate both in chemically precipitated products and in solution (as ⁠). Deposited Mg-based precipitation products may serve to reduce corrosion damage either by acting as physical barriers to the corrosion process or by in-turn affecting the solution chemistry upon later dissolution. It was observed previously that when a solution was saturated with MgO, the OCP was drastically shifted to lower potentials.²⁵  If in one wetting event the deposition of Mg corrosion products takes place, then in another wetting process the dissolution of these products can cause a favorable change in the solution chemistry for mitigation of AA2024-T351 corrosion. In this way, continued repartitioning of Mg into the scribe would be less favorable if previous repartitioning worked to reduce the corrosion conditions that would otherwise lead to localized chemistries favorable for formation of Mg corrosion product.⁴⁹  This hypothesis may help explain why less repartitioning is seen for MgORP/TC compared to MgRP/TC (Figures 6 and 7). Aqueous Mg²⁺ can restrain the increase in pH proximate to cathodic sites by consuming OH⁻ upon precipitation of Mg(OH)₂, preventing caustic attack of Al. The results of this investigation merit a future investigation into the characteristics of Mg²⁺-release as a corrosion inhibition strategy. Such a study is underway.

This investigation sought to advance current understanding of the long-term protection performance of Mg-based coatings. It was especially desirable to make observations from these long-term field exposure studies that would contribute toward developing a mechanism that explains and predicts Mg-based primer behavior. An accurate mechanism for protection by MgORP or MgRP after cathodic protection has terminated must take into consideration the following points: (1) the Mg in MgRP/TC is globally converted to Mg(OH)₂ in increasing amount throughout exposure (Figures 9 and 12), (2) the MgO in MgORP/TC depletes without evidence of other products forming (Figures 10 and 12), (3) MgRP can cathodically protect the substrate while MgORP cannot (Figures 13 through 18), (4) Mg²⁺ from MgRP/TC repartitions into the scribe in increasing amounts throughout the course of exposure (Figures 6 and 7), and (5) Mg²⁺ from MgORP/TC also repartitions into the scribe, but to a lesser extent than MgRP/TC (Figures 6 and 7). Mg²⁺-release from MgRP and MgORP is capable of providing “chemical protection” by methods to be fully explored in future work.

• •

  The amount of crystalline elemental Mg pigment in MgRP/TC samples decreases with exposure time but is not fully depleted after 2.5 y, or even 4.25 y of marine coastal exposure at KSC (Figures 9 and 12).

• •

  The amount of MgO pigment in a topcoat finished MgO sample decreased greatly over a 2.5 y outdoor exposure period but was not fully depleted either (Figures 10 and 12). The depletion of MgO without transformation to corrosion product indicates that MgO chemically dissolves into solution.

• •

  2.5 y exposed MgRP/TC exhibited residual electrochemically active galvanic coupling to the AA2024-T351 substrate (Figure 17), indicating prolonged cathodic protection capacity.

• •

  The presence of crystalline Mg(OH)₂ after oxidation of Mg in the MgRP/TC system (confirmed by XRD and Raman spectroscopy) indicates that Mg(OH)₂ is the preferred corrosion product for Mg oxidation in the primer for these atmospheric marine coastal field exposure conditions (Figures 8, 9, and 12).

• •

  The conversion of Mg to Mg(OH)₂ and the lack of conversion of MgO to Mg(OH)₂ was rationalized by the different dissolution rates of the two pigments. Mg is hypothesized to dissolve quickly enough to form the critical solution chemistry needed for Mg(OH)₂ formation at the interface, whereas MgO dissolution is outpaced by aqueous diffusion.

• •

  MgRP/TC has a larger extent of repartitioning of Mg²⁺ into the scribe than MgORP/TC, the concentration of which increased throughout a 4.25 y exposure (Figure 7).

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  Dissolved from MgO/Mg(OH)₂/Mg may provide additional chemical protection effects that suppress corrosion of the AA2024-T351 substrate at coating defects such as scratches. This forms the basis for additional research.

Financial support provided by the Office of the Undersecretary of Defense Corrosion University Pilot Program, directed by Mr. Daniel Dunmire under contract #FA7000-14-2-0010. UVA is most grateful to U.S. Naval Air Systems Command personnel Craig Matzdorf, Frank Pepe, Victor Rodriguez-Santiago, and Julia Russell for materials and sample preparation. The Energy Frontier Research Center supported development of MgO dissolution hypothesis. Ryan Katona drafted the 3D file used to print the custom impeller used in the transient pH monitoring experiment.