ACCELERATED CORROSION TESTING OF GALVANIC COUPLES James F. Dante, Josh Averett, Fritz Friedersdorf, and Christy Vestal Luna Innovations 706 Forest St. Suite A Charlottesville, VA 22903 dantej@lunainnovations.com ABSTRACT New materials and structural designs are required for advanced aircraft functionalities. Long standing standardized corrosion test methods (e.g. ASTM B117) are regularly called out in procurement documents to qualify these new materials. However, it is becoming increasingly evident that these test methods can be misleading resulting in approval of material systems that have very poor in service performance and/or the rejection of promising new technologies. In this work, we examine critical differences in the galvanic corrosion behavior between silver and aluminum alloys under bulk immersion and simulated atmospheric exposures. Possible cathodic reactions and their relative importance under the different exposure conditions will be described. The effect of these reactions on the overall galvanic current will also be explained. Finally, the implications of these findings on the selection of corrosion abatement technologies will be addressed. Keywords: galvanic, accelerated testing, corrosion, silver, aluminum INTRODUCTION In the design of military and commercial assets, great care is taken to avoid dissimilar metal contact that may result in galvanic corrosion in severe environments. However, complete exclusion of galvanic contacts is seldom achieved. Designers and maintainers must resort to coatings, sealants, and other protection systems to reduce the occurrence or severity of galvanic attack. Severity of possible galvanic attack is often tested using electrochemical techniques under immersion conditions. Selection of inhibitor systems for the reduction in galvanic corrosion is also a result of laboratory testing using accelerated corrosion chamber environments. However, as material technology has progressed, accelerated corrosion testing has not. As a result, there are instances where protection systems pass laboratory tests but fail in operational environments. Conversely, there are promising technologies that are never fielded because of failures under the unrealistic conditions found in accelerated corrosion test environments. Under atmospheric corrosion conditions, a significantly different localized environment exists at the surface of a corroding metal (1),(2). Because of the very thin electrolyte layer, oxygen mass transport 1
is significantly reduced increasing the cathodic reaction rates and raising the Open Circuit Potential (OCP). Both of these effects will impact galvanic corrosion. For inhibitors (e.g. Cr 2 O -2 7 ) that are used to decrease the oxygen reduction kinetics, higher concentration of inhibitor would need to be used to compensate for the increase in oxygen reduction kinetics under atmospheric conditions. Additionally, higher values of OCP may result in the formation of corrosion products not observed under immersion conditions. Therefore, electrochemical evaluations under bulk electrochemical conditions may result in misleading conclusions regarding galvanic corrosion rate and the effectiveness of various inhibitors to inhibit galvanic corrosion. In the presence of chloride, Ag electrodes can undergo very rapid dissolution under oxidizing conditions (3). In the presence of oxidizing species that may be found under atmospheric conditions, Ag can be oxidized and lead to the formation of AgCl complexes (4). It is also known that the presence of Cl - ions significantly lowers the oxidation potential of Ag which acts to reduce the oxidizing power of solution required to form AgCl (5). When combined in a lap joint with Al2024-T3, Ag seems to remain relatively free of corrosion in accelerated test environments compared with outdoor field exposures (6). This may be a result of significant differences in the concentration of oxidizing species under the relatively wet conditions found in exposure chambers. As a result of these processes, there is the possibility of two cathodic processes: oxygen reduction and AgCl reduction. The focus of this work is gain an improved understanding of the corrosion mechanism between Ag and Al7075-T6 under both immersion and simulated atmospheric conditions. While there are numerous alloy combinations to choose from, a galvanic couple containing Ag has interesting and unique properties. The specific identification of the cathodic processes that occur on Ag will be addressed. Further, the relative importance of each possible cathodic reaction to galvanic corrosion between Ag and Al7075-T6 will be explored. Finally, the influence of exposure conditions (immersion or atmospheric) on the cathodic reactions on Ag and their implication for galvanic corrosion will be examined. EXPERIMENTAL Materials Galvanic couples between pure Ag and Al7075-T6 were studied. Ag rod or sheet (99.9%) was used throughout this study. Non-clad Al7075-T6 was used for electrochemical characterization and in measurements of galvanic corrosion under both immersion and atmospheric conditions. The affect of various chloride concentrations on the reduction kinetics were measured. All chemicals were reagent grade. In most cases, chloride was added to a 60mM Na 2 SO 4 base solution. Sulfates are often found in pollutant chemistries in relative high abundance and thus serve as a suitable electrolyte. The sulfate base solution allowed the effects of solution resistance to be minimized for solutions with lowest chloride concentration. Electrochemical Testing Electrochemical testing was performed under both immersion and simulated atmospheric conditions. The atmospheric cell is described in detail in the following section. Measurements under immersion conditions were performed in a three electrode electrochemical cell using a Saturated Calomel Electrode (SCE) reference electrode. During polarization experiments, a Pt mesh electrode was used as a counter electrode. For exposure under simulated atmospheric conditions, a Pt wire electrode was used as the counter electrode. Measurements of galvanic current were taken by shorting the Ag and Al7075-T6 together through a potentiostat and measuring. The mixed potential of the galvanic couple was measured by a separate SCE electrode. Under immersion conditions, the surface area of both electrodes of the galvanic current was 1cm 2. All electrochemical measurements were performed using a PAR 263A potentiostat. Cyclic polarization scans were performed to determine key cathodic reaction processes on the Ag electrode. Scan rates of either 5mV/sec or 10 mv/sec were used. Scans from -1,000mV (SCE) to as high as +350mV (SCE) under both aerated and deaerated conditions were used to identify peaks associated with oxygen reduction and AgCl reduction. A base solution containing 60mM Na 2 SO 4 was maintained for each scan. The purpose of these additions was to avoid IR drop effects at the lowest 2
Impedance (ohms) NaCl concentrations. No IR interrupt was used during any of the electrochemical testing. Sulfate species are prevalent in pollutant particulates and are therefore not an unrealistic addition. Argon (99.999%) was used to deoxygenate the flat cell during experiments under deaeration conditions. The effects of dichromate additions on the cathodic reaction on Ag were studied using a rotating disk electrode. Cyclic polarization scans were performed under at a rotation rate of 500rpm. Under these hydrodynamic conditions where mass transfer limited current was controlled, the role of the inhibiting ion on oxygen reduction was more clearly observed. Linear polarization scans were performed at a rate of 0.2mV/sec to determine the formation potential for AgCl as a function of NaCl concentration in 60mM Na 2 SO 4. NaCl concentrations varied from 0.6mM NaCl 600mM NaCl. The potential was scanned from -750mV below the Open Circuit Potential (OCP) to approximately 1V (SCE). Electrochemical Impedance Spectroscopy (EIS) scans were performed within the atmospheric cell to measure the exposure time required for surface wetting of the filter paper separator. Scans were performed from 100KHz down to 1Hz using a 10mV AC potential amplitude at the OCP. Values of impedance magnitude where the phase angle = 0 were taken as a measure of the series resistance of the filter paper separator. a) b) 1.00E+07 Figure 1. (a) Disassembled atmospheric cell to shown the relative positioning of the Ag and Al7075-T6 electrodes and the filter paper. (b) Assembled atmospheric cell inside of the controlled temperature/humidity chamber showing the SCE reference sensor mounting. (c) impedance of the filter paper member between the Ag and Al7075-T6 electrodes as a function of exposure time 1.00E+01 0 200 400 600 800 1000 c) Exposure Time (Minutes) Atmospheric Simulations A special cell was designed to make measurements under simulated atmospheric conditions (Figure 1). Ag and Al7075-T6 samples are separated by filter paper (Whatman; GB004) that was soaked in a solution containing 60mM Na 2 SO 4 + 6mM NaCl and dried for 1 hour at 60 o C. The pure cellulose filter paper (1.0 mm thick) served as a reservoir for the aggressive salt solution as well as a salt bridge for the reference electrode. A SCE reference electrode was inserted through the top of the cell assembly and directly contacts the filter paper salt bridge. The system was then wetted by exposure to 30 o C at 90% relative humidity (RH). A potentiostat was used to measure the galvanic current between the two metals and the mixed potential was measured relative to the galvanic couple using the SCE. The measured current was converted to current density by normalization to the exposed area of the Al7075-T6 electrode (6.18cm 2 ). In further studies, the cell in Figure 1 was slightly 1.00E+06 1.00E+05 1.00E+04 1.00E+03 1.00E+02 3
modified to accommodate a loop shaped Pt mesh counter electrode. An external switch was attached such that galvanic contact between the Ag and Al7075-T6 samples could established or broken in-situ. Additionally, the Pt counter electrode could be switched into the circuit. This arrangement provided the ability to perform polarization measurements after a galvanic couple had been in contact for a specified time. To verify wetting of the filter paper membrane, EIS measurements were made using the Ag working electrode and a Pt wire loop counter electrode (Figure 1). It is apparent that at exposure times shorter than 1 hour, insufficient wetting has occurred within the atmospheric cell to sustain electrochemical measurements. Therefore, all electrochemical measurements under simulated atmospheric conditions were carried out after an exposure of 1.5 hours. It should be noted that at the highest current densities measured during linear polarization scans, a significant IR drop voltage error exists (0.1V). However, in the voltage range of the galvanic potential, the voltage error is only 10mV. RESULTS Cathodic Reactions on a Ag Electrode The two possible cathodic reactions at the Ag electrode are oxygen reduction reaction (Reaction 1) and AgCl reduction (Reaction 2). Ag/AgCl is used as a reference electrode because of its temperature stability, reversibility and high exchange current density (7). This second phenomenon is a result of the fact that the oxidation/reduction kinetics of reaction 2 are very rapid. Thus, it may be expected that under oxidizing conditions that form AgCl, the reduction of AgCl may be available to significantly participate in the cathodic reaction in a galvanic couple. Given that many inhibitor packages are designed to slow the oxygen reduction reaction, it is worthwhile to elucidate the role of the AgCl reduction process in chloride containing environments. A comparison of solutions with 0.6mM NaCl is shown in Figure 2(a) and (b). Scans were run to either +100mV or +350mV respectively. Scans were performed under aerated (ambient aeration) and deaerated (Argon purge) conditions. Regardless of scan range, a reduction peak can be seen in the region of -200mV. This peak is likely associated with the reduction of oxygen in solution since this peak is not present under deaerated conditions. Upon polarization to higher potentials (+350mV), a second peak is observed in the solution containing 0.6mM NaCl. The oxidation and complimentary reduction peak are likely associated with the formation of a AgCl salt (5). Note that the peak current density of the reduction peak is on the order of 100mA; similar to peak current density in solutions containing only Na 2 SO 4. While the oxygen reduction peak and steady state cathodic current are a function of oxygen levels, the AgCl oxidation/reduction peak are not. The effect of chloride additions to the sulfate based electrolyte was examined (Figure 3a). The linear polarization data indicates the oxidation of Ag to form AgCl followed by a second oxidation potential where AgO is formed. As the chloride concentration increases beyond 6mM NaCl, the formation of AgCl becomes the predominant oxidation event obscuring AgO formation. Based on this data, cyclic polarization scans were limited to a potential range of +350mV (SCE). Cyclic polarization scans (Figure 3b) were recorded in order to characterize both the oxidation and reduction behavior of AgCl. In the absence of chloride, only the peak associated with oxygen reduction is observed. As higher concentration of chloride are added, the peak current density of the oxidation and reduction peaks associated with AgCl increase. At 6mM NaCl, the peak current density associated with AgCl reduction is 1 order of magnitude larger than that for the oxygen reduction reaction. Further, the formation potential of the AgCl moves to lower values of potential as the NaCl concentration increases. (1) (2) 4
Potential (V SCE) Current Density (A/cm 2 ) Current Density (A/cm 2 ) Current Density (A/cm 2 ) 5.0E-05 0.0E+00 1.5E-04 1.0E-04-5.0E-05 5.0E-05-1.0E-04-1.5E-04-2.0E-04 deaerated aerated 0.0E+00-5.0E-05-1.0E-04-1.5E-04 deaerated aerated -2.5E-04-1.00-0.80-0.60-0.40-0.20 0.00 0.20 Potential (V vs. SCE) a) b) -2.0E-04-1.00-0.80-0.60-0.40-0.20 0.00 0.20 0.40 Potential (V vs. SCE) Figure 2. Cyclic polarization scan (10mV/sec) of Ag in 60mM Na2SO4 + 0.6mM NaCl. The scan was reversed at (a) +0.1V (SCE) and (b) 0.35V (SCE). 6.0E-04 4.0E-04 0mM NaCl 0.6mM NaCl 6.0mM NaCl 2.0E-04 0.0E+00 AgCl formation AgO formation -2.0E-04-4.0E-04-6.0E-04 a) b) 0.3-8.0E-04-0.500-0.300-0.100 0.100 0.300 0.500 Potential (V vs. SCE) c) 0.2 0.1 0-0.1-0.2 OCP of Ag AgCl Formation Potential -0.3 OCP: 10e-4M chromate OCP: 10e-3M chromate OCP: 10e-2M chromate -0.4-1 0 1 2 3 log(nacl Concentration) (mm) Figure 3. (a) Effect of chloride additions on the linear polarization behavior of Ag exposed to a 60mM Na 2 SO 4 base solution (natural aeration, scan rate = 0.2mV/sec). (b) Effect of chloride additions on the cyclic polarization behavior of Ag exposed to a 60mM Na 2 SO 4 base solution (natural aeration, scan rate = 10mV/sec). (c) OCP and AgCl formation potential as a function of chloride and chromate additions (natural aeration, scan rate = 0.2mV/sec). 5
Current (A/cm 2 ) Current (A/cm 2 ) The OCP and AgCl formation potentials (derived from Figure 3a) were recorded as a function of chloride concentration in the base Na 2 SO 4 solution (Figure 3c). Additionally, for the solution containing 6mM NaCl, the change in OCP as a function of added Na 2 Cr 2 O 7 concentration was determined. The formation potential of AgCl follows Nernst behavior as a function of chloride concentration with a slope of -58mV/log(Cl - ) as expected for the single electron reaction (5). However, the OCP is also reduced as chloride concentration increases implying that in bulk solution with oxygen present, AgCl will not be spontaneously formed. As dichromate is added, the OCP increases. Since there is no effect of dichromate on the formation potential of AgCl, additions of the anion results in the formation of AgCl at OCP conditions. This explains the appearance of a large reduction peaks observed during cathodic polarization of Ag foils exposed to chloride + dichromate electrolytes(8). It should be noted that the 60mM Na 2 SO 4 in the electrolyte also acts to increase the OCP. It should further be noted that deaeration of the electrolyte did not affect the formation potential of AgCl. Effect of Dichromate additions on Cathodic Reactions Kinetics for Ag Cyclic polarization scans (scan rate = 5mV/sec) were performed to determine the effect of Na 2 Cr 2 O 7 additions on both the oxygen reduction and AgCl reduction reactions (Figure 4). Scans were performed in a 60mM Na 2 SO 4 solution using a rotating electrode (500rpm) to more clearly demonstrate the effects of inhibiting anion additions. Dichromate ions clearly inhibit the oxygen reduction reaction (Figure 4a). This is consistent with the effects of dichromate on copper and copper bearing intermetallic particles on Aluminum alloys (9),(10). It also appears that dichromate may inhibit the formation of AgO. Dichromate ions effectively reduce the oxygen reduction reaction upon additions of 6mM NaCl to the base solution. However, the presence of Cr 2 O 7-2 ions appears to enhance the AgCl reduction peak (Figure 4b). As shown in Figure 3c, the presence of dichromate ions increases the OCP of Ag in the presence of NaCl. Since the presence of dichromate additions do not affect the formation potential of AgCl, it must be concluded after examination of Figure 4b that reaction kinetics for the formation of AgCl are increased in the presence dichromate ions. 1.5E-04 1.0E-04 5.0E-05 0.0E+00-5.0E-05-1.0E-04-1.5E-04-2.0E-04-2.5E-04 no chromate 10e-4 chromate 10E-3 chromate 10E-2 chromate -3.0E-04-1.000-0.700-0.400-0.100 0.200 Potential (V SCE) 2.0E-03 1.0E-03 0.0E+00-1.0E-03-2.0E-03 No Chromate 10e-4 chromate 10e-3 chromate 10e-2 chromate -3.0E-03-1.000-0.700-0.400-0.100 0.200 a) b) Potential (V SCE) Figure 4. Effect of Na 2 Cr 2 O 7 on the cyclic polarization behavior of Ag in 60mM Na 2 SO 4 (a) without NaCl and (b) with 6mM NaCl. Scan rate = 5mV/sec. Galvanic Corrosion between Ag and Al7075-T6 Silver was galvanically connected to an Aluminum 7075-T6 electrode in bulk solution containing 60mM Na 2 SO 4 + 6mM NaCl and the galvanic current measured over one hour (Figure 5). In the figure, positive current indicates corrosion of the Al7075-T6 electrode. Long term current density on the order of 5x10-5 A/cm 2 was observed. Comparison to a galvanic couple where the Ag was replaced with AgCl reveals that a current transient of 5x10-3 A/cm 2 was present. While this transient was a factor of 100 greater than that of the Ag/Al7075-T6 couple, it only lasted for 100 seconds (Figure 5b). The appearance of the transient current is consistent with the fact that AgCl reduction can significantly affect 6
Current (A) Current (A) the corrosion rate of aluminum. The galvanic potential of both the Ag/Al7075-T6 and the AgCl/Al7075- T6 couple was near -720mV (SCE) which was the OCP of uncoupled Al7075-T6. As observed in Figure 3c, the electrochemical potential must remain above +100mV (SCE) in order for AgCl to be generated in the presence of 6mM NaCl. The large thermodynamic driving force for AgCl reduction and the high exchange current density of Reaction 2, therefore, result in the rapid depletion of AgCl. Further, because of the low galvanic potential, long term galvanic current is governed by oxygen reduction kinetics only. 1.E-02 1.E-02 AgCl reduction 1.E-03 Ag 1mM dichromate AgCl 1.E-03 Ag 1mM dichromate AgCl 1.E-04 1.E-04 1.E-05 0 1000 2000 3000 4000 1.E-05 0 200 400 600 Time (s) a) Time (s) b) Figure 5. Galvanic current between Ag (and AgCl) and Al7075-T6 after (a) 1 hour and (b) the first 10 minutes of (a). Tests were conducted in 60mM Na 2 SO 4 + 6mM NaCl. This fact is further supported by the long term galvanic current observed between Ag and Al7075-T6 upon addition of 60mM Na 2 Cr 2 O 7. Under conditions, where silver in not coupled to aluminum, high concentrations of dichromate result in enhanced oxidation of the silver. However, under galvanic coupling conditions where the mixed potential is close to that of the uncoupled Al7075- T6 alloy, oxygen reduction reactions dominate on the Ag electrode. The inhibition of the long term galvanic current is, therefore, a direct consequence of the fact that Cr 2 O -2 7 was shown to reduce the oxygen reduction kinetics on Ag (Figure 4a). Corrosion under Simulated Atmospheric Conditions Cathodic polarization curves on Ag foils were generated at 30 o C and 90% RH using the atmospheric corrosion cell (Figure 6). The filter paper separator was soaked in 60mM Na 2 SO 4 + 6mM NaCl prior to drying and cell assembly. Curves were generated for various exposure times to the simulated environment in the uncoupled (Figure 6a) and coupled (Figure 6b) state. The very low currents observed after a 10 min exposure to the atmospheric cell can be rationalized by examining Figure 1 which indicates that surface wetting is not complete prior to 45 minutes. After longer exposure times, several striking observations should be noted. Examination of Figure 3c reveals that as chloride concentration increases, both the AgCl formation potential and the OCP of Ag decrease. Thus under immersion conditions, AgCl cannot be formed without the addition of strong oxidizing species. An electrochemical potential approaching 0.0V (SCE) is observed in Figure 6a. Further, a reduction peak just negative of the OCP is observed with high cathodic reaction rates again at lower potentials. A chloridized Ag electrode was exposed in the atmospheric cell and found to have an OCP roughly equivalent to the OCP of the pure Ag electrodes. Therefore, the observed OCP of Ag is at the formation potential for AgCl within the atmospheric cell and the subsequent reduction peak is associated with the AgCl reduction process. Extrapolation of the AgCl formation potential data from Figure 3c to the OCP of the Ag electrode in the atmospheric cell (approximately -0.037V (SCE)) yields a chloride concentration of 4.2M. At 76% RH, the deliquescence point for NaCl, a completely saturated 5.5M NaCl solution concentration would be predicted (11),(12). As RH increases to 90%, the concentration is expected to decrease to approximate 3.2M NaCl; in reasonable agreement with the value predicted from Figure 3b (13). 7
Potential (V SCE) Potential (V) Ag-7075-T6 Intersecting Current RH=90%, Temp = 30C 0.000-0.100-0.200 0.400 0.200 0.000 20 Minute Exposure 40 Minute Exposure 3 Hours Exposure 16 Hours Exposure -0.300-0.400-0.500-0.600-0.700 10 minutes 1 hour 16 hour -0.200-0.400-0.600-0.800-1.000-1.200 AgCl formation a) Current (A/cm 2 Current (A/cm ) b) 2 ) Figure 6. Cathodic polarization curves for a Ag electrode exposed in the atmospheric cell at 30 o C and 90%RH. Data was collected after various exposure times for (a) uncoupled Ag electrode (b) scans after decoupling the Ag electrode from Al7075-T6. The filter paper separator was soaked in 60mM Na 2 SO 4 + 6mM NaCl prior to drying and assembly of the atmospheric cell. In Figure 6b, the Ag and Al7075-T6 were coupled for the specified times. The couples were decoupled and a cathodic scan was immediately performed on the Ag electrode. While the cathodic current density remained high at more negative potentials, no AgCl reduction peak was observed. Additionally, the OCP immediately after decoupling the two electrodes is well below the formation potential of AgCl within the atmospheric cell. Thus, during coupling no AgCl can form precluding any cathodic kinetics associated with AgCl reduction under atmospheric conditions after longterm exposures. A comparison of the electrochemical behavior of Ag under immersion and atmospheric conditions is shown in Figure 7a. As mentioned previously, the AgCl reduction peak is only observed under atmospheric exposure conditions. The major difference in the observed electrochemical behavior is that the mass transport limited oxygen reduction current is increased by a factor of 10 under atmospheric conditions. The galvanic current measured in the atmospheric cell is approximately one order of magnitude higher than found under immersion conditions (Figure 7b). This, combined with the fact that the galvanic corrosion potential predicted from Figure 7a is below the formation potential for AgCl, is consistent with idea that galvanic current between the Ag and the Al7075-T6 substrates is dominated by the oxygen reduction reaction during both immersion and atmospheric exposures. For comparison, the anodic polarization curve of Al7075-T6 in the atmospheric cell is shown in Figure 7a. The anodic polarization curve for Al7075-T6 under immersion conditions is not included for clarity but it is similar to the curve shown with a limiting current density a factor of 10 lower and an OCP 100mV higher. The increase in the mass transport limited current density for oxygen results in a significant jump in the mixed galvanic potential (near -0.775 under immersion and -0.600mV < E < - 200mV under atmospheric conditions) (Figure 7a). This trend is observed during galvanic testing. Actual values of galvanic current and potential will shift based on the relative area of the Ag and Al7075-T6 electrode (i.e. increasing the relative surface area of Ag will increase the galvanic potential and current). In Figure 7c and Figure 7d, the difference in attack morphology can be seen. Under immersion conditions, localized corrosion is observed while under atmospheric conditions, severe localized corrosion occurs that takes on a uniform appearance. Under immersion conditions, corrosion occurs above the potential for passivity breakdown. However, under atmospheric conditions, it is not clear if corrosion occurs as a result of passive film breakdown or chemical dissolution of the passive film. Chemical dissolution will occur at high values of ph. Given the high cathodic current densities that 8
Potential (V SCE) Current (A/cm 2 ) occur at the Ag electrode, it is possible that extremely alkaline conditions exist leading to the possible of oxide dissolution. This phenomenon will be explored further in future publications. 0.600 0.400 0.200 0.000 Ag (immersion) Ag (atmospheric) Al 7075-T6 AgCl reduction 1.0E-02 1.0E-03 1.0E-04 Ag/Al 7075-T6 (bulk) Ag/Al 7075-T6 (atmospheric) -0.200-0.400 Oxygen Reduction 1.0E-05-0.600 1.0E-06-0.800-1.000 I c immersion I c atmospheric 1.0E-07 a) -1.200 1.E-08 1.E-06 1.E-04 1.E-02 log(i) (A/cm 2 ) b) 1.0E-08 0 1,000 2,000 3,000 4,000 Time (seconds) c) d) Figure 7. (a) Anodic and cathodic polarization behavior of Al7075-T6 and Ag under different exposure conditions. (b) Comparison of the galvanic current between a Ag and Al7075-T6 electrode under different exposure conditions. (c) Type of localized attack observed under immersion conditions. (d) Type of attack observed under simulated atmospheric conditions. Considerations for Inhibitor Selection Both the oxygen reduction and AgCl reduction reactions may participate in the overall cathodic reaction driving the corrosion of a less noble Al7075-T6 substrate. While the oxygen reduction reaction is strongly affected by deaeration, the formation and reduction of AgCl is not. Thus, for occluded geometries where oxygen depletion occurs, corrosion rates may still be high as a result of the AgCl reduction mechanism. However, by coupling a Ag substrate to an Al7075-T6 substrate, a mixed galvanic potential well below the AgCl formation potential results precluding the AgCl reduction reaction. Under limited circumstances where oxidizing environments exist (atmospheric conditions or under immersion conditions in the presence of strong oxidizing inhibitors like Cr 2 O 7-2 ), corrosion of the Ag can occur prior to the formation of a galvanic couple with Al7075-T6 resulting in a high rate (albeit short lived) AgCl reduction mechanism. Corrosion of aluminum at high electrochemical potentials will result from the formation a galvanic couple with a more noble metal like Ag. A decrease in the oxygen reduction kinetics of the more noble metal results in little change of galvanic potential in chloride solutions but does reduce the corrosion rate. However, localized corrosion of the aluminum will continue to occur until the galvanic potential falls below the pitting potential of aluminum. Coupling Ag with Al7075-T6 under immersion conditions in the presence of Na 2 Cr 2 O 7 results in a small decrease in galvanic current (Figure 5) but passive behavior is not observed (passive current density < 1x10-6 A/cm 2 ). Under atmospheric environments where the oxygen reduction kinetics are very rapid, corrosion appears to be controlled by the limiting oxidation rate of Al7075-T6. In this case, as cathodic inhibitors 9
Potential (V) Corrosion Current (A/cm 2 ) are added, large potential changes can occur with only minor changes in galvanic current (Figure 7). One of several processes needs to occur for inhibitors to work under atmospheric environment. First, enough cathodic inhibitor must be added such the galvanic potential falls below the region where current limiting oxidation processes dominate the galvanic corrosion rate. Additions of Na 2 Cr 2 O 7 to the filter paper separator yield only a slight decrease in the oxygen reduction kinetics (Figure 8a) on Ag under atmospheric environments. Interestingly, direct modification of the Ag surface provides an even greater reduction in the cathodic reaction kinetics on the Ag surface than dichromate additions. A second factor that influences the inhibitor effectiveness is the galvanic potential generated. While Na 2 Cr 2 O 7 additions only have a minor effect on oxygen reduction compared with various Ag surface treatments, the reduction in galvanic current from the dichromate and the surface treatments are similar (Figure 8b). The galvanic potential for a couple between Ag and Al7075-T6 under simulated atmospheric conditions in the presence of various surface treatments is: untreated Ag = -0.370mV (SCE), treatment B = -0.721mV (SCE), and dichromate = -0.275mV (SCE). Note that while the dichromate additions do not significantly affect the oxygen reduction kinetics, the mixed galvanic potential is relatively high resulting in oxygen reduction currents similar to those of surface treatment B at the much lower galvanic potential (Figure 8a). In the case of dichromate additions, the galvanic current is reduced by its effect on galvanic potential while for the two surface modifications; the galvanic current is reduced as a result of their ability to decrease in the oxygen reduction rate. It is also possible that inhibitors and surface treatments can affect the anodic behavior of Al7075-T6 in the transpassive state to reduce the galvanic corrosion rate. While the AgCl reduction reaction has little role in the galvanic corrosion process, it is interesting to note that both surface treatments inhibit the formation of AgCl (Figure 8a). This property would be useful in preventing the oxidation of Ag resulting in a decrease in its conductivity (4). Other inhibitors (such as benzotriazole (BTA)) and its derivatives are known to influence Ag oxidation(3) and may be expected to have a similar effect on the cathodic polarization behavior of Ag (3). 0.000 AgCl reduciton 1.E-02-0.200-0.400-0.600-0.800-1.000 Untreat Ag chromate surface treatment A surface treatment B oxygen reduciton 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 1.E-08 Untreated Ag Chromate surface treatment A surface treatment B 1.E-09-1.200 1.E-10 0 2,000 4,000 6,000 Current (A/cm a) 2 ) b) Time (S) Figure 8. (a) Cathodic polarization curves of Ag exposed to or treated with various inhibitors. (b) Galvanic current density between a Ag and Al7075-T6 substrate during exposure to or treatments with various inhibitors. The filter paper separator was soaked in 60mM Na2SO4 + 6mM NaCl that was dried prior to assembly in the atmosphere exposure cell. 10
CONCLUSIONS A galvanic couple between Ag and Al7075-T6 result in the noble Ag acting as the cathode while the aluminum undergoes transpassive dissolution. Cathodic reaction that promote corrosion of the Al7075-T6 substrate include oxygen reduction and (under limited circumstances) AgCl reduction. Comparison of the electrochemical behavior of Ag under immersion and simulated atmospheric conditions reveals a 10 fold increase in the oxygen reduction kinetics and an increased OCP under atmospheric conditions. The increase in OCP results in the formation of AgCl under atmospheric conditions unlike the situation under immersion where AgCl formation only occurs in the presence of oxidizing species. The increase in oxygen reduction kinetics is responsible for enhanced galvanic corrosion currents under atmospheric conditions. Additions of Na 2 Cr 2 O 7 reduce oxygen reduction kinetics but appear to increase the kinetics for AgCl formation. Additionally, the oxidizing dichromate ions increase the OCP of Ag in NaCl solutions resulting in AgCl formation. Under atmospheric conditions, dichromate inhibitors and surface modifications of the Ag electrode act to reduce the oxygen reduction reaction kinetics. This effect is small for dichromate compared with direct surface modifications of the Ag. Dichromate reduces the galvanic current of an Ag/Al7075-T6 couple based on its ability to increase the galvanic potential. Surface modifications of Ag, on the other, lower galvanic current in the couple by decreasing oxygen reduction kinetics. References 1. Dante, J. F. and and Kelly, R. G. The Evolution of the Adsorbed Solution Layer During Atmospheric Corrosion and its Effects on the Corrosion Rate of Copper. J. Electrochem. Soc. 1993, Vol. 140. 2. Dunn, D., et al. Corrosion of Iron Under Alternating Wet and Dry Conditons. 1999, p. Paper 223. 3. Brusic, V., et al. Corrosion and Protection of a Conductive Silver Paste. J. Electrochem. Soc. 1995, Vol. 142. 4. Graedel, T. E. Corrosion Mechanisms for Silver Exposed to the Atmosphere. J. Electrochem. Soc. 1992, Vol. 139. 5. Thompson, W. T., Kaye, C. W. Bale, M. H. and D., and Pelton A. Uhlig's Corrosion Handbook. [ed.] W. R. Revie. 2 nd edition, 2000, p. 129. 6. Acevedo-Hurtadoa, P.O., et al. Characterization of Atmospheric Corrosion in Al/Ag Lap Joints. Corrosion Science. 2008, Vol. 50. 7. Newman, John S. Electrochemical Systems. 2 nd edition, 1991. 8. Johnson, J. Galvanic Corrosion of Aluminum. Research in Progress Symposium. 2008. 9. Sehgal, A., et al. Pit Growth Study in Al Alloys by the Foil Penetration Technique. J. Electrochem. Soc. 2000, Vol. 147. 10. Clark, W. J., et al. A Galvanic Corrosion Approach to Investigating Chromate. J. Electrochem. Soc. 2002, Vol. 149. 11. Cole, I. S., et al. A Study of the Wetting of Metal Surfaces in Order to Understand the Processes Controlling Atmospheric Corrosion. 2004, Vol. 151, 12, pp. B627-B635. 12. Tang, I. N. and Munkelwitz, H. R. 1993, Vol. 27, p. 467. 13. Trueman, T., et al. The Development of a Corrosion Prognostic Health Management System for Australian Defence Force Aircraft. Advanced Materials Research. 2008, Vol. 38. 11