L. A. Pawlick, R. G. Kelly
Center for Electrochemical Science and Engineering
Department of Materials Science and Engineering
University of Virginia
Charlottesville, VA 22903
The electrochemical behavior of aluminum alloy AA5052 in methanolic solutions containing low concentrations of acid, chloride, sulfate and water has been studied. In all solutions investigated, the alloy exhibited spontaneous passivity. The addition of 1 mM acid increased the corrosion potential dramatically, whereas 1 mM chloride decreased the pitting potential and 1mM sulfate had no measurable effects. Water appears to decrease the pitting potential due to the enhancement of aluminum ion hydrolysis at incipient pits. No important second order interactions were observed. The effects of acid on the corrosion potential and rate are explained in terms of mixed potential theory.
As the use of alcohol in automotive fuels increases due to environmental concerns, so does the need to characterize and understand the corrosion of metals and alloys in alcoholic solutions. Of particular importance are the effects of low levels of the impurities commonly expected in these fuels such as acid, chloride, sulfate, and water. In recent work on iron in methanolic solutions, Brossia and Kelly first developed a quantitative characterization of the electrochemical phenomenology of iron in methanolic solutions including the effects of common impurities  which led to a description of the mechanisms underlying the accelerating effects of acid and the inhibitory effects of water on the open circuit corrosion rate . One of the key elements of that work involved the use of statistical experimental design  to allow a quantitative analysis of the main (first-order) effects of the various solution additions, second-order interactions and an estimate of the experimental error. The error estimation permitted the statistical significance of each effect to be quantitatively assessed.
Brossia et al.  were able to explain the two primary empirical observations concerning the effect of acid on the corrosion of iron in methanol [4-6]: large increases in corrosion rate (by a factor of 15 due to a 1 mM acid addition ) and the corrosion potential (by 200 mV due to a 1 mM acid addition ). Brossia et al.  showed that the effects of acid were due to two separable phenomena; acid activates the originally passive iron surface, and proton reduction has substantially faster kinetics than oxygen reduction in acidified methanol. This latter effect dominates, leading to an increase in the corrosion potential with a subsequent increase in corrosion rate.
The addition of water inhibits the corrosion of iron in acidic methanol dramatically [1-6]. Brossia et al.  determined that proton reduction was under substantial mass transport control at the corrosion potential of iron. They then showed that the addition of water to acidified solutions reduced the corrosion rate predominantly by inhibiting the mobility of the proton, thus reducing the proton diffusivity and hence the diffusion limited current density of the dominant cathodic reaction in acidified methanol. This reduction in diffusion limited current density led directly to the decrease in the corrosion rate. The reduction in proton mobility is due to the preferential solvation of protons by water relative to methanol . At low concentrations of water, this limits the proton hopping that is important for conductivity.
In an extension of that work, the electrochemical behavior of aluminum alloy AA5052 has been studied and analyzed. By studying a valve metal such as aluminum which passivates with a thick film, information on the effect of the nature of the passive film on the corrosion of materials in methanol can be gained. In addition, AA5052 is being considered as a construction material for future automobile fuel tanks. Thus, an assessment of the effects of impurities on its corrosion behavior would have practical importance as well.
Limited previous work on the corrosion of aluminum alloys in methanolic solutions has been performed. Mansfeld  and Palit et al.  focussed on the corrosion of aluminum alloys in strongly acidified methanolic solutions. Upon anodic polarization of AA6061 above 0 V(SCE) in 0.1 N sulfuric acid, Mansfeld observed pitting with gas evolution from the pits. He proposed that the pitting was due to the presence of the sulfate ion. Palit et al.  also observed pitting of pure Al in acidic methanol in solutions open to air. Hronsky  measured weight loss for pure Al in 0.3 M HCl. Severe pitting was observed under open circuit conditions. Wing et al.  and Lash  also used coupon testing to investigate the behavior of various cast aluminum alloys in M85 (15% fuel, 85% methanol with 1.1mM acid, 0.1% water, and 0.07mM chloride). They found substantial weight losses and pitting of the cast Al alloys.
In order to better understand the corrosion of aluminum alloys in methanol, the present study was conducted. The corrosion and electrochemical behavior of AA5052 in a variety of methanolic solutions was studied and the effects of the addition of several impurities were quantified and explained in terms of the previous work on iron . Previous observations in this area are also rationalized within the framework of mixed potential theory.
Materials - Aluminum alloy AA5052 was supplied by Kobe Steel in the form of 0.50 mm thick sheet. Its composition was (in wt.%) 2.46 Mn, 0.22 Fe, 0.19 Cr, 0.1 Si, 0.02 Ti, remainder Al. Disk electrodes were cut from the sheet and wet polished with successively finer silicon carbide paper to a 600 grit finish.
Solutions - All test solutions were based on spectrophotometry grade, "Photrex" reagent methanol (J.T. Baker). All solutions contained 0.1 M anhydrous sodium perchlorate (Aesar) as a supporting electrolyte. Other solution additions included water, chloride, sulfate, and/or acid. The chloride was added as anhydrous lithium chloride (Fisher), the sulfate as anhydrous sodium sulfate (Aldrich), and acid as sulfuric acid. The water content of the solutions was measured via Karl Fischer titration (Mettler DL-18). A full factorial design was used to investigate the effects of the various impurities. Two levels of concentration were used, zero and 1 mM, except for water. The inherent water level of the methanol and that added from perchlorate led to a minimum water content for solutions of <0.06 wt.%. The effects of water were studied by the addition of water to a concentration of wt.%. No efforts were made to remove dissolved molecular oxygen as this would tend to evaporate large quantities of methanol, changing the concentrations of the dissolved substances. Previous work has identified the relevant cathodic reactions at occurring in the different solutions.
Testing Procedures - All experiments were conducted at room temperature. Electrochemical measurements were conducted with an E.G.&G. Princeton Applied Research (PAR) Versastat(TM) controlled by the PAR Model 352 software. A silver/silver chloride (SSC) electrode immersed in a compartment containing methanol with 0.1 M LiCl and 1.5 mass% water was used as the reference electrode. The reference electrode was separated from the working electrode solution by a Vycor(TM) frit. The SSC electrode has a potential of -29 mV vs. an aqueous saturated calomel electrode. The SSC was found to be very stable with respect to time and minimized the liquid junction potential . Cyclic polarization scans were conducted at a scan rate of 0.5 mV/s starting at an initial potential of -0.8 V(SSC). A vertex current density of 5 mA/cm2 was used. Automatic current interruption was used for on-line correction for ohmic drop.
Analysis Procedures - Each curve was analyzed for corrosion potential, corrosion current density and pitting potential. The corrosion current density was determined by use of PARcalc(TM) and checked manually by Tafel extrapolation. To quantitatively determine the significance of the effects of added impurities and the experimental error, the results of the full factorial design were analyzed with Number Cruncher Statistical Software (NCSS) ver.5.03 (licensed by J. L. Hintze). Performing duplicate experiments for each condition and comparing the results of all experiments containing the solution species of interest to all experiments that did not contain that species allowed for the separation of effects due to single impurities as well as any synergistic effects between species. Upon completion of the tests, each effect was statistically analyzed to determine its significance as well as to determine the experimental error.
In the base solution, AA5052 did not exhibit an active-passive transition as shown in Figure 1. This lack of an active-passive transition near the corrosion potential can be termed spontaneous passivity, and this term is used throughout this manuscript to refer to such behavior. This passive region was limited at +0.35 V(SSC) by pitting as shown in the photomicrograph of Figure 2. The effects on the polarization behavior of the addition of individual impurities (1 mM acid or 1 mM sulfate or 1 mM chloride or 0.5 wt.% water) are shown in Figure 3. Acid increased the open circuit potential and decreased the pitting potential of the AA5052, and increased the open circuit corrosion rate slightly. The addition of water led to a decrease in the pitting potential, while sulfate had little effect. Chloride decreased the pitting potential.
A summary of the first order effects of the impurities on the electrochemical parameters that characterize the corrosion process in both low and high water contents is shown in Figures 4 and 5. None of the solution species had a dramatic effect on the corrosion rate, either alone or in combination. No significant synergistic effects were observed. In all solutions studied in which acid was present, the corrosion potential rose dramatically. Acid was the only solution addition studied which affected the corrosion potential to a statistically significant degree. A solution containing acid and water led to an increased OCP and a decreased pitting potential as shown in Figure 6. None of the solution additions had a significant effect on the repassivation potential, with all values close to -0.44 V(SCE).
In order to investigate the importance of hydrolysis on the stabilization of pits, a study was performed in which AlCl3 was added and its effects on the acid concentration and the corrosion of the AA5052 were studied. The initial water concentration was 500 ppm, and the open circuit potential of the AA5052 was -0.5 V(SSC). Upon addition of the 0.05 M AlCl3, a white precipitate formed and the solution fizzed. The open circuit potential of the AA5052 fell to -1.1 V(SSC) and the polarization resistance decreased from 897 to 4.3 ohms-cm2. Post-test inspection of the surface showed that the attack was pitting, not uniform corrosion. The diffusion limited current density on Pt increased after the addition of the AlCl3 to approximately 1.3 mA/cm2, indicative of a acid concentration of approximately 60 mM. Water was then added to the solution to increase its concentration to 0.5 wt.%. This addition resulted in the reduction in the diffusion limited current density for proton reduction on Pt to 0.929 mA/cm2. The corrosion potential of the AA5052 increased to -0.722 V(SSC) and the polarization resistance also increased to 491ohms-cm2.
In all solutions studied, AA5052 exhibits spontaneous passivity, indicating that the air-formed film is stable at and near the open circuit potential (Figure 1). The correspondence of the measured pitting potential to the stabilization of localized attack was established via post-test examination (Figure 2). While the effects of sulfate, chloride, acid and water on the open circuit corrosion parameters (i.e., corrosion rate and corrosion potential) were qualitatively similar to those observed for iron previously [1,2], the effects are much smaller quantitatively. Of particular interest was the small effect of low levels of acidity. At these same levels of acid, iron spontaneously activated under open circuit conditions . These levels might reasonably be expected in a methanol fuel . Thus, it might appear that AA5052 tanks would perform better from a corrosion standpoint than steel in such an application. However, it should be noted that the presence of 1 mM acid did increase the corrosion potential of the AA5052 substantially (ca. 200 mV) while lowering its pitting potential by 100 mV (Figure 3). This decrease in the range of passivity increases the probability of stable pitting occuring under open circuit conditions.
As was also observed for iron , sulfate had no effect on the corrosion behavior of AA5052 (Figures 4 and 5). This is contrary to the conclusions of Mansfeld  who speculated that sulfate was the cause of the pitting of a variety of alloys (including AA6061) in acidified methanol. The experiments in that work were not designed to separate the effects of acid from those of the sulfate. In the present work, the use of sodium sulfate and a statistical experimental design allowed the individual effects of each species to be determined, indicating that AA5052 will pit in neutral methanolic solutions in the absence of sulfate upon sufficient anodic polarization. The low pitting potential observed by Mansfeld of 0 V(SCE) was most likely due to the high acidity (0.2 M) present in his solutions.
As shown in Figure 3, the addition of 1 mM acid increased the corrosion potential of AA5052 by 100 mV. This increase results from the introduction of an additional cathodic reaction (proton reduction) with faster kinetics than oxygen reduction (which is the predominant cathodic reaction in neutral methanol ). Unlike iron, the small acid addition does not activate the surface of the AA5052, so the increase in corrosion rate is quite modest and due solely to the slight increase in passive current density with potential (Figure 1). The addition of 1 mM acid also decreases the pitting potential of the AA5052 by 200 mV, which combined with the 100 mV increase in the corrosion potential, significantly decreases the passive range of the alloy (Figure 6). It would be expected that sufficient increase in the acid concentration would lead to the equivalence of the corrosion and pitting potentials, leading to pitting under open circuit conditions as observed by Hronsky  for pure Al in strongly acidified methanol. The high chloride content of Hronsky's solutions would also be expected to contribute to a decrease the pitting potential, as shown in this work. Wing and Evarts  also observed pitting of AA356 under open circuit conditions in a simulated 85% methanol fuel containing low levels of chloride, acid and water.
Water has a very small effect on the open circuit corrosion rate of AA5052, even in the presence of acid. This is not unexpected, due to the fact that the alloy surface remains passive. Under these conditions, the decrease in the limiting current for proton reduction due to the addition of water  will not have an effect on the corrosion rate, i.e., the corrosion potential of AA5052 is in the activation control region of the proton reduction reaction in methanol.
The deleterious effect of water on the pitting potential was unexpected. Generally, water is considered to improve the ability of the passive film to form in nonaqueous solutions, particularly under acidic conditions [1-6]. One explanation for the present results showing a clearly deleterious effect on water on the pitting potential is an increase in the stabilization of pitting that could result from hydrolysis of Al ions produced at metastable pits. It has become generally accepted that the pitting potential observed in potentiodynamic polarization scans is related to the conditions under which pits can propagate stably, rather than the potential at which they can initiate. Under such conditions, the high level of acidity required for localized corrosion site propagation must be achieved by hydrolysis of Al3+ at the incipient pit. As the water content of the solution increases, the hydrolysis becomes less reactant limited and stabilization of pitting occurs at lower potentials.
This proposal was tested by the study of the addition of AlCl3 on the acid content and the corrosion of the AA5052. The addition of the AlCl3 caused fizzing and the formation of a white precipitate. The hydrolysis of the aluminum salt would occur via:
AlCl3 + 3H2O = Al(OH)3 + 3H+ + 3Cl-.
The reaction proceeded as written as evidenced by the large increase in acid content (from 0 to 60 mM) for the 50 mM AlCl3 added. The large decrease in the open circuit potential and spontaneous pitting of the AA5052 upon the addition of the AlCl3 supports the contention that the stabilization of the pitting occurs by hydrolysis. The 500 ppm of water initially present is more than sufficient to allow the hydrolysis to occur to completion. The 60 mM of acid (and 150 mM chloride) led to the formation of stable pits which propagated at a very high rate due to the reduction on protons on the areas which did not pit. The massive decrease in the polarization resistance could not be used to calculate a penetration rate, due to the localized nature of the attack, but in conjunction with the post-test inspection and drop in the open circuit potential, can be used to support the contention that the pitting intiated upon addition of the AlCl3. The addition of further water did decrease the corrosion rate, as signified by the increase in polarization resistance from 4.3 to 491 ohms-cm2.
Thus, water plays several roles in organic solutions. It can decrease the mobility of the proton and thereby decrease corrosion rates when active corrosion occurs, as has been shown for iron  and for the actively pitting AA5052 in this study. In addition, for metals which strongly hydrolyze, water additions can lower the pitting potential in neutral solutions by enhancing the stability of metastable pits. The extent of the effect of water on the pitting potential in neutral organic solutions would be expected to depend upon the hydrolysis rates of the different metal ions in methanolic solutions, as the diffusion limited current density for proton reduction will be independent of substrate. Based on previous work on iron , in which neither water nor acid had deleterious effects on the pitting potential, it appears that hydrolysis effects are more important for aluminum. It would be of interest to use methanol solutions as test vehicles for studies of metal ion hydroylsis and its relation to localized corrosion.
The authors would like to acknowledge the technical discussions with C.S. Brossia (University of Virginia) as well as the financial support of Kobe Steel and equipment support from EG&G Princeton Applied Research. The assistance of J. Vining in the preparation of the HTML version of this manuscript is also gratefully acknowledged.
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