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2010BEHAVIOR OF AL-ZN-IN ANODES AT ELEVATED TEMPERATUREGRANT GIBSONGIBSON APPLIED TECHNOLOGY AND ENGINEERING, LLC16360 PARK TEN PLACE, STE 206 HOUSTON TX 77084gtgibson@ABSTRACT There has been little theoretical and experimental data on the behavior of Al-Zn-In anodes in elevatedtemperature seawater. The majority of the work was aimed at giving the cathodic protection engineer somedirection on the performance of anodes in harsh environments without discussion of the theoretical aspects ofaluminum activation and failure at elevated temperatures. A discussion of previous research is reviewed withrespect to aluminum anode performance in seawater at elevated temperatures. Experiments were conducted ona commercially available aluminum anode at 20, 40, 60, and 80 C in 3.5% NaCl. Current capacity, corrosionmorphology, and hydrogen evolution results are presented and a discussion on anode activation mechanisms isreviewed in relation to the results. This paper focuses on aluminum zinc indium activated anodes.ANODE CHEMISTRY DEVELOPMENTAs a sacrificial anode, aluminum is a very highly favored metal because it possesses a thermodynamicpotential of –1.663V SHE, a high electrochemical equivalent, low density, and a high theoretical current capacity(2980 A ⋅hr/kg). In addition, aluminum is not a precious metal making it economically feasible to use as asacrificial anode. The drawback of aluminum is that its theoretical driving potential is not realized. Aluminumadopts a potential of –700mV SCE in seawater due to the formation of a protective oxide layer 1. This potential isnot active enough to polarize steel to its protection potential. Another drawback is aluminum corrodes by pitting ina seawater environment. While most of the current produced by pitting may well be used in protection of astructure, the anode may become perforated and may lose aluminum by mechanical detachment. Thus, thecurrent capacity of the anode would be diminished.The first aluminum anodes used were binary alloys of aluminum-zinc 2,3,4 and aluminium-tin 4,5. Doremus andDavis 4 give the nominal composition of an Al-Zn alloy as: Zn, 5.0 to 6.0 weight percent; Fe, 0.17% max; Cu,0.02% max; Si, 0.10% max; and Al, remainder. It was claimed to have a capacity of 1540 to 1598 A ⋅hr/kg and anoperating potential of –1.00V Ag/AgCl. The Al-Zn anode was comparable to zinc and was commonly used as areplacement for zinc when weight considerations were paramount. Hine and Wei 5 reported an Al-Sn anode withan operating potential of -1.30V SCE, but upon coupling to a structure, the anode would become polarized tounacceptable levels. It was later found that Al-Sn anodes depended strongly on composition of material and heattreatment. With close manufacturing control, Al-Sn anodes with current capacities of 1050 A ⋅hr/kg and operatingpotentials of –1.1 to –1.2V SCE were realized. Even though these aluminum alloys gave a slight advantage overzinc, they were not readily accepted in the field because of the lack of published data on their performance.Then in 1966, Newport and Reding 6 reported the results of tests on 2500 alloys investigating the effects ofalloying aluminum with many different elements. They found that alloying with Cu and Mn resulted in a potentialmore noble than unalloyed aluminum. Zn, Cd, Mg, and Ba shifted the potential of aluminum negative direction by©2010 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACEInternational, Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.Paper No.10396100 to 300mV and Ga, Hg, Sn, and In shifted the potential by 300 to 900mV. They found that thedepassivators, or activators, had some common features. The elements had low melting points, a low solubility inAl (except Zn and Mg), and most activating elements did not form intermetallic compounds with Al. Currentefficiency tests showed that none of the binary alloys possessed suitable efficiencies, but it was found that ternaryalloy compositions could greatly enhance the efficiency. Ternary alloy trials resulted in an Al-Zn-Hg alloy that had an operating potential of –1.05V SCE, a current capacity of around 2800 A⋅hr/kg, and a uniform corrosion pattern. They also stressed the control of the purity of the aluminum stock used to produce the alloys. Lowering the purityfrom 99.9% to 99.7% resulted in a loss of 25% in efficiency, but little change in operating potential. Schrieber andReding9 confirmed the laboratory results in field testing in seawater for up to 660 days.In the same month, Sakano, Toda, and Hanada7 reported on a Al-Zn-In sacrificial anode. They also stressedthe importance of purity in the aluminum stock noticing a difference in current capacity by switching from 99.99%purity to 99.85% purity. The best results for a viable anode were obtained with 99.85% purity with 0.025% In,2.52% Zn, and 0.01% Cd. The cadmium was added to soften the corrosion product and improve the corrosioncharacteristics. They do not give the preliminary results showing the effects of different alloy compositions.Since 1966, three ternary alloys have evolved namely tin8, mercury6,9, and indium7,10,11,12,13,14,15 which are allalloyed with zinc. The tin based alloys are no longer preferred because they require very accurate compositions,heat treatments, their parameters vary heavily with current density, suffer non-uniform corrosion, and adherentcorrosion products16. While mercury anodes out-perform both tin and indium anodes, they are no longer usedbecause mercury is hazardous to the environment17,18. Indium based anodes are now the preferred materialbecause they are non-hazardous, experience uniform dissolution, and are easy to manufacture.Most recently, Umay et al19 investigated the interrelation between zinc and indium in aluminum anodes.Anodes with a compositional range of zinc between 1.5% and 5.5% and indium between 0.01% and 0.04% werecast using 99.75% pure aluminum. The impurity content was 0.1% iron, 0.07% silicon, and 0.0005% copper.Figure 1 depicts the anode efficiency as a function of indium and zinc content. The graphs show that at eachzinc content there is an indium content that produces a maximum efficiency. The 1.5, 3.5, and 5.5% zinccompositions with various indium contents produce optimized efficiencies that are local and highly dependent onindium concentration. The 4.5% zinc composition with indium concentrations of 0.017% and 0.026% producesmore of a plateau at an efficiency of 82%. In terms of dissolution morphology, at low indium contents, pits werelocalized, narrow, and deep. At intermediate indium concentrations, the pits were homogeneously distributed andshallow with a smooth pit bottom. At the higher indium concentrations, pit distribution was homogeneous and pitstructure was shallow with deep intergranular corrosion at pit bottoms. The compositional limits are near those ofthe commercial anodes marketed today20, but with slight differences. The difference in anode performance by Umay et al and present commercial anodes can be attributed to the impurity of the stock material.The realization of high performance anodes depends highly on the purity of the aluminum stock with indiumand zinc being the main alloy addition. Much work has been directed in the past two decades to find smalladditions to the stock to counter the deleterious effects of using lower purity aluminum, which would reducemanufacturing costs.Iron and copper are the two most detrimental impurities. They act as residual flaws on the surface of theanode. Also, copper and iron are cathodic to the aluminum matrix and have high exchange current densities forthe electrolytic evolution of hydrogen21. A selection of some of the elements found in aluminum is given in Table 1. The potentials of some of the intermetallics formed in the aluminum matrix are given in Table 222.Table 1. Exchange Current for hydrogen evolution21.Element -Logi o,HIron 5.6Copper 7.8Aluminium 8.0 Zinc 10.5Indium 11.3Mercury 12.3It is those impurities with lower hydrogen overvoltages than aluminum and more noble potentials that hamper the current capacity. Zinc and mercury both have higher hydrogen overvoltages than aluminum although mercury is cathodic to aluminum it is not detrimental. Iron and copper have low hydrogen overvoltages and are highly cathodic to the aluminum matrix. It can be envisioned that as lower purity aluminum stock is used, an increase in cathodic residual flaws is present, and more current is wasted due to hydrogen evolution on the surface of impurities rather than going towards protecting the structure.Table 2. Potential of intermetallics and elements in a NaCl-H2O2 Solution22.Intermetallic Potential (V) Vs. 0.1N CalomelCu -0.11Si -0.17CuAl2 -0.44FeAl3 -0.47FeMnAl12 -0.75 Al (high purity) -0.76Zn -1.26 The effects of copper, iron, and silicon on the current capacity of an Al-Zn-Sn anode developed by Ponchel and Horst8 are shown in Figure 2. As shown, copper has by far the most detrimental effect at weight percentages as low as 0.01%. Iron can be tolerated up to concentrations of around 0.1%. Silicon has a more gradual effect and can be tolerated at higher concentrations. It was also mentioned that nickel has an effect similar to copper on the current capacity. The work was done on Al-Zn-Sn anodes and the data may be subject to many errors as mentioned earlier16. The aim was to show the effects and importance of individual impurities at very low levels. Hence, it is not surprising that Ponchel and Horst8 set strict controls on the purity of aluminum stock: Fe, 0.1% max.; Si, 0.1% max; Cu, 0.009% max.In contrast, Sakano et al7 (Al-Zn-In) found that iron is beneficial for the current capacity up to a certain level. Exceeding this level would result in a loss in capacity. Copper was found to be very detrimental at a concentration of 0.019%. They also point out copper affects the corrosion characteristics, causing pitting and adherence of corrosion products. In regards to silicon, they found that it decreased the efficiency to 86% from around 92%. Silicon had the harmful effects of lowering the anode potential, irregular corrosion patterns, and lengthy induction periods (time for anode to reach operating potential). With the above observations, Sakano et al set their impurity limits as Fe, 0.1% max; Si, ~0.04%; Cu, <0.02%.R. F. May12 investigated the beneficial effects of iron in high performance Al-Zn-In anodes. More uniform corrosion was observed when specific additions of iron were made to high purity stock. This effect was offset by a loss in efficiency. May argues that using a higher purity stock drives up the cost of anode and also reduces the anode activation pattern. From Table 2, adding small amounts of manganese would alloy with the iron to produce intermetallics that would be near the potential of the matrix, thus reducing the local cell action effects of the iron. May suggests that the iron intermetallic phases, or residual flaws, promote initial dissolution. He coins the term “activation initiators.” May concludes that by adding manganese to the alloy, higher iron levels can be tolerated, and uniformity of anode attack can be maintained. A specific alloy composition was not given, but May does mention European Patent 018712723. It specifies a maximum effect with a manganese weight percent between 0.01% and 0.20%.Zamin24 studied Al-Mn binary alloys and the effect of iron concentration on corrosion resistance. He found that Mn increases the corrosion resistance of Al and increasing the Mn concentration can alleviate the detrimental effect of iron. He found that the presence of Mn alone has no effect but depends solely on the Mn/Fe ratio. Work by Klinghoffer and Linder25 showed that a ratio of 1:1 was the most beneficial. Balancing the Mn/Fe ratio, they were able to produce an Al-Zn-In (Zn, 4.0%; In, 0.24%) anode with up to 0.20% Fe. A summary of their work is shown in Figure 3. The current capacity increases up to a ratio of 1:1 and then decreases dramatically. The operating potential is not affected until a ratio of 1:1 is exceeded where it becomes more noble.Googan26 studied aluminum and aluminium-1.0% Zn with iron additions between 0.05% and 1.0%. The work found that iron is mostly segregated as a grain boundary intermetallic phase FeAl3. He proposes the “Iron-Dissolution-Electrodeposition” process in which dissolution of FeAl3 results in ferrous ions depositing to form a layer of pure iron on the alloy surface. The pure iron deposit stimulates the cathodic reaction kinetics, which results in increased local cell action and ultimately a loss in efficiency. Seeking a solution to the problem, Googan found the 0.5% germanium overcomes the deleterious effects of the iron. In contrast to manganese, the germanium does not alter the FeAl3 intermetallic but instead interferes with the deposition of the ferrous ions. The phenomenon would then inhibit the production of low hydrogen overvoltage iron deposits.In 1978, Smith, Reding, and Riley10 developed commercially available anode1 consisting of Al-Zn-In-Si. They did not report on the iron or copper contents but did state that the silicon sequesters the iron to form an intermetallic that is less cathodic than pure iron or silicon. The silicon works in the same way as manganese in that it is the ratio of silicon to iron which is important. Presently, Al-Zn-In alloys can be produced with up to 0.5% iron with 85% efficiency17.1 Galvalum IIIKobayashi and Tamura15 reported the results of tests of various alloy additions. Figure 4 shows the effect of silicon on the operating potential and current capacity of Al-Zn-In alloy2 which has an iron impurity concentration of 0.14%. The silicon ranges between 0.07% and 0.4%. A maximum appears in the current capacity at about 0.15% silicon. It can be concluded that the optimum situation is an iron-silicon ratio of 1:1, which is close to that of the manganese.BEHAVIOR OF ANODES AT ELEVATED TEMPERATUREAt present there has been little theoretical investigation into the behavior of aluminum based anodes in elevated temperature seawater. The majority of the work was aimed at giving the cathodic protection engineer some direction on the performance of aluminum based anodes in harsh environments. The main environment studied was seabed mud. Elevated temperature seawater tests are few and far between and their results are rarely discussed. A review and an in-depth study of indium activated anodes in mud has been completed by Erricker27 and will not be discussed here. This section presents results of indium activated anodes in elevated temperature seawater and brine.Murai, Tamura, and Miura28,29 studied a Al-Zn-Sn-Bi-Ga alloy in seawater from 0 to 80°C at 3 and 10 A/m2 for 240 hours. The anode potential remained constant at –1.080V SCE from 10 to 50°C at 3 A/m2. At 10 A/m2, the anode polarized to –1.055V SCE and remained constant from 20°C till around 60°C. The potential became more noble above 60°C reaching approximately -1.020V SCE at 80°C, with not much difference between 3 and 10 A/m2. They attribute the change in potential at higher temperature to the growth of boehmite. The current efficiency was 95% between 0 and 40°C. At 70°C, the current efficiency is 88% for 3 A/m2 and 91% for 10 A/m2. When the seawater temperature was increased to 80°C, the current efficiency decreased rapidly to 40% for 3 A/m2 and 51% for 10 A/m2. The corrosion pattern was identical over the temperature range 0 to 60°C showing general attack. Above 70°C, the corrosion pattern was more localized. They do not discuss any possible explanation for the rapid loss in efficiency above 70°C.Smith, Reding, and Riley10 tested an Al-Zn-In-Si alloy in 75°C 15% brine. At a current density of 2.1 A/m2, the alloy delivered 2000 A⋅h/kg and an operating potential of –1060 mV SCE. The same alloy delivered 2500 A⋅h/kg and –1080 mV SCE in full seawater tests. Additional studies in low salinity seawater showed that the operating potential of the alloy became more positive as the salinity decreased. This indicates that the potential of the alloy might be related to resistivity of the environment much like the breakdown potential is on the chloride activity. No change in current capacity was noted with change in salinity.Schrieber and Murray30,31 report the results of 30 day tests on Al-Zn-In-Si and Al-Zn-Hg anodes at 38, 66, and 93°C using current densities of 2.2 A/m2 and 6.5 A/m2 in 3 and 7% NaCl. In general, the anode potential became more noble with increasing temperature. Increasing the brine concentration lowered the operating potential. At 2.2 A/m2, the current capacity increases slightly with increasing temperature from 38 to 66°C, whilst the current capacity is relatively unchanged for the higher current density over this temperature range. In agreement with Murai et al28,29, the current capacity decreases rapidly above a temperature near 70°C. The data for 2.2 A/m2 is summarized in Figure 5. The corrosion morphology was uniform for both alloys at 38 and 66°C. In line with the decrease in current capacity at 93°C, the morphology was uneven and localized. Schrieber and Murray do not discuss possible mechanisms for the behavior of the anodes with increasing temperature.Kobayashi and Tamura32 reported similar results on an Al-Zn-In alloy. They conducted their tests in artificial seawater between 5 and 100°C at 10 A/m2 for 240 hours. The current capacity for the alloy was approximately 2620 A⋅h/kg between 5 and 65°C. Above 80°C, the current capacity decreased to about 1000 A⋅h/kg at 100°C. The operating potential of the alloy was –1.10V SCE until 50°C. Above 50°C, the operating potential increases to –1.00V SCE at 100°C. They state that the growth of boehmite may cause the operating potential to become more noble. Corrosion morphology change with temperature was not discussed.Wroe and May33,12 only reported results for 5 and 95°C. At the elevated temperature, Al-Zn-In alloy gave a current capacity of 474 A⋅h/kg and an operating potential of –1.074V SCE. They claim that a high magnesium, low manganese alloy delivers 1125 A⋅h/kg and –1.05V SCE at 95°C. These results are compared against those2 Alanode IIIat 5°C which gave values of –1.120V SCE and 2634 A⋅h/kg for the Al-Zn-In and –1.130V SCE and 2794 A⋅h/kg for the high Mg low Mn alloy.Fischer34 studied the effects of temperature and depth on the behavior of Al-Zn-In anodes in natural seawater. Current densities from 0.5 to 5 A/m2 and pressures from 1 to 50 bar were used in a 60 day test. In agreement with previous cited work, the operating potential of the anode increased with increasing temperature. The current capacity was largely dependent on the current density and temperature. At 0.5 A/m2, increasing the temperature from 10°C to 70°C increased the current capacity from about 2000 A⋅h/kg to around 2200 A⋅h/kg. At 5 A/m2, increasing the temperature from 10°C to 70°C gradually decreased the current capacity from about 2080 A⋅h/kg to 1400 A⋅h/kg. The results at 0.5 A/m2 are in agreement with Schrieber and Murray30,31 , being that the current capacity increased with temperature for low current densities. Fischer did not test above 70°C. At ambient temperature and pressure, Fischer observed an even distribution of pits over the anode surface. At higher temperatures, the corrosion morphology was much more irregular with deep large pits and grooves. In general, at higher temperature, the corrosion morphology was more localized and increasing the pressure further localized the attack.At present, not much work has been directed to the mechanism behind the behavior of aluminum-based anodes at higher temperatures. From the above discussion, some observations need to be addressed. First, the operating potential becomes more noble with increasing temperature. Second, the current capacity either increases with increasing temperature for low current densities or decreases with temperature if the current density is above some value. Third, the corrosion morphology changes from uniform to very localized with increasing temperature.EXPERIMENTAL WORKSacrificial anode parameters form the core of cathodic protection design. The two most important electrochemical parameters are the operating potential and the current capacity. Other factors are the corrosion morphology, long term output characteristics, and anode structure. Short-term electrochemical tests are used to measure the electrochemical properties of anode material, but the most accurate test is a long term field test35. The problems with long-term tests are the time to complete the test and the ability to obtain reproducible results. Thus, anode producers have a vested interest in short term testing of sacrificial anodes.The most popular of the various tests is the galvanostatic test which involves the passage of a constant current through the anode test specimen for a specified time whilst measuring its potential. The current capacity is calculated by the weight loss of the sample and the total amount of charge passed (A⋅h). The weight loss method measures the sum of all the sources of inefficiency. A different approach for measuring efficiency using the galvanostatic test is to measure the volume of hydrogen evolved from the sample over a known period of time.Both current capacity tests were performed on the anode material alloy C3 and alloy X at room and elevated temperatures. Two types of alloys were tested: alloy C (commercially available chemistry) and for comparative purposes alloy X (non-commercial alloy – made for scientific research). The nominal composition of the alloys is given in Table 3. Alloy X is nominally the same as alloy C but with a lower composition in indium. It was aimed that in comparing the two alloys some insight into the activation mechanism of indium would be gained.The alloys were received as sticks and samples were machined using a band saw and finished to the desired height with a lathe. After the samples were cut, preparation was performed as follows. First, the samples were scrubbed using a nylon brush with soap and water. Afterwards, the samples were cleaned with acetone and a nylon brush. Finally, the samples were dried in an oven for 15 minutes at 120°C.A general schematic diagram of the weight loss galvanostatic test is shown in Figure 6. The main components are the test sample, electrolyte bath, cathode, coulometer, and galvanostat.All tests were performed in 15 liters of de-ionized water with 3.5% NaCl. The NACE36 and DNV37 prescribe using artificial seawater or circulated real full strength seawater. Due to the amount of tests and volume of electrolyte needed, this was deemed uneconomical. The solution was at room temperature which varied between 17 and 22ºC. The tests performed at room temperature are designated as 20ºC.3 Alloys were provided by Impalloy Alloys Ltd. Alloy C is a generic name for a commercially available alloy Impalloy III. Alloy X is a generic name for an alloy specially cast for experimental purposes and is not commercially available.Table 3. Nominal compositions of alloys (weight percent). Element Alloy C Alloy XZinc 4.67 4.91 Indium 0.0175 0.0039Iron 0.083 0.047Silicon 0.86 0.049Copper 0.0024 0.0007Gallium 0.0082 0.0121Titanium 0.0198 0.0239Aluminum Balance BalanceExperimental Procedure15 liters of electrolyte per tank were prepared the day before the test was to begin. On the following day, the pH was set to 8.2 by addition of HCl or NaOH. Next, the samples and copper plating rod were weighed. The samples were placed in the centre of the cathodes, which were placed in the centre of the tanks. The sample level was centered between the bottom of the tank and the electrolyte level. With all the components of the test connected in series, the current was set with the galvanostat as to give the desired current density. The potential of the anode was measured with a saturated calomel electrode (SCE) connected to a saturated KCl agar-agar Luggin probe and a high impedance digital voltmeter. The SCE reference electrode remained at room temperature for all tests. From the start of the test, the potential was measured at 1,3,24, 96, and 168 hours. When the test was complete (168 hours), the galvanostat was turned off and the samples were removed from the electrolyte. They were cleaned using distilled water and a nylon brush, rinsed in acetone, and dried at 120°C for 15 minutes. The copper plating rod was rinsed with distilled water, then acetone, and also dried at 120°C for 15 minutes. The copper plating rod and the samples were re-weighed and the current capacity was calculated by the following formula:)( )( 8433.0)/( grams Sample of Loss Weight grams Rod Copper Gain Weight kg h A Capacity Current ⋅=⋅ The samples were finally categorized and stored in plastic bags for later examination.Elevated Temperature Tests - ExperimentalFigure 7 shows the internal components of the heat jacket, the top cover, and a photograph of the heat jacket and temperature controller. The internal container for the electrolyte was a carbon steel cylinder of 15.5cm inner diameter, 25.5cm depth, and wall thickness of 0.5cm. 4 liters of 3.5% NaCl electrolyte was used. Surrounding the steel cylinder was galvanized steel mesh and aluminum foil. Heater tape was formed around the mesh and foil and itself fully enclosed by a layer of aluminum foil. The next layer was a covering of fiberglass cloth wrap followed by a layer of pink housing fiberglass insulation. Lastly, steel mesh was wrapped around to maintain shape. The exterior was covered by packaging tape to prevent contact with the fiberglass and for aesthetic reasons.The top was machined from a polypropylene sheet of 2.5cm thickness and diameter of 17.7cm. A lip was also machined to the inner diameter of the steel cylinder to insure a snug fit. The centre of the top was bored to a diameter of 4.5cm. Three other holes (1.1cm diameter) were drilled and tapped for cable glands. The centre bore allowed installation and removal of the sample from the experimental kit. The three cable glands allowed a thermometer and a Luggin probe to be fitted and the third was a spare.The temperature was controlled by a capillary thermostat. The thermostat is operated from a remote bulb connected by a 1m capillary. The temperature range for the thermostat was 0 to 120°C. The thermostat, indicator light, and connections to heater tape were installed in a housing fitted with a 3-amp fuse. The final product is shown in Figure 7.The procedure for the elevated temperature experiments are the same as for the room temperature experiments including duration of test. The pH of the electrolyte was adjusted to 8.2 before heating. It usually required 3 to 7 hours for the desired temperature to be reached. The samples were immersed in the solution after it reached the desired temperature. The SCE reference electrode remained at room temperature for all tests. This may give rise to a minor thermal liquid junction potential. Ashworth and Fairhurst 38 reported the magnitude to be ≤5mV. The error would be present in all tests and was deemed acceptable. The temperatures studied were 40, 60, and 80°C.Hydrogen Evolution Tests - ExperimentalHydrogen evolution tests were designed to quantify the amount of inefficiency due to local cell action. The efficiencies measured could then be compared to the weight loss experiments with the aim of separating the total efficiency loss into mechanical loss, negative difference effect, and local cell action.Hydrogen evolution experiments at room temperature were performed using the weight loss apparatus described above. An inverted 50mL burette fitted with a funnel was used to collect the hydrogen gas. The samples were started at the desired current density. Hydrogen collection was started 24 hours after the start of the test. The burette and funnel were placed symmetrically over the sample with funnel level with the top of the sample. The electrolyte was drawn into the burette with a pipette filler. Collection time was between 3 to 24 hours depending on the amount of gas collected. The efficiency was calculated by the following formula⎥⎦⎤⎢⎣⎡⋅+=(min))(132 %T mL V Current Applied Current Applied Efficiencywhere V is the change in volume of gas collected over the time period T .For elevated temperatures, the apparatus shown in Figure 8 was used. A 5 litre Quick Fit jar was fitted with steel mesh. A 5-inlet Quick Fit top was used to hold the titanium rod to support the sample, support the burette, and allow electrical connection to the cathode. The whole jar was then submerged into an elevated temperature bath thermistatically controlled to ±2°C. The samples were run for 24 hours prior to gas collection. The temperature of the burette was measured by taping a thermocouple to the burette in the area of gas collection and the constant of 132 was adjusted for the temperature.EXPERIMENTAL RESULTSThis section presents the results of how current density and temperature affect the current capacity, operating potential, and corrosion morphology of alloy C and alloy X. The current capacity was measured by the weight loss and hydrogen evolution methods over the current densities of 0.1 to 6 A/m 2. For comparative purposes, alloy X was tested only at 3 A/m 2 at elevated temperature. The aim was to gain some insight into the mechanism of indium activation, the mechanism of efficiency loss, and assess the feasibility of using a commercially available anode alloy at elevated temperature.Results at 20°CThe operating potential for alloy X versus current density after 7 days exposure is shown in Figure 9. Averaged over all the current densities, alloy X had a mean operating potential of -977±3 mV SCE. One sample was run at a current density of 60 A/m 2 and had an operating potential of -930mV SCE showing that a considerable current density is needed to polarize the alloy.The addition of indium (Impalloy III) decreased the operating potential by over 125mV to -1110±3 mV SCE. The operating potential versus current density after 7 days exposure for alloy C is also shown in Figure 10. Again, the operating potential is independent of the current density measured. In line with alloy X, running alloy C at 60 A/m 2 resulted in polarization of ~43mV to -1070mV SCE.Operating potentials measured in this study agree well with those given in the literature. Doremus and Davis 4 & Hine and Wei 5 reported the potential of an Al-5%Zn alloy to be -780mV SCE. Sakano et al 7 reported a potential of -1100mV SCE for an Al-Zn-In alloy in 1966. Schrieber and Murray 31 measured a value of -1.108mV AgCl Seawater for an Al-Zn-In-Si anode in artificial seawater. This is close to a value of -1090mV SCE reported。