Al-Zn-In harcanan anotların elektrokimyasal özelliklerine çinko ve indiyum'un etkisi

Abstract

Aluminum alloys have becx)me a popular sacrificial anode material in cathodic protection, because of their high electrochemical equivalent, availability and reasonable price. Although aluminum has a very (-) potential in EMF series, naturally formed dense and protective oxide film shifts its potential to more (+) values and prevents its use as an anode material in the püre form. in early industrial application, to obtain high (-) potential values, püre Al alloyed with %5.5 Zn was used. Even though this alloy had a (-) potential close to that of Zn, it was not economical due to its very low current efficiency. Since then other minör alloying elements such as Hg, Sn, in were added to the alloy to overcome this deficiency. in present, Al sacrificial anode compositions have been fixed as Al-Zn-Hg, Al-Zn-Sn, Al-Zn-In alloys. However, Al-Zn-Hg anodes causes pollution. Al-Zn- Sn alloys requires heat treattnent for optimum perfbrmance characteristics. Al-Zn- In anodes, not having these disadvantages became the most widely used aluminum sacrificial anodes. To obtain high (-) potentials and high current efficiency, Al-Zn-In alloys with different compositions have been developed and generally patented. it is known that Zn and in have a synergistic effect in activating Al. However, for optimum properties, there is no systematic knowledge about how much indium must be added for a given Zn concentration. The aim of this study is to investigate systematically anode perfbrmance characteristics of different zinc and indium containing Al-Zn-In alloys and determine the relation between the performance of the alloy and its zinc and indium content. Activation of Al-Zn-In anodes occurs with local breakdovra of passivating oxide film in other words with pitting. Pitting behavior of Al-Zn-In alloys with different zinc and indium content were tested with cyclic polarization method. Also some alloys with critical indium contents for a given Zn concentration were exposed to galvanostatic current efficiency tests. The surfaces of corroded alloys were examined with scanning electron microscope. To contribute to the activation mechanism of indium, experiments in saturated A1C13 solutions were also conducted and the results were evaluated with respect to local acidification theory. overvoltage values of Al-Zn-In alloys were determined in saturated AICI3 solutions by using galvanostatic techniques. In galvanostatic experiments, programmable electrometer, Keithley 617, was added to potentiostatic experiment circuit. During the experiments, 1 mA/cm2 current density was applied until a fixed potential was obtained. When potential reached a fixed value, current was interrupted and men potential change was observed. During the experiments, potential versus time data was measured by a programmable electrometer. Potentiodynamic polarization curves in saturated AICI3 solutions showed that Zn had nearly 60 mV and indium nearly 370 mV more positive corrosion potentials than that of aluminum in pit-like solution. Also unlike aluminum, both zinc and indium exhibited low anodic overvoltage values in anodic region of the polarization curve. From the cathodic region of the polarization curve of indium, it was clear that indium had a higher hydrogen overvoltage in pit-like solution. From the galvanostatic experiments in saturated AICI3, it was found that the addition of indium to Al-%4.5Zn alloy increased the anodic overvoltage slightly. Addition of indium under the critical concentration (the concentration at which hysteresis band width increased and repassivation potential decreased sharply) did not change the corrosion potential in pit-like solution. But the addition of indium more than the critical concentration decreased corrosion potential. So it can be said that more dramatic decrease in pitting potentials in cyclic polarization experiments is due to the fact that indium decreased the corrosion potential inside the pit more dramatically. The pitting potentials obtained from cyclic polarization curves and theoretical pitting potentials calculated from the Galvele model were in accordance (with a deviation of 20 mV). The original results obtained from this study can be summarized as follows: 1- Zinc and indium has a synergistic effect in activating aluminum. For a given Zn content to cause a more dramatic decrease in potential, the necessary In/Zn ratio decreased with a logarithmic relation. 2- Data obtained by cyclic polarization technique, namely pitting potential, repassivation potential and hysteresis band width values can be useful tools in evaluating the potential and even, dissolution morphology characteristics of Al- Zn-In alloys. 3- In decreasing the corrosion potentials of Al-Zn-In alloys, even small concentrations of indium showed a substantial effect. However to decrease the pitting and repassivation potentials dramatically and to improve the dissolution morphology, critical In concentrations for a given Zn content were needed. 4- Pitting potential values of Al-Zn-In alloys showed a good correlation with the theoretical pitting potentials calculated according to localized acidification theory proposed by Galvele. After a critical indium content, sudden decrease in pitting potentials was caused by sudden decrease of corrosion potential inside the pits. 5- In Al-Zn-In alloys, when an indium content of %0.04 was reached, the alloys gave similar potential behavior, regardless of their Zn content. 6- Among the investigated Al-Zn-In alloys, Al-4.5Zn-0.026 In alloy gave the optimum properties considering potential, %efficiency and dissolution morphology Other results obtained in this study are: 7- Pits observed in the specimens showing wide hysteresis band was richer from the point of zinc and indium content. In this type of pits, indium concentrations were higher on the pit bottoms compared to bulk concentrations. Widening of hysterisis band and the change of pit morpology after a critical concentration of indium may be due to the formation of an indium salt film at the pit bottom and the dissoltution of the alloy through this film. 8- Low efficiency values of Al-Zn-In alloys containing no or insufficient amount of indium was due to intense hydrogen evolution, while low efficiencies of Al-Zn-In alloys with excessive indium was due to increased intergranular corrosion. 9- From the point of activation, probably, besides the indium in solid solution, indium precipitates contributed to the activation of the alloy. overvoltage values of Al-Zn-In alloys were determined in saturated AICI3 solutions by using galvanostatic techniques. In galvanostatic experiments, programmable electrometer, Keithley 617, was added to potentiostatic experiment circuit. During the experiments, 1 mA/cm2 current density was applied until a fixed potential was obtained. When potential reached a fixed value, current was interrupted and men potential change was observed. During the experiments, potential versus time data was measured by a programmable electrometer. Potentiodynamic polarization curves in saturated AICI3 solutions showed that Zn had nearly 60 mV and indium nearly 370 mV more positive corrosion potentials than that of aluminum in pit-like solution. Also unlike aluminum, both zinc and indium exhibited low anodic overvoltage values in anodic region of the polarization curve. From the cathodic region of the polarization curve of indium, it was clear that indium had a higher hydrogen overvoltage in pit-like solution. From the galvanostatic experiments in saturated AICI3, it was found that the addition of indium to Al-%4.5Zn alloy increased the anodic overvoltage slightly. Addition of indium under the critical concentration (the concentration at which hysteresis band width increased and repassivation potential decreased sharply) did not change the corrosion potential in pit-like solution. But the addition of indium more than the critical concentration decreased corrosion potential. So it can be said that more dramatic decrease in pitting potentials in cyclic polarization experiments is due to the fact that indium decreased the corrosion potential inside the pit more dramatically. The pitting potentials obtained from cyclic polarization curves and theoretical pitting potentials calculated from the Galvele model were in accordance (with a deviation of 20 mV). The original results obtained from this study can be summarized as follows: 1- Zinc and indium has a synergistic effect in activating aluminum. For a given Zn content to cause a more dramatic decrease in potential, the necessary In/Zn ratio decreased with a logarithmic relation. 2- Data obtained by cyclic polarization technique, namely pitting potential, repassivation potential and hysteresis band width values can be useful tools in evaluating the potential and even, dissolution morphology characteristics of Al- Zn-In alloys. 3- In decreasing the corrosion potentials of Al-Zn-In alloys, even small concentrations of indium showed a substantial effect. However to decrease the pitting and repassivation potentials dramatically and to improve the dissolution morphology, critical In concentrations for a given Zn content were needed. 4- Pitting potential values of Al-Zn-In alloys showed a good correlation with the theoretical pitting potentials calculated according to localized acidification theory proposed by Galvele. After a critical indium content, sudden decrease in pitting potentials was caused by sudden decrease of corrosion potential inside the pits. 5- In Al-Zn-In alloys, when an indium content of %0.04 was reached, the alloys gave similar potential behavior, regardless of their Zn content. 6- Among the investigated Al-Zn-In alloys, Al-4.5Zn-0.026 In alloy gave the optimum properties considering potential, %efficiency and dissolution morphology Other results obtained in this study are: 7- Pits observed in the specimens showing wide hysteresis band was richer from the point of zinc and indium content. In this type of pits, indium concentrations were higher on the pit bottoms compared to bulk concentrations. Widening of hysterisis band and the change of pit morpology after a critical concentration of indium may be due to the formation of an indium salt film at the pit bottom and the dissoltution of the alloy through this film. 8- Low efficiency values of Al-Zn-In alloys containing no or insufficient amount of indium was due to intense hydrogen evolution, while low efficiencies of Al-Zn-In alloys with excessive indium was due to increased intergranular corrosion. 9- From the point of activation, probably, besides the indium in solid solution, indium precipitates contributed to the activation of the alloy. overvoltage values of Al-Zn-In alloys were determined in saturated AICI3 solutions by using galvanostatic techniques. In galvanostatic experiments, programmable electrometer, Keithley 617, was added to potentiostatic experiment circuit. During the experiments, 1 mA/cm2 current density was applied until a fixed potential was obtained. When potential reached a fixed value, current was interrupted and men potential change was observed. During the experiments, potential versus time data was measured by a programmable electrometer. Potentiodynamic polarization curves in saturated AICI3 solutions showed that Zn had nearly 60 mV and indium nearly 370 mV more positive corrosion potentials than that of aluminum in pit-like solution. Also unlike aluminum, both zinc and indium exhibited low anodic overvoltage values in anodic region of the polarization curve. From the cathodic region of the polarization curve of indium, it was clear that indium had a higher hydrogen overvoltage in pit-like solution. From the galvanostatic experiments in saturated AICI3, it was found that the addition of indium to Al-%4.5Zn alloy increased the anodic overvoltage slightly. Addition of indium under the critical concentration (the concentration at which hysteresis band width increased and repassivation potential decreased sharply) did not change the corrosion potential in pit-like solution. But the addition of indium more than the critical concentration decreased corrosion potential. So it can be said that more dramatic decrease in pitting potentials in cyclic polarization experiments is due to the fact that indium decreased the corrosion potential inside the pit more dramatically. The pitting potentials obtained from cyclic polarization curves and theoretical pitting potentials calculated from the Galvele model were in accordance (with a deviation of 20 mV). The original results obtained from this study can be summarized as follows: 1- Zinc and indium has a synergistic effect in activating aluminum. For a given Zn content to cause a more dramatic decrease in potential, the necessary In/Zn ratio decreased with a logarithmic relation. 2- Data obtained by cyclic polarization technique, namely pitting potential, repassivation potential and hysteresis band width values can be useful tools in evaluating the potential and even, dissolution morphology characteristics of Al- Zn-In alloys. 3- In decreasing the corrosion potentials of Al-Zn-In alloys, even small concentrations of indium showed a substantial effect. However to decrease the pitting and repassivation potentials dramatically and to improve the dissolution morphology, critical In concentrations for a given Zn content were needed. 4- Pitting potential values of Al-Zn-In alloys showed a good correlation with the theoretical pitting potentials calculated according to localized acidification theory proposed by Galvele. After a critical indium content, sudden decrease in pitting potentials was caused by sudden decrease of corrosion potential inside the pits. 5- In Al-Zn-In alloys, when an indium content of %0.04 was reached, the alloys gave similar potential behavior, regardless of their Zn content. 6- Among the investigated Al-Zn-In alloys, Al-4.5Zn-0.026 In alloy gave the optimum properties considering potential, %efficiency and dissolution morphology Other results obtained in this study are: 7- Pits observed in the specimens showing wide hysteresis band was richer from the point of zinc and indium content. In this type of pits, indium concentrations were higher on the pit bottoms compared to bulk concentrations. Widening of hysterisis band and the change of pit morpology after a critical concentration of indium may be due to the formation of an indium salt film at the pit bottom and the dissoltution of the alloy through this film. 8- Low efficiency values of Al-Zn-In alloys containing no or insufficient amount of indium was due to intense hydrogen evolution, while low efficiencies of Al-Zn-In alloys with excessive indium was due to increased intergranular corrosion. 9- From the point of activation, probably, besides the indium in solid solution, indium precipitates contributed to the activation of the alloy.