Anti-Corrosion Methods and MaterialsThe kinetics of anodic and cathodic polarization of aluminium and its alloysDr. Zaki Ahmad

Article information:To cite this document:Dr. Zaki Ahmad, (1986),"The kinetics of anodic and cathodic polarization of aluminium and its alloys", Anti-CorrosionMethods and Materials, Vol. 33 Iss 11 pp. 4 - 11Permanent link to this document:http://dx.doi.org/10.1108/eb020492

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The kinetics of anodic and cathodic polarization of aluminium and its alloys by Dr. Zaki Ahmad, Mechanical Engineering Department University of Petroleum & Minerals Dhahran 31261, Saudi Arabia

The relatively complex corrosion mechanism of aluminium has been studied by several authors. Corrosion of aluminium occurs only when the metal protective oxide layer is damaged and when the repair mechanism is prevented by chemical dissolution1. Polarization methods have been extensively used to investigate the mechanism of localised corrosion and processes that lead to localised corrosion. The potential-pH diagrams are shown in Fig. 1A. In using potentiostatic techniques, the potential is controlled and current is determined as the independent variable2. Potentiostatic and potentiody-namic techniques have been applied by several authors to study the corrosion of aluminium in different environment3_8. Both anodic and cathodic polarization curves have been used to interpret the kinetics of pitting corrosion of aluminium in chloride containing environments. Both the anodic and cathodic process are complex and the interpretation of the anodic and cathodic polarization curves of aluminium is often tedious. The situation arises partly from the fact that the role of film formation on the kinetics of corrosion is not clearly understood. Previously there is not established mechanisms of initiation and propagation of pits in aluminium and its alloys. Several parameters such as pitting potential, breakdown potential, active passive transition potential, related to the pitting process of aluminium, are full of controversy9_11. Numerous references on the above can be found in literature12_15).

It is the purpose of this paper to show how the anodic and cathodic polarization curves can be used to interpret the corrosion processes.

Anodic polarization A typical anodic polarization curve is shown in Fig. 1B. The curve is marked by two important regions, A and B. The region A shows a gradual increase in the current with the applied potential. As soon as region A is crossed, a sharp rise in the current is observed. This is characteristic of region B. The break in the curve is due to the onset of pitting as a consequence of breakdown of the oxide layer. Cyclic polarization technique have been extensively used for investigating pitting corrosion of aluminium. In order to be able to interpret the anodic polarization curves it is essential to understand their characteristics. Another anodic polarization curve is shown in Fig. 2. Epp represent the protection potential and Ep, the critical pitting potential. This diagram can be divided in three regions.

1. Region A: In this region pitting initiates and propagation takes place on and above a critical pitting potential designated by Ep, where a marked increase in the anodic dissolute current occurs.

2. Region B: This is the region between Ep (pitting potential) and Epp(protection potential). In this region new pits will not initiate but the existing pits would continue to propagate.

The protection potential (Epp) is defined as the poten

tial below which there would be no propatation and initiation of pits. There is, however, a disagreement on the existence of a protection potential and pits have been observed to propagate below Epp16_17.

3. Region C: This region represents immunity to pitting and at potentials more negative than Epp pitting is not likely to propagate. However, pitting of a different nature (alkaline) has been observed below Epp16_17). The relative position of corrosion potential with respect to pitting potential has been used in interpretation of the pitting behavior of aluminium alloys. The corrosion resistance of aluminium depends on the formation of protective films; such films are formed when the metal is anodically polarized in solution where pH is not too far removed from neutrality. The breakdown of the protective film would occur when the anodic potential exceeds a certain value. It is generally agreed that the breakdown of the hydrate oxide is caused by halide ions. The initiation of pitting is shown in a E vs. I diagram, discussed above, by a sharp increase in the current density. This is shown in Fig. 2. The break in the curve shown corresponds to pitting potential (Ep), the potential at which pits initiate. The existence of a pitting potential is, however, disputed18_19. As the corrosion behavior of aluminium up to the point of film breakdown is controlled by the anodic process, it is to be expected that anions rather than cations would play a major role up to the point of breakdown20. It has been observed that pitting would occur only if the critical pitting potential is lower than the corrosion potential21. Pitting would be inhibited if Ecorr is lower than Ep. Previous data suggests that during anodic polarization, film thickening occurs in all solute except chloride. It was found that anodic treatment thickened the film by about 28 A with 0.1 N solution of chromate (pH + 8.41), sulphate (pH + 7) but no evidence of oxide thickness was obtained with 0.1 N chloride (pH + 7)22. Under conditions of steady state the film breakdown is balanced by film repair. The initial process during anodic polarization is repair of hydrated oxide film and replacement of the oxide film by a non steady state film23. Curves ABC, A'B'C and A"B"C represent anodic polarization curves for aluminium dissolution as a function of pH (Fig. 3). At low potentials (AB, A'B' and A"B") the rate of the anodic reaction is independent of the electrode potential but increases with increasing pH. The rate of corrosion in this region is controlled by the rate of mass transport of hydroxide ions to the oxide-solution interface4. It has been suggested that the above reaction controls the rate of corrosion4. The pitting potential of aluminium becomes less noble with increasing chloride concentration.

The presence of small amounts of cathodic alloying elements leads to an increase in the pitting potential more positive than the pitting potential24. The critical pitting potential is also increased by inhibitor addition (Fig. 4.). The anodic behavior of aluminium has been used to study the mechanism of pitting and intergranular corrosion. Galvele25 observed that pitting of aluminium

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is related to a critical potential which does not depend on induction period. The behavior of 99.9 A1 when potential is changed from the passive range to the pitting range is shown in Fig. 5. It was observed that on holding the specimen at a low potential, the current decreased to a very small value, whereas at a higher potential (more noble) within the pitting range, the current increased by two or three orders of magnitude. This suggests that if pitting were to be independent of potential, once the pitting starts, it would not be possible to stop it by decreasing the potential. It has been shown by polarization technique that commercial aluminium is passive within the potential limits (0.76 mVSEC to 1.38 mVSE-C)16. Pitting occurs above the higher limit, whereas the lower limit marks the point where the film becomes unprotective. The upper limit marks the pitting potential whereas the lower limit marks the active to passive transition.

In recent studies conducted the lower limit has been found to be 1.5 mVSCE17. It has been observed that in both acid and basic solutions of halides two distinct rate controlling reactions occur. Below the critical pitting potential the diffusion of aluminium ions appear to be the rate controlling reaction. The growth of the film (barrier type) proceeds by diffusion of the metal ions through N-type oxide (excess metal ions)26. Above the critical pitting potential, which is independant of pH, anodic polarization does not proceed further. If pH is increased due to increased alkalinity (OH ion production) the protective film dissolves. The shape of the anodic polarization curves indicate whether the film is being formed, repaired, or destroyed, hence, the kinetics of pitting can be studied by anodic polarization curves. lt has been suggested that the magnitude of pitting potential is dependent on the thermo-dynamic potential for the formation of 26. The anodic reaction takes place nearly on all the surfaces, whereas the cathodic reaction takes place only on impurities and grain boundaries27_28). The cathodic reaction stops as soon as the cathodic sites are destroyed and the film achieves an equilibrium thickness.

The following are the main anodic reaction29:

3H2O A1(OH)3 AI + 2H2O A1 (OH) +3H+ + 3e

H2O A12O3

The film breakdown as a consequence of formation of complexes such as

The films formed on aluminium have a dominant influence on the kinetics of anodic reaction. The nature and formation of films has been the subject of numerous investigations30_35. It has been widely accepted that duplex type of films are formed. On immersion in water oxide film can grow up to a thickness of 10 A . The duplex film on aluminium (Fig. 6) consist of a protective inner barrier oxide layer and a bulk (outer oxide) layer. The composition of amorphous bulk hydrated oxide film is dependent on the interaction corroding medium.The role of pH is important, as it affects the solubility (Fig. 6a). Also it has been suggested that film formation is completed in three stages, the first being the formation of amorphous aluminium hydroxide AI(OH)3, the second mono-hydrate orthorhombic crystalline boehmite A12O3, 3H2O [or A1O (OH] and the third being the trihydrate monoclinic bayerite Al2O3 or hydragellite [ Al2O3, 3H2O] [36]. It has been suggested that the. overall corrosion rate is affected by the thickness and nature of the bulk film, which in most instances is a film of oxide layer which takes place are reflected by the change in the shapes of the anodic polarization curve. Pitting proceeds by disruption of the outer oxide layer by

chloride or other halide ions due to the formation of AICI4 complex as stated above. The dissolution of the oxide film is slower in acids than in alkaline solutions4. In alkaline solutions it has been observed that the rate of anodic reaction decreases with time at constant pH and constant solution velocity4. The rate of anodic process also appears to be influenced by the rate of corroding specimen to the solution volume and rate of refreshment of bulk solution.

In addition to the kinetics of corrosion, the anodic polarization curves have been used to study the effect of alloying elements on the corrosion resistance of aluminium.

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Effect of alloying elements The effect of addition of alloying elements of Cu, Mn and Cr on the anodic polarization of Al 99.7 in Arabian Gulf water is shown in Figs. 7-937. Copper addition increases the pitting potential, hence, it weakens the resistance to pitting. Thus, a shift of Ep to more noble value does not always signify a higher resistance to pitting. The pitting resistance of Al 99.7 containing Cr and Mn is superior to that of Al 99.7 containing 0.3 or 0.3 copper. Al 99.7 containing 0.2 chromium shows a higher potential and a higher pitting resistance then Al 99.7 containing Mn.

The anodic polarization curves of pure aluminium and various binary alloys in 1 N-NaCI are shown in Fig. 10. The pitting potentials of pure aluminium and aluminium alloys are shown in Table 1. The relation between pitting potential of binary aluminium alloys and content of alloying element in 1 N-NaCI solution at P > 10.0 is shown in Fig. 11. It is observed from Fig. 11 that increasing Mg content lead to a decrease of pitting potential Incremental addition of Mn shifts the pitting potential slightly in the noble direction, whereas, Fe addition does not cause any appreciable change. Nickel and copper addition shift the potential in the noble direction as suggested earlier. The following order for some binary aluminium alloys in NaCI was obtained in terms of decreasing pitting potential38 from the anodic polarization studies:

AI-Cu>AI-Ni>AI-Si>AI-Mn>AI-Cr>AI-B>AI-Zr>AI-Ti> Al-Mg> Al-Be

The variation of pitting dissolution current for the some binary alloys is shown in Fig. 11. The following order in terms of decreasing resistance to pitting has been reported for the following alloying elements:

Fe>Mg>Mn>Si>Cu.

In recent studies conducted by the author it has been observed that Mg increases and Cr decreases the pitting potential whereas Si does not have a major influence on the pitting potential of AI-2.5 Mg alloy37. Anodic polarization curves have also been used in studying the effect of ion implantation on the corrosion resistance of commercially pure aluminium39. The polarization curves of unimplanted aluminium in 0.01 M Na2SO4 and 1000 ppm CI are shown in Fig. 12. Similar curves in chloride free

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environment are shown in Fig. 13. The cathodic regions of the two sets of curves, the corrosion potential and passive current densities, are same for both. However, in CI containing environment breakdown of curve occurs at a certain critical potential and a sharp rise in the anodic current density is observed. The surface of unimplanted alloy shows no pitting in CI free environment, whereas in CI environment it is badly pitted. It has further been observed that the corrosion potential of pure aluminium is raised by more than 500-700 mVSCE by implanting Mo. The breakdown potential is also increased by more than 200 mVSCE39. The results therefore show that anodic polarization studies can be useful in studying the effect of ion implantation. The anodic polarization studies can, therefore, be used with advantage to study the effect of different parameters on the corrosion resistance of aluminium alloys. The technique is by no means perfect and it is open to controversy and criticism. Concentrated efforts would be required to make this technique quantitative and more reliable.

Cathodic Polarization Cathodic reactions occur on metals presumably by migration of electrons through the surface oxide films and subsequent interaction of those electrons with hydrogen ions and dissolved oxygen at the film/solution interface. Corrosion of aluminium and its alloys proceeds similarly by reduction of hydrogen and oxygen and the rate of corrosion is controlled by the diffusion limited current density for cathodic reduction. A schematic representation of the cathodic polarization diagram is shown in Fig. 14. The corrosion current density (I corr) is obtained by extrapolating the linear Tafel region of the curve to the steady state corrosion potential (Ecorr). The exchange current density is obtained by extrapolation of the linear part of the curve to the reversible hydrogen potential. Although aluminium corrosion has received much attention in the last decade, its electrochemical behavior is not fully understood.

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The cathodic corrosion, particularly alkaline pitting of aluminium in unbuffered chloride media has been investigated by potential controlled methods16. The polarizi-bility of aluminium is increased by the presence of impurity rich sites on the surface. As soon as the film is broken down the cathodic reduction process establishes itself. The process of reduction occurs possibly by migration of electrons through the oxide film and its interaction with hydrogen ion and dissolved oxygen as discussed above. It has been established that cathodic reaction occurs over the localised cathodic sites40. During cathodic polarization hydrogen is evolved which results in the production of excess of OH ions adjacent to the working electrode surface. The high local pH converts the air formed oxide film to the hydrated oxide.

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The degree of hydration depends on temperature, time, ion concentration and pH.

Figure 14A shows the main points of a typical potenti-ostatic polarization curve in air saturated solution of NaCI and Fig. 14B shows the cathodic polarization curves of Al alloys in sodium chloride clearly. The probable reaction is

H2O + e OH + H (ads) (1)

The increase of pH at the cathode has been experimentally verified. The non Tafel portion of the slope appears to be controlled by the limited cathodic current and anodic reaction. The limiting cathodic current may be caused by one of the following:

2H + 2e Hz (2)

2HO + O2 + 4e 40H (3)

The concentration of hydrogen ions is controlled by reaction (2). Although oxygen may have a greater effect on the corrosion rate of aluminium, the reduction of hydrogen ion predominates. Dissolved oxygen, however, increases the tendency of the high purity aluminium to pit. It has been suggested that pure aluminium is insensitive to the effect of oxygen, whereas it exerts a greater effect on the corrosion of aluminium alloys. The effect of oxygen on aluminium alloys in chloride solutions merits further study17. The solubility of the hydroxide film depends largely on the magnitude of the basicity or acidity. If oxygen is excluded from the solution reaction (1) would be controlling reaction. On replacing oxygen by nitrogen the amount of current was not found to increase substantially which showed that reaction (3) was not controlling the limited cathodic current. It has also been shown that the limiting current is greater if oxygen is absent as the amorphous film of hydroxide is loosely adherent in the absence of oxygen and does not provide a good resistance to aluminium ion flow. On the other hand, the film formed in the presence of oxygen is tenacious and compact and it therefore provides a greater degree of protection.

Under the open circuit condition, the cathodic current polarize the surface to the pitting potential. During cathodic polarization the meal dissolution proceeds mainly by alkalinization. At a certain potential the rate of alkalinization (product of OH ions) become too high to allow oxide to be formed and therefore the rate of corrosion is accelerated. The presence of impurities and cathodic site in aluminium has been verified. As soon as the cathodic sites are covered by oxide, repassivation takes place41. If the exposed cathodic area is too small, the available cathodic sites may not be sufficient to maintain corrosion, and the surface may, therefore, stay passive. Passivation may also be caused by parting of the cathodic sites from the surface of the metal. The solubility of the oxide film tends to increase with increased alkalinity and consequently the rate of dissolution increases. It has been suggested that cathodic reaction in alkaline solution is independent of pH (4). Between pH 4-10 the rate of attack is reported to decrease because of the deposit of hydroxide, but at pH values higher or lower than the above range the rates are constant in water23. At higher OH concentration, the rate is limited by OH diffusion.

In alkaline solutions the corrosion current and corrosion potential is determined by the intersection of the anodic polarization curve with the cathodic polarization curve (Fig. 15). With increased OH concentration the transport rate of anode process increases and the intersection of the anodic and cathodic polarization curves

shifts to more negative potentials and higher current densities4. On the other hand with increasing acidity the corrosion current increases with a greater discharge of hydrogen. The rate of transport controlled anodic reaction would become small compared to the cathodic reaction and the potential would shift in the more noble direction close to the pitting potential41. For instance the pitting resistance of two alloys can be compared by cathodic polarization studies.

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In order to be able to protect aluminium corrosion it is important to know the upper and lower potential limits beyond which the alloy does not pit. The upper limit for commercially pure alumimium in aqueous chloride is reported to be 0.76 mVSCE and the lower limit 1.38 mVSCE17. It has been suggested that the lower limit is the result of shift of reactions from solution metal oxide interface to the metal-metal oxide interface at that potential [42]. The increased rate of general corrosion between 1.65 mVSCE appears to be related to the ionic conductivity rather than electronic conductivity at potential greater than 1.38mVSCE [17]. It has been suggested that between the two limits the pitting of a different nature, alkaline pitting, take place whereas above the upper limit alkaline pits propagate due to local alkalini-zation of the surface, however, once the cathodic sites are destroyed, the surface repassivates.

Aluminium undergoes pitting corrosion at zero net current when oxygen is present43. The failure of aluminium surface to remain passive at high cathodic polarization appears to be due to the increased rate of hydration of the oxide film with decreasing potential. The above suggestion for a lower potential bond for aluminium contradict the existence of a protection potential for commercially pure aluminium. For modified aluminium alloys a distinct protection potential (Epp) has been observed by the authors44 and pitting is not found to occur below 1200 mVSCE.

The fact that pitting would not develop if the corrosion potential were below the pitting potential, has been utilized to predict the pitting performance of aluminium alloys45. More negative values of corrosion potential indicate a decreasing susceptibility to pitting in alkaline solution because the increased rate of corrosion at more negative potential makes it difficult to polarize the alloy to its breakdown potential. If the rate of general corrosion is high, pitting would not take place. The criteria that pitting would not occur or the resistance to pitting would not be high if Ecorr was more negative than Ep, does not, however, provide a quantitative basis, and it cannot be used to differentiate between the pitting resistance of alloys slightly differing in composition. It has been suggested that Ep is not a function of the alloy content and it varies with the type of surface preparation46. More accurate prediction can be made if the results obtained from potential-time studies and metal log raphic observations are also considered. Results based on Ep value alone can be misleading. For instance, copper shows a high value of Ep, however, it is prone to pitting. The values of Ep obtained by potentiostatic induction-time method47 appear to be more accurate as shown by recent studies.

The cathodic polarization curve can be used to compare the corrosion resistance of alloys at least qualitatively as shown in Fig. 15. It can be observed from the figure that the rate of corrosion of alloy 6061 is higher than alloy 5454 because of the enhanced cathodic reaction4. The high rate of dissolution of alloy 6005 maintains the alloy in the range of pitting potential. The limit of pitting resistance of aluminium and its alloys in chloride solution is determined by the position of the pitting potential with respect to the curves for cathodic polarization. Useful information on the pitting resistance of aluminium alloys and the mechanism of pitting can therefore be obtained from cathodic polarization curves. Inspite of the advances in the electrochemical techniques made in the last decade, a standard technique is still to be realised. The film formation process in alumi-

continued on page 17

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The kinetics of anodic and cathodic polarization of aluminium and its alloys continued from page 10

nium is highly complex and not completely understood. There is still controversy on the existance and significance of pitting potential, protection potential and active-passive transition potential. With the growing interest in aluminium as an economic material and the development of new technique it is expected that some of the controversies would be resolved.

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References 1. Roebuck A.H., and T.R. Pritchell, Materials Protection, July 1966, 16. 2. Sundarrajah, J., and T. Ramchari, Corrosion, 17, 1961, 39-41t. 3. Bonewitz, R.A., Corrosion, NACE, 29, 1973, 215. 4. Bohlmann, E.G., and F.A. Posey, Proc. 1st Int. Symp. on Water Desalination, Washington, D.C., October 1961, 306. 5. Ahmad, Z., and S. Rashidi, Proc. Third Middle East Corrosion Conf., Bahrain, May 16-18, 1983, 229-241. 6. Nisancloglu, K., and H.Holton, Corrosion Science, 18, 1978, 835-849. 7. Broli, A., H.Holton, and H. Sigurdson, Werkstoffe und Korrosion, 26, 1975, 629. 8. Broli, A., and H. Holton, 7th Scandanavian Corrosion Congress, Trondheim, May 1975. 9. Schwenk, N.W., Corrosion, 20, 1964, 129. 10. Galvele, J.R., J.B. Lumden, and Staehle, J. Electrochem Soc., 125, 1978, 1204. 11. Richardson, J.A., and G.C.Wood, Corrosion Science, 10, 1971, 318. 12. Nisancioglu, K., and H.Holton, Corrosion Science, 18, 1978, 835. 13. Szlarska-Smialowska, Z., Corrosion Science, 12, 1972, 527. 14. Galvele, J.R., J. Electrochem Soc., 122, 1977, 1296. 15. Nisancioglu, K., and H. Holton, Werkstoffe und Korrosion, 30, 1979, 105. 16. Nisancioglu, K., and H. Holton, Corrosion Science. 19, 1979, 337. 17. Ahmad, Z., cummunication with Osterreich Forschungszen-tum, 1985. 18. Wood, G.C., and W.H. sutton, Localised Corrosion, Ed. R.W. Staehle, NACE, 1974, 526. 19. Galvelle, J.R., and S.M. DeMichelli, Corrosion Science, 10, 1970, 795. 20. Anderson, P.J., and M.E. Hocking, J. Appl. Chem., 3, June 1956, 312. 21. Haynie, F.H., and S.J. Ketcham. Corrosion, 19, 1963, 403t. 22. Lorking, R.F., and J.E. Mayne, J. Apple. Chem., 11, 1961, 179.

23. Reader, W., and P.B. Vermileyon, Corrosion Science, 65, 1969, 561. 24. Miller, I.L., and J.R. Galvelle, Corrosion Science, 17, 1977, 179. 25. Galvelle, J.R., and S.M. DeMichelli, Corrosion Science, 19, 1970, 793. 26. Dillon, R.L., Corrosion, 15, 1950, 132. 27. Dibari, G.A., and H.J. Read, Corrosion, 27, 11, 1971, 483. 28. Petrocelli, J.V., J. Electrochem Soc., 12, 1952, 99. 29. Schmitt, C.R., Rev. Coat. Corrosion, 4, 1, 97, (95). 30. Hart, R.H., J. Electrochem. Soc., 30, 1957, 57. 31. Heine, M., D.S.Keir, and M.J.Pryor, J. Electrochem. Soc., 24, 1, 1965, 112. 32. Ruther, W.E., and J.E. Draley, Corrosion, 12, 1956, 31. 33. Troutner, V.H., Corrosion, 15, 1951, 9t. 34. Fraker, A.C., and A.W.T. Ruff, Corrosion, NACE, 27, 4, 1971, 151. 35. Maclennan, D.F., Corrosion, 17, 1961, 105. 36. Becerra Alcibides, and Darby ron, Corrosion, NACE, 30, 5, 1974, 153. 37. Ahmad, Z., Effect of Si, Mn, and Cr on the Corrosion Resistance of aluminium in Arabian Gulf Water (Int. Report Nov. 1985). 38. Morioka Susuma, Sawada Yoshinoby, and Sugimoto katshisa, Corrosion (Japan), 34, 1970, 312. 39. Al-Saffar, A.H., Ashworth, V, et al., Corrosion Science, 20, 1980, 127. 40. Pryor, M.J., and D.S. Keir, J. Electrochem Soc., 19, 1955, 102. 41. Bonewitz, R.A., and J.D. Vernik, Mat. Perf., 14, 8, 1975, 16. 42. Kunze, J., Corrosion Science, 7, 1967, 273. 43. Nisancioglu, K., Scandanavian Corrosion Congress, Helsinki, Proceedings, 1964, 418. 44. Pessall, N., and C. Liu, Electrochem Acta, 16, 1971, 1987. 45. Bonewitz, R.A., Corrosion, 29, 1973, 6. 46. Meissner, H., private communication, 1984. 47. Ahmad, Z., Progress Report # 2, 1985, University of Petroleum & Minerals, Dhahran, Saudi Arabia.

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