Some Corrosion Characteristics of Aged Aluminum Alloy 6061 in Neutral and Alkaline Solutions

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1 Some Corrosion Characteristics of Aged Aluminum Alloy 6061 in Neutral and Alkaline Solutions K.El-Menshawy 1, A.A.El-Sayed 1, M.E.El-Bedawy 1, H. A.Ahmed 2 and S.M.El-Raghy 2 1- Metallurgy Department, Atomic Energy Authority, Cairo, Egypt 2- Faculty of Engineering, Cairo niversity, Egypt ABSTRACT The composition and microstructure of the alloy as well as the corrosive nature of the environment play an important role in modifying the corrosion resistance of aluminum alloys. This work was intended to study the influence of the solution ph on the corrosion characteristics of 6061 aluminum alloy, (Al-Mg-Si alloy containing 0.22 wt% Cu) after aging at, and C for different periods of time. The investigation was performed by applying potentiodynamic polarization technique in neutral deaerated 0.5 % M NaCl solution as well as in alkaline NaOH solution (ph = 10). Analysis of the potentiodynamic polarization curves showed similar dependence of I corr and cathodic current densities on the aging treatment in both solutions. It was found that E corr values in the NaCl solution were shifted in the more noble direction while in the NaOH solution they were shifted in the more active direction compared to the solution treated condition. The magnitude of the noble or active shift in E corr increased with aging time for the underaged specimens and decreased towards E corr of the solution treated condition for the overaged specimens in both solutions. The I corr values were higher in the alkaline solution for the same aging treatment. Keywords: corrosion / Aluminum alloy 6061 /. 1-INTRODCTION When aluminum surfaces are exposed to the atmosphere, a thin invisible, strongly adherent oxide film forms immediately which protects the metal from further oxidation. This film gives aluminum and its alloys their high resistance to corrosion. rotection by this film is limited to environments in which it is stable or only slightly soluble. According to ourbaix diagram, passivity of aluminum is limited to the ph range from about 4 to 9. Above and below this range, aluminum and its alloys normally exhibit uniform corrosion attack. However, in the ph range where the oxide film is stable, the protective effect of the film is greatly influenced by its integrity. Any defects or discontinuities in the passive film will expose the underlying metal to the surrounding corrosive medium and will lead to the onset of localized corrosion [1]. In heat treatable aluminum alloys series 6xxx (Al-Mg-Si), it was found that, the susceptibility to localized corrosion (pitting and / or intergranular (IGC)) and the extent of attack are mainly controlled by the type, amount and distribution of the precipitates which form in the alloy during any thermal or thermomechanical treatments performed during manufacturing processes [2-6]. Depending on the composition of the alloy and parameters of the heat treatment process, these precipitates form in the bulk of the grain, or in the bulk as well as grain boundaries. As indicated by several authors, the precipitates formed by heat treatment in AlMgSi alloys containing Cu (series 6xxx) are the Q phase (Al 4 Mg 8 Si 7 Cu 2 ), ß phase (Mg 2 Si) and free Si if Si content in the alloy exceeds the Mg 2 Si stoichiometry [7-9]. As far as the protective oxide film is concerned, the precipitates formed in or near the surface of the alloy are incorporated in the oxide film and determine, to large extent, the corrosion characteristics of the alloy. Therefore, the corrosion mode and the degree of attack on heat treatable aluminum alloys are controlled by the ph of the corrosive environment and the precipitation behavior [3, 6]. -51-

2 Ambat and Dwarakadasa [10] indicated that the corrosion characteristics of two heat treatable aluminum alloys 8090 (Al-Li-Zn-Mg-Zr) and 2014 (A1-Cu) in a certain aging condition are greatly influenced by the solution ph in a wide range of chloride ion concentrations extending from 1 to 10 wt % NaCl. The objective of the present investigation is to study the effect of aging treatments in the temperature range C on the corrosion behavior of aluminum alloy 6061 in neutral and alkaline solutions. Following solution heat treatment or any aging treatment the alloy specimens were quenched in ice water. The quenched solution treated specimens were kept at subzero temperature before artificial aging to prevent or minimize the effect of natural aging before the artificial aging process. Material 2-EXRIMENTAL The study was carried out on 10 mm thick flat plates of 6061 AA received in the peak-aged T6 temper. The chemical composition is shown in Table (1). The plates were cut into samples of 10 mm width and 15 mm in length by electric discharge machine. Table (1): Alloy composition [wt%] Element Mg Si Cu Fe Cr Mn Zn Ti Al Wt.% Bal. Heat-treatment procedures Solution heat treatment of the as received samples was carried out at 550 C for 2 hrs, followed by quenching immediately in ice water. The specimens were kept at - 5 C to prevent or minimize the effect of natural aging before any artificial aging treatment. Some of these specimens were tested in the as quenched condition and the other specimens were artificially aged at three aging temperatures (,, C) for different aging times as shown in Table (2). After any aging treatment the alloy specimens were also quenched in ice water and kept at - 5 C until they were taken for corrosion testing. Table (2): parameters temperature ( C ) periods (min) 90, 180, 360, 1800, 2880, 5040, 6000, 10, 20100, , 50, 180, 330, 1020, , 10, 17, 50, 180, 420 Electrochemical Corrosion testing rior to corrosion behavior studies, the samples were ground on SiC grinding papers from 240 till 1000 grade, followed by polishing on polishing lapped cloth using 1 µm diamond suspension. Then the polished samples were degreased with ethanol before immersion in the test solution. The electrochemical corrosion behavior of the samples was studied by applying the potentiodynamic polarization technique using a potentiostat (Electrochemical Impedance Analyzer, Model 6310) interfaced to a computer and a three-electrode cell with the sample as a working electrode of exposed area 100 mm 2, a saturated calomel reference electrode (SCE) and two graphite rods as the counter electrode. -52-

3 After the specimen was immersed in the test solution, a delay time of 180 s was applied to allow the specimen surface to achieve a steady state before the potential scan was commenced. The scan rate for the potentiodynamic polarization experiments was 0.5 mv/s. otentiodynamic polarization curves were recorded in neutral 0.5 % mol NaCl solution. For Al-alloys in aerated NaCl solutions, the breakdown potential (E br ) is near the corrosion potential (E corr ). Therefore, in order to study features of the anodic polarization curve and to determine the breakdown potential (E br ) accurately, the NaCl solution was deaerated by purging high purity nitrogen gas (N 2 ) through the solution for 2 hrs prior to the test. The potential scans in the NaCl electrolyte were started at -250 mv with respect to Open Circuit otential (OC) and continued to mv with respect to OC. The effect of different heat treatment conditions on the corrosion characteristics of the alloy was also studied in the alkaline NaOH solution of ph 10. In the NaOH electrolyte, the scans were started from (-500 mv to +100 mv) with respect to OC. 3-RESLTS Overlaying of the potentiodynamic polarization curves in neutral NaCl solution and alkaline NaOH solution (ph = 10) for alloy specimens given the same aging treatment indicated certain effects of solution ph on the corrosion characteristics of the alloy in all aging conditions. Examples of the overlayed curves are given in Fig. (1) a c for samples aged at C in the underaged (), peak aged () and overaged (O) conditions while Fig. (2) a c shows the overlayed curves for specimens aged at, and C in the underaged condition. As shown in previous work [11], these aging regions correspond to the hardness changes with aging time at a given aging temperature where the peak aging condition refers to the aging time which gives maximum hardness value at this temperature. -53-

4 Fig. (1): potentiodynamic polarization curves in neutral NaCl and alkaline NaOH (ph = 10) solutions for samples aged at C in (a) the underaged, (b) peak aged and (c) overaged conditions. -54-

5 (b Fig. (2): potentiodynamic polarization curves in neutral NaCl and alkaline NaOH (ph = 10) solutions for samples aged at (a), (b) and (c) C in the underaged condition. Generally, for all aging conditions illustrated in Fig. 1 and 2, testing of the aged specimens in alkaline solution NaOH led to a change in the slope of the cathodic polarization curves, an -55-

6 increase in the limiting cathodic current densities and a shift in the E corr in the active direction relative to those tested in the neutral NaCl solution. Similar effects were reported in literature for aged aluminum alloys AA 8090 and 2014 [10] where increasing solution ph from 6 to 11 changed the slope of the cathodic polarization curves and shifted E corr in the active direction. Some corrosion characteristics in neutral NaCl and in alkaline NaOH solutions derived from the potentiodynamic polarization measurements are listed in Tables 3 and 4 for some underaged, peak aged and overaged conditions. It is observed from table 3 that increasing the aging time in the underaged region towards the peak condition ennobled E corr, increased I corr and shortened the passive region compared to the solution heat treated condition, while in the NaOH solution (Table 4) increasing the aging time in the same range shifted E corr in the more active direction and increased the I corr values. Table (3): The corrosion characteristics in NaCl solution Temp. ºC Time, min ????? Condition O O I corr, na/cm E corr vs. SCE, mv E br vs. SCE, mv assivity region, mv Solution treated at 550 ºC for 2 hr Table (4): The corrosion characteristics in NaOH solution Temp., ºC Time, min ????? Condition O O Solution treated at 550 ºC for 2 hr E corr vs. SCE, mv I corr, 10 3 na/cm

7 4-DISCSSION As mentioned above in section 1, increasing the aging time in the underaged region increases the volume fraction of the cathodic precipitates particularly the ß phase and the Cu containing Q phase leading to the development of closely spaced fine distribution of these precipitates. This will increase the cathodic area fraction on the specimen surface and consequently the cathodic reaction rate will be increased. In turn, the anodic reaction will be derived at a higher rate giving increasing I corr values with aging time in the underaged region. Because of the fine distribution of precipitates in this region, it is expected that other fresh cathodic particles are more rapidly reached by the corroding zone around these particles and this will lead to more rapid dissolution rate of the matrix reflected as higher I corr values as manifested in Tables (3) and (4). Moreover, increasing the volume fraction of precipitate particles with aging time for the underaged tempers is expected to cause the shortening in the passive potential range with aging time and the minimum value of the passive range was recorded near the peak aging condition for the alloy specimens tested in NaCl solution (Table (3)). Clearly, the increase in volume fraction of precipitates with aging time will increase their population in or near the surface and this will retard the passive film repair kinetics leading to more rapid pitting corrosion. Thus, the I corr values in both solutions showed similar dependence on aging time despite the fact that the corrosion current density values at any aging condition are remarkably higher in the alkaline solution than in the neutral NaCl solution. Solution ph normally influences the corrosion behavior of aluminum by changing the stability of the passive film [1]. Therefore, the higher I corr values in the NaOH solution for all aged conditions, compared to the neutral NaCl solution (Tables (3) and (4)) are attributed to the highly unstable nature of the passive oxide film in the NaOH solution which has a ph value outside the stability region of the film. The corrosion potential in NaOH solution showed negative shift with aging time to peak condition compared to that of the solution treated specimen (Table (4)). This is contrary to the noble shift observed when the specimens were tested in neutral NaCl solution (Table (3)). Generally, the corrosion potential of an aluminum alloy in a given corrosion environment is determined by the composition of the surface. In neutral solutions the passive film on aluminum is stable and the cathodic surface precipitates, Cu- rich Q- phase and Mg 2 Si, exist as breakdown points in the film. Therefore, the noble shift in E corr for the specimens tested in neutral NaCl solution and the increase in magnitude of this shift with increasing aging time in the underaged range (Table (3)) may be attributed to the increase in the cathodic area fraction on the specimen surface due to increased density of precipitates on the surface with aging time. This can also enrich the surface in Cu. Similar noble shift in E corr for Al 6061 metal matrix composite was observed after potentiodynamic polarization in a similar test solution [12]. It has also been reported that Cu is one of the elements which can shift E corr of Al in the noble direction in the neutral NaCl solution [13]. In the alkaline solutions, the passive film on Al and its alloys is unstable [1] and the matrix of the alloy constitutes the predominant area fraction of the specimen surface exposed to the solution. Thus, for the solution heat treated specimen in alkaline solution, the E corr is determined by the matrix composition which is a solid solution of aluminum with Mg, Si and Cu. The negative shift in E corr upon aging (Table (4)) may be explained in terms of the depletion of solute elements in the matrix caused by the precipitation of the Mg 2 Si and the Cu rich Al 4 Mg 8 Si 7 Cu 2 phases. In such alkaline medium Al exists in solution as AlO - 2 ions while in the neutral solution Al(OH) 3 is precipitated on the surface decreasing the corrosion current density for the specimens tested in NaCl solution (Tables (3) and (4)). The change in slope of the cathodic polarization curve with ph of the test environment (Figs. (1) and (2)) is attributed to the change in the cathodic reaction at different ph levels [10]. In alkaline solution the corrosion of Al proceeds mainly by water reduction according to the reaction; Al + 3 H 2 O + OH - - 3/2 H 2 + Al(OH) 4-57-

8 In neutral solution, however, oxygen reduction will be the main cathodic reaction. In the present work, the oxygen concentration in the NaCl solution was considerably decreased by deaeration before the polarization experiments. This, in addition to the high availability of (OH) - ions at ph = 10 is believed to be the reason for the change in slope of the cathodic polarization curves and the higher cathodic current densities observed in the alkaline solution (Figs. (1) and (2)). The acceleration of the cathodic reaction and the unstable nature of the passive film in the alkaline solution NaOH (ph = 10) can explain the negative shift in E corr observed with increase of solution ph from about 6 to 10 as shown in Figs. (1) and (2). The specimens overaged at and showed reverse shift in the values of E corr, I corr and the cathodic current densities towards the values of the solution heat treated specimen after testing in both neutral and alkaline solutions. Coarsening in size and distribution of the precipitates during overaging is the most probable reason for these reverse changes in the overaged specimens. Coarsening in size and distribution of the precipitates is normally associated with development of larger precipitate-free areas as well as back tracing of some of the alloying elements to the matrix enriching it in the alloying elements. Thus, with progress in overaging, the composition of the matrix gradually approaches that of the solution heat treated condition. Consequently, the magnitude of the noble or active shift in E corr in the NaCl and NaOH solutions, respectively as well as the cathodic current density values and I corr values decrease towards those of the solution treated condition in both solutions (Table (3), (4)). It must be mentioned that these systematic changes were observed for the specimens aged at and C. However, the specimens aged at C showed some anomalous behavior in both solutions. In the NaCl solution, although the changes in I corr values for the three aging conditions followed similar trend as these aged at and C, the underaged specimens exhibited only slight noble shift in E corr of about 30 mv compared to the solution treated specimens. On the other hand the overaged specimens did not show significant change in E corr or E br compared to those of the peak aged condition (Table (3)). In the alkaline NaOH solution, the I corr values for the three aging conditions at C are lower than I corr for the solution treated specimen in the same test solution (Table (4)). This behavior may be ascribed to the different precipitation and coarsening kinetics at C relative to those at and C. Based on knowledge available in the relevant literature [14], no significant changes in matrix precipitation were observed upon increasing the aging time to min at 100 C for a peak aged aluminum alloy 6013 and grain boundary precipitation was observed only after aging in the temperature range C for min. This leads to the suggestion that aging at C for the aging times applied in the present work (maximum of min) for alloy 6061 containing Cu does not lead to significant changes in the precipitation process particularly after the peak aging condition. This was mainly reflected in the absence of remarkable changes in I corr values in the alkaline solution for the specimens aged at C in the three aging conditions (Table (4)). at this temperature and its effect on the corrosion behavior of alloy 6061 in both solutions needs further work. 5-CONCLSIONS The results obtained from the present work indicate significant role of the aging parameters on the corrosion behavior of alloy 6061 AA in alkaline as well as in neutral solution. The following conclusions have been drawn; 1- Analysis of the potentiodynamic polarization curves showed similar dependence of I corr and the cathodic current densities on the aging treatment in neutral deaerated 0.5 % mol NaCl solution and in the alkaline NaOH solution. 2- The changes in E corr values exhibit different trends in the two test solutions. The E corr values were shifted in the more noble direction in the NaCl solution while they were shifted in the more active direction in the alkaline solution for all aging conditions compared to the solution treated condition. The -58-

9 magnitude of the shift increased with the aging time in the underaged region to the peak condition and decreased again for the overaged samples. 3- Some exceptions were noted for the specimens aged at C which were attributed to different precipitation and coarsening kinetics compared to those at and C. 4- For all aging conditions, testing in alkaline solution NaOH changed the slope of the cathodic polarization curves, increased the limiting cathodic current densities, shifted E corr in the active direction and increased the corrosion current densities I corr compared to those in the neutral NaCl solution. 6-REFERENCES (1) M. ourbaix, 'Atlas of Electrochemical Equilibrium Diagrams in Aqueous Solutions', NACE, Houston, Texas (1966) p (2) G. Svenningsen, M.H. Larsen, J.H. Nordlien, K. Nisancioglu, Effect of high temperature heat treatment on intergranular corrosion of AlMgSi(Cu) model alloy, Corros. Sci. 48 (2006) (3) G. Svenningsen, M.H. Larsen, Effect of artificial aging on intergranular corrosion of extruded AlMgSi alloy with small Cu content, Corros. Sci. 48 (2006) (4) G. Svenningsen, M.H. Larsen, Effect of thermomechanical history on intergranular corrosion of extruded AlMgSi(Cu) model alloy, Corros. Sci. 48 (2006) (5) G. Svenningsen, J.E. Lein, A. Bjorgum, J.H. Nordlien, K. Nisancioglu, Effect of low copper content and heat treatment on intergranular corrosion of model AlMgSi alloys, Corros. Sci. 48 (2006) (6) M. H. Larsen, J. C. Walmsley, Significance of low copper content on grain boundary nanostructure and intergranular corrosion of AlMgSi(Cu) model alloys, Mater. Sci. Forum (2006) (7) H. Zhan, J. M. C. Mo, F. Hannour, L. Zhuang, H. Terryn, J. H. W. de Wit, The influence of copper content on intergranular corrosion of model AlMgSi(Cu) alloys, Materials and Corrosion 59 (2008) (8) M. H. Larsen, J. C. Walmsley, O. Lunder, Mathiesen, RH, Kemal Nisancioglu, Intergranular corrosion of copper-containing AA6xxx AlMgSi aluminum alloys, J. Electrochem. Soc. 155 (2008) C550-C556. (9) M. H. Larsen, J. C. Walmsley, O. Lunder and Kemal Nisancioglu, Effect of Excess Silicon and Small Copper Content on Intergranular Corrosion of 6000-Series Aluminum Alloys, J. Electrochem. Soc. 157 (2010) C61-C68. (10) E. S. Dwarakadasa, Studies on the influence of chloride ion and ph on the electrochemical behavior of aluminum alloys 8090 and 2014, J. Applied Electrochemistry 24 (1994) (11) M. E. El-Bedawy, M.Sc., Effect of aging on the corrosion of aluminum alloy 6061, Faculty of Engineering, Cairo niversity, (12) H. J. Greene and F. Mansfeld, Corrosion protection of aluminum metal-matrix composites, Corrosion Science 53 (1997) (13) M. Reboul,. Meyer, in roceedings of the fourth International Aluminium-Lithium Conference, Vol. II, (edited by G. Champier, B. Dubost, D. Mianny and L. Sabetay), France, June (1987), pp. 3. (14) R. Braun, Investigations on the long-term stability of 6013-T6 sheet, Mater. Characterization 56 (2006)