Al-Cu alaşımlarının akım ve potansiyel kontrollü anodik davranışlarına yapı ve bileşimin etkisi

Abstract

Bu çalışmada Al-Cu alaşımlarının anodik polarizasyon davranışlarında bakır içeriğine bağlı olarak gözlenen değişikliklerin, iki farklı deney yöntemi (potansiyodinamik ve galvanodinamik) ve çözeltiye alınmış, yaşlandırılmış ve hızlı katılaştırılmış numuneler kullanılarak incelenmesi ve malzemenin bileşim ve yapısının anodik davranışına nasıl yansıdığının belirlenmesi amaçlanmıştır. Deneysel çalışmalarda, %99.99 safiyetteki saf alüminyumdan dökülmüş olan çeşitli bileşimlerdeki Al-Cu numuneler hazırlanmıştır. Al-Cu alaşımlarında oluşan çökelti pariküllerinin histerizis genişliğine olan etkisini araştırmak amacıyla, bu numuneler çözeltiye alınmış ve yaşlandırılmıştır. Aynı amaçla hızlı katılaştırılmış AI-%1Cu, AI-%3Cu ve 2017 Al-Cu alaşımları da kullanılmıştır. Deneyler sonucunda; oyuklanma potansiyeli Al-Cu alaşımlarında matriks içerisinde çözünmüş bakır bileşimine bağlı olarak değiştiği saptanmıştır. Alaşımın bakır içeriğinin artışı ile oyuklanma potansiyeli daha pozitif değerlere doğru kaymaktadır. Galvanodinamik deneyler sonucunda, potansiyodinamik deneylerde elde edilen histerizis bandından farklı bir histerizis bandı ortaya çıkmıştır. Bu deneylerde potansiyelin serbest değişken olması nedeniyle akım histerizisi yerine potansiyel histerizisi görülmüştür. Potansiyel histerizisinin başladığı akım yoğunluğunun (dönüş kademesinde potansiyelin düştüğü akım yoğunluğu) aynı zamanda direnç artışının fazlalaştığı nokta olduğu bulunmuştur. Buna göre direncin artmaya başladığı akım yoğunluğunun %Cu oranına göre değişiminde çözeltiye alınmış numunelerde belirgin bir fark yok iken yaşlandırılmış numunelerde %0.5Cu oranından sonra düşüş görülmüştür. Gerek galvanodinamik deneylerde gerekse potansiyodinamik deneylerinde elde edilen sonuçlarda %0.5Cu miktarı Al-Cu alaşımının oyuklanma davranışı açısından kritik bir oran olarak belirlenmiştir. Bu bileşimin üzerinde alaşımın yaşlandırma ısıl işleminin hem oyuklanma potansiyelinde hem de yüzey filminin direncinin değişimi üzerinde etkili olduğu saptanmıştır.

The Effect of Structure and Composition on The Current and Potential Controlled Anodic Behavior of Al-Cu Alloys SUMMARY Although aluminum is thermodynamically a very active metal, it is resistant to corrosion because of passive film formation on its surface. Aluminum and its alloys as the other passivating metals are sensitive to localized corrosion which starts with the initiation of the local rupture of the protective oxide film. Pitting, crevice and intergranular corrosion are the main types of localized corrosion. Copper is one of the primary alloying element in high -strength Al- alloys (2000 series) and It is used as additional alloying element in other high strength Al- alloys such as Al-Li-Cu and Al-Zn-Mg-Cu. The main objective of copper addition to aluminum alloys is to improve the mechanical properties of alloys by making them age hardenable Although copper improves the mechanical properties of aluminum alloys, it has a detrimental influence on corrosion behavior of aluminum alloys in chloride solutions. Al-Cu alloys are more sensitive to pitting corrosion than pure aluminum. These alloys can also be sensitive to intergranular corrosion and stress corrosion cracking depending on alloy composition and aging heat treatment. AI2CU intermetallics precipitated at the grain boundaries during aging make the alloys prone to intergranular corrosion and stress-corrosion cracking. The pitting potential of Al-Cu alloys changes with amount of dissolved copper in matrix. Pitting potential of the alloy shifts more electropositive values with increasing copper content in alloy.(16) The pitting properties of Al-Cu alloys are effected by aging heat treatment. Presence of 0' and 0" particles, which are formed during aging process, decrease the pitting potential of the alloy. It was observed that in the case of the formation of 0' phase during the aging process, 0' phase (disc VI shaped) was oriented preferentially along (110) planes. 0' particles are concentrated along slip planes of the matrix and it was found that pits formed along {111} planes were concentrated around 0' particles. Presence of GP zones do not have effect on the pitting potential of the alloy (16). Anodic polarization curves are widely used in the determination of pitting properties of metal and alloys. The pitting potential (Enp) and repassivation potential (Epp) can be determined by the evaluation of the polarization curves. The determined potential values can be used in the evaluation of pitting resistance of the alloys. These curves are practically meaningful in the pitting corrosion resistant alloys such as stainless steels and they are widely used to determine the resistance to pitting corrosion, depending on alloy composition and environment. Unlike the pitting resistant materials, the determination and evaluation of the pitting and repassivation. potential of the aluminum and its alloys are not significantly practical. Because, in chloride containing environments, aluminum and its alloys are readily subjected to pitting and it is not possible to prevent pitting of aluminum alloys by means of alloying. For example, it is known that pitting potential of alloy shift more positive values by increasing copper content. This can be interpreted as aluminum-copper alloys are more resistant to pitting corrosion. However, it is known that aluminum-copper alloys are subjected to more severe attack by pitting. In this study, solution treated, aged and rapid solidified specimens were used in order to investigate the effect of copper content on the anodic polarization behavior of Al-Cu alloys by using potentiodynamic and galvanodynamic techniques. The evaluation of pitting behavior of alloys can be performed by two different ways. These are potentiodynamic and galvanodynamic techniques. In potentiodynamic technique, a constant potential changing with a certain scan rate is applied and corresponding current density is measured. While in galvanodynamic technique, the current passing through corrosion cell is controlled. A potential hysteresis is generally observed in cyclic potentiodynamic polarization technique. The hysteresis width showed an increase with increasing copper content of Al-Cu binary alloy. The relation between hysteresis behavior and copper content of alloy was attributed to formation secondary phase particles in aluminum matrix (3). In this study both potential and current controlled techniques were used for the evaluation of the anodic behavior of Al-Cu alloys in %3.5 NaCI solution. VII The experiments were carried out using as-cast Al-Cu alloys prepared from 99.99 % purity aluminum and proper amount of copper. In order to investigate the effect of precipitates on the anodic polarization behavior, cast alloys were solution heat treated and homogenized by heating at 500 °C for 72 hours. Solution treated alloys were aged at 160 °C for 120 hours Rapid solidified Al- 1Cu, AI-3Cu and 2017 Al-Cu alloys were also used in the potantiodynamic polarization experiments The solution heat treated and aged Al-Cu alloys containing 0.01 % Cu, 0.03 % Cu, 0.1 % Cu, 0.5 % Cu and 1 % Cu were tested in 3.5 (wt)% NaCI. The same experiments were repeated with rapid solidified alloys containing 1 % Cu, 3 % Cu and 2017 aluminum alloy. Cyclic potentiodynamic polarization experiments were conducted in nitrogen deaerated 3.5 % NaCI solutions at 25 °C. The electrochemical measurement system consisted of a potentiostat (EG&G Potentiostat/Galvanostat Model 273) controlled by EG&G Model 332 Softcorr Corrosion Software loaded Apple lie computer. The experiments were performed by 20 mV/min scan rate. The potential was reversed when current density reached 400 |LiA/cm2. Current controlled electrochemical experiments were performed galvanodynamically in 3,5 % NaCI solution for solution heat treated and aged alloys with 1%Cu, 0.5 %Cu, 0.25 %Cu and 0.1 %Cu content. The system used in cyclic galvanodynamic polarization experiments was consisted of Wenking LB 75L potentiostat and Wenking Model VSG 83 scan generator. In order to measure the electrode potential vs. scanned current, A/D converter (PCL-71 1S) which has contained 8 input channels with common ground was used. This A/D converter was controlled by a PC-compatible computer. A software controlling the A/D converter was written with MS-Quick Basic compiler. In potentiodynamic experiments: Results indicated that the pitting potentials shifted more positive values for both solution heat treated and aged samples by increasing copper content in the alloys. In addition, the pitting potentials of aged alloys containing over 0.5 %Cu showed a definite decrease with respect to the pitting potentials of solution heat treated samples. The variation of repassivation potential of alloy with copper content was similar to that of pitting potential. Both repassivation potential and pitting potential for aged alloys decreased with copper content over 0.5%Cu with respect to solution heat treated alloys. VIII Hysteresis width didn't change significantly for both solution heat treated and aged alloys but when copper content of the alloy exceeds 0.1%, it increased sharply. Increase in the pitting potentials of the rapid solidified specimens showed that the pitting potential was effected by type of solidification. It is possible that pitting resistance of the Al-Cu alloys can be improved by the rapid solidification treatment. This result is confirmed by the other authors (1, 23, 28). In galvanodynamic experiments: The obtained hysteresis width were not the same for the experiments conducted galvanodynamically and potentiodynamically. In galvanodynamic experiments, potential hysteresis was observed instead of current hysteresis. It was found that the current density at which potential hysteresis initiate corresponds to the point where the increase of the surface resistance become more significant In solution treated alloys the resistance decrease with copper content was found to be linear but in aged alloys 05%Cu a sudden decrease at the resistance was observed The conclusions can be summarized as follows; 1. The pitting potential of Al-Cu alloys increase with increasing copper content. 2. A sudden increase in hysteresis width of aluminum copper alloys was observed starting from 0.1 %Cu in potentiodynamic experiments. This behavior is not affected by the aging heat treatment. Rapid solidification of the alloys resulted in an increase in the pitting potential of the alloys but rapid solidification didn't make an effect on the hysteresis width. These results indicate that the observed hysteresis widening is related to copper content of the alloy but not to aging heat treatment. 3. In the potentiodynamic experiments, the pitting potential and repassivation potential of the aluminum copper alloys are both effected by the aging heat treatment in alloys containing 0.5%Cu and over. Aging heat treatment shifted the Enp and Erp to more negative values. IX 4. Interpretation of galvanodynamic experiments with respect to resistance change gave promising results for examining the effect of heat treatment in the Al-Cu alloys. An abrupt change in resistance (versus Cu content) was observed in 0.5% Cu containing aged alloys.