CORROSION BEHAVIOR OF HEAT-TREATED 7068 ALUMINUM ALLOY

Abstract

7xxx series (Al-Zn-Mg-Cu) aluminum alloys are high-strength precipitation hardening alloys. They have various applications in automobile and aircraft industries owing to the desired combination of their physico-mechanical properties. Additionally, their excellent workability, high specific strength, and higher heat and electric conductivity make them suitable materials for the fabrication of structural and industrial components. Moreover, compared to other light alloys, they are advantageous materials for the manufacturer due to the low fabrication costs, and well-developed fabrication techniques. However, high stress corrosion cracking (SCC) susceptibility can limit their use in various applications. Even though there has been much study on these alloys, researchers are still working to improve them by taking into account the alloying elements and heat treatment process, their effects on mechanical properties, SCC behavior, and the manufacturing process in order to widen their scope of applications. The peak aged temper condition (T6, T651) of 7xxx series alloys shows better strength, while it exhibits high susceptibility to stress corrosion cracking along with other localized forms of corrosion like pitting and exfoliation corrosion. To prevent these detrimental consequences, over-ageing heat treatment (T76, T7651) is practiced although it decreases the strength by up to 10-15%. Therefore, to overcome these drawbacks and in order to achieve a combination of better strength along with superior corrosion resistance, three-stage heat treatment process which is known as retrogression and re-ageing (RRA) treatment came into existence. The final morphology, size, and distribution of precipitates during RRA process are very different as compared to aforementioned conventional heat-treatment processes. Amongst the new 7xxx series alloys, high zinc (>7 wt%) containing 7068 alloy is one of the highest strength alloys and is projected to be useful for various applications in the aerospace and automotive industry. However, its response to T6 and different RRA treatment parameters in terms of microstructural evolution, GBP chemistry, and PFZ characteristics is unknown. Further, correlation of microstructural features resulting from different T6 and RRA procedures with its mechanical and corrosion behavior is yet to be ascertained. This is especially important since over-aging condition that is normally recommended for optimized corrosion performance of other 7xxx series alloys, does not ensure adequate corrosion performance in high zinc alloys. Therefore, present dissertation work deals with determining optimized T6 and RRA process parameters in order to obtain a combination of superior corrosion resistance and better strength. Furthermore, the passive film breakdown mechanism in borate buffer solution and metastable pitting behavior are studied. Various techniques like electron microscopy, differential scanning calorimetry (DSC), potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis are employed in order to analyze the effect of heat treatment process parameters on the corrosion behavior. Microstructural features of high zinc 7068 aluminum alloy resulting from different ageing treatments during T6 heat treatment process are correlated with mechanical properties, localized corrosion behaviour, and SCC susceptibility. Localized attack manifested in dissolution of second phase precipitates which occurs from selective leaching of magnesium and aluminum. The micro-galvanic effect of iron-containing precipitates on the matrix dissolution is less as compared to Al2MgCu phase. 7068 alloy exhibited comparable SCC resistance in both peak aged and over-aged conditions, which is attributed to the absence of precipitate free zones (PFZ) in the former. Besides, Peak and overaged show discontinuous grain boundary precipitates and fewer Al2MgCu/α-Al micro-galvanic couples. Thus, over-ageing process that reduces strength, is not necessary for satisfactory SCC performance in this alloy. Mechanical and corrosion behavior of high-strength, high-zinc (>7 wt%) containing 7068 aluminum alloy is investigated after employing different RRA treatments. The effect of pre-aging conditions on the distribution of copper, zinc, and magnesium, the volume fraction of η′ phase, and the width of PFZ have been investigated. Microstructural and compositional features are correlated with hardness, uniform corrosion, SCC, and intergranular corrosion resistance. A combination of two opposite effects, that is, the presence of nobler, high-copper containing grain boundary precipitates and micro-galvanic effect of PFZ along with the distribution of alloying elements, that is, Cu, Zn, and Mg govern the electrochemical behavior of RRA treated 7068 alloy. Specimens that were pre-aged at 100 °C and retrogression treated at 180 °C exhibited retarded reduction reaction kinetics in polarization studies, and provided better SCC resistance. Besides the microchemistry of grain boundary precipitates, matrix precipitate size affected the oxygen reduction kinetics. An increase in copper content of discontinuous boundary precipitates did not monotonously lead to improved SCC resistance. Thus, optimum pre-aging and RRA conditions are identified for this high-zinc 7xxx series alloy, and it is found that specimen pre-aged at lower temperature exhibited higher strength and better corrosion resistance.