Influence of Heat Treatment and Scandium Addition on the Electrochemical Polarization Behavior of Al-Zn-Mg-Cu-Zr Alloy
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TRODUCTION
TYPE 7010 Al alloy is a relatively new high-strength alloy as compared to that of the conventional type 7075 Al alloy. The lower level of iron and silicon impurities in type 7010 Al alloy than as in type 7075 Al alloy enables it to exhibit better toughness than the latter.[1] However, high-strength Al alloys are known for their susceptibility to localized corrosion such as pitting, intergranular corrosion (IGC), and environmentally assisted cracking (EAC).[2–5] Our recent research work showed that microstructural changes brought out through multistep overaging enhanced the EAC resistance of type 7010 Al alloy, with only a minimal loss in the tensile strength.[2] It is also known that scandium addition to Al alloys improves the strength through grain refinement[6] as well as the weldability.[7,8] Our recent research work on scandium containing type 7010 Al alloy showed not only improvement in the tensile strength levels but also high EAC resistance.[9] This behavior was attributed to the M. BOBBY KANNAN, Senior Research Fellow, and V.S. RAJA, Professor, are with the Corrosion Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. Contact e-mail: [email protected] Manuscript submitted February 20, 2006. Article published online September 22, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
recrystallization inhibition effect caused by the presence of scandium. However, it has to be noted that not much work has been carried out on the electrochemical polarization behavior of type 7010 Al alloy to understand how aging[10] and scandium addition affect the localized corrosion. Moreover, there are conflicting views on the anomalous anodic polarization behavior of the alloy under different heat-treated conditions. Hence, this work was carried out to understand the influence of both the heat treatments and scandium addition on the electrochemical polarization behavior of type 7010 Al alloy and to correlate the polarization behavior to localized attacks such as pitting and IGC. Since only a limited literature exists on the electrochemical polarization behavior of type 7010 alloy,[10] literature related to type 2xxx series alloys and other related type 7xxx series alloys is reviewed here to lend perspective to the present work. A literature review of related copper containing high-strength Al alloys of type 2xxx and 7xxx series shows that they are prone to pitting and IGC in the peak-aged condition. While they become resistant to IGC on overaging, this heat treatment does not enhance the pitting corrosion resistance of these alloys.[11–14] Electrochemical polarization data have been used to understand both these aspects of corrosion. A few authors reported that the VOLUME 38A, NOVEMBER 2007—2843
polarization curves of copper containing peak-aged 2xxx and 7xxx series alloys obtained in 3.5 wt pct NaCl show two distinct breakdown potentials.[12–15] On the contrary, they observed only one breakdown potential in the polarization curves of the overaged alloy. The authors have correlated
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