Chapter 14 - Corrosion and Stress Corrosion of Aluminum–Lithium Alloys

• Published in: Aluminum-Lithium Alloys pp. 457 - 500
• Main Authors: Holroyd, N.J.H., Scamans, G.M., Newman, R.C., Vasudevan, A.K.
• Format: Book Chapter
• Language: English
• Published: Elsevier Inc 2014
• Subjects: Al–Li alloys; corrosion; fracture and exfoliation; intergranular corrosion; stress corrosion; intergranular corrosion; corrosion; fracture and exfoliation; stress corrosion; Al–Li alloys
• ISBN: 9780124016798; 9780124016989; 0124016790; 0124016987
• DOI: 10.1016/B978-0-12-401698-9.00014-8

Industrial interest in wrought heat-treatable aluminium-lithium (Al–Li) based alloys dates back to around 1919 in Germany. However the exploitation of these alloys has historically been limited by their mechanical property anisotropy and concerns over their localized corrosion resistance and temperature stability. Recently, in the last ten years, alloy and process development has resulted in alloy compositions and thermomechanical treatments that potentially can overcome these issues. To put these developments in perspective we have reviewed the corrosion characteristics of first, second and third generation alloys with an emphasis on localized corrosion (intergranular and exfoliation) and stress corrosion cracking (SCC). Intergranular corrosion susceptibility of Al–Li–Cu and Al–Li–Cu–Mg alloys increases with copper content, and the depth of attack increases with ageing, i.e. UA<PA<OA. For alloys that contain zinc the mode of attack is less truly intergranular and more intra-granular or inter-subgranular in nature. Susceptibility to exfoliation corrosion has a similar temper dependence when assessed by conventional EXCO testing, but not following either exposure to a “real-life” outdoor marine environment or to a less aggressive test such as MASTMAASIS. In these tests there are two regimes of exfoliation susceptibility corresponding to under- and over-aged tempers, and an intermediate regime, including peak-aged tempers, with an improved resistance, i.e., UA>PA<OA. SCC initiation from smooth surfaces is rare for copper-free Al–Li alloys irrespective of temper or whether exposure is under continuous or alternate immersion conditions. For copper-containing alloys, initiation remains difficult for smooth specimens under continuous immersion conditions, but occurs readily under conditions of alternate immersion. It is likely that crack initiation is controlled by an anodic dissolution dominated process, but crack propagation may be controlled by either anodic dissolution or a hydrogen related process. Test results reviewed to date on thin plate samples show that the lower lithium content third generation Al–Li alloys, with suitable zinc and/or silver additions and thermomechanical processing can provide an acceptable resistance to localized corrosion (IGC and Exfoliation) and SCC in near to peak aged tempers. For this conclusion to apply to thicker products (>~30 mm) further analysis of corrosion test results is required.