Remedial treatments for stress corrosion cracking in Al-Zn-Mg-Cu alloys

S. P. Knight, S. P. Lynch, N. Birbilis, B. C. Muddle

Research output: Chapter in Book/Report/Conference proceedingConference PaperOther

2 Citations (Scopus)

Abstract

Stress corrosion cracking (SCC), occurring in aqueous and moist air environments, continues to be a problem for 7xxx series aluminium alloys, particularly with respect to maintenance of ageing aircraft. Reducing the relative humidity of the environment was found to decrease SCC velocities in the 7079, 7022, and 7075 peak-aged Al alloys. Decreasing the humidity had a greater effect on SCC velocity for the low-copper 7079 and 7022 alloys compared with the high-copper 7075 alloy. This result may be caused by differences in the oxide-film that develops at the crack-front during SCC in moist air. Remedial heat-treatments, such as overaging and retrogression and re-ageing (RRA), are known to be effective at decreasing SCC velocity in alloys containing >1 wt% Cu. However, after 30 or more years of research there is still no consensus on the mechanism of improvement. The present work shows overaging and RRA were effective at decreasing SCC velocity for 7075 and 7022 alloys, also for 7079 following a slow quench from the solution treatment temperature. For 7079 material that was rapidly quenched from solution treatment temperature, overaging and RRA treatments produced no change in SCC velocity despite significant changes in microstructure. In aqueous chloride solutions, crack growth rates between 7079, 7075, and 7022 varied by 2 orders of magnitude in the peak-aged (T651) condition, and by 4 orders of magnitude in the overaged condition. There were no obvious differences in microstructure (grain boundary precipitate size and spacing, precipitate free zone width, intragranular precipitate size and spacing) between the alloys studied in either the peak-aged condition or the overaged condition. Results from analytical transmission electron microscopy suggest that grain boundary precipitate composition determines the rate of cracking for 7xxx alloys in aqueous environments.

Original languageEnglish
Title of host publication49th Annual Conference of the Australasian Corrosion Association 2009
Subtitle of host publicationCorrosion and Prevention 2009
Pages612-623
Number of pages12
Publication statusPublished - 1 Dec 2009
EventAustralasian Corrosion Association (ACA) Conference 2009: Corrosion and Prevention - Coffs Harbour, Australia
Duration: 15 Nov 200919 Nov 2009
Conference number: 49th

Publication series

Name49th Annual Conference of the Australasian Corrosion Association 2009: Corrosion and Prevention 2009

Conference

ConferenceAustralasian Corrosion Association (ACA) Conference 2009
Abbreviated titleCP 2009
CountryAustralia
CityCoffs Harbour
Period15/11/0919/11/09

Keywords

  • Aluminium alloy.
  • Overaging.
  • Retrogression and reageing
  • Stress corrosion cracking.

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