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This is an informal report intended primarily for internal or limited external distribution. The opinions and conclusions statea are those of the author and may or may not be those of the Laboratory. Work performed under the auspices of the U.S. Department of Energy by the 6 Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. __U_[II_ flF TF_t_OOCG_F.._T !$ UNLIMITETi['q. DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completene_ or usefulaess of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commerdni products, proces_ or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of Califoraia. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. a r This report has be,m reproduced directly from the best available copy.

Alloys of AA 5182 are commonly used in the automotive industry to provide weight reductions in vehicle chassis. The strength of such alloys is based on the Mg content, with Mg causing the precipitation of a β-Al8Mg5 phase along the length of the grain boundaries in the microstructure. These precipitates are vulnerable to intergranular corrosion (IGC) in acidic environments. For this reason, the proportions of Mg in the microstructure must be limited, with >3 wt-Mg causing intergranular attack. The alloys used in this study had an Mg content of 4·43, so were vulnerable to IGC. Standard nitric acid mass loss tests were applied to determine the level of corrosion resistance in two samples subjected to various treatments. Using ‘five parameter’ analysis, the importance of the grain boundary plane and thermomechanical history to IGC resistance was determined. ‘Special’ planes were identified in the microstructure and their impact on IGC resistance was assessed. From such analysis, it was found that a high deformation level of 50 together with a simulated batch annealing heat treatment yielded the highest proportions of ‘special’ boundaries within the microstructure. ‘Special’ boundaries in this study were identified to be low angle boundaries, <111> twist boundaries and <110> tilt boundaries, which are associated with a reduced energy compared to random high angle boundaries. As precipitation generally occurs along high energy boundaries, ‘special’ boundaries reduce precipitate formation, thus reducing IGC intensity. Future implications include the introduction of a higher proportion of ‘special’ boundaries into the microstructure, allowing for an increase in Mg content in these alloys.