Experimental Validation of Long Term Thermal Exposure Sensitization of AA5xxx Alloys

Zhu, Yakun (Department of Metallurgical Engineering, University of Utah) | Kar, Soumya (Department of Metallurgical Engineering, University of Utah) | Free, Michael L. (Department of Metallurgical Engineering, University of Utah) | Cullen, David A. (Materials Science & Technology Division, Oak Ridge National Laboratory) | Allard, Lawrence F. (Materials Science & Technology Division, Oak Ridge National Laboratory)

OnePetro 

ABSTRACT:

In the present work, β-phase formation along grain boundaries has been evaluated by scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) analysis methods. The thickness of β-phase along grain boundaries, measured by STEM for a 1-year thermally treated sample, was estimated to be 15 - 20 nm. Etching effects of phosphoric acid solution and ammonium persulfate solution on the Al matrix are discussed. The effect of ASTM G67 nitric acid attack on different surfaces was investigated. Mass loss data was collected for several AA5xxx alloys using long-term (~ 12 months), constant temperature (40 to 70°C) exposure tests. Other samples were treated in thermal exposure furnaces with cyclic temperatures (40 to 45°C and 50 to 70°C) to represent the heating cycles in service conditions.



INTRODUCTION

Although the presence of different intermetallic precipitates in the matrix of 5083 alloys improves mechanical properties1,2, one type of precipitate, Al3Mg2, or β-phase, compromises corrosion resistance3, 4. The β-phase has a corrosion potential of around -1.29V (saturated calomel electrode) which makes it typically more active than the AA5083 (UNS A95083) Al matrix, which has a corrosion potential of -0.73V (saturated calomel electrode). Thus the β-phase is preferentially attacked by corrosive environments4, 5, 6. The β-phase is usually associated with intergranular corrosion (IGC) and stress corrosion cracking (SCC) 6, 7. β-phase formation reduces the service life and quality of aluminum parts. Considerable research has been conducted to understand the mechanisms of β-phase-related corrosion phenomena4, 5, 6, 7, 8, 9, 10, 11, 12. Samples for SEM imaging were mounted in epoxy resin, polished to 1 ~ 0.02 micron, and then etched using a phosphoric acid solution (10 vol. % or 1.72 M phosphoric acid) to make the β-phase visible as etch trenches in secondary electron images of the cross-section samples.