Intergranular Corrosion in Sensitized AA5083 Under Alternating Wet and Dry Conditions

AA5XXX aluminum-magnesium alloys have become a popular choice for high performance marine structures, especially for the US Navy.  The combination of their high strength-to-weight ratio, uniform corrosion resistance, and low cost make these alloys well-suited for shipbuilding.  Despite their excellent resistance to uniform corrosion, these alloys can become susceptible to intergranular corrosion (IGC) due to sensitization or precipitation of the more anodic β-phase (Al₃Mg₂) along the grain boundaries after long periods of exposures to service temperatures of as low as 50^oC.^[1-2]

A significant amount of research has been conducted on the behavior of AA5083 in the context of full immersion exposure in marine environment.^[3-6]  IGC propagation depths were found to be influenced by the exposure time, applied potential, degree of sensitization (DoS), and orientation of propagation. These studies also revealed that an incubation period is necessary for β-phase pitting to develop into IGC in chloride-containing solutions under potentiostatic conditions.  Although during service some areas of these marine structures will be constantly submerged, other parts will only be exposed to atmospheric exposure, and certain surfaces will be subjected to alternating conditions of wetting and drying.  This study examines the IGC behavior of AA5083 under various alternating exposure conditions.  Specifically, whether these alternating exposures may accelerate or impede IGC propagation in comparison with continuous full immersion was investigated.  Because environmental conditions such as relative humidity, film thickness, and electrolyte concentration influence atmospheric corrosion, it is imperative to define a dry cycle environment. 

Preliminary tests involved a series of 4 sets of alternating wet and dry cycles on AA5083-H131 specimens sensitized to 50 mg/cm².  Every wet or dry cycle lasted for 25 h, for a cumulative wet or full immersion of 100 h.  During each wet cycle, the specimens were potentiostatically held at -0.73 V_SCE in 0.6 M NaCl solution, pH 8.3.  The specimens were each subjected to a different dry cycle environment: (a) no electrolyte in 10% RH, (b) no electrolyte in 90% RH, and (c) fresh 0.6 M NaCl solution at open circuit potential.  As compared to that continuously exposed to -0.73 V_SCEfor 100 h, the IGC depths after the cumulative alternate immersion exposures, in Figure 1, are substantially lower, which could be due incubation at the beginning of each wet cycle.  Additionally, these results also implied that during dry cycle, IGC can still propagate given the appropriate RH levels and/or electrolyte conditions.  To avoid damage contributions from these factors, the 10% RH was chosen as the most conservative dry cycle environment.

In order to study the influence of cycle time on the IGC damage depths, additional experiments involving various resident times in wet and dry environment were conducted.  For each test, the resident time was equal for every wet and dry cycle.  For all the tests, the total cumulative wet or full immersion time was 100 h.  Initial results, in Figure 2, show that the shorter cycles achieved lower steady state current densities that decreased with each cycle.  Moreover, an incubation time to reach a steady state current density was observed at each cycle.  To probe the time to reach steady state current density, experiments with dry cycles as short as 30 min in between 25-h wet cycles were conducted.  IGC depths were characterized at the end by using suitable metallographic techniques, and image analyses to track effects of cycle time and incubation on the physical damages.  

The results of this study provide insights into the importance of alternating wet and dry cycles on the IGC propagation of sensitized Al-Mg alloys.  The study is part of a larger effort to understand IGC and IGSCC damage in AA5XXX series applied in marine environments.

Acknowledgements

Office of Naval Research under the contract ONR: N00014-08-10315

References

[1] G.M. Scamans, N.J.H. Holroyd, C.D.S. Tuck, Corros. Sci., 27 (1987) 329-347.

[2] R.H Jones, D.R. Baer, M.J. Danielson, J.S. Vetrano, Metall. Mater. Trans A, 32 (2001) 1699-1711

[3] S. Jain, M.L.C. Lim, J.L. Hudson, J.R. Scully, Corros. Sci., 59 (2012) 136-147.

[4] M.L.C. Lim, J.R.Scully, R.G. Kelly, Corrosion, 69 (2013) 35-47.

[5] M.L.C. Lim, R.G. Kelly, J.R. Scully, “Critical Electrochemical Conditions for Intergranular Corrosion in Sensitized AA5083-H131”, in NACE Department of Defense Virtual Corrosion Conference 2013.

[6] Y. Yuan, “Localized Corrosion and Stress Corrosion Cracking of Aluminum-Magnesium Alloys” (Ph.D. diss., University of Birmingham: Birmingham, U.K., 2005)