X-Ray Tomography of Intergranular Corrosion Penetration in Sensitized AA5083-H131
Authors: Lim Mary Lyn C. , Kelly Robert G. , Scully John R. 
Published: February 2020

The increased demand for speed, range, and fuel economy in the competitive field of commercial shipping and the expanding demands of the US Navy have emphasized the need for lightweight structural materials that possess a challenging combination of material property requirements. Aluminum-magnesium alloys (AA5XXX) are an ideal lightweight substitute for steel in marine systems due to their high strength-to-weight ratio, good weldability, and excellent uniform corrosion resistance. However, alloys containing more than 3 wt.% of Mg, when exposed to service temperatures for extended periods of time, can become susceptible to intergranular corrosion (IGC) due to sensitization or the precipitation of the more anodic β-phase (Al3Mg2) at the grain boundaries.[1]

From a practical engineering perspective, the incidence, distribution, and propagation rate of IGC, as influenced by metallurgical, electrochemical, and geometrical factors, are important as IGC can develop into more severe forms of damage such as intergranular stress corrosion cracking (IGSCC). Because IGC is a sub-surface form of corrosion, it is essential to understand what occurs within the bulk material, below the visible surface. IGC spreading on the surface allows for depth penetration to occur on multiple parallel sites, subsequently causing simultaneous propagation of multiple IGC fissures. These individual IGC fissures can branch out in the perpendicular directions, eventually forming IGC networks. Previous work revealed that the incidence and rate of spreading and propagation of IGC in AA5083 is governed by multiple factors: β-phase morphology and distribution, propagation direction, degree of sensitization (DoS), applied potential and current density, and exposure time.[2-5]However, these observations of IGC were obtained through activity occurring on the material surface, in the form of IGC spreading, or in conventional, 2D post-mortem, destructive analysis of metallographic cross-sections of the specimen for IGC depth penetration. Although valuable insights were acquired from these conventional 2D visualization techniques, the true 3D complex geometries associated with IGC could be inaccurately reflected, specifically with regards to networking or connectivity. IGC fissures that appear separated in 2D may actually be connected in 3D.
In recent years, X-ray computed tomography (XCT) has been an important emerging technique to visualize 3D internal structures of materials, allowing improved characterization, both qualitatively and quantitatively, of degradation and damage accumulation processes such as pitting, IGC, and cracking.[6-7] This technique is based on the variations in absorption coefficients as the X-ray beam penetrates through the specimen.[8] The absorption coefficient is associated with the density and the atomic number of the material within the specimen, enabling 3D visualization of defects and providing quantitative information.
This presentation explores the IGC damage evolution and kinetics in sensitized AA5083-H131 specimens with different geometries, which produced varying extent of IGC connectivity. The specimen geometries included wire (no spreading), thin foil (limited spreading), and the 3D specimen with planar surface exposed (both spreading and penetration). Multiple potentiostatic and galvanostatic tests were conducted in 0.6 M NaCl solution for different exposure times with IGC penetration in the longitudinal (rolling) direction. XCT was utilized to non-destructively characterize IGC depth and degree of connectivity as a function of applied potential, applied current density, and exposure time. Initial experimental results (Figure 1) show the extent, both in a qualitative and quantitative manner, of IGC damage in sensitized AA5083-H131 captured via XCT.
The results of this study provide further knowledge regarding the extent and dependence of IGC propagation kinetics with respect to the factors studied, which will be useful in life prediction modeling and in efforts to understand the underlying mechanisms controlling IGC and IGSCC in AA5XXX. This study is part of a larger effort to develop predictive models that estimate and forecast IGC and IGSCC damage progression in AA5XXX applied in marine environments. It also aids in the development of mitigation strategies and optimization of structure design for this class of alloy.

Acknowledgements

Office of Naval Research under the contract ONR: N00014-08-10315 with Dr. Airan Perez as contract monitor.

References

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