PSI - Issue 52

D. Kujawski et al. / Procedia Structural Integrity 52 (2024) 293–308 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

302 10

over 4 additional hours of exposure Cabrera (2010). Since this seminal work, numerous authors have studied thin oxide film growth in gaseous and aqueous environments, and while modifications have been proposed, they essentially build on the Mott (1939), (1940) and Cabrera-Mott (1949), (2010) model. The Cabrera-Mott (1949) model applies to the growth of thin, flaw free, oxide films, but in both gaseous and aqueous environments oxide films frequently grow thicker than this model predicts. This is the result of a much slower growth mechanism that deposits metal ions in hard solid phases on the environment side of the initial film as illustrated in Fig 6. This change from oxide growth to oxide deposition results in a change in the structure and morphology of the phases present and the outer (precipitated) layer is very heterogeneous with fissures that in some cases, extend all the way down to the barrier layer as illustrated in Fig. 6. The structure and morphology of the phases present in this layer indicates that it is formed by the precipitation and growth of solid phases from metal ions in aqueous solution. At a crack tip, metal ions would be solvated by the formation of aquo-metal ion complexes Vasudevan et al. (2022). These complexes typically have 6 water molecules bound to a central metal ion and this increase in size and results in an increase in viscosity. As these aquo-metal ion complexes transport from the crack tip, they begin to decompose forming hydroxide ion complexes (hydrolysis). The linking of the hydroxide ions in these complexes (olation) further increases the size and viscosity of these complexes. The end-product phases must nucleate and grow in these gels. The time required for the kinetics of these reactions will depend on the particulars of alloy and environment chemistry, but our survey of the literature found that these processes tend to be slow compared to fatigue testing frequencies Vasudevan et al. (2022). Therefore, when a crack closes during a fatigue experiment, the outer, precipitated, layer shown in Fig. 6 will not yet have formed, and only the barrier layer and solution rich in hydroxide ion complexes will be present in the region of the crack tip Vasudevan et al. (2022).

Fig. 7 Schematic of a typical passivating film on a metallic alloy boldly exposed to humid air or water long enough to reach steady state film thicknesses. It appears that the oxides in the barrier layer may fail both of the requirements above for causing OICC. First, its growth at room temperature is limited by solid state diffusion and the Carbera-Mott limit to values on the order of 1 to 20 nm. These factors will not allow oxide volumes to grow to the required values for OICC during a load cycle. Second, recent research indicates that these thin films can be ductile. For example, Yang et al. (2018) used an environmental transmission electron microscope (ETEM) and found that at room temperature thin oxide films of this type can be quite ductile and deform in a viscous manner. This behavior was attributed to the suppression of the glass transition temperature, but they also reported less ductile behavior for thicker films and higher strain rates Yang et al. (2018). Therefore, at this time it is unclear how these layers will behave during a fatigue cycle. They may deform in a ductile manner, like that reported by Yang et al., (2018) or they may fracture in a brittle manner. In either case, bare metal will