Long-term integrity of the passive film of nickel-based alloys

In the absence of severe localized corrosion conditions, nickel-based alloys containing chromium are protected against fast corrosion by a chromium — rich oxide adherent film commonly known as a ‘passive film’ at the exposed surface. Typical examples are the thin, adherent passive oxide films observed on sample surfaces after short-term polarization tests and long-term immersion tests (Orme, 2005; NWTRB, 2002). Film thicknesses were in the range of a few nanometers (10-9 meters, nm, 3.9 x 10-10 inch) and tended to be rich in chromium (III) oxides (Cr2O3 and/or NiCr2 O4). A thick outer layer was also observed on top of the inner chromium-rich oxide layer. The outer layer was typically porous and consisted mostly of nickel oxide and the oxides of some other alloying elements. The chromium-rich oxide is considered to protect the bare metal against rapid corrosion in the long term, i. e. geological timeframes. A cross-sectional view of the passive film formed on the surface of an annealed nickel-based alloy is presented in Plate II (between pages 448 and 449). It is important to understand whether or not the passive layer persists for a long period of time (Ahn et al., 2008a). A number of issues have been studied to determine whether the protective layer remains stable in the long term. For example, if the protective layer grows continuously, the stress may build up at the interface of the bare metal and the protective layer, and the protective layer may spall off. However, the subsequently exposed bare metal would repassivate. Certain metalloids such as sulphur may be segregated at the interface during the anodic dissolution of the bare metal surface. A potential mechanism of the breakdown of the passive film induced by enrichment of sulphur at the metal-passive film interface is presented in Fig. 7.1 (Marcus, 19952 . When the surface concentration of the segregated sulphur exceeds a critical value, the protective layer will become unstable. The bare metal exposed as a result of the unstable protective layer may repassivate after dissolution of the accumulated sulphur layer. Other impurity elements such as silicon in the alloys or solutions may also affect the long-term stability of the protective layer. Microbially-influenced corrosion may also destabilize the protective layer. However, the bare metal surface formed after the desta­bilization of the protective layer could repassivate.

An important related issue is the accuracy in measuring very low general corrosion rates. General corrosion rates on the order of nm/year are



7.1 Mechanism of the breakdown of the passive film induced by enrichment of sulphur at the metal-passive film interface (Marcus, 1995). Used with permission from Taylor and Francis.

difficult to measure accurately. The accuracy is important because the rates must be extrapolated to a very long time period to calculate the extent of general corrosion and assess when the package would fail.

Once the passive film becomes unstable without repassivation during disposal, either high general corrosion or localized corrosion such as crevice corrosion or pitting corrosion would occur. For localized corrosion to be initiated, if there is no existing (propagating) pitting or crevice corrosion, the corrosion potential needs to reach the breakdown potential for highly corrosion-resistant alloys (such as nickel-based alloys) (Ahn et al., 2008b, 2013; ASM International,1993). This condition is determined by the severity of the evolved groundwater chemistry. More conservatively, at the corrosion potential below the repassivation potential, even the existing (propagating) pitting or crevice corrosion would be arrested. The breakdown potential or repassivation potential generally increases with higher concentration ratios of oxyanions such as nitrates to chloride (Dunn et al., 2005). Even if the localized corrosion occurs, it is not expected to open up entire areas of a container surface. The cathodic capacity of the outside of an active crevice or pit, from the separated cathodic area from the active area, would limit localized corrosion propagation fronts (Shukla et al., 2007). Some studies show only pit growth rather than uniform dissolution in the crevice area of highly corrosion-resistant alloys (Ahn et al., 2008a). Based on the cathodic capacity limitation, a maximum of 20% of the surface area is likely to be open (He et al., 2011).

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