On both alloy categories crevice propagation can be supported by both external and internal cathodic processes. The kinetics of the external process (O2 reduction) can be followed electrochemically using a galvanic coupling technique, while the internal process (H+ reduction), although detectable by its influence on the O2 reduction kinetics, cannot be directly followed in real time. Which of these two reactions is dominant is very dependent on alloy composition and microstructure.
For Ti alloys, the microstructure is very dependent on the added alloying elements (e.g., Ni, Pd, Ru) and the content of unavoidable impurities such as Fe, and perhaps also Mo and Cr. Their distribution in the uncorroded alloy can be determined using atomic probe tomography and their redistribution, as a consequence of a period of crevice propagation, by dynamic secondary ion mass spectrometry.
By comparison to Ti alloys, Ni-based alloys (in particular Ni-Cr-Mo alloys) are free of segregated secondary phases, and possess high quality, low energy grain boundaries. These two qualities can be demonstrated using electron backscatter diffraction, transmission electron microscopy and electron energy loss spectroscopy. However, while the quality of the grain boundaries is a key feature in controlling the initiation of crevice corrosion, it is the alloy composition which exerts the dominant influence on the accumulation and distribution of corrosion damage by depositing Mo oxides at propagating sites, a process that can be demonstrated by correlating the damage distribution, determined by confocal laser scanning microscopy, with the accumulation of molybdates, confirmed by microRaman spectroscopy.
Using this combination of electrochemical, microscopic and spectroscopic techniques we are presently determining the influence of alloy composition on the relative importance of internally and externally supported crevice propagation. Also, we are developing the technique of X-ray photoelectron spectroscopic imaging to demonstrate the self-healing capability conferred on these alloys by the inclusion of Mo.