Doctor of Philosophy
Dr. David.W. Shoesmith
To enhance its corrosion resistance in aggressive media, Ni is alloyed with various amounts of Cr and Mo along with small amounts of other alloying elements such as W, Cu, and Fe. While the resulting alloys (known as Ni superalloys) show excellent passive behaviour, the function of individual alloying elements in resisting localized corrosion processes, in particular crevice corrosion is not fully understood. This study focuses on the electrochemistry and corrosion of a series of Ni-Cr-Mo (W) alloys with various Cr and Mo contents. Several electrochemical and surface characterization techniques were used to investigate the role of major alloying elements on maintaining passivity and protecting the alloy under crevice corrosion conditions.
To initiate crevice corrosion, either galvanostatic or galvanodynamic polarization was used. Using these techniques to apply an electrochemical current the cathodic reaction on the counter electrode is controlled simulating the cathodic reaction needed to drive the anodic crevice corrosion reaction. A comparison of the crevice corrosion behaviour, controlled galvanostatically, of C22 (Ni‑22Cr-13Mo-3W), BC1 (Ni-16Cr-22Mo) and C625 (Ni-21Cr-9Mo) in 5 M NaCl solution at 150°C shows that crevice initiation is mainly controlled by the Cr content of the alloy while both Cr and Mo (Mo +W) synergistically determine the crevice activation rate. Once the crevice is activated, the corrosion damage propagation profile is dominantly influenced by the Mo (Mo + W) content of the alloy and also by the applied current. Higher currents and a higher Mo + W content lead to shallower and more laterally distributed corrosion damage. A series of weight change measurements on BC1, galvanostatically crevice corroded to a constant applied charge, show that internal proton reduction plays a key role in supporting active alloy dissolution, with more than 50% of the corrosion damage supported by this reaction. The results indicate that the alloy resists initiation of crevice corrosion, but once initiation has occurred it can continue to propagate spontaneously.
The properties of the oxide films anodically formed on the alloys were investigated before and after a period of dissolution at pH 7 and 9 using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The purpose of these experiments was to determine the properties of the passive film formed on the alloy after transpassive dissolution. While Cr is the main element in maintaining passivity, the corrosion resistance of the reformed passive film after transpassive break down is enhanced mainly by Mo.
Investigation of the crevice corrosion damage morphology beneath corrosion products, showed intergranular corrosion initiated preferentially on high energy random grain boundaries. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses of grain boundaries showed needle shaped inclusions enriched in oxygen and depleted in nickel were present on these boundaries but not on the dominant ∑ 3 boundaries.
Ebrahimi, Nafiseh, "The Influence of Alloying Elements on The Crevice Corrosion Behaviour of Ni-Cr-Mo Alloys" (2015). Electronic Thesis and Dissertation Repository. 3316.