The surface chemistry associated with pitting corrosion on a nickel-based alloy, Inconel 600, has been studied using imaging x-ray photoelectron spectroscopy. The irregularity of the sample surfaces necessitated a more elaborate background correction than is usually applied in such imaging. Areas of active pitting were found to contain corrosion products that are high in chromium oxide and depleted in nickel and iron. Sites of anodic activity on the surface were able to be defined more clearly using principal component analysis. Elements that are dissolved preferentially are deposited cathodically within well-defined regions whose distance from the anodic pit appears related to the open-circuit potential. Copyright
INTRODUCTIONThis paper has two themes: a study of corrosion mechanisms that assist the pitting of Inconel 600 under hydrothermal conditions; and the assessment of XPS and Auger electron spectroscopy (AES) imaging methods to characterize such processes. X-ray photoelectron spectroscopy imaging has been of increasing interest to the surface analysis community with the emergence of commercial spectrometers equipped with the capability for resolving the origin of photoelectrons spatially separated on a surface by a few microns. Chemical specificity is the distinct advantage of photoelectron spectroscopy imaging over the equivalent AES and secondary ion mass spectroscopy (SIMS) techniques, but the low photoelectron emittance could lead to unacceptably low image contrast, particularly if a narrow bandpass is required to resolve closely placed chemical states. Also, the high backgrounds associated with some XPS spectra could lead to false contrast from areas with different extrinsic loss contributions. These latter challenges are particularly severe on rough nickel alloy surfaces affected by pitting corrosion; chemical shifts of interest are <2 eV and the extrinsic loss structure for the most intense metal 2p lines is high. Thus, part of this work has sought background correction remedies that could be applied to such images. Also, the most effective use of chemical shift information has been sought, in light of the need to acquire such images over times considerably longer than those used for other surface techniques. Finally, advantages of XPS imaging other than chemical effects are explored for these particular surfaces. Localized corrosion is initiated and sustained by electrochemical couples that drive an anodic dissolution of metal and a cathodic deposition of corrosion products at sites adjacent to the anode. The chemical potential driving the reaction is created by local differences in impurity or defect concentration. Thus, microscopic surface analysis is quite important to any diagnosis of the causes of the corrosion failure. The present case concerns pitting attack during its use as a nuclear boiler heat-exchanger alloy in contact with pressurized water at 280°C. Of primary interest is the mechanism that gave rise to the corrosion cell, the size of the cell and evolution of the cell as the pitti...
