Studying Role of Microstructure and Minor Alloying Elements in Cr-Dominated Passive Films with Photoemission Spectroscopy and Microscopy 157 views

The excellent corrosion resistance of nickel-chromium alloys and more complex Cr containing alloys relies on Cr as the primary protective element and synergistic interactions from minor alloying elements. Characterizing the chemistry, structure, and homogeneity of the Cr-dominated passive films and model oxides is critical to understanding how Cr interacts with minor alloying elements and gaining insight into the underlying mechanisms. Observing the passive layer of oxides and hydroxides that governs corrosion is challenging because it is only a few nanometers thin. To probe the chemical structure and the morphology of oxide films, photoelectron spectroscopies are used extensively for their chemical specificity and nanometers-scale information depth. The first stage of this work examines the role of molybdenum as a minor alloying element in NiCr alloys. Mo is known to improve the resistance of NiCr alloys to localized corrosion, but there is still debate about the mechanism of action. Quantitative X-ray photoelectron spectroscopy (XPS) combined with electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV) and atomic emission spectro-electrochemistry (AESEC) are used to characterize the structure and corrosion resistance of NiCr and NiCrMo alloys. Minor alloying reduces pitting corrosion and Mo has multiple effects on the passive film chemistry of NiCr alloys during aqueous corrosion: promoting Cr2O3, reducing film stratification, and making the composition of the passive film less sensitive to different electrochemical conditions. Early-stage oxidation studies of NiCr and NiCrMo alloys employ X-ray photoemission electron microscopy (XPEEM) to characterize oxide chemistry and morphology simultaneously. Interpreting the large amount of complex image data XPEEM produces is a unique challenge for data analysis. To address the big image data problem, quantitative image analysis techniques were used to identify Cr2O3 islands. High-dimensional X-ray absorption spectroscopy XPEEM images were analyzed with principal component analysis (PCA) and non-negative matrix analysis (NNMA) and cosine similarity were used to analyze hyperspectral images. For binary NiCr alloys, Cr2O3 is observed to nucleate and grow in a layer+island mode. The inclusion of Mo changes the oxide growth mode to a layer-by-layer growth mode due to the interaction of Mo and Cr in the oxide. The uniform oxide that forms may be linked to Mo’s role in preventing local breakdown of the passive film. The second stage of this work uses the methods developed for characterizing NiCrMo passive films and extends them to an AlCrFeMnMoNiTi compositionally complex alloy (CCA). This CCA has an FCC matrix with an L21 intermetallic phase for mechanical strengthening, and both phases contain passivating elements. To understand localized corrosion in this system, the chemistry and distribution of oxides across the phases needs to be better understood. This is especially challenging because of the large number of elements present, with many possible synergistic or anti-synergistic interactions. The methods developed in the first stage of this work fit naturally into this role. The composition and bonding air-formed native oxide is characterized on each individual phase and across the phase boundaries with XPEEM and complementary time-of-flight secondary ion mass spectrometry. The composition of the passive film is consistent across different domains and has a sharp transition in chemistry at the interface. Nanoscale inclusions enriched in Ti and Al are also observed, which may have negative implications for corrosion resistance. Finally, the early-stage oxidation behavior of this alloy is studied with controlled oxidation to better understand which oxides form and understand the structure and bonding in the initial oxide layer. Selective oxidation of each element is driven by the thermodynamics driving force for oxide formation and bulk concentration, and the composition of the oxide stays constant throughout oxidation. The oxide composition of the early-stage oxide is quantified and provides important insights into the chemistry of the native oxide.

PHD (Doctor of Philosophy)

Compositionally complex alloys; Metal Oxidation; Metal Corrosion; Xray photelectron spectroscopy; Spectromicroscopy; Image analysis

English

2025-11-07