Electrochemical Passivation of Ni-Cr and Ni-Cr-Mo Alloys: The Fate of Alloying Elements and Implications of Oxide Dopants and Defects Towards Passivation and Breakdown

Ni-Cr-based alloys are regarded amongst the most corrosion resistant variety used in commercial and industrial applications. This benefit is due to the interaction of Mo alloyed in synergy with Cr and other minor elements (i.e. W). The role of Mo in stabilization and repassivation is understood, but its role in oxide formation and growth and dopant behavior is controversial. The exact atomistic mechanism by which electrochemical localized corrosion initiation is hindered or even prevented has, however, not been thoroughly studied. The first step towards achieving this understanding can be attained by a more comprehensive investigation of the role(s) of Mo and W in passive film formation and growth. Understanding the exact fate of alloying elements is also a challenge in multi-component alloys. The influence of Mo on growth rate and electronic doping of semiconducting passive films has similarly been limited in previous literature. The overall objectives of this thesis are to (1) establish an in operando electrochemical framework for investigating the kinetics of atomic scale aqueous passivation for multi-element alloys, (2) apply this framework in tandem with in-depth surface characterization in order to investigate the electrochemical passivation behavior of selected Ni-Cr and Ni-Cr-Mo alloys in acidic and alkaline sulfate and (3) chloride environments with more complex kinetic processes, (4) uncover the specific role of Mo dopants during Ni-Cr alloy passivation by promoting film growth and encouraging repassivation, and (5) demonstrate the influence of solute capture during fast electrochemical reactions on the electrochemical stability and electronic properties of passive films.
First, single frequency electrochemical impedance spectroscopy (SF-EIS) was applied for novel measurements of film growth during potentiostatic and galvanostatic polarization through analysis of an oxide as an electrical constant phase element. The unique application of online inductively coupled plasma-mass spectrometry (ICP-MS) for evaluation of element-specific contributions to dissolution and oxidation was also presented. From both of these techniques, the oxidation current density was extractable and more in-depth analysis yields the role of individual elements towards passivation or dissolution reaction kinetics. The material parameters required for broad and precise application of these techniques were additionally obtained for the alloys in additional sulfate and chloride-containing environments. The combination of methods enabled in operando tracking of the total current densities for (i) oxidation, (ii) cation ejection by multiple paths, and (iii) oxide film growth during non-steady state passivation.
The electrochemical passivation behavior of selected Ni-Cr and Ni-Cr-Mo alloys were studied in acidic and alkaline sulfate media through application of the previously established techniques and the addition of surface science methods. Specifically, the influence of Cr and Mo alloying on passivity and the inhibition of localized film dissolution were probed using the in operando electrochemical techniques described above. Complementary in situ and ex situ analysis of chemical and atomic structure were conducted using polarized neutron reflectometry (PNR), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), ellipsometry, and 3D atom probe tomography (3DAPT) to investigate the oxide composition, thickness, and morphology in detail, along with additional application of Mott-Schottky analysis to EIS data (MS-EIS) for indication of the variation of electronic properties during potentiostatic growth processes. The specific roles of pH and Mo on passivation kinetics are presented and the results provide an increased understanding of both thermodynamics and electronic factors on electrochemical measurements without the complication of chloride-assisted localized corrosion. The environment pH was found to have an impact on the relative chemical stability of Ni-Cr passive films but not noticeably for Ni-Cr-Mo ones. In addition, Mo was observed to act as an electronic dopant, increasing growth kinetics and modifying the concentration of point defects in oxides.
The initial understanding of Ni-Cr-Mo alloy passivation obtained for sulfate environments was extended to those containing chloride. There, localized corrosion was significantly increased via pitting, crevice corrosion, and general film dissolution assisted by chloride anion incorporation at the film/solution interface. In these environments, the influence of Mo was especially visible as it bolstered the resistance of films to such corrosion events and, upon their initiation, facilitated repassivation. The specific roles of pH and Mo during passivation and breakdown kinetics are highlighted, providing an insight into the fate of the elements which comprise the alloys, and their effects on passivation behavior. It was observed that early oxidation of both Ni and Cr-species occurred in the presence of acidic electrolyte. Preferential dissolution of Ni2+ at later times enabled gradual Cr3+ enrichment within the surface film. However, greater relative stability of NiO and Ni(OH)2 was observed in the alkaline condition. Upon alloying with Mo, Cr3+ became increasingly enriched in the surface film during anodic polarization. Oxides were interpreted to consist of non-stoichiometric solid solutions formed via solute capture.
The specific role of Mo towards increasing an alloy and oxide’s resistance to passive film breakdown initiation of localized corrosion and promoting repassivation at a constant potential in acidic, chloride-containing environments was studied in depth. The influence of increasing Mo concentrations on the chemical, physical, and electronic properties of the passive films grown were identified through additional application of similar techniques: DC electrochemistry including metastable pitting analysis, SF-EIS, XPS, and MS-EIS. The results provide an increased understanding of the influence Mo cations, when acting as substitutional dopants within a passive film, can have during several stages of passivity: nucleation, growth, breakdown, and repassivation. The 9 wt% Mo alloy yielded the optimal corrosion resistance and excessive doping can occur for higher Mo base alloy concentrations. This phenomena results in poor repassivation due to a decline in the surface enrichment of available, solute-captured Mo4,6+ cations.
These findings concerning the role of alloying elements toward passivation processes were extended to examine specifically galvanostatic, or “rate-controlled”, passive film growth and the implications of passivation kinetics and solute capture on the electrochemical stability, electronic properties, structure, and early breakdown initiation of passive films. This investigation into the implications of “fast” versus “slow” passive film growth utilized systematic investigation by galvanostatic SF-EIS and subsequent metastable pitting analysis, MS-EIS, XPS, and AFM. The effect of solute capture on modifying the composition and distribution of metal cations within the passive films, along with the beneficial effect of Mo capture in Ni-Cr-Mo compared to Cr in Ni-Cr are presented. It was found that the higher valency of Mo dopants enabled greater film resistivity to breakdown and enabled faster repassivation following any metastable events by inhibiting the diffusion of detrimental metal cation vacancies. Growth at slower rates produced films with segregation of Ni2+ and Mo4,6+ to the film/electrolyte interface, leaving Cr3+ enriched at the metal/film interface.
This thesis contributed to the scientific understanding of the passivation mechanisms for Ni-based alloys in acidic and alkaline chloride-containing environments, and the additional influence of Mo as a minor alloying element on growth rate and ultimately the corrosion initiation process through its influence specifically on electrochemical stability, structure, and electronic properties of electrochemically grown oxide films. This progress shed light on the precise alloying element attributes which enable passivity and their function in a solute capture-type, or conversely a phase-separated, oxide.