Unraveling the role of tungsten as a minor alloying element in the oxidation NiCr alloys

Abstract

Ni-based superalloys offer a unique combination of mechanical properties, corrosion resistance and high temperature performance. Near ambient pressure X-ray photoelectron spectroscopy was used to study in operando the initial steps of oxidation for Ni-5Cr, Ni-15Cr, Ni-30Cr and Ni-15Cr-6W at 500 °C, p(O2)=10−6 mbar. The comparison of oxide evolution for these alloys quantifies the outsized impact of W in promoting chromia formation. For the binary alloys an increase in chromia due to Cr-surface enrichment is followed by NiO nucleation and growth thus seeding a dual-layer structure. The addition of W (Ni-15Cr-6W) shifts the reaction pathways towards chromia thus enhancing oxide quality. Density functional theory calculations confirm that W atoms adjacent to Cr create highly favorable oxygen adsorption sites. The addition of W supercharges the reactivity of Cr with oxygen essentially funneling oxygen atoms into Cr sites. The experimental results are discussed in the context of surface composition, chemistry, reactant fluxes, and microstructure.

The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni–Cr–Mo Alloys

“…Reports suggest that Mo suppresses the dissolution of Cr, [20,[23][24] stabilizes the passive film in the case of local corrosion attack known as pitting corrosion. [19,25,26] favoring the Cr 2 O 3 formation, [27][28][29] and has beneficial properties when it comes to re-passivation of the surface. [26,30,31] Ni alloys are less extensively studied than stainless steel, and there is still a lack of understanding of the corrosion mechanisms.…”

“…[32] NiO and MoO 2 are good catalysts for the oxygen evolution reaction (OER), [33][34][35][36][37][38][39] which adds another dimension of complexity to understanding the degradation of Ni-Cr-Mo alloys since OER is known to be coupled with dissolution and degradation in other related materials. [40][41][42][43][44][45] The oxidation and corrosion of Ni alloys have previously been studied with electrochemical measurements, [20,31] X-ray photoelectron spectroscopy (XPS), [23,[27][28][29][46][47][48] scanning tunneling microscopy, [21,30,49] time-of-flight secondary-ion mass spectrometry, [50,51] transmission electron microscopy and energy loss spectroscopy, [52,53] operando neutron reflectivity, [54,55] and inline inductively coupled plasma mass spectrometry. [56,57] providing valuable information of the electrochemical behavior, the passive film, and metal dissolution.…”

Elemental partitioning and corrosion resistance of Ni–Cr alloys revealed by accurate ab-initio thermodynamic and electrochemical calculations

“…For the purpose of long-term passivation of Ni-Cr alloy surfaces, the growth of (oxy)hydroxides should be suppressed and/or that of denser oxides promoted. The alloying elements can be tuned to chemically activate the oxide formation 37,41,116,117 , or to promote the nucleation of metastable Cr 2 O 3 by providing a corundum structure template 118 . Some surface treatment methods can also be exploited to realize highquality surface passivation, e.g., pre-oxidation in oxygen atmospheres 100,101 and deposition of oxide coatings 119 .…”