Passivation and localised corrosion susceptibility of new Al–Cr–Fe complex metallic alloys in acidic NaCl electrolytes

Structure and selected properties of Al-Cr-Fe alloys with the presence of structurally complex alloy phases

The aim of the study was to supplement the data on the Al65Cr20Fe15 alloy with binary phase structure and the Al71Cr24Fe5 alloy with multiphase structure prepared with two different cooling rates from the liquid state. The presence of the structurally complex Al65Cr27Fe8 phase was confirmed by neutron diffraction, scanning electron microscopy with the analysis of chemical composition and transmission electron microscopy. Additionally, the Al8Cr5 phase with γ-brass structure was identified for Al71Cr24Fe5 alloy in both cooling rates from the liquid state. Due to the interesting features of structurally complex alloys, the wear resistance, magnetic properties, and corrosion products after performing electrochemical tests were examined. Based on pin-on-disc measurements, a lower friction coefficient was observed for the Al65Cr20Fe15 alloy (µ ≈ 0.55) compared to the Al71Cr24Fe5 multiphase alloy (µ ≈ 0.6). The average hardness of the binary phase Al65Cr20Fe5 alloy (HV0.1 = 917 ± 30) was higher compared to the multiphase Al71Cr24Fe5 alloy (HV0.1 = 728 ± 34) and the single phase Al-Cr-Fe alloys described in the literature. Moreover, the beneficial effect of rapid solidification on hardness was demonstrated. The alloys Al65Cr20Fe15 and Al71Cr24Fe5 showed paramagnetic behavior, however rapidly solidified Al71Cr24Fe5 alloy indicated an increase of magnetic properties. The studied alloys were characterized by the presence of passive layers after electrochemical tests. A higher amount of oxides on the surface of the Al71Cr24Fe5 alloy was recorded due to the positive effect of chromium on the stabilization of the passive layer.

Relationship between Phase Occurrence, Chemical Composition, and Corrosion Behavior of as-Solidified Al-Pd-Co Alloys

The microstructure, phase constitution, and corrosion performance of as-solidified Al₇₀Pd₂₅Co₅ and Al₇₄Pd₁₂Co₁₄ alloys (element concentrations in at.%) have been investigated in the present work. The alloys were prepared by arc-melting of Al, Pd, and Co lumps in argon. The Al₇₄Pd₁₂Co₁₄ alloy was composed of structurally complex ε_n phase, while the Al₇₀Pd₂₅Co₅ alloy was composed of ε_n and δ phases. The corrosion performance was studied by open circuit potential measurements and potentiodynamic polarization in aqueous NaCl solution (3.5 wt.%). Marked open circuit potential oscillations of the Al₇₀Pd₂₅Co₅ alloy have been observed, indicating individual breakdown and re-passivation events on the sample surface. A preferential corrosion attack of ε_n was found, while the binary δ phase (Al₃Pd₂) remained free of corrosion. A de-alloying of Al from ε_n and formation of intermittent interpenetrating channel networks occurred in both alloys. The corrosion behavior of ε_n is discussed in terms of its chemical composition and crystal structure. The corrosion activity of ε_n could be further exploited in preparation of porous Pd–Co networks with possible catalytic activity.

Anodizing of Self-Passivating W xTi1- x Precursors for W xTi1- xO n Oxide Alloys with Tailored Stability

TiO₂ and WO₃ are two of the most important, industrially relevant earth-abundant oxides. Although both materials show complementary functionality and are promising candidates for similar types of applications such as catalysis, sensor technology, and energy conversion, their chemical stability in reactive environments differs remarkably. In this study, anodic barrier oxides are grown on solid-solution W _xTi_{1- x} alloy precursors covering a wide compositional range (0 ≤ x ≤ 1) with the goal of creating functional oxides with tailored stability. A strong Ti-cation enrichment in the surface region of the grown W _xTi_{1- x}O _n layer is observed, which can be controlled by both the anodizing conditions and precursor composition. For Ti concentrations above 50 at. %, a continuous nanometer-thick TiO₂ protective coating is achieved on top of a homogeneous W _xTi_{1- x}O _n film as evidenced by X-ray photoelectron spectroscopy and transmission electron microscopy analyses. A comprehensive electrochemical assessment demonstrates a very stable passivation of the surface in both acidic and alkaline environments. This increase in chemical stability correlates directly with the presence of this protective TiO₂ film. The results of this work provide insights into the oxidation behavior of W_{1- x}Ti _x alloys, but more importantly demonstrate how controlled oxidation of self-passivating alloys can lead to oxide alloys with thin, protective surface layers that otherwise would require more sophisticated deposition methods.