Anti-corrosion mechanisms of complex concentrated alloys

Abstract

Alloys with superior corrosion resistance are desirable for various engineering applications in corrosive environments such as seawater. Nevertheless, developing corrosion-resistant alloys based on conventional strategies has reached a stagnant stage due to the fundamental limitation of the single-passivation mechanism. Complex concentrated alloys (CCAs) offer multiple degrees of freedom to design corrosion-resistant alloys that go beyond single-passivation, which are promising towards elevated corrosion resistance. Therefore, understanding the anti-corrosion mechanisms of CCAs, particularly when diverse components interact during passivation, is crucial for providing reliable guidelines. The first part of the research investigated the anti-pitting mechanism of the CoCrFeMnNi alloy, which is susceptible to chloride conditions despite containing 20 wt.% Cr content. The results reveal that the Cr-depletion zone caused by MnO·Cr2O3, which is formed by two principal elements, is to blame for its poor anti-pitting properties. Subsequently, the methods of adding Si, Al, and Ti for removing the Cr-depletion were systematically studied in detail with a deep understanding of the mechanism. The effectiveness of the method originates from lower Gibbs formation energies of the Si, Al, and Ti oxides than that of Cr2O3. The addition of Si shows the best enhancement of anti-corrosion properties and is thus employed to develop other CCAs. Second, a new alloy of Fe38.8Cr21.7Co18.7Mn17.6Si3.2 CCA was developed by removing the Cr-depletion zone and its corrosion resistance in 3.5 wt.% NaCl was investigated. It is found that a Cr-based passive layer and a Mn-based passive layer are activated in an uninterrupted sequence, protecting the CCA up to ~1700 mV (SCE), showing a fundamental breakthrough over conventional Cr-passivated alloys. From a combination of passivation sequence and chemical homogeneity, a sequential dual-passivation strategy was finally proposed and successfully applied in developing another alloy with similar superior corrosion resistance. Finally, Mn passivation of the Fe38.8Cr21.7Co18.7Mn17.6Si3.2 CCA in 0.1M H2SO4, for the first time, was studied. It reveals Mn passivation occurs at ~1350 mV (SHE), after the dissolution of Cr-based passive film. Mn passivation originates from the selective oxidation of dissolved Mn2+ and Mn3+ ions to stable MnO2. The Mn-based passive film at ~1500 mV (SHE) has a thickness of ~170 nm but is defective. The unique passivation of Mn extends the alloy to the application in acidic conditions at high potentials. Overall, the thesis provides a sequential dual-passivation strategy for designing anti-corrosion alloys with fundamental breakthroughs and highlights addressing potential local inhomogeneity from the combination of multiple elements.