Development of Corrosion-Resistant Multi-Principal Element Alloys

耐腐蝕多主元合金的設計開發

Student thesis: Doctoral Thesis

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Award date18 Aug 2021

Abstract

Corrosion of metallic materials is a critical issue in a variety of industries, including oil production, mining processing and aerospace industry, because it degrades the proper function of materials and structures by accelerating their failure during service time. If not mitigated, corrosion could lead to significant environmental damage, economic loss, and even human injury or death. Therefore, exploring underlying corrosion mechanisms and developing corrosion-resistant materials are of great importance.

The multi-principal element alloys (MPEAs), such as medium entropy alloys (MEAs) and high entropy alloys (HEAs), eschew the ‘base element’ concept, providing a new route to design chemically complex alloys. Unlike conventional alloys, multi-principal element alloys contain four or more principal elements in equal or near-equal atomic ratio and attract a great deal of interests due to their superior mechanical properties. However, fewer studies focus on their anti-corrosion properties of these alloys, and the understanding of their corrosion mechanisms is poor at the fundamental level. Therefore, here we developed several corrosion-resistant multi-principal element alloys and provided a comprehensive explanation of their corrosion resistance.

We firstly developed a nanostructured eutectic high entropy alloy (EHEA) of composition FeCrNiCoNb0.5 (atomic %), which comprises a lamellar nanostructure with FCC and Laves phase. Due to the formation of a compact amorphous high entropy oxide film and the dual-phase nanostructure, this EHEA exhibits unique combination of a low corrosion current density, a large passivation region and a superior repassivation ability in 1 M NaCl, outperforming the variety of conventional alloys and other high entropy alloys hitherto reported. The outcome of this study indicates that the notion of EHEAs could be explored to design corrosion resistant chemically complex alloys.

Microstructure refinement in a metal could impede the motion of flow defects and therefore improve its strength. However, the microstructure size effect on corrosion resistance of metal, particularly multi-principal element alloys, is still an open issue. Hence, we tuned the corrosion resistance of FeCrNiCoNb0.5 EHEAs by changing their microstructure sizes. The improved passivation ability of the EHEAs with refining microstructure size is attributed to the increases of active sites on phase boundaries, which facilitate passive film growth. More importantly, we further propose that the residual stress near the FCC-Laves phase boundary is vital to passivation, which has not been reported in the literature.

Among all possible corrosion mechanisms, pitting corrosion is common to different metals, which can be initiated from the damage of surface oxide layers due to the localized chemical attack of aggressive species, such as chlorides (Cl-) and hydrogen ions (H+). Therefore, we finally designed a series of MEAs MoxCrNiCo (atomic %) (x ranges from ~0.4 to ~1.0) alloys containing FCC and sigma phases, which exhibit excellent corrosion resistance in low-pH solutions containing Cl- ions (e.g. HCl). In sharp contrast to traditional alloys, the anti-corrosion properties of MEAs do not deteriorate with reducing pH (increasing H+ ion concentration). Conversely, the transpassivation potential of our alloys increases with decreasing pH. Density functional theory (DFT) calculations were performed to gain insights into the mechanism for the high passive film growth rate on our MEAs. Moreover, we also carried out CALPHAD to help understand the stability of passive film in the acidic environment. The promising properties indicate that these MEAs can be used as replacement of traditional alloys for the industries, where the prevention of pitting corrosion is vital.