Interplay of water and reactive elements in oxidation of alumina-forming alloys

Abstract

High-temperature alloys are crucial to many important technologies that underpin our civilization. All these materials rely on forming an external oxide layer (scale) for corrosion protection. Despite decades of research on oxide scale growth, many open questions remain, including the crucial role of the so-called reactive elements and water. Here, we reveal the hitherto unknown interplay between reactive elements and water during alumina scale growth, causing a metastable 'messy' nano-structured alumina layer to form. We propose that reactive-element-decorated, hydroxylated interfaces between alumina nanograins enable water to access an inner cathode in the bottom of the scale, at odds with the established scale growth scenario. As evidence, hydride-nanodomains and reactive element/hydrogen (deuterium) co-variation are observed in the alumina scale. The defect-rich alumina subsequently recrystallizes to form a protective scale. First-principles modelling is also performed to validate the RE effect. Our findings open up promising avenues in oxidation research and suggest ways to improve alloy properties.

High-temperature oxidation behaviour of AlxFeCrCoNi and AlTiVCr compositionally complex alloys

“…According to the well-established Wagner's theory [30], an oxidation process would follow a parabolic rate law, if the growth of scale is diffusion controlled, and therefore, it is convenient to suggest that the Al0.9FeCrCoNi CCA is capable of forming of a protective oxide scale on its surface during oxidation at 700 and 900°C. It is noted that Wagner described the formation of oxide scale as an electrochemical process with a cathode at the scale/gas surface and an anode at the alloy/scale interface [1,2,30,31].…”

“…At elevated temperatures, alloys tend to react with the environment and form different "unwanted" products (e.g., oxides, nitrides, carbides, and sulphides), and if oxidation is unhindered, such reactions may wholly consume the alloy. As such, the ability of high-temperature alloys to form an adherent, slow-growing and protective surface oxide (scale), nominally consisting of chromia (Cr2O3) and alumina (Al2O3), is essential for sustained high temperature and harsh environment exposure without premature failure [1,2]. Of the two family of high-temperature materials, i.e., alumina-and chromia-formers, alloys forming protective Al2O3 scales such as iron (Fe)-based (FeCrAls) [1], nickel (Ni)-based (NiCrAls) [3], cobalt (Co)-based (CoCrAls) [4] and nickel aluminates (NiAl)-type materials [5] are considered for applications with operating temperatures >900ºC.…”

“…As such, the ability of high-temperature alloys to form an adherent, slow-growing and protective surface oxide (scale), nominally consisting of chromia (Cr2O3) and alumina (Al2O3), is essential for sustained high temperature and harsh environment exposure without premature failure [1,2]. Of the two family of high-temperature materials, i.e., alumina-and chromia-formers, alloys forming protective Al2O3 scales such as iron (Fe)-based (FeCrAls) [1], nickel (Ni)-based (NiCrAls) [3], cobalt (Co)-based (CoCrAls) [4] and nickel aluminates (NiAl)-type materials [5] are considered for applications with operating temperatures >900ºC. This is because α-Al2O3 scale possess several characteristics that are highly desired for the high-temperature oxidation performance of alloys including: (i) they grow very slowly, (ii) thermodynamic stability and (iii) chemically inert [1,2].…”

“…Despite sustained research and development in the field of high-temperature materials for many decades, there remains an ongoing desire for materials with improved high-temperature capabilities owing to demands for higher process efficiencies [1]. Presently, compositionally complex alloys (CCAs) are currently being considered as promising candidate materials for high-temperature applications owing to their high melting points, high-temperature strength (creep resistance), wear resistance and microstructural stability [6].…”

Real-time dissolution of a compositionally complex alloy using inline ICP and correlation with XPS

“…It is likely due to the fact that Al was largely oxidised during sputtering (> 50 %). It is also possible related to the slow kinetics of Al oxide formation, such slow-growing of alumina (Al2O3) has been reported in Al-containing high temperature alloys and such slow-forming, dense oxide is beneficial for their oxidation property 22 .…”