Compatibility of alumina forming alloys with LiNO₃-containing molten salts for solar thermal plants

Highlights

• Alumina forming alloys (AFA) with potential for use in solar thermal plants.

• Auspicious behavior of Ni-based AFA to the lithium-containing molten salt at 550 °C.

• New molten salt mixture with lithium as heat transfer fluid for solar thermal plants.

Abstract

The next generation of solar thermal plants will increase the operating temperatures; thus, new structural materials with better performance than the currently used should be required. Alumina forming alloys (AFA) are an alternative since they have been reported as highly resistant to corrosive environments, including molten salts. In this study, two AFA (OC4 and HR224) were exposed to a ternary lithium-containing nitrate molten salt mixture (57 wt.% KNO₃–30 wt.% LiNO₃–13 wt.% NaNO₃) at 550 °C for 1000 h to determine their corrosion compatibility through gravimetric and complementary techniques. The mass gain results revealed a good performance of both alloys, allowing them to be recommended for use in solar thermal plants. However, HR224 showed lower weight change attributed to a thin layer of non-porous and continuous corrosion products composed of nickel oxide and aluminum-nickel spinel, which act as protective compounds. On the contrary, OC4 showed a thick multi-layer structure of highly porous, rough, and irregular corrosion products composed mainly of iron oxides and spinels.

Introduction

Considering the progress of the technologies and industries associated with solar energy, the demand for more resistant structural materials and more efficient heat transfer fluids (HTF) suitable for the next generation − which involves broader temperature ranges (over to 565 °C) − has significantly increased [1]. To date, the best option in the case of structural materials has been different grades of stainless steel; however, they lose their anticorrosive behavior when exceeding 600 °C [2]. Therefore, other alloys like corrosion resistance alloys (CRA) have gained attention, but their high cost has prevented their widespread use [3]. In this regard, alumina forming alloys (AFA) appear as a promising alternative since they have shown outstanding properties such as higher oxidation resistance, creep resistance, and tensile strength. They also have good weldability and lower cost [3], [4], [5], [6]. Regarding HTF, the current state-of-the-art promotes new molten salt mixtures by two alternatives, adding additives to the existing industry standard mixture (Solar salt, a binary nitrate mixture) or its replacement by carbonates and chlorides [7,8].

AFAs can be either based on Fe or Ni, and their high oxidation resistance originates from aluminum. When present in concentrations over 2.5% by weight, Al could promote the development of a dense and protective scale on the surface [4]. This scale corresponds to Al₂O₃, which can be present in several polymorphic stables (α) and metastable phases (γ and θ). α-Al₂O₃ is obtained at extensive oxidation times and temperatures, whereas γ and θ-Al₂O₃ are formed at low temperatures [9], [10], [11]. α-Al₂O₃ is thermodynamically stable and has been demonstrated to be a more effective barrier to the attack and penetration of corrosive ions than γ and θ-Al₂O₃ since the latter two contain higher concentrations of lattice defects and can induce crack formation [3,6,[12], [13], [14], [15], [16], [17], [18]]. However, they can be transformed into α-Al₂O₃ by adding chromium or iron to Ni-based AFAs or by increasing their content in Fe-based AFAs [19]. So far, several AFAs exposed to molten salt at high temperatures have been studied; Fernández et al. [6,7,20] have investigated both Fe (OC4 and OCT) and Ni-based AFAs (In702 and HR224). OC4 was exposed to nitrates molten salt at 390 °C for 2000 h and carbonate molten salt mixture at 650ºC for 1000 h. OCT and In702 were exposed to a nitrate molten salt mixture at 550 °C for 1000 h and to a carbonate molten salt mixture at 650 ºC for 1000 h (separately). HR224 was exposed to a carbonate molten salt mixture at 650 ºC for 1000 h. The results of these studies revealed the importance of aluminum in the alloys' corrosion performance since OC4 at a low temperature almost did not show attack from the molten salt due to the formation of a protective layer based on aluminum and chromium. The authors also highlighted the beneficial effect of increasing aluminum and nickel concentration in the alloys for improving corrosion resistance; indeed, In702 showed the best performance of all the alloys. In702 was also evaluated by Gomez-Vidal et al. [14,15] in a binary chloride molten salt mixture, obtaining less promising results than in exposure to nitrates and carbonates molten salts. Besides, Gomez-Vidal et al. [14,15] reported that AFA's corrosion resistance could also be increased pre-oxidizing alumina to obtain only α-Al₂O₃. The results obtained for three AFAs (In702, Haynes 224, and Kanthal APMT) were auspicious since the corrosion rates obtained were in the recommended ranges for solar thermal plants. In the same context, Ding et al. [13] studied two pre-oxidized Fe-based AFAs in a binary chloride at 700 ºC for 500 h (in an inert atmosphere), obtaining good results. In particular, no spallation and depletion of the alumina layer was observed, which was attributed to its excellent adherence, dense and uniform structure.

Concerning molten salts, the addition of lithium to molten nitrates has been proposed as it broadens the range of thermal stability and, therefore, the operating temperatures of solar thermal plants [21]. In particular, Fernández et al. [10,[13], [14]] added LiNO₃ to Solar salt, studying its effect by both laboratory and plant scale experiments and reported enhancements in energy density, confirming the viability of their addition on a large scale. The subsequent studies of the group focused on the compatibility of lithium-containing molten salts with structural metallic materials [22], [23], [24]. A1 and T-22 were exposed to 20 wt.% LiNO₃–52 wt.% KNO₃–28 wt.% NaNO₃ at 390 °C for 2000 h; the stainless steel grades AISI430 and 316 were exposed to three molten salts mixtures (mixture A: 30 wt.% LiNO₃–10 wt.% KNO₃–60 wt.% NaNO₃, mixture B: 30 wt.% LiNO₃–10 wt.% Ca(NO₃)₂–60 wt.% NaNO₃ and mixture C: 10 wt.% LiNO₃–10 wt.% Ca(NO₃)₂–60 wt.% KNO₃–20 wt.% NaNO₃ at 390 °C for 1000 h), the results revealed lower corrosion resistance than AFAs due to corrosion products' detachment attributed to their mismatched growth and fragile behavior. Thereby, this research aims to determine the compatibility of two AFAs (Fe and Ni-based) with the promising ternary nitrate molten salt (57 wt.% KNO₃–30 wt.% LiNO₃–13 wt.% NaNO₃) in exposure to 550 °C for 1000 h. An isothermal gravimetric test assessed the alloys' corrosion performance, and complementary techniques studied corrosion products' structure. The obtained results were also compared with the reported so far to establish the potentialities of using either AFAs and lithium-containing molten salts in solar thermal plants.