High temperature corrosion beneath carbonate melts of aluminide coatings for CSP application

Highlights

• Corrosion of P91 by molten carbonates at 650 °C takes place at very fast rates.

• Dynamic conditions lead to higher corrosion degradation of coated and uncoated samples.

• Non-uniform corrosion takes place on FeAl coated systems.

• Mn from the substrate reaches the corrosion scale surface on coated and uncoated P91.

Abstract

Slurry iron-aluminide coatings deposited by spraying on 9 wt% Cr P91 alloy as well as uncoated P91 were exposed isothermally at 650 °C to a ternary molten salt mixture based on a Na, K and Li carbonate eutectic, under static and dynamic conditions. Uncoated P91 evidenced considerable mass gains and extensive spallation in both conditions. Indeed, P91 developed a very thick fast growing multilayered oxide scale which included LiFeO₂, LiFe₅O₈ and (Fe,Cr)₃O₄. Under dynamic conditions, the metal loss was higher that when the test was carried out statically but this was not reflected in the gravimetric measurements likely due to spallation of the scales in both cases. The coated systems performed better than the uncoated material up to at least 1000 h according to metallographic inspection. However, the aluminide coating showed non-uniform attack and on the corresponding zones, a thick layer likely consisting of LiFeO₂ developed over an internal oxidation zone corresponding to all of the coating initial thickness. K was also detected within the internal oxidation zone suggesting that the coating was internally attacked at least by K containing species (Li cannot be detected by EDS). K and perhaps Li seem to diffuse along the grain boundaries of the coating, leading to internal oxidation responsible for the degradation. On the non-degraded zones, the coating maintained the initial microstructure as very low coating/substrate interdiffusion occurred. A 20 wt% average Al content at the surface does not seem to be high enough to sustain a protective oxide.

Introduction

Among renewable energy technologies, concentrated solar power (CSP) technology shows great potential, converting sunlight into thermal energy at high conversion efficiencies [1]. Compared to other renewable energy sources, its hybridization potential and unique ability to store heat for producing power beyond daylight hours, renders CSP one of the most potentially competitive technologies in the market [1,2].

One of the key components in the thermal storage system is the thermal energy storage (TES) medium, which must provide a practical and low-cost solution to enable the technology to be dispatchable [3]. Molten salts are currently the most used storage fluids in commercial central tower technology CSP plants, specifically an alkali metal nitrates mixture composed of 60 wt% NaNO₃ and 40 wt% KNO₃ called Solar Salt [4]. The molten salts are pumped to the receiver during sunny periods, stored in the so called “hot tank” (~565 °C) and from there pumped to a heat exchanger for steam production. The returning, cooler salts are also stored as a liquid in the “cold tank” (~290 °C) from where they are pumped back into the receiver when the sun shines [5]. Indeed, the Solar Salt has a freezing point of ~214 °C and the upper operating temperature is limited to 580 °C due to salt decomposition [6]. In order to achieve higher operating temperatures (≥600 °C) for advanced and more efficient power cycles, nitrate mixtures must be replaced by other halide (chlorides, fluorides) or carbonate salt mixtures [7]. Molten chlorides have higher decomposition temperatures than molten nitrates but are extremely corrosive to metal alloys [[6], [7], [8]], especially in presence of oxygen or water, so controlled salt purification and pre-melting procedures under inert atmospheres are required to remove impurities and avoid reactions and accelerated corrosion [7]. Alkali carbonates salts containing Li are less corrosive than halide salts, and can operate at temperatures up to 800 °C without decomposing and most notably, do not require delicate and expensive preparation under inert atmosphere. However, these salts mixtures can be expensive as the price of Li is high (around $7.50/Kg [9]) due to the lithium-ion batteries market demand [10]. Mehos et al. [7] made some estimations of the market price of various salt compositions and concluded that a carbonate ternary mixture (K/Na/Li) is almost three times more expensive than the Solar Salt. Nevertheless, despite the high price, numerous authors (such as Turchi et al. [10]) highlight the great potential of these salts for CSP applications in particular due to the higher heat capacity, as less amounts are required for the same storage capacity when compared for instance with the Solar Salt.

Within carbonate-based salt mixtures, a molten eutectic ternary mixture consisting of 32 wt% Li₂CO₃, 33 wt% Na₂CO₃, and 35 wt% K₂CO₃ seems to offer a promising working temperature range [8]. Encinas-Sánchez et al. [11] demonstrated a working temperature range of 394.49 °C – 820.30 °C. Turchi et al. [10] studied the use of this ternary mixture at 650 °C in a molten-salt power tower configuration. These authors observed good thermal properties, highlighting its higher volumetric heat capacity (ρCp) of 3.60 kJ/l.°C at 600 °C as compared to that of the Solar Salt (2.65 kJ/l.°C at 600 °C). ρCp is an important factor in determining the volume of the storage tanks, as the tank size required for a given storage capacity is inversely proportional to the energy density ρCpΔT, where ΔT is the temperature difference between the “hot” and “cold” storage tanks [10]. However, corrosiveness of this molten salt mixture is a major constraint [3] and requires close attention to avoid incidents and prevent process failures [8]. Corrosion studies of carbon, ferritic and stainless steels in contact with molten carbonates for CSPs have been addressed in a number of research works [3,8,[12], [13], [14]] and Ni-based alloys [15] and austenitic steels have received particular attention because of their strength and resistance to oxidation. Because of the high price of Ni-based alloys, and looking for the formation of Cr-rich protective oxide scales, austenitic materials have gained interest [10].

Coatings on lower cost ferritic and austenitic steels maybe an alternative and there are several studies of protective coatings in contact with molten salts [[16], [17], [18], [19], [20], [21], [22], [23]]. For instance Agüero et al. studied the behavior of aluminides produced by Al Ion Vapor Deposition (IVD) or spraying an Al slurry on AISI 310 followed by a diffusion heat treatment for molten carbonate fuel cells (MCFCs). They showed that these coatings were very protective from the eutectic 62 mol% Li₂CO₃ – 38 mol% K₂CO₃ mixture at 650 °C for at least 1000 h [16,17]. In addition, the quasi-crystalline approximant AlCoFeCr as well as FeCrAl coatings deposited by plasma spray on AISI 310 also exhibited excellent behaviour for 1000 h at 700 °C employing the same carbonate eutectic melt [18]. Recently, Gomez-Vidal et al. [19] evaluated the protective character of thermally sprayed MCrAlX coatings (M: Ni, and/or Co; X: Y, Hf, Si, and/or Ta) in molten carbonate environments, consisting of eutectic carbonate mixtures (both binary and ternary) at 600 °C. The coatings were able to mitigate corrosion, reducing the rate of the base materials loss from 2500 to 34 μm year⁻¹. However, results showed some internal porosity due to the intrinsic morphology of thermal spray processes that could lead to corrosion. In addition, aluminide coatings have shown good corrosion properties in both molten nitrates [20,21] and carbonates [17,18] mixtures. Ni et al. [22] also showed that in molten K/Na/Li carbonates at 650 °C, the aluminide reacts with Li and forms a thin layer of corrosion-resistant LiAlO₂ and Gonzalez-Rodriguez et al. [23] reported that a minimum of 14 wt% of Al in a FeAl intermetallic is required to form a continuous LiAlO₂ scale in a Li/K carbonate melt at 650 °C. Moreover, they found that previous heat treatments of the alloy surface play a key role on the improvement of the corrosion behavior (corrosion rate reduction over 50%).

Slurry aluminide coatings are obtained by means of applying a paint and constitute a simple, practical and low cost process [24]. Other than molten carbonates [16,17] and nitrates [21,25,26], slurry aluminide coatings deposited on ferritic steels have demonstrated excellent resistance to several corrosive environments such as steam [27] and boiler coal and biomass combustion atmospheres [28,29]. Al slurries can be applied on the inner surfaces of tubes employing the already developed industrial process Valior™ [30].

In this work, slurry iron aluminide coatings containing 20 wt% of Al were applied on 9 wt% Cr P91 alloy. Both coated and uncoated samples were exposed isothermally at 650 °C to the above mentioned molten eutectic ternary mixture consisting of 32 wt% Li₂CO₃, 33 wt% Na₂CO₃, and 35 wt% K₂CO₃, under static and dynamic conditions. The coated samples in the as-annealed state as well as the exposed samples along with uncoated P91 were characterized by means of optical microscopy, FESEM and XRD to understand the corrosion behavior of those materials.