Corrosion Behavior of Structural Materials for Potential Use in Nitrate Salts Based Solar Thermal Power Plants

Abstract

Commercial and economic success of concentrated solar power (CSP) plants requires operating at maximum efficiency and capacity which necessitates the use of materials that are reliable at high temperatures. This study investigates the corrosion behavior of structural alloys in molten nitrate salts at three temperatures common to CSP plants. Corrosion behavior was evaluated using gravimetric and inductively-coupled plasma optical emission spectroscopy (ICP-OES) analysis. Surface oxide structure and chemistry was characterized using X-ray diffraction and Raman spectroscopy. Electrochemical behavior of candidate structural alloys Alloy 4130, austenitic stainless steel 316, and super-austenitic Incoloy 800H was evaluated using potentiodynamic polarization characteristics. Gravimetric and ICP-OES analysis indicated that Alloy 4130 exhibited the least corrosion resistance at 500 • C compared to SS316 and 800H. However, at 300 • C, the three alloys exhibited similar weight gain. Electrochemical evaluation of these candidate materials was observed to correlate well with the corrosion behavior observed from gravimetric and ICP-OES analysis. This study identifies that all three alloys exhibited acceptable corrosion rate in 300 • C molten salt, while elevated salt temperatures require the more corrosion resistant alloys, stainless steel 316 and 800H. Characterization of the sample surfaces revealed the presence of spinels at lower temperatures, while Fe 2 O 3 was the dominant iron oxide at higher temperatures for each alloy. Like all power plants, maximizing the conversion efficiency of solar energy in a concentrated solar power (CSP) plant is essential to minimizing cost and resources while maximizing energy output. The choice of heat transfer fluid is a fundamental parameter in maximizing efficiency of an energy source. A heat transfer fluid that can also function as an energy storage medium for solar energy while being chemically compatible with the structural components of the plant is an ideal candidate for implementation.Significant research had been performed from 1982 to 1985 on the use of molten salt as a heat transfer medium in solar power applications. Sandia National Laboratory in New Mexico conducted the Molten Salt Electric Experiment (MSEE) which was the first power tower system to use molten nitrate salt as the primary working fluid to produce electricity. This experiment proved the feasibility of using molten salt as a heat transfer medium in a central receiver design along with a two-tank storage system designed to address intermittency. 1The MSEE used a molten nitrate salt composition of 40% KNO 3 and 60% NaNO 3 by weight which is the composition used for the study presented herein.Corrosion of structural components is determined by the properties of the heat transfer fluid and corrosion resistance of the components in contact with that heat transfer fluid. Depending on the corrosion resistance of the alloy to a high-temperature, oxidizing environment, the alloy will either form an adherent, compact oxid...

Surface morphology analysis of oxide layers formed on 10CrMo9-10 steel used in the power industry

“…Many research centers performs research related to the oxidation of materials [7][8][9][10][11][12][13][14][15][16][17][18][19][20], especially metallic materials [21][22][23][24]. Corrosion is an issue of high interest to scientists [25][26][27][28][29][30]. In particular, the problem of high-temperature corrosion is particularly important in the case of long-term elements operation in power stations or combined heat and power stations [9,21,[31][32][33][34][35][36][37][38][39].…”

Corrosion Studies of Low-Alloyed FeCrAl Steels in Liquid Lead at 750 °C

“…However, today CSP plants are limited to operational temperatures below 600 °C. The limitations are mainly due to the disintegration of conventional liquid heat transfer fluids (HTFs), such as nitrate-based solar salts, but also by the corrosion rates of commercial construction materials that will increase with temperature [1][2][3][4][5][6]. These drawbacks have initiated a search for HTFs that are better suited at high temperatures and high heat fluxes, and here, liquid lead has been suggested as a potential candidate.…”

Scale Formation and Degradation of Diffusion Coatings Deposited on 9% Cr Steel in Molten Solar Salt

“…Hence, the current development trend focuses on CSP plants with TES (e.g., Crescent Dunes Solar Power Plant, USA) by the utilization of heat transfer fluids (HTF) with high thermal capacities. Molten nitrate salts, particularly the non-eutectic mixture of 60 wt.% NaNO 3 -40 wt.% KNO 3 (known as "solar salt") are qualified as state-of-the-art HTF for CSP applications owing to their beneficial thermophysical properties for TES [1,4,[6][7][8][9][10][11]. The minimum temperature of the operation range of solar salt (around 290 • C) is limited by the liquidus temperature (lying around 240 • C) to avoid solidification of the salt, since continuous molten salt flow is required [2,12].…”

“…More importantly, both decomposition reactions result in an increased concentration of oxidizing agents in the salt melt, which is basically an ionic electrolyte at higher temperatures and conveys oxidizing agents to metallic surfaces, while metal cations can be dissolved into the salt melt [2,10,13]. For instance, Cr is a stable oxide former for various industrial applications; however, Cr cations present in the Cr-rich oxide scales can further oxidize and form chromate or dichromate species (e.g., via Reaction 3 [7,15]) that are highly soluble in the molten salt [9,11,16]. The formation of these species not only increases the extent of Cr-depletion, but also generate major environmental and health issues due to chromates being highly toxic [6].…”

“…Furthermore, commercial solar salt contains a wide variety of impurities, such as chlorides and sulfates, which can increase the corrosion rate, as shown in [17,18]. Due to the above stated reasons, the corrosion resistance of structural materials employed in the receiver tubing system and the hot storage tank is a decisive factor for plant design and lifetime [4,6,[9][10][11]19].…”