Abstract

The corrosion degradation of metallic components in Concentrated Solar Power Plants (CSP) with molten salt poses significant threat to the continuous safe operation at high temperature. Updated understanding of the corrosion behavior, mechanisms and the properties of corrosion products formed are needed to explain and optimize the corrosion performance of metals. In this study, the corrosion behavior of 321 SS, 347 SS, Alloy 625 and Alloy 825, are investigated in nitrate salt for 7, 14 and 28 days under isothermal; at 565°C, and thermal cycling conditions (between 565 and 290°C) in air. Several corrosion and advance surface evaluation techniques were implemented to characterize the corrosion process. The results show that the corrosion rate after thermal cycles is lower than in isothermal condition. This was due to increased spallation of corrosion products under isothermal test condition than during thermal cycling tests; particularly in stainless steel samples after 28 days. Multilayer corrosion products were observed on the surface of stainless steels and Ni-based alloys from both isothermal and thermal cycling tests. For stainless steels, an outer layer of sodium iron oxide, intermediate iron oxide and an inner layer of iron chromium spinel were found. NiO and Cr-rich oxides that were detected on the surface of Ni-based alloy, serve as protective layers against further corrosion processes.

Introduction

The drive towards renewable energy, requirement for reduction in fossil fuel consumption and emission of carbon dioxide has received substantial attention from governments and researchers worldwide during the last few decades [1]. The exploration of renewable sources of energy has been grouped into wind, geothermal, tidal and solar energy. Solar energy has shown great promise due to the abundant amount of energy reaching the Earth [2, 3]. Electricity generation from solar irradiation can be achieved by photovoltaic (PV) and photothermal conversion [4]. Concentrated solar power plant (CSP) has played a significant role in large-scale solar thermal electricity generation from photothermal conversion [5], due to high potential efficiency, low operation cost and low environmental impact. In current CSP systems, molten salts (especially mixture of NaNO3 and KNO3) are widely used as heat transfer fluids (HTF) and sensible heat storage materials to collect, transfer and store energy in heat collectors, heat transfer pipes and hot storage tanks, respectively. The high operating temperature of next generation CSPs (typically ranging from 550-850°C) with molten salt inevitably poses a significant corrosion threat to all exposed metallic components [6]. Most candidate materials are more vulnerable to high temperature oxidation [7] and intergranular corrosion (IGC) at the metal-salt interface [7].

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