ABSTRACT

Molten salts have emerged as viable candidates for thermal energy storage in Concentrated Solar Power (CSP) applications. Candidate chloride salts offer the advantages of being readily available and stable at high temperatures, thus opening up the possibility for increased power generation efficiency. However, molten chloride salts are corrosive; therefore, proper materials selection for plant hardware is vital. Current CSP plants use stainless steels and nickel-base alloys as materials of construction because of the desirable combination of mechanical properties and corrosion resistance. In this research project, the focus was on the corrosion behavior of two different stainless steels (UNS S30400 and UNS S31600) and a carbon steel, i.e., UNS G10180. These were tested at 700°C in a molten NaCl-KCl-MgCl2eutectic salt in static air and flowing argon. Electrochemical techniques were used to characterize the corrosion behavior of these materials. The morphology of the attack was determined using scanning electron microscopy coupled with energy dispersive spectroscopy (EDS). X-ray diffraction was used to characterize the corrosion products formed on the surface of the substrate. Based on these results, the candidate salt was deemed to be unsuitable for this application. In addition, all of the candidate alloys had unacceptably high corrosion rates.

INTRODUCTION

Solar energy is a rapidly expanding alternative to fossil fuels that is predominantly harvested through the use of photovoltaics (PVs). The functionality of PVs is restricted to the periods when direct sunlight is available. One method of overcoming this constraint to energy generation is through the use of concentrated solar power (CSP). CSP is emerging as an attractive alternative to PVs because of the persistence in providing energy to the grid during periods when the sunlight is absent. This is facilitated by storing energy in a heat transfer fluid (HTF).

Molten salts have been identified as ideal heat transfer fluids in CSP applications because they contain many desirable thermal properties, e.g., high heat capacities, stability over a wide range of temperatures, etc. The Andasol plant in Spain and the Solano plant in Arizona currently use nitrate mixtures as heat transfer fluids.1 Stainless steels and nickel-base alloys are the materials of choice for molten salt containment in many current CSP plants because they provide sufficient corrosion resistance when exposed to nitrate salts, the currently preferred HTF.2 Chloride mixtures are a potentially attractive alternative because they offer higher thermal stabilities and are more economically viable than nitrates. However, molten chloride salts are corrosive, and therefore, proper salt & containment material selection are essential.

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