Electrochemistry of High Temperature Corrosion in Molten Chloride Salt

Abstract

Technological interest for the use of molten chloride salts as heat transfer and storage media in advanced nuclear reactors [1] and in concentrated solar power plants is increasing. However, alloy corrosion is the critical materials-compatibility issue for structural materials exposed to these media. Driving forces for corrosion in molten salts are impurities, temperature gradients, and activity gradients [2]. Impurities in molten salts are H2O, OH-, O2, metal oxides, and polyvalent metal ions [3].We have studied the corrosion behavior of Fe-Cr-Ni model alloys in LiCl-KCl-MgCl2 salts; the eutectic has a compromise between raw material cost, performance, and melting point for use as a coolant in a molten salt reactor. Water is soluble in high temperature molten salt; it reacts with the F- and Cl- to produce HF and HCl, which are highly corrosive – so a residual amount of water enhances corrosion rates dramatically.Water and oxygen contents of salts were removed via step-wise heat treatment under Ar gas to avoid hydrolysis. We achieved the removal of residual H2O and HCl by convecting the melt through Mg ribbons with a very high surface area. We developed a Mg|Mg2+ reference electrode for this system, provided that MgCl2 is part of the eutectic and the water concentration is low enough. It is found to be nearly perfect, as seen from electrochemical measurements.The residual water content and corrosion products such as Ni2+, Fe2+, Cr2+ in molten salts are measured electrochemically by cathodic polarization of Pt. Whilst the main focus of this study was alloying effects on corrosion, interesting data were obtained on the anodic behaviour of deposited Mg, suggesting a new level of complexity in the Mg dissolution mechanism. We will also talk about the open circuit potentials of various metals (e.g., Ni, Fe, Cr, ...) vs Mg|Mg2+, and the extent to which thermodynamics can be useful in the interpretation of the resulting data.Dealloying is the selective electrolytic dissolution of less-noble element(s) from a homogenous metallic solid solution [4]. We have studied the fundamentals of alloying effects, specifically critical alloy compositions for dealloying in model Fe-(Cr)-Ni alloys.We found that up to a specific temperature, classical dealloying happens in molten salts. Fe and Cr preferentially oxidize, and a bicontinuous microporous Ni layer forms. We discovered that the parting limit for dealloying, 55-60 at.% of the less-noble element(s) (Fe and Cr), dropped by several percentages compared with aqueous solutions because chloride salts increase surface diffusivity of the more-noble component, Ni, at the surface.We have also found that under some electrochemical conditions, superficial dealloying occurs at 62 at.% of Ni and even at 68 at.% Ni. The latter case, similar to the case with brass [5], will be explained by equilibrium of Ni from the alloy surface with Ni ion in molten salts, which deliberately was added to the salt; the exchange current increases mobility of Ni. This leaves exposed Fe or Cr sites on the surface, that can dissolve.The operating mass transport process for this type of dealloying in molten salts is "percolation dealloying”. We will also talk about the transition to a mechanism controlled by lattice vacancy diffusion which will happen at very high temperatures. References MacPherson, H.G., The molten salt reactor adventure. Nuclear Science and engineering, 1985. 90(4): p. 374-380.Sridharan, K. and T.R. Allen, Corrosion in molten salts, in Molten Salts Chemistry. 2013, Elsevier p. 241-267.Delpech, S., et al., Molten fluorides for nuclear applications. Materials Today, 2010. 13(12): p. 34-41.Newman, R.C., Dealloying, in Shreir's corrosion (4th ed.). 2010, Elsevier. p. 801-809.Newman, R.C., T. Shahrabi, and K. Sieradzki, Direct electrochemical measurement of dezincification including the effect of alloyed arsenic. Corrosion science, 1988. 28(9): p. 873-886.