Improving the corrosion resistance of ferritic-martensitic steels at 600 °C in molten solar salt via diffusion coatings

Behavior of Ni20Cr Alloy in Molten Nitrate Salts

“…In general, in Solar Salt, at immersion times greater than 500 h, the effect of temperature on the corrosion process of the alloy was observed, namely, increasing the temperature increased the mass gain, which indicates an increase in the metallic dissolution process and, therefore, a greater presence of corrosion products. This is justifiable given that corrosion in molten nitrates occurs because of their corrosive ions (O 2− , NO −2 ) formed by the thermal decomposition of nitrate ions (NO −3 ) whose presence is accelerated at temperatures above 500 °C [ 6 , 12 ]. This shows the direct relationship between the metal dissolution process and the ability of the alloy to self-heal with both temperature and immersion time.…”

“…The mass gain detected at the highest test temperature is lower than those reported for other nickel-based alloys [ 5 ], and it has also been suggested that nickel-based alloys are passivated in molten nitrate salts [ 7 ]. In general, the behavior represents the combination of different processes that occur on its surface, namely, metallic dissolution, growth of oxides, precipitation of corrosion products, and scaling, and its tendency can indicate the kinetics and mechanism of the process of corrosion [ 12 , 17 ].…”

“…The importance of the basicity of the melt on the dissolution mechanism of the metallic surface and its protective layers has already been previously documented [ 21 , 22 , 23 , 24 , 25 , 26 , 27 ]. In the case of Solar Salt, in the molten state, it experiences the following decomposition reactions, and its reaction rate increases with increasing temperature [ 1 , 12 , 28 , 29 , 30 ]: …”

“…The nitrite ion (NO −2 ) formed after decomposition can undergo secondary decomposition reactions according to [ 1 , 12 , 28 , 29 , 30 ]: …”

“…Many of the materials used are Fe-based or Ni-based, and their corrosion resistance depends on the development of a protective oxides layer (Cr 2 O 3 , Al 2 O 3 ) and the presence of other alloying elements that mainly improve their mechanical properties [ 2 , 3 , 7 , 9 ]. It has been observed that the content of both Ni and Cr improves the corrosion resistance; with a higher content of these elements, the corrosion resistance is increased [ 2 , 6 , 10 , 11 , 12 ]. Therefore, the various studies carried out to date have indicated that nickel-based alloys show excellent performance in various HFT fluids [ 4 , 5 , 7 ].…”