10,000 h molten salt corrosion testing on IN617, uncoated and aluminide ferritic steels at 580 ºC

Abstract

Long term testing of two ferritic-martensitic steels (P91 and VM12-SHC) with and without slurry deposited aluminide coatings containing 20 wt.% of Al at the surface was carried out by exposing these materials to the Solar Salt, a eutectic mixture composed of 60 % NaNO3 - 40 % KNO3 at 580ºC. This salt is currently used in operating thermal solar power plants as heat transfer and storage fluid. Tubes made of expensive Ni based alloys are employed to mitigate corrosion. The tested uncoated ferritic materials exhibited very high corrosion rates developing thick, easily detached scales. IN617 was also tested as a reference and experienced very low corrosion up to 5,000 h, but after 10,000 h a 50 ȝm thick, mostly NiO scale had developed with Na0.6CoO2 crystals deposited on top. There was also evidence of significant Cr depletion at the alloy surface. Carcinogenic CrVI was found in the Solar Salt melt in which the three uncoated alloys were immersed. In contrast, the two coated ferritic steels did not show evidence of degradation after 10,000 h and the most significant microstructural change was the development of a very thin protective NaAlO2 layer on their surface. An industrial process to deposit these coatings on the inner surfaces of pipes has already been developed.