Suppression of molten salt corrosion by plasma sprayed Ni3Al coatings

Corrosion behaviour of stainless steel 347 was investigated in a molten nitrate salt (60 wt% NaNO3 + 40 wt% KNO3) immersion at 565 °C for up to 3000 h. A growth of stratified oxide layers consisting of NaFeO2, Fe2O3 and Fe3O4 was observed on the stainless steel surface with a constant gravimetric corrosion rate of ~ 0.4 µm/year. The feasibility of using Ni3Al coatings deposited by means of air plasma spray for suppression of corrosion was investigated. Ni3Al coatings were observed to undergo a fast oxidation with a corrosion rate of ~ 2.7 µm/year in the first 500 h, and subsequently stabilise between 500 and 3000 h with no observable changes in microstructure, composition and weight at a corrosion rate of ~ 0.02 µm/year. The results presented in this study strongly suggest that Ni3Al coating suppresses the formation of oxide layers on the surface of stainless steel substrates and can be used as protection against corrosion in the presence of molten nitrate salts, which is of relevance to thermal energy storage applications.

Effects of Additives Containing Cyanopyridine on Electrodeposition of Bright Al Coatings from AlCl3-EMIC Ionic Liquids

The Al coatings were electrodeposited on the Cu substrate from AlCl3-EMIC ionic liquid (AlCl3:EMIC = 2:1 molar ratio) containing three cyanopyridine additives with different positions of the substituent group on the pyridine ring, which were 2-cyanopyridine, 3-cyanopyridine, and 4-cyanopyridine. The effects of cyanopyridine additives on the deposition potential, morphology, brightness, and corrosion properties of Al coatings were investigated. It was considered that the deposition potential of Al shifted to more negative overpotentials, the quality of Al coatings was promoted, and the corrosion property was improved by the cyanopyridine additives to a varying degree. Especially in the presence of 4-cyanopyridine, the flattest mirror bright Al coating was obtained, which had the smallest homogeneous nanocrystal grain size and strongest Al (200) crystallographic orientation. The average roughness Ra value was as low as 31 nm compared to that in the absence of cyanopyridine additives, which was 417 nm. Furthermore, the corrosion current density of the bright Al coating was three orders of magnitude lower than the rough Al coating, which resulted from the dense nanocrystal structure.