The Investigation Of Stainless Steel Corrosion In Chloride Melts

Abstract:

Alkali halide based molten electrolytes have a wide range of potential applications for electrowinning and electrorefining of metals and also for pyrochemical reprocessing of spent nuclear fuels. However, the industrial scale use of fused salts is limited by the problem of finding suitable corrosion resistant materials stable in contact with molten media. Corrosion of metals in molten salts was studied for a number of years, but there is no single point of view concerning mechanism of stainless steal corrosion in halide melts. In the current work we used a variety of methods for studying the phenomenon of stainless steel corrosion including gravimetric and chemical analysis, surface examination, electrochemical measurements and in situ high temperature electronic spectroscopy. Corrosion behavior of stainless steel types AISI 316L, 316Ti and 321 were studied here at 750 0C in melts based on NaCl-KCl equimolar mixture. Time of the corrosion tests was varied from 1 to 30 h and several cylindrical samples of each type of steel were used in each experiment. After conducting the experiments the weight decrease of the samples was determined and averaged. Quenched melt samples taken after each experiment were analyzed to determine the content of the elements of interest using ICP MS method (Elan 9000, Perkin Elmer) and X-ray fluorescence spectroscopy (ARL QUANT’X). The surface of steel samples after corrosion tests was examined using metallographic analysis (Olympus GX-71F) and Х-ray microanalysis (JSM 6490). All the electrochemical measurements were performed using high-temperature three-electrode cell and Autolab 302N potentiostat/galvanostat with Booster 20A amplifier. The electronic absorption spectra (EAS) were recorded between 200 and 1500 nm using an original set-up based on Avantes AvaSpec-2048-2 and NIR256-1.7 fiber optic spectrometers. In a separate series of experiments anodic dissolution of a number of individual elements representing the major components of stainless steels was first studied and the EAS of iron, chromium, nickel, molybdenum, manganese and titanium chloro-species were recorded. It was found, that the major corrosion products of steels are iron, manganese and chromium species. Prolongation of anodic dissolution leads to increasing chromium-to-iron ratio in the melt. Titanium in steels forms very stable carbonitride species that aren’t dissolved during anodic oxidation. We conclude that under applied anodic potential the corrosion of stainless steels in molten chlorides is electrochemical in nature. At the initial period the exchange reaction between steel components and alkali metal cations takes place in parallel with the electrochemical process. In case of absence of anodic polarization the following mechanism of stainless steel corrosion in molten chlorides under inert atmosphere was proposed: 1) chemical interaction between the alloy and the salt intensified by the formation of microgalvanic pairs; 2) sensitization, i.e. formation of chromium and molybdenum carbide-containing phases in steel as a result of heating to 750 0C; 3) additional formation of galvanic pairs between the grains of austenitic alloys and the carbide phases at the grain boundaries resulting in enhanced intergranular corrosion.

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