Etude de la corrosion de différents aciers par le mélange fondu de nitrate-nitrite alcalins

Abstract

Electricité de France, with the assistance of the Ecole Nationale Supérieure de Chimie de Paris, has undertaken studies on salt chemistry and steel behaviour in a molten (Na, K)NO 3(Na, K)NO 2 mixture in the framework of the Themis programme; this salt is used as a coolant and storage fluid in this solar thermal electric power plant. The thermodynamic equilibria between the chemical components of the salt and its gas blanket have been identified; a method has been determined to fix the oxoacidity of the mixture and consequently the nitrogen oxide content of the salt circuit. Corrosion tests performed on specimens have confirmed the good resistance of the AISI 316L stainless steel at 500 °C (the temperature of the hottest tube of the boiler). In contrast, these tests have revealed that low alloy steels show a different behaviour at 450 °C (the nominal temperature of salt circuits) depending on the oxoacidity of the molten salt: there is linear corrosion of steel without chromium in an oxoacid medium, whereas parabolic corrosion occurs in the other cases. Chloride pollution of the salt mixture has a very negative effect on these types of steel. The corrosion of steels occurs together with nitriding which increases as the rate of oxidation of the specimen increases. Parallel to this study of the corrosion of steel, a fundamental study of the corrosion of iron was undertaken from both the thermodynamic and the kinetic points of view (between 420 and 500 °C). It showed that corrosion was basically the result of the oxidation of iron by the NO 3 −NO 2 − mixture; nitriding is only a secondary phenomenon. Experimental compilation of the potential-oxoacidity diagrams (E- pO 2−) was used to characterize the different oxides involved in this corrosion. The electrochemical characteristics were studied using an impedance meter by examining the influence of the acidity of the mixture on the impedance. This yielded information on the kinetics of formation of the various oxide layers and allowed them to be identified by their electrical properties (resistance, capacitance, permittivity etc.) and by their passivating power.