Anodic Behavior Modeling of the C-276 Alloy in Chlorinated Media Simulating the Hydrothermal Conditions of Organic Waste Treatment

Abstract

Experimental current-potential curves corrected from the contribution due to the medium oxidation are used to propose a dissolution-passivation model of the nickel base alloy C-276. This model is controlled by the major presence of nickel and shows a reaction chain mechanism with a bifurcation and two competitive determinant branches for the setup of the protection of the alloy. Considering a Langmuir-type adsorption, we formulate a current-potential law which leads, via a mathematical optimization calculation using Nelder's simplex, to the eight kinetics parameters involved in the five considered elementary steps. The respective influence of the different branches can be determined. It is used in association with the scanning electron microscopy profiles obtained at fixed potentials, to evaluate the limited reliability of the C-276 alloy as a plant reactor material for waste treatment under hydrothermal conditions, either sub-or supercritical oxidation. This evaluation relies on the comparison with the titanium T60 whose anodic behavior and performances have already been published. The determination of the kinetic parameters also underlines the role played by the oxygen source ͑H 2 O 2 ͒ on the mechanism, a role partly hidden by strict observation of the current-potential experimental curves.The context of this study has already been stated and is briefly recalled here. [1][2][3][4] The principle is to develop elements of appreciation for an efficient choice of a material to be used in plant reactors for organic waste treatment under hydrothermal oxidation, either sub͑temperature Ͻ 373°C͒ or supercritical ͑temperature Ͼ 373°C͒ conditions. They should also be able to orientate the choice in the case of chloride formation for pH Ͻ 5.To our knowledge, available information on this subject and especially on nickel-based alloys, such as the C276 one, come from gravimetric measurements and corrosion facieses. 5 These studies show a high reactivity of nickel base alloys and an expected tendency to localized attacks. Despite this observation, which reveals the invalidity of these alloys, they still remain frequently used, without any attempts of a kinetic and/or mechanistic investigation. However, only such an approach can provide some comprehension of the phenomenon and lead to the identification of the right solution able to stop or at least slow down the material reactivity. The main goal of the paper is to conceive a model for the reaction mechanisms as for titanium alloys in the same experimental conditions. 1,6 It is also related to previous works that were carried out on other nickel-base alloys ͑I800͒ 7,8 and pure nickel 9 in environments simulating the conditions of primary and secondary media in PWR nuclear plants ͑for high pHs and minor quantities of chloride ions͒. The similitude of the current-potential curves led us to use the same type of model already efficient for titanium metal 6 and pure nickel by taking into account some reference works on this material generally performed in sulfuric acid media and ...