Chemical and Crystal Characterization of the Oxide Film Formed in 304L SS Under Autoclave Conditions, With and Without C
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Chemical and Crystal Characterization of the Oxide Film Formed in 304L SS Under Autoclave Conditions, With and Without Crevice Formation Ángeles Díaz Sánchez1, Aida Contreras Ramírez1, Carlos Arganis Juárez1 1 Instituto Nacional de Investigaciones Nucleares. Carretera México-Toluca s/n, La Marquesa Ocoyoacac, México C.P.52750 ABSTRACT The principles of Stress Corrosion Cracking (SCC) are supported in the behavior of the oxide film formed into a crack; in fact the active dissolution of metal atoms after a film rupture and until fill repassivation is the base of slip dissolution model which is a good model to justified the crack tip advance in stainless steels (SS) used in vessel internal components for the nuclear industry. This paper shows the analyzed made at the oxide film formed on samples of 304L SS sensitized and non sensitized, under autoclave conditions (288°C, 8MPa) with and without crevice geometric formation, using SEM, XRD and Raman Spectra. The crevice and no crevice condition allow establish the difference of an oxide formed on a free surface (no crevice) and the oxide formed on the wall in a crack (crevice); the chemical and physical properties of oxide film can alter the mechanism and kinetics of SCC process, so the difference between these two conditions will give more information about the behavior of the oxide film. INTRODUCTION Stress corrosion cracking (SCC) is a generic problem in high temperature water, with most alloys employed suffering from cracking under some conditions like AISI 304L used in internals components of Boiling Water Reactors (BWR); this phenomena requires the simultaneous interaction of stress, a susceptible metallurgical condition and a certain aggressive environment. The high levels of oxygen and hydrogen peroxide generated under an operational Normal Water Condition (NWC) in a BWR, promote an Electrochemical Corrosion Potential (ECP) that is enough to initiate and propagate SCC in susceptible materials (+200 mV vs SHE) [1]. Changes in water chemistry have been the principal solution for mitigate this cracking, and the change included a more reducing environment by application of Hydrogen Water Chemistry (HWC) by injection of gaseous hydrogen to the feed water when the ECP decrease below -230 mV vs SHE; however, and even though the operational problem is being solved, many aspects of the mechanism that control the stress corrosion cracking remains unclear. One of the proposals used to explain the stress corrosion cracking is the slip dissolution model that is consider from the standpoint of electrolyte resistance and/or dissolved metal ion concentration in the crack. The ohmic potential (IR) drop and dissolved metal ion concentration in a crack with a pre-existing active path and passive walls in a conductive environment were predicted to be dependent upon stress and crack velocity. The electrochemical crack tip advance based on active dissolution will stop once fell repassivation occurs, and these steps depend of the chemical composition and structure of surface film formed
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