Oxide Film Characterization after the crack propagation in CT specimens of AISI 304L under Hydrogen Water Chemistry Cond
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Oxide Film Characterization after the crack propagation in CT specimens of AISI 304L under Hydrogen Water Chemistry Condition. 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 Stress Corrosion Cracking (SCC) in a general term describing stressed alloy fracture that occurs by crack propagation in specifically environments, and has the appearance of brittle fracture, yet it can occur in ductile materials like AISI 304L used in internal components of Boiling Water Reactors (BWR). The high levels of oxygen and hydrogen peroxide generated during an operational Normal Water Condition (NWC) promotes an Electrochemical Corrosion Potential (ECP), enough to generate SCC in susceptible materials. Changes in water chemistry have been some of the main solutions for mitigate this degradation mechanism, and one of these changes is reducing the ECP by the injection of Hydrogen in the feed water of the reactor; this addition moves the ECP below a threshold value, under which the SCC is mitigated (-230mV vs SHE). This paper shows the characterization by Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Raman Spectroscopy of the oxide film formed in to a crack propagated during a Rising Displacement Test method (RDT), on Hydrogen Water Chemistry (HWC) conditions: 20 ppb O2, 125 ppb H2, P=8MPa, T=288°C, using a CT specimen of austenitic stainless steel AISI 304L sensitized. The characterization allowed identifying the magnetite formation since an incipient way, until very good formed magnetite crystals. INTRODUCTION During the last years, SCC of type 304 L stainless steels has been a major concern in BWR under NWC conditions containing 100 ppb to 300 ppb of oxygen (O2), hydrogen peroxide (H2O2) and 10 ppb to 20 ppb of hydrogen (H2); this susceptibility has been attributed to the oxidizing water chemistry environment (+200mV vs SHE). As a practical solution, H2 is added to the feedwater to mitigate this degradation mechanism, creating an HWC condition that reduces the dissolved O2 levels, and decrease the ECP below a critical value (-230mV vs SHE) at which SCC susceptibility is decreased markedly [1]. However, the field experience establishes that the underlying mechanism is not easy to understand and a combined effect of: environment, stress, metallurgical condition and the behavior of the oxide film formed on the material surface can contribute to generate SCC [2]. One of the most important models to understand the mechanism in sensitized stainless steels, has been the theory of the slip-oxidation model that is based on mechanic oxide film rupture and localized dissolution along an active path [3]; the oxide film should permit an active dissolution in the crack tip, but an adequate coated protection in the rest of the crack surfaces avoiding a general corrosion process. It is evident that the chemical composition, and properties of passive film formed are a crucial factor
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