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ABSTRACT The purpose of this work was to examine the creep activation energy (Qc) in the chemical-mechanical correlation model for stress corrosion cracking (SCC),1-3 which is expressed by: [Equation available in full paper] (1) where tf is the time to failure, m is a strain rate exponent (a constant depending upon the material), Qc is the activation energy of creep (a constant depending on the material), R is a gas constant, T is absolute temperature, is the applied stress, t* is a correlation coefficient depending on environments and materials, and is an environmental parameter. Equation (1) was obtained based upon the assumption that SCC is a process dependent upon both creep and chemical reactions. The role of chemical reactions in SCC is incorporated in and t*. The role of creep is incorporated in Qc, m, and in the case of constant loading. has a value between 0 and 1, and the value of indicates the contribution of environment relative to the creep. A smaller value of indicates a larger contribution of the environment, 0 indicates a mainly corrosion process, and 1 indicates a dominant creep. The physical meaning of was examined previously for the SCC system of stainless steels (SS) in concentrated chloride solutions at applied potentials.3 It was demonstrated that the decrease of from 1 reflects enhancement of the creep strain rate by the environment and that depends significantly upon the repassivation parameter of the films on the SS. Effects of various environmental factors on and t* have been discussed previously.1-3 In contrast, Qc and m are constants independent of the environment, based upon previous analyses. Further, if Qc is the creep activation energy, then its value would vary for different materials. Previous analyses on SCC of austenitic SS in water,1 in concentrated chloride solutions at open-circuit potentials,2 and at applied potentials3 have shown that Qc is a constant ~ 123 kJ/mol in the tempera-
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ABSTRACT The purpose of the present work was to analyze the effects of stress ( ) and electrochemical potential (E) on stress corrosion cracking (SCC) of stainless steels (SS) in concentrated chloride solutions using the chemical-mechanical correlation model, as described in previous work.1-2 The chemical-mechanical correlation model is given by: [Equation available in full paper] (1) where tf is the time to failure; m is the strain rate exponent, a constant depending on the material; Qc is the activation energy of creep, a constant depending on the material; R is a gas constant; T is absolute temperature; is the applied stress; t* is the correlation coefficient depending upon the materials and environments; and is the environmental parameter. Equation (1) defines tf of SCC as functions of and T in terms of the creep parameters m and Qc, the environmental factor , and the material-environmental factor t*. The creep parameters have been shown to be material constants independent of the environment in previous analyses of SCC in hightemperature water and in concentrated chloride solutions, where values of Qc/m between 19 kJ/mol and 20.5 kJ/mol, respectively, were obtained.1-2 incorporates the chemical contribution and is sensitive to variables related to the chemical reactions involved in SCC. The value of for SCC is limited between 0 and 1, = 1 indicates a pure creep process, and 0 indicates a pure corrosion process. The present work analyzed the effects of E on the parameters Qc/m, , and t* in Equation (1). Previously obtained values of Qc/m (19 kJ/mol to 20.5 kJ/mol) were based upon the SCC data at open-circuit potentials (OCP). Since the value of Qc/ m is independent of environment based on the chemical-mechanical correlation model, no effect of E on Qc/m was expected. However, whether Qc/m is dependent on E was examined in this analysis. The parameters of and t* were expected to depend upon E since both have been shown to be affected generally by environmental factors. Dependencies of and t* were reviewed in relation to the anodic dissolution and repassivation
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ABSTRACT Experimental data from published literature on stress corrosion cracking (SCC) of austenitic stainless steels (SS) as influenced by temperature and stress in concentrated chloride solutions were analyzed using the chemical-mechanical correlation model. Effects of stress and temperature on SCC were analyzed using 63 sets of data for times to failure (tf) from 17 references. The data followed a systematic pattern which was described by the chemical-mechanical correlation model. An equation of tf as a function of stress and temperature was developed using an environmental parameter b that depends upon temperature, molybdenum concentration, and yield strength of the SS. INTRODUCTION The objective of the present work was to analyze the effects of temperature and stress on time to failure (tf) for austenitic stainless steels (SS) exposed to concentrated chloride solutions to determine the utility of the chemical-mechanical correlation model. The chemical-mechanical correlation model was proposed for analyzing stress corrosion cracking (SCC) of sensitized austenitic SS in high-temperature water and revealed a unique pattern.1 This pattern was examined in detail by analyzing 63 sets of experimental data from 17 references.2-19 Experimental data were based on tf as a function of applied stress or temperature for austenitic SS in concentrated chloride solutions at open-circuit potentials. This paper includes four sections dealing with the quantitative development of the correlation model. The first section summarizes the analytical framework. The second describes the data correlations based on least-square methods. The third section analyzes the correlated relations with the chemical-mechanical correlation model, and the final section discusses the utility of the correlation model in general.