Initiation of stress corrosion cracking for pipeline steels in a carbonate-bicarbonate solution
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INTRODUCTION
STRESS corrosion cracking (SCC) of steels in carbonate-bicarbonate solutions has been responsible for failures at a refinery catalytic cracking unitt~ and of natural gas pipelines. [2-61Such service SCC occurs within a narrow potential region corresponding to the active to passive transition,t6-10] in which SCC is caused by anodic dissolution after strain breaks the corrosion product film on the specimen surface or at the crack tip. Less attention has been paid to SCC of pipeline steels at cathodic potentials, where SCC is caused by hydrogen embrittlement. ParkinsI71 has shown that in service, the crack initiation process dominates the component lifetime because cracks propagate rapidly once initiated. However, a detailed study of SCC initiation is still lacking. The linearly increasing stress test (LIST) is a new testing methodology developed by Atrens et al.I"l It is similar to the constant extension rate test (CERT) with the essential difference that LIST is load controlled, whereas CERT is displacement controlled. The LIST is a rapid method to evaluate SCC behavior, particularly to determine the SCC initiation stress. I~,~2,~31 The LIST technique has been used in the present research to study the SCC behavior of a range of pipeline steels in a carbonate-bicarbonate solution. The aim was to study the SCC susceptibility, especially to measure the SCC initiation stress, and to relate SCC behavior to the material properties, applied potential, stress rate, and surface condition. II.
EXPERIMENTAL PROCEDURE
The materials were commercial pipeline steels, X65(A), X65(B), X65(C), X52(A), X46(A), and X42(A), with chemical compositions and mechanical properties as shown in Tables 1 and 2. Potentiodynamic polarization measurements were carried
Z.F. WANG, Research Fellow, and A. ATRENS, Associate Professor, are with the Department of Mining and Metallurgical Engineering, The University of Queensland, Brisbane Qld 4072, Australia. Manuscript submitted May 23, 1995. 268~-VOLUME 27A, SEPTEMBER 1996
out at 20 ~ 50 ~ and 70 ~ in the absence of load, to evaluate the expected potential range for SCC using the methodology of Parkins. "4,~51 The specimens were in the form of rods, 11.29 mm in diameter (i.e., 1 cm 2 of surface area exposed to the solution). Before testing, the specimen surface was abraded with 800-grit silicon carbide paper. The potential was controlled with reference to a saturated calomel electrode (SCE) which was placed in a beaker containing the test solution at room temperature and was connected to the actual test solution through a Luggin probe. The specimen was held at - 1 3 0 0 mV(SCE) at room temperature for 5 minutes to remove surface oxide films, heated to the test temperature, and allowed I hour for stabilization. The polarization tests were started from the free corrosion potential, scanning in the anodic direction to 100 mV(SCE) at the rate of 10 mV/min and 1 V/rain. The SCC experiments were conducted in 1N N a 2 C O 3 + 1N NaHCO 3 solution at 70 ~ in a standard cell, as described
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