PSI - Issue 42

Mimoun Elboujdaini et al. / Procedia Structural Integrity 42 (2022) 1033–1039 Mimoun Elboujdaini / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Environment-induced cracking is a complex failure process in which crack development, initiation, and propagation, in a structure or component results from the synergistic reactions of the engineering material, under a total stress (applied plus residual), with the environment. All the metallurgical and microstructural parameters of the material; the chemical, electrochemical and microbial parameters, and the temperature of the environment; and the mode, amplitude and spectrum of the stress can interact and influence, individually or jointly, the cracking process. While stress corrosion cracking (SCC) of high-pressure gas/oil transmission pipelines is usually associated with electrolyte of pH above 9, and the SCC is essentially intergranular (IG), due to the shielding effect of some types of coating to the applied cathodic protection current, solutions with pH around or below neutral can be developed in coating disbondment, and transgrannular (TG) SCC of pipelines can occur. Transgranular SCC of pipelines in near-neutral pH solution has been identified in the last few decades or so. It is realized that, compared with high-pH IGSCC of pipelines, more restricted loading conditions, cyclic or dynamic, are required for crack growth to occur. Significant progress has been achieved in understanding the crack propagation behavior ; however, little is known regarding the characteristics of crack initiation and the early stages of propagation, especially any microstructural correlations of the early stages of crack development. The cracking behavior of a linepipe steel is investigated using initially smooth and polished specimens under four point cyclic bending in a dilute simulated ground water of near-neutral pH. Crack development is monitored by interrupting the tests and microscopically examining the change of surface morphology . The role of inclusions in providing crack initiation sites is identified, and the influence of maximum stress level and stress ratio on the crack kinetics is examined. 2. Experimental The Grades X-65 to X-80 steels were used in this study. Flat coupon bend specimens with dimensions of 60 x 20 x 7 mm were cut from the pipe without flattening, and the length of the coupon was in the transverse direction of the pipe so that, in the tests, crack development is in the longitudinal direction of the pipe, as observed in field service. The test surface of the specimen was polished to less 1µm diamond finish, using non-aqueous coolant and oil-based diamond suspension, to avoid dissolving non-metallic inclusions during polishing . A benchmark was made on the specimen surface remote from the central area so that the micro-features and cracks could be located with respect to this benchmark along X and Y coordinates. In some specimens, PTFE tape was also used to wrap a part of the specimen in some experiments, to simulate the boundary of a coating disbondment. Before the test, the morphology of the test surface was microscopically examined or documented using acetate replicas. Tests were conducted in NS-4 solution, a simulated dilute ground water, which has been found to be associated with transgranular SCC of pipelines, containing (mg/L) KCl 122, NaHCO 3 483, CaCl 2 .2H 2 O 181, MgSO 4 .7H 2 O 131. The solution gradually acidified during saturation with either pure CO 2 or 5% CO 2 balance N 2 . The pH stabilized at 6.9 with 5% CO 2 balance N 2 . The tests were interrupted after the desired times for microscopic examination and/or acetate replication of the specimen surface. Observation and/or replication were made directly on the specimens and on the acetate films to the surface after peeling them off. Cracks could be sized and located on X and Y coordinates with reference to the benchmark using an optical microscope connected to an image analyzer. A scanning electron microscope (SEM) was

also used for the inspection. 3. Results and Discussion 3.1. Metallography of Cracks

Non-metallic inclusions were observed on the surfaces of the specimens. The energy dispersive X-ray (EDX) microanalysis showed that the inclusions were mainly aluminate, with notable amounts of calcium (Ca) and sulfur (S), both of which varied widely between the inclusions. In some cases, the CaS may envelop the oxide/aluminate