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In this research, the surface and cross section of an API 5L X70 pipeline steel was investigated by SEM observation and EDS analysis in order to determine type and morphology of inclusions. Then, the electrochemical hydrogen charging technique using 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate was utilized to create hydrogen induced cracks (HICs) in X70 steel. After hydrogen charging experiments, the cross section of hydrogen charged steels was polished up to 1 µm then etched with 2% nital solution to find HIC cracks. The HIC crack initiation and propagation was studied by Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) techniques. The results showed that there are various types of inclusions including oxide, sulfide and nitride inclusions at the cross section of as-received X70 steel; however, the accumulation of inclusions was higher at the center of cross section than in other regions. But, in hydrogen charged specimens, only some special inclusions, such as sulfide and nitride type inclusions, can initiate HIC cracks. Oxide inclusions and precipices has not any role in HIC crack initiation and propagation, but they reduce the fracture toughness of X70 steel. Moreover, the EBSD results showed that the crack initiates in an area with weak or random texture. Other effective parameters in crack growth, such as orientation of grains involved with HIC crack, was discussed.

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In this study stress corrosion cracking behavior of X70 pipeline steel after shielded metal arc welding has been investigated. For this purpose, the alloy was welded by low, medium and high rates of heat input. Microstructures of specimens were studied using light and electron microscopes. Stress corrosion cracking susceptibility was evaluated using slow strain rate tests in C2 simulated soil solution in free corrosion potential. Scanning electron microscopy was used to examine the fracture surfaces. Corrosion of different areas of weld was evaluated by potentiodynamic polarization test method. Results showed that by increasing heat input, tensile strength and hardness of welded metal reduces, larger heat affected zone (HAZ) is produced and the grain size is increased. Slow Strain Rate Test (SSRT) results showed that increasing the welding heat input will reduce the susceptibility to SCC. SEM images obtained from samples confirmed brittle fracture of specimens tested in the corrosive environment. It was also observed that the produced cracks might grow in transgranular mode. Potentiodynamic polarization tests showed the lowest rate of anodic dissolution for welded samples with higher heat inputs. Overall, results showed that by reducing heat input the susceptibility to SCC increases; this is due decrease in areas which are sensitive to crack growth and initiation.

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This study investigates the effect of pulse Cathodic protection as compared to conventional Cathodic protection on corrosion and electrochemical conditions under disbonded coating of X-52 pipeline steel. In this regard, a laboratory cell that can simulate the underlying conditions by applying cathodic protection was designed and made. Coupons with dimensions of 20×10×5 mm, made from X-52 pipeline steel were placed under the simulated coating disbondment while the length and gap size of disbondment was 200 mm and 3 mm respectively. By placing the cell in a simulated soil solution C2, conventional & pulse CP of -870mVSCE was applied to the open mouth (OM) of simulated coating disbondment. For pulse Cathodic protection, frequencies of 1000, 5000, and 10000 Hz were utilized. Result showed for conventional CP, potential failed sharply to Open Circuit Potential (OCP) at 7 cm from the OM of the simulated coating disbondment. For pulse Cathodic protection, at the frequency of 10 kHz, in contrast to conventional CP, no drop in potential was observed under the simulated coating disbondment. In fact, the potential gradient which normally is observed under simulated coating disbondment with conventional CP being applied was not observed. Aslo, investigating the chemical and electrochemical conditions under coating disbondment, showed that there was more uniform distribution of pH and potential with increase in frequency of pulse Cathodic protection, as compared to the conventional Cathodic protection being applied.

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According to the latest survey literature on destructive factors of steel pipelines, hydrogen induced cracking has been recognized as a key factor. During this type of destruction, the hydrogen atoms, produced from the corrosion process between the pipe surface and hydrogen sulfide, accumulate on the surface of the pipe and penetrate into the steel microstructure over time. These atoms are stacked in structural defects, such as inclusions, grain boundaries, hydrogen traps, dislocations, and are combined together to form hydrogen molecules. This process creates a high pressure and provides the basis for starting hydrogen induced cracks. Many factors contribute in initiation and propagation of these cracks, such as nonmetallic inclusions, hydrogen traps, orientation of grains and type of grain boundaries. In this paper, it is attempted to accurately discuss the mechanism of failure by hydrogen induced cracking and their affected parameters using the obtained results.

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The effect of pulse cathodic protection (CP) on CP penetration depth under  tape coating disbondment of pipeline steel  has been investigated in this  study and results have been compared with conventional CP. In this regard, a laboratory cell with the length of 2000 mm that can simulate the underlying conditions by applying CP was designed and made. For investigation of crevice width on CP penetration depth coupons with dimensions of 2000×10 mm and 2000×50 mm made from API X-52  pipeline steel were placed under coating disbondment with the gap size of 3 mm and different widths. By placing the cell in a simulated soil solution C2, the  potential of 1000mVSCE through  two methods of conventional & pulse CP with the frequency of 10000 Hz was applied to the open mouth (OM) of the simulated coating disbondment. Result showed by applying conventional CP for both the crevice widths, potential failed sharply to Open Circuit Potential (OCP) near to the OM while for pulse CP, in contrast to conventional CP, which CP penetration depth was  increased. At the crevice with the width of 50 mm CP level was measured more than the width of 10 mm, such that the entire disbondment length was protected by CP.  The lower CP level for the crevice with the width of 10 mm could be due to concentration polarization resistance which restricted the penetration of the positive current within the crevice. Also, pH value under the coating disbondment with applying pulse CP at the crevice with the width of 50 mm was measured more than the pH value for the disbondment with 30 mm width other due to higher CP level under this disbondment. In summary, results showed despite the conventional CP, applying pulse CP with high frequency could be an effective method in increase of the CP penetration depth under tape coating disbondments.