Hydrogen-Induced Cracking of Linepipe Steels Part 1Threshold Hydrogen Concentration and pHK(1)

• Published in: Corrosion (Houston, Tex.) Vol. 49; no. 1
• Main Authors: Revie, R.W., Sastri, V.S., Hoey, G.R., Ramsingh, R.R., Mak, D.K., Shehata, M.T.
• Format: Journal Article
• Language: English
• Published: NACE 01-01-1993; NACE International
• ISSN: 0010-9312; 1938-159X

ABSTRACTThe susceptibility of sour service linepipe steels to hydrogen-induced cracking (HlC), also known as stepwise cracking (SWC), depends on metallurgical as well as environmental factors.1 The former includes alloying elements, microstructure, strength, segregation, and shape of nonmetallic inclusions; the latter includes partial pressures of hydrogen sulfide and carbon dioxide, temperature, pH, and the presence of aggressive species such as chloride.Hydrogen atoms, produced as a result of corrosion of the inside wall, diffuse through the pipe wall and are trapped at heterogeneous sites in the steel, thus leading to blistering and HIC. Permeation of hydrogen through the pipe walls can be measured by an electrochemical device strapped to the outer wall of the pipe. Hydrogen concentration at the inside pipe surface (COH) can be calculated using permeation current density, diffusion equations, wall thickness, and the diffusion coefficient.2 For HIC to occur, COH must equal or exceed the threshold hydrogen concentration (CthH). The National Association of Corrosion Engineers has developed a standard test (TM-02-84) for evaluating stepwise cracking of linepipe steels.3 In this test, standard coupons cut from the pipe (20 mm by 100 mm by wall thickness [WT]) are immersed in hydrogen sulfide (H2S) saturated synthetic seawater solution for 96 h. The coupons are then sectioned and examined for HIC. The correlation between the diffusible hydrogen extracted from the coupons and HIC and the existence of CthH were established by Ikeda et al.4 To evaluate and compare the relative resistances of the steels to HIC, two parameters for each steel were determined experimentally: Threshold Hydrogen Concentration­The concentration of diffusible hydrogen in the steel above which cracking occurs, and Threshold pH­The pH below which cracking occurs. The steels were also characterized by chemical and 17