Hydrogen induced damage in pipeline steels

Abstract

The hydrogen induced cracking (HIC) resistance of several grades of plate steels was investigated using electrolytic hydrogen charging. HIC generated by electrolytic charging was also compared to the industrial standard test for HIC, the NACE standard TM0284. The electrolytic charging (EC) apparatus was designed to optimize the reproducibility of the HIC results and the robustness of the components during long charging times. A characterization study on the EC apparatus was undertaken. Alterations to applied current density and charging time were conducted on a highly susceptible plate steel, 100XF, to assess HIC damage as a function of charging conditions. Intermediate current densities of 10 to 15 mA/cm2 produced the greatest extent of cracking without significant corrosion related surface damage. The hydrogen charging time did not greatly affect the extent and depth of cracking for test times between 24 to 48 hours. Thus, for subsequent experiments, the applied current density was set to 15 mA/cm2 and the charging time was set to 24 hours. Plate steel grades X52, X60, X70, and 100XF were prestrained in tension to various levels and then electrolytically charged with hydrogen or tested with the NACE standard TM0284 test (solution A) saturated with H2S(g) to induce HIC. Prestrain was introduced to assess its impact on HIC. Hydrogen damage was quantified with the crack ratios defined in the NACE Standard TM0284. The results from the EC and NACE methods were very comparable to one, with respect to the magnitude of cracking and the trends between alloy and pre-strain conditions observed. Both methods showed that HIC substantially increased for the high strength 100XF steel compared to the lower strength alloys. This is consistent with NACE recommendations for HIC resistance steels, which suggests that alloy strength should be less than 116 ksi (800 MPa) or 248 HV (22 HRC). The HIC results were largely independent of the pre-strain levels imposed within the statistical accuracy of the evaluation method employed. The total, irreversibly trapped, and diffusible hydrogen amounts were measured or estimated for each condition using a LECO interstitial analyzer and the American Welding Society method for measuring diffusible hydrogen concentrations. The total amount of diffusible hydrogen was highest for the 100XFalloy and lowest for the X52 alloy. The amount of trapped hydrogen was similar for all the alloys, implying that the number of irreversible trap sites were comparable. However, the diffusible hydrogen content was greatest for the 100XF alloy and lowest for the X52 alloy, which is believed to be related to the relatively high amount of grain boundary area and high dislocation density of the 100XF alloy. A qualitative analysis on the effect of microstructure and nonmetallic inclusions on HIC was performed and produced results that confirmed findings from literature. Cracking was observed around nonmetallic inclusions such as sulfides and oxides in the metal matrix. For materials in which both inclusion types were present, X60 and X70, HIC originated and was observed most often around sulfide type inclusions.