Quantitative Measurement of Hydrogen Diffusion and Trapping Parameters For Linepipe Steel under Constant Load in Sour Environment

To understand clearly the cracking mechanism of the stress-oriented hydrogen induced cracking (SOHIC), it is inevitable to measure quantitatively the amount of diffusible and trapped hydrogen and to correlate them with the cracking characteristics. There have been lots of efforts to measure the diffusivity and permeability of hydrogen in the steel under tensile straining condition using the electrochemical hydrogen permeation technique (HPT). However, the different experimental conditions such as the strain rate, kind of aqueous solution and applied electrochemical parameters often lead to unrealistic data. It is hence necessary to apply the actual corrosion condition to the HPT for the investigation of hydrogen effects on the cracking resistance which is represented as the threshold stress measured by NACE TM0177A method.
 In this study, the API X65 grade linepipe steel having high SOHIC resistance is used. Most experimental conditions of the HPT are determined as the same way as the ISO17081 standard. The NACE-A solution is selected as the hydrogen charging solution and the hydrogen is injected by free corrosion of the specimen in the testing solution in order to simulate the same corrosion environment of the TM0177A method. Applied stresses are smaller than the threshold stress to prevent the breakdown of the testing cell and specimen. In this study, the permeability, apparent diffusivity, apparent solubility of diffusible hydrogen and the amount of irreversibly trapped hydrogen are measured by the modified HPT, and they are correlated with the SOHIC resistance to investigate the role of hydrogen on the cracking.