Hydrogen Content and Charpy Toughness of Pipeline Steels with Different Hydrogen Charging Processes

Abstract

Hydrogen embrittlement of pipeline steels has become a major design concern for the transportation of pure hydrogen gas or hydrogen blends using pipeline, especially at high design stresses. Quantification of the effects requires measurement of hydrogen content in test samples and suitable test controls to simulate the practical service conditions. In this work, the total hydrogen content in pipeline steels pre-charged using electrolytic and gaseous methods was measured using the inert gas fusion (LECO) analysis. The analysis results showed that an average of approximately 0.2 ppm hydrogen existed in the as-received X65 steel specimens without either electrolytic or gaseous hydrogen charging. The electrolytic pre-charging in 0.1 M NaOH solution with 150 mg/L As₂O₃ was effective to introduce hydrogen into the X65 steel, and the highest total hydrogen content of 1.4 ppm was achieved at a charging current density of 2.5 mA/cm² and charging time of one hour. The highest total hydrogen content achieved by the gaseous charging technique in pure H₂ at 10.3 MPa pressure at room temperature for 15 days was 0.4 ppm. Pd surface coating promoted hydrogen absorption into the steel and led to almost doubled total hydrogen contents for both charging techniques. Ex-situ Charpy tests of electrolytically pre-charged X65 specimens at room temperature showed approximately maximum 20% reduction in Charpy absorbed energy (CVN) compared to uncharged specimens. The discrepancy in the pre-charging time needed to reach the saturation effect (i.e., one hour for LECO vs. five hours for Charpy) can be attributed to the different sample geometry and dimensions for the LECO and Charpy tests.