Materials behavior in extreme conditions: influence of large amounts of H2S on steel toughness in low temperature environments

In the presence of H2S, carbon and low alloy steels may suffer hydrogen damage and hydrogen embrittlement. This is the reason why so-called sour service materials must be used in compliance with international accepted standards, used worldwide in oil & gas field, when fluids are classified as sour. When the temperature is low, the effect of hydrogen on the reduction of toughness and ductility of the materials could represent a risk. Oil and natural gas transporting pipes in low temperature field, in fact, during winter time, at the shutdown and the subsequent re-starting operations, can reach very low temperatures (-40°C) in presence of high H2S content and high pressure. The aim of this paper is to study the combined influence of hydrogen and low temperature on mechanical properties of two pipeline materials, X65 and F22. An extensive experimental testing program was carried out, on as received materials and after hydrogenation. For each test condition Charpy, J-R curves and crack propagation tests were carried out varying the temperature. Two hydrogen charging techniques have been considered: electrochemical charging technique [1] and gaseous exposure in H2S containing environment by means of autoclave exposure. Tests carried out showed that hydrogen affects materials fracture toughness at low temperatures. Some differences have been found between the two applied charging modes and such phenomena have been interpreted with the interaction between hydrogen and lattice traps (reversible vs. irreversible).