The effect of impurities, added to hydrogen environment, on hydrogen embrittlement (HE) was investigated in association with the effect of material strength. Addition of CO and O2 deactivated the HE. O2 prevented the HE with lower concentration than CO. Material with higher strength required larger amount of impurities to prevent the HE. Reduction of hydrogen uptake was the primary result of the addition of the impurities, but a certain amount of hydrogen uptake occurred when the HE was completely inhibited. Discussion regarding essential HE mechanisms for the fracture morphologies was required to interpret the effect of material strength.
Susceptibility of hydrogen embrittlement (HE) depends on several variables concerning material, environment and mechanical conditions. For example, HE manifested by reduced fracture toughness is promoted with increase in material strength (Sandoz, 1972; Nelson and Williams, 1973; Gerberich, Chen and John, 1975; Loginow and Phelps, 1975; Bandyopadhyay, Kameda and McMahon, 1983; Moody, Robinson and Garrison, 1990; Nibur, Somerday, San Marchi, Foulk, Dadlfarnia, Sofronis and Hayden, 2010). Gas impurities can inhibit HE (Kussmaul, Deimel, Fischer and Sattler, 1998; Gangloff and Somerday, 2012). Relatively low loading rates cause considerably severe HE (Kondo and Kubota, 2011; Macadre, Artamonov, Matsuoka and Furtado, 2011). Several candidate mechanisms that work either alone or synergistically have been developed for HE such as hydrogen enhanced localized plasticity (HELP) (Beachem, 1972; Birnbaum and Sofronis, 1994), brittle fracture of hydride (Westlake, 1969; Hardie and McIntyre, 1973), and hydrogen-induced decohesion (Pferi, 1926; Troiano, 1960; Oriani, 1972). Meanwhile, hydrogen uptake into the materials, hydrogen diffusion and hydrogen accumulation at defect are the common processes to activate these HE mechanisms. When considering crack propagation in gaseous hydrogen environment, dissociation of hydrogen molecules into hydrogen atoms is essentially necessary for the hydrogen uptake. Bare Fe surface that is created by the crack propagation aids this dissociation reaction owing to its catalytic action (Lennard-Jones, 1932). If environmental hydrogen gas contains gas impurities, and the impurities have a higher selectivity than hydrogen molecules in terms of adsorption on the Fe surface, the impurities can cover the Fe surface faster and more effectively than the hydrogen molecules. It results in deactivation of the catalytic action of the Fe surface (Staykov, Yamabe, and Somerday, 2014). As a result, HE is prevented. In fact, the addition of O2 and CO inhibited HE of pipeline steels in not only fracture toughness test (Kussmaul, Deimel, Fischer and Sattler, 1998; Gangloff and Somerday, 2012) but also fatigue test (Frandsen and Marcus, 1975; Somerday, Sofronis, Nibur, San Marchi and Kirchheim, 2013).
