Hydrogen Electrode Reaction and Hydrogen Embrittlement of Mild Steel in Hydrogen Sulfide Solutions Available to Purchase

Effects of strain rate, applied potential, pH, temperature, and hydrogen sulfide concentration on sulfide cracking of mild steel were examined at constant strain rates by means of an Instron-type machine. Fractographic observations were also carried out on a scanning electron microscope. Kinetic studies were also made to elucidate the hydrogen electrode reaction mechanism for mild steel and the effect of hydrogen sulfide on the reaction. Amount of hydrogen absorbed in mild steels stressed at a constant strain rate was measured and relationship of hydrogen embrittlement of mild steel with hydrogen electrode reaction and hydrogen absorption in hydrogen sulfide solutions was investigated. Fracture surfaces were transgranular quasi-cleavage and nucleation sites of cracks appeared to be carbides in the pearlite, inclusions and/or grain boundaries. The elementary reactions of hydrogen electrode reaction in acidic solutions are proton discharge and recombination of adsorbed hydrogen atoms and the rate determining step is the proton discharge. The backward reaction rates of both elementary reactions can be disregarded. The dissolved species which promotes hydrogen embrittlement in hydrogen sulfide solutions is undissociated molecular H₂S. The molecular H₂S adsorbed on steel is assumed to act as a bridge-forming liqand for the proton discharge, which thereby accelerates the discharge reaction and hydrogen entry into steel and consequently promotes hydrogen embrittlement. The activation energy for the cracking was estimated as 8800 cal/mole.