Mechanism of the Iron-Hydrogen Sulfide Reaction at Elevated Temperatures

The mechanism of the iron‐hydrogen sulfide reaction was studied in the 500°–760°C temperature range for a total gas pressure of 1 atm. The rate of attack was essentially constant (linear law), and the corrosion scale was composed of two layers: a thin dense layer adjacent to the metal and a porous layer with a coarse crystalline structure. These two features which differentiate this reaction from the iron‐sulfur vapor reaction are interpreted on the basis of two main effects: ( a ) partial blocking of reaction sites by hydrogen produced during attack, and occurrence of a reaction at a constant number of reaction sites; and ( b ) continuous recrystallization of the dense scale into a porous scale offering no barrier to transport of reactants. This mechanism is self‐regulating. The following evidence is presented: ( a ) linear Arrhenius plots were obtained; ( b ) essentially the same rate of attack was observed for hydrogen‐hydrogen sulfide and helium‐hydrogen sulfide mixtures having the same partial pressure of hydrogen sulfide; and the dependence of rate on hydrogen sulfide pressure was accounted for; ( c ) the attack rate was not affected by the coating of specimens with a dense layer of varying thickness obtained by preliminary attack in sulfur vapor; ( d ) the iron‐sulfur reaction, which obeys the parabolic law of attack, was inhibited by hydrogen diffusing from the back of the specimen surface; ( e ) recrystallization of the dense layer into the porous one was observed for specimens aged in helium; and ( f ) aging of iron sulfide with a fine crystalline structure (prepared by sulfur attack) in the corresponding hydrogen ‐hydrogen sulfide equilibrium mixture caused the scale structure to become somewhat coarse. Discussion of experimental methods covers the following points: ( a ) measurements of the rate of attack with a quartz spring balance in hydrogen‐hydrogen sulfide and helium‐hydrogen sulfide mixtures; ( b ) techniques for attack in sulfur vapor followed by hydrogen sulfide attack; ( c ) use of a mild steel cartridge containing lithium aluminum hydride in the hydrogen diffusion studies; and ( d ) scale thickness studies.