ROLE OF HYDRIDES IN HYDROGEN ENTRY INTO STEEL FROM SOLUTIONS CONTAINING PROMOTERS

The saturation solubility of hydrogen in steel has been investigated by cathodically charging steel specimens in solutions of different pH values containing various promoters. Hydrogen has been determined volumetrically after vacuum extraction at 300°C. It has been established that the [H]sat vs. pH vurve for each promoter rises to a maximum [H]max and then decreases with increase in pH. This observation has led to the conclusion that the hydride of the promoter is the dominant species in the mechanism of promoter action, and support for this view is provided by the fact that [H]max increases with the bond strength (or binding energy) of the promoter hydride. Hydrogen entry into steel in the presence of promoters is facilitated by formation of a promoter hydride, which thereby weakens the Fe{single bond}H bond. From the values of [H]max the order of effectiveness of the promoters studied is S > P > Se > Te > As which is contrary to the results obtained by previous workers. Nevertheless it should be emphasized that promoter action is dependent upon the pH of the solution, the precise experimental conditions of charging and the method used to evaluate hydrogen entry. © 1969.