The Role of Alloyed Molybdenum in the Dissolution and the Passivation of Nickel‐Molybdenum Alloys in the Presence of Adsorbed Sulfur

The effects of Mo on the dissolution and the passivation of a Ni‐2 atom percent (a/o) Mo(100) alloy in 0.05 M H 2 SO 4 were investigated with or without adsorbed sulfur. Sulfur was preadsorbed on the alloy surface in H 2 S ‐ H 2 gas mixtures. Complete monolayers of S ( i.e. , 43 × 10 − 9 g / cm 2 ) were formed with p H 2 S / p H 2 = 2 × 10 − 4 and T = 600 ° C . The surface coverages by sulfur were measured using the sulfur 35 radioisotope ( 35 S) and the chemical state was analyzed by electron spectroscopy for chemical analysis. Potentiodynamic and potentiostatic measurements were performed, and the concentration of sulfur on the surface was measured as a function of the amount of dissolving molybdenum. The effect of S is found to be very weak compared to the one previously observed on Ni and Ni‐Fe alloys, which shows that molybdenum counteracts the detrimental effects of adsorbed sulfur. The surface concentration of sulfur on the Ni‐Mo alloy decreases sharply during the active dissolution of the alloy, contrary to what was previously observed on Ni and Ni‐Fe alloys. Mo destabilizes the adsorbed sulfur. The mechanism is based on a surface reaction in which Mo bonds to and removes S on the surface. The experimental results are well fitted by a model where the removal of an S atom on the surface requires the formation of Mo k S clusters which are subsequently dissolved away. The best fit is obtained with k = 2 , which indicates that two molybdenum atoms must bond to a sulfur atom on the surface to remove it.