This Bureau of Mines study used rigorously maintained, ultra-high- purity electrochemical systems for in situ characterization of fe- 18cr surfaces for corrosion studies in well-defined aqueous environments that heretofore were unavailable by normal or conventional laboratory techniques. Contrary to data in the literature, fe-18cr does not spontaneously corrode in 1n h2so4 solutions of ultra-high-purity; instead, it forms a state that is dependent on the deaerating gas. In he-saturated solution the behavior is one of long-term inactivity at potentials that are usually associated with passivity. In h2-saturated solution a steady-state, open-circuit potential is spontaneously formed that is within -1 mv of the reversible hydrogen electrode potential for that solution. This noncorroding state in h2-saturated solution is termed "stability" because even though corrosion is thermodynamically possible at -1 mv, the electrode is kinetically stable and exhibits, over a narrow potential range, equilibrium behavior that is associated with the h+/h2 exchange reaction. In situ electrochemical measurements of the fe-18cr solution interface indicated that stability was dependent not on the presence of surface metal-oxygen species, but on the absence of corrosion- inducing species. Without altering the ultra-high purity of an h2- saturated solution, it was determined that corrosion could be induced and stability restored by in situ electrochemical control of the surface concentration of hydrogen atoms. A mechanism has been proposed to account for the hydrogen atom dependency.