  During aqueous corrosion, atoms in the solid react chemically with oxygen,
leading either to the formation of an oxide film or to the dissolution of the
host material. Commonly, the first step in corrosion involves an oxygen atom
from the dissociated water that reacts with the surface atoms and breaks near
surface bonds. In contrast, hydrogen on the surface often functions as a
passivating species. Here, we discovered that the roles of O and H are reversed
in the early corrosion stages on a Si terminated SiC surface. O forms stable
species on the surface, and chemical attack occurs by H that breaks the Si-C
bonds. This so-called hydrogen scission reaction is enabled by a newly
discovered metastable bridging hydroxyl group that can form during water
dissociation. The Si atom that is displaced from the surface during water
attack subsequently forms H2SiO3, which is a known precursor to the formation
of silica and silicic acid. This study suggests that the roles of H and O in
oxidation need to be reconsidered.
