An Unexpected Role of H During SiC Corrosion in Water

TL;DR

This study reveals a novel mechanism in SiC water corrosion where hydrogen, not oxygen, initiates bond breaking due to a metastable hydroxyl group, challenging traditional oxidation models.

Contribution

It uncovers a reversed role of H and O in SiC corrosion, highlighting hydrogen's active role in early water attack through a newly identified metastable hydroxyl species.

Findings

Hydrogen actively breaks Si-C bonds during early corrosion stages.

A metastable bridging hydroxyl group facilitates water dissociation.

Displaced Si forms H2SiO3, a precursor to silica formation.

Abstract

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.