Initiation of corrosion in metal substrates coated with plasma-deposited hydrogenated amorphous silicon alloy ceramic thin films

In recent years, silicon alloy ceramic thin films deposited by the plasma-enhanced chemical vapour deposition (PECVD) technique have been investigated as corrosion barriers for metal substrates. However, conflicting observations as to the cause of substrate corrosion when coated with these layers warranted further investigation. Hence, this study attempted to elucidate the origin and cause of localised substrate corrosion for several types of PECVD silicon alloy films. It was of particular interest to determine the effect of Cl\+sp--sp\ and SO\+v\4\-v\\+sp\2-\-sp\ on the synthetic forms since Cl\+sp--sp\ is generally more aggressive than SO\+v\4\-v\\+sp\2-\-sp\ and increases the propensity for breakdown of naturally formed films. The virgin films were characterised using SEM to identify any defects. The films were then examined after anodic polarisation and long-term exposure in an environmental chamber. It was revealed that film performance is dependent on both film and substrate composition, but not directly on the anion species. Corrosion initiation sites consisted of typical pinholes, minute cracks and also regions where no defects could be detected using SEM. (Abstract quotes from original text)