Rust

Rust was produced when the filings of the sentient iron died. BM Chemicals made liquids that could prevent this. (COMIC: The Metal Eaters)

The Ninth Doctor quipped that a Dalek would get rusty as it withstood water from the sprinklers at GeoComTex. (TV: Dalek)

, who purported himself to be a native of the Mondasian colony ship, claimed that his world was rust. (TV: World Enough and Time)

The Dalek Prime Strategist was, as a mark of a prior failure, confined to a Dalek War Machine casing which had begun to rust over when he met the Tenth Doctor. (COMIC: Defender of the Daleks)

In early 2020, the Thirteenth Doctor encountered a Dalek reconnaissance scout whose rebuilt casing was heavily rusted, only traces of its original blue and silver paint remaining. (TV: Resolution)