Rust

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

In the early days of the Dalek Empire, the Dalek City was infected with a rust plague that was spread by the Black Dalek Leader. After deducing this, the Dalek Prime sent the Black Dalek to the recasting furnace, ending the outbreak. (COMIC: Plague of Death) To one side of the Dalek City was a great Sea of Rust. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

During the 25th century Dalek invasion, the Daleks sent to invade Venus risked rusting over in the humid climate. They countered this by coating their casings in red lead. (PROSE: Red for Danger)

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) A service duct on Floor 0507 was rusted shut. To evacuate the people, Bill Potts, since upgraded into a CyberMondan, used her weapon to cut through it. (TV: The Doctor Falls)

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)

Both Ashad and his Cyberguards had heavily rusted cyber-bodies. (TV: Ascension of the Cybermen)