Rust was produced when the filings of the sentient iron died. BM Chemicals made liquids that could prevent this. (COMIC: The Metal Eaters [+]John Canning, TVC comic stories (Polystyle, 1970).)
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 [+]unclear authorship, The Daleks comics (City Magazines, 1965).) 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 [+]George Mann, Justin Richards and Cavan Scott, Dalek: The Astounding Untold History of the Greatest Enemies of the Universe (Ebury Publishing, 2017).)
During the 2400s Dalek invasion of the solar system, 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 Dalek Book (Dalek annual, Souvenir Press, 1964).)
In 2012 inside the Vault, the Ninth Doctor quipped that a Dalek would get rusty as it withstood water from the sprinklers at Geocomtex. (TV: Dalek [+]Robert Shearman, adapted from Jubilee (Robert Shearman), Doctor Who series 1 (BBC One, 2005).)
Razor, who purported himself to be a native of the Mondasian colony ship, claimed that his world was rust. (TV: World Enough and Time [+]Steven Moffat, Doctor Who series 10 (BBC One, 2017).) 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 [+]Steven Moffat, Doctor Who series 10 (BBC One, 2017).)
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 [+]Jody Houser, Time Lord Victorious release order (Titan Comics, 2020).)
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 [+]Chris Chibnall, Doctor Who New Year Special 2019 (BBC One, 2019).)
Both Ashad and his Cyberguards had heavily rusted cyber-bodies. (TV: Ascension of the Cybermen [+]Chris Chibnall, Doctor Who series 12 (BBC One, 2020).)