Definition
A non-protective form of iron oxide (Fe2O3) that forms on ferrous metals exposed to oxygen and moisture. Unlike a tight protective oxide layer, red rust is porous and flaky, allowing moisture and oxygen to continue reaching the underlying metal so corrosion progresses unchecked until the part is weakened or destroyed.
Plain English
The familiar reddish-brown crust that appears on iron and steel parts when they get wet. It does not protect the metal underneath -- it keeps eating into it.
Context Anchor
Seen during aircraft inspections on steel parts such as bolts, control cables, engine mounts, landing gear parts, and other ferrous metal components.
Derivation
From the reddish-brown color of iron oxide. Called 'red' to distinguish it from other corrosion products such as black rust (a denser, sometimes protective oxide) and white rust (zinc oxide on galvanized parts).
Why Pilots Care
Indicates active corrosion that can weaken structural components if not addressed, potentially leading to part failure.
Grounding Statement
If a steel aircraft part has red-brown powder, flakes, or rough spots, treat it as a sign that corrosion may be active.
Intuition Check
Do not assume red rust is only cosmetic. On iron or steel aircraft parts, red rust means corrosion is present and the part may need maintenance attention.
Example Sentence 1
During the preflight, the pilot noticed red rust forming on a weld joint of the engine mount and wrote it up for the mechanic to inspect.
Example Sentence 2
Red rust on the propeller hub required immediate treatment to prevent further material loss.