Definition
A thin, dull film of corrosion that forms on the surface of certain metals — most commonly silver, copper, brass, and some aluminum alloys — when the metal reacts with sulfur, oxygen, or other compounds in the surrounding air. Unlike heavy corrosion, tarnish is usually surface-deep and does not significantly weaken the metal, but it can interfere with electrical contact and indicates that the protective surface is being attacked.
Plain English
A dull, discolored film that builds up on metal when it reacts with the air. It's the same kind of darkening you see on old silverware. On aircraft parts it usually looks like a haze or stain on the metal's surface.
Context Anchor
Seen during aircraft cleaning, inspection, and corrosion-control work, especially on exposed metal parts or hardware.
Derivation
From the Old French 'ternir,' meaning to make dull or dim. The original sense — losing brightness — still describes exactly what tarnish does to a metal surface.
Why Pilots Care
Tarnish signals active corrosion that can reduce electrical conductivity, weaken material strength, or interfere with proper contact in critical systems.
Intuition Check
Do not read tarnish here as damage to someone’s reputation. In aircraft maintenance, tarnish means a physical film or discoloration on metal.
Example Sentence 1
The mechanic cleaned the tarnish off the battery terminals to restore a solid electrical connection.
Example Sentence 2
Tarnish on the propeller blade edges prompted a closer inspection for hidden corrosion.