Definition
An electrochemical form of corrosion that occurs when two different metals are in direct contact in the presence of an electrolyte such as water or salt-laden moisture. The metals form a small electrical cell, and one of them (the more chemically active, or anodic, metal) corrodes faster than it would on its own, while the other is largely protected.
Plain English
When two different kinds of metal touch and get wet, one of them eats away faster than normal. The contact between the two metals plus the moisture sets up a tiny battery effect, and the weaker metal pays the price.
Context Anchor
Seen in aircraft maintenance, especially around fasteners, skins, fittings, battery areas, and places where moisture can collect between different metals.
Derivation
From Latin 'corrodere', meaning 'to gnaw away.' The 'dissimilar-metal' part simply names the cause: it is corrosion that only happens because the two metals are not the same. Knowing that helps the term make sense — it is corrosion driven by the difference between the metals.
Why Pilots Care
Unchecked dissimilar-metal corrosion can weaken structural parts and compromise aircraft safety over time.
Analogy
It is like creating a tiny unwanted battery between two different metals. If moisture connects them, one metal becomes the part that gets used up.
Grounding Statement
Picture an aluminum skin held with a different metal fastener, with salty moisture trapped between them; the area around the fastener may begin to corrode.
Intuition Check
Dissimilar-metal corrosion does not happen just because two metals are different. The usual problem is different metals plus contact plus conductive moisture.
Example Sentence 1
The mechanic found dissimilar-metal corrosion where a steel bolt passed through an aluminum bracket that had been exposed to rainwater.
Example Sentence 2
Installing a nonconductive barrier between the magnesium casting and steel bracket prevented dissimilar-metal corrosion on the engine mount.