Definition
A form of corrosion that attacks the boundaries between the individual metal grains (crystals) within an alloy, rather than the surface of the metal itself. It is caused by a difference in chemical composition between the grain boundaries and the grains themselves, often resulting from improper heat treatment or prolonged exposure to high temperatures. Because the damage occurs internally along grain boundaries, the metal can lose significant strength while still appearing sound on the outside, sometimes producing surface lifting, flaking, or exfoliation as the corrosion progresses.
Plain English
A type of metal rot that eats away at the tiny seams between the crystals inside the metal. The outside can still look fine while the inside is being weakened.
Context Anchor
Seen in aircraft inspection and maintenance, especially when checking aluminum parts, sheet metal, and other metal components for hidden corrosion damage.
Derivation
From Latin inter (between) and granum (grain). Literally 'between the grains' — pointing to where the corrosion is happening: at the boundaries between the metal's internal grains.
Why Pilots Care
Can lead to sudden cracking or failure of load-bearing aluminum parts such as wing spars or fuselage skins with little external warning, directly affecting airworthiness.
Analogy
Think of a tiled floor where the tiles still look good, but the grout between them is crumbling. The damage is along the boundaries, not across the face of each tile.
Intuition Check
Do not assume corrosion always means visible surface rust or pitting. Intergranular corrosion can be hidden inside the metal, along the boundaries between its tiny internal grains.
Example Sentence 1
During the wing inspection, the technician found surface flaking that turned out to be intergranular corrosion in the aluminum skin.
Example Sentence 2
Regular nondestructive testing helps detect intergranular corrosion before it causes structural cracking under flight loads.