Preferential chemical or electrochemical attack that progresses along the grain boundaries of a polycrystalline material because those boundaries differ in composition or microstructure from grain interiors.

Principle

Grain boundaries can be sites of precipitate formation, solute segregation, second-phase depletion or altered electrochemical potential, producing continuous or discontinuous paths that are anodic (or more reactive) relative to grain interiors and so concentrate corrosion along boundaries.

Demonstration

Illustrative scenario: austenitic stainless steel that has been heated through a sensitization range forms chromium carbides at boundaries, depleting chromium locally and permitting attack in aggressive solutions. Observable outcome: penetration following boundary network and loss of intergranular cohesion. Uncertainty: the precise boundary chemistry and environment required for attack can vary with alloy and thermal history.

Misapplication

Calling any non-uniform corrosion 'intergranular' when attack is actually transgranular pitting or general surface attack; attributing a boundary-following appearance to mechanical cracking without chemical evidence.

Consequence

Loss of grain-to-grain cohesion, rapid through-thickness penetration or network attack that undermines load-bearing capability even when gross thickness loss is small; inspection metrics must target boundary penetration rather than average corrosion rate.

Reversal

Transgranular corrosion, where attack progresses through grains rather than along grain boundaries, typically when grain interiors are less noble or when precipitates are distributed uniformly.

Boundary

Applies to polycrystalline solids with distinct grain boundaries (metals, some ceramics); does not describe corrosion of amorphous materials, coatings that mask boundaries, or purely mechanical intergranular cracking without chemical attack.

Semantic Tension

Often conflated with 'grain-boundary corrosion' or with intergranular fracture; the tension is between a chemical/electrochemical mechanism (this term) and mechanical separation of boundaries (fracture).

Synthesis

Intergranular corrosion is a chemically driven, boundary-focused degradation phenomenon: when microstructural features at grain boundaries create a local electrochemical advantage for dissolution, corrosion preferentially follows the boundary network and can rapidly compromise structural integrity despite modest overall material loss.