intercrystalline

Metals are not solid throughout. They are made of crystals, or grains, packed together like irregular stones in a wall. The boundaries where these grains meet are called grain boundaries. Intercrystalline describes anything that occurs along these boundaries: cracks, corrosion, embrittlement, or other damage that follows the grain structure rather than cutting across it.

In contrast, transgranular failure cuts straight through the grains themselves, indifferent to their edges. Intercrystalline failure is often weaker and faster because grain boundaries are already zones of weakness. Atoms are less densely packed there, and impurities tend to collect at these interfaces during solidification or heat treatment. A stress that would merely bend or dent a metal elsewhere can initiate a crack along a grain boundary and propagate it quickly.

Intercrystalline corrosion is particularly dangerous in stainless steels. When these alloys are heated to certain temperatures, chromium carbides form along the grain boundaries, depleting the chromium in the surrounding metal. This denuded zone corrodes readily in chloride or acidic environments, creating a network of corroded paths between the grains. This failure mode has caused failures in welded vessels, fasteners, and pipework in service.

Intercrystalline stress corrosion cracking (ICSCC) occurs when a metal experiences both sustained tensile stress and a corrosive environment. Austenitic stainless steels in hot chloride solutions, or carbon steels in caustic alkaline solutions, are vulnerable. The crack follows grain boundaries, making it harder to detect by visual inspection than a single transgranular fracture.

Prevention relies on metallurgical control: proper heat treatment to avoid sensitization, use of stabilized or low-carbon grades of stainless steel, selection of alloys resistant to the expected environment, and stress relief or shot peening to reduce residual tension. Post-weld heat treatment (PWHT) at specific temperatures can restore chromium distribution and close the window of vulnerability.