intergranular corrosion
/ in-ter-GRAN-yoo-lar /
A metal is not one smooth solid but a mosaic of tiny crystal grains, and the seams where those grains meet — the grain boundaries — are slightly disordered, mismatched regions, like the grout lines between tiles. In intergranular corrosion the attack runs preferentially along these boundaries, eating narrow channels into the metal along the seams while the grain interiors stay almost intact. Taken far enough it can loosen whole grains until the metal crumbles like sugar, even though the surface may still look sound.
The most important case is 'sensitized' stainless steel. Stainless resists corrosion because dissolved chromium lets it grow a protective film, but if the steel is held in the roughly 500 to 800 degrees C range — as happens in a band beside a weld — chromium and carbon combine and precipitate chromium carbide (Cr23C6) right on the grain boundaries. That greedily pulls chromium out of the thin zone next to the boundary, dropping it below the roughly 11 percent needed to stay passive. The chromium-depleted boundary strip is now the weak anode against the still-protected grain faces, so corrosion tunnels along the boundaries. When this happens in the heat-affected band parallel to a weld, it is called weld decay.
The fixes attack the sensitization directly. Use a low-carbon grade such as 304L or 316L, so there is too little carbon to form much carbide; use a 'stabilized' grade like 321 (titanium) or 347 (niobium), whose added elements grab the carbon first and spare the chromium; or give the part a solution heat treatment (heat high, then quench fast) to redissolve the carbides. The lesson is broader than stainless: a heat process that is otherwise harmless can quietly ruin corrosion resistance by rearranging what sits on the grain boundaries.
A stainless-steel tank welded with an ordinary high-carbon grade can develop a leaking line running a few millimeters back from each weld: the metal directly at the weld got hot enough to re-dissolve carbides, and the far metal never sensitized, but the band in between sat in the 500-800 degrees C window long enough to precipitate carbides and corrode through along its grain boundaries.
Weld decay appears in a band beside the weld, not at the weld itself — the giveaway of thermal sensitization.
Sensitization changes almost nothing you can see or weigh; the steel looks and measures normal, yet its grain boundaries have quietly lost the chromium that made it stainless — which is why low-carbon or stabilized grades are specified for welded parts even at extra cost.