Grain boundary carbide
Accumulation of carbon-metal compounds along crystal grain boundaries. This can deplete protective elements like chromium near grain boundaries.
When welding austenitic steel, a problem can occur in the heat-affected zone: at certain temperatures, chromium combines with carbon to form carbides. This happens at the grain boundaries, and that's exactly where the chromium gets depleted. The steel loses its corrosion protection because the protective passive layer can no longer form properly. This is called sensitization, and the steel can then rust or even disintegrate along the grain boundaries. To avoid this, best use steel with low carbon, you recognize it by the 'L' in the designation, or use stabilized grades. When welding itself: keep the heat low, weld quickly, and don't let the part stay hot too long. If the material is already damaged, only solution annealing with subsequent rapid quenching helps to redistribute the chromium evenly.