What Does Grain-Boundary Corrosion Mean?
Metals are made of atoms which are arranged in metallic crystals called grains. A metal can have numerous grains with crystal boundaries between them. These boundaries are known as grain boundaries. Grain boundaries are zones that have short-range disorders that determine the different properties of engineering materials.
Grain boundaries have less density on the atomic scale, a property that implies the presence of atomic holes, which atoms can diffuse. This makes the zone prone to oxidation and corrosion due to the movement of particles in and out of the holes. When oxygen diffuses through the grain boundaries, it reacts with steel to form iron oxide at the boundaries.
Corrosionpedia Explains Grain-Boundary Corrosion
In the case of stainless steel, carbon diffuses into the austenite grain boundary to form chromium carbides. These carbides deplete the regions adjacent to the chromium grain boundaries very fast. Chromium is known to add corrosion resistance to the material, and when carbon depletes it, the resistance of the metal decreases. This makes the boundaries sensitive to corrosion, a condition known as sensitization.
Sensitization opens the metal up for an attack called an inter-granular attack, which is enhanced by the segregation of impurities at the boundary. Attack at the grain boundaries depends on the level of sensitization, and the aggressiveness found in the corrosive environment.
In steel, sensitization that occurs due to the precipitation of chromium carbides at the grain boundary depends on the temperature. A temperature range of 450-900 degrees Celsius can precipitate chromium carbide. This condition can also be called "weld decay" if the temperatures are due to stress relief, annealing or occur during the welding process.
The problem of sensitization can be dealt with at an early stage. It can be reduced when different grades of carbon are used, preferably low carbon grades. It can also be stabilized by the addition of niobium or titanium, which has greater affinity to the formation of carbides than chromium. This is useful as the precipitation of carbides from the material will not affect the chromium content.