Definition - What does Grain Dropping mean?
Grain dropping is the dislodgement and loss of a grain or grains (crystals) from a metal surface as a result of intergranular corrosion.
Rapid attack at the grain boundaries can result in grains "dropping" or falling out of the metal surface, resulting in the disintegration of the metal. In practical application, the loss of cross-section thickness and the introduction of cracks can have severe consequences for applications like pressure containment.
Corrosionpedia explains Grain Dropping
Grain dropping is used to describe a type of intergranular corrosion. Intergranular corrosion is rapid corrosive attack of immediately adjacent grain boundaries with little or no attack of the grains.
In austinitic stainless steel, for example, chromium is the primary alloying element that makes it corrosion resistant; the chromium-depleted regions are susceptible to preferential corrosion attack. It is believed that this occurs because the chromium content immediately adjacent to the carbide may be below that required for the stainless steel alloy. If the carbides form a continuous network on the grain boundary, then corrosion can produce possible grain dropping.
Dissolution of anodic grain boundaries starts from the surface and advances along the grains' interfaces. The process results in grain dropping through deterioration of the bonding between the grains and drop of mechanical properties.
Grain dropping causes loss of entire grains and structural damage. The damage caused by preferential corrosion of alloy-depleted regions ranges from grain dropping to shallow pitting at localized sites depending on the distribution and morphology of intermetallic precipitate particles at grain boundaries. Therefore, grain dropping is the cause of mass loss, which results from the actual intergranular corrosion propagation along grain boundaries from the surface into the material.
Understanding Corrosion in Pumps and How to Deal With It