What Does Leaching Mean?
Leaching is the preferential removal of one element from a solid solution, such as the loss of zinc from brass (dezincification). Leaching is also the process of extracting minerals from a solid by dissolving them in a liquid, either naturally or through an industrial process.
Leaching causes reduction of mechanical strength and can result in structure failure. Since the original shape and dimensions of components and pipes remain unaffected, leaching often gives little sign of the extent of corrosion from the outwards appearance of the material.
Corrosionpedia Explains Leaching
Leaching is the selective corrosion in which one element is removed from a solid solution alloy. The gradual loss of zinc from brass (dezincification) is perhaps the most well-known example of this type of corrosion, but aluminum can also be leached from aluminum bronzes (dealuminification) and nickel from 70/30 cupronickel alloys (denickelification).
In each case, initial corrosion dissolves both components of the alloy, but the more noble metal, copper in this case, is then precipitated from the solution at the surface. This leads to increased solution of the parent alloy due to galvanic effects and hence further deposition of copper. The overall effect is to reduce the surface and underlying regions of a component to a spongy mass of material with significantly reduced mechanical strength, leading to possible collapse under normal working stresses.
A common form of leaching is the graphitization of cast irons. In slightly acidic waters, both flake graphite (gray) and nodular graphite (ductile) irons are corroded due to the anodic behavior of the matrix with respect to the cathodic graphite. This results in the conversion of the structure to a weak porous mass of corrosion product and graphite residue. In water pipes, both internal and external graphitization may occur where soil chemistry is aggressive.
The graphitized surface can be easily penetrated by a screwdriver or knife and the extent of the damage revealed by an examination under a microscope. Where it is cost effective, graphitization is avoided by the use of high-nickel austenitic cast irons.
The tendency to this form of attack can be decreased by additional alloying such as the addition of arsenic to brass and nickel to aluminum-based bronzes.