The removal of one element from a solid solution alloy is often called leaching. The gradual loss of zinc from brass (dezincification) is perhaps the most well known example of this type of corrosion, but aluminium can also be leached from aluminium 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, is then precipitated from 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 much reduced mechanical strength, leading to possible collapse under normal working stresses.
The tendency to this form of attack can be decreased by additional alloying such as the addition of arsenic to brass and nickel to Al-bronzes. Leaching and other examples of the selective attack are illustrated schematically in figure 1.
Figure 1. Leaching (top) and selective corrosion (bottom)
Graphitisation of Cast Iron
A common form of leaching is the graphitisation of cast irons. In slightly acidic waters both flake graphite (grey) and nodular graphite (ductile) irons are corroded due to the anodic behaviour 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. However, there is often little sign of the extent of this damage from the outwards appearance of the material, since the original shape and dimensions of components and pipes remain unaffected. This highlights the importance of correct application of ultrasonic testing in the assessment of condition of cast iron sections that may have suffered this form of attack.
In water pipes both internal and external graphitisation may occur where soil chemistry is aggressive. Corrosion mechanisms will also be subject to the influence of microbiological activity. In some cases, in effluent lines and older water mains, pipe sections can be almost fully graphitised whilst still holding water. They have been severely weakened, however, and are prone to sudden failure if water pressure changes, if supporting soil moves or vibration from overhead traffic increases.
The graphitised surface can be easily penetrated by a screwdriver or knife and the extent of the damage revealed by a examination under a microscope. Where it is cost effective graphitisation is avoided by the use of high nickel austenitic cast irons.