Define Cast iron substructure - Inspection of bridge?

Cast Iron also has the problem of corrosion due to atmosphere, which is similar to that of steel. However, in acidic or brackish water graphitic corrosion occurs. In this case, parts of corrosion products are formed underneath the normal surface layers. There is little visual evidence of the attack since the original dimensions and shape of the component are retained.

However, the corroded metal, bound together by graphitic ions, which are part of original make up of grey cast iron, is soft, porous and of very low mechanical strength. It can easily be cut with a knife or a light chisel and if lightly tapped with a hammer does not ring like a sound metal. This process is called graphitisation and is usually a slow process.