Question

While brass valves are commonly used underground in water distribution systems, the valves in one area begun to cracking. The metal was found to be pinkish

color in some spots instead of the original yellow. Name the type of corrosion and explain its mechanism.

Answer 1

This type of corrosion may called as selective leaching or dezincification when brass corrodes, it can undergo dezincification, a process in which zinc is lost and copper is left behind. Mild dezincification may simply cause a cosmetic change, namely, the colour of the surface turning from yellow to pink, but severe dezincification can lead to the weakening of brass and even its perforation.

Mechanism...

The general mechanism is as follows the entire alloy is dissolved and later one of its constituent is re-plated from solution , copper is deposited as a fine copper dust which zinc is leached out of brass leaving behind a highly porous mass ( zinc has a high tendency to dissolution while copper has a high tendency to plate E◦ Zinc = -0.76 volt , E◦ Copper = 0.33 volt ) Dezincification cause only a slight change in appearance like color and does not affect the dimensions.