Post Treatment of Galvanized Coating to Improve Corrosion Resistance

In electromotive force (EMF) series,zinc occupies a place much lower than iron (E0Zn = -0.76V and for iron E0,Fe = -0.44V). Thus thermodynamically, Zn is very unstable comp ared to iron and in practice, it is indeed fond to corrode sacrificially in contact of iron in most of the envir-onments.In dry environment having plenty of supply of oxygen and carbon di oxide, the corrosion rate of zinc, however, is about 25 times less than iron. lt is a very surprising fact and reason is attributed to the formation of passive layer on the zinc surface. Zinc surface combines
with Oxygen to form ZnO. If CO, is also available, the upper layer is converted into zinc carbonate. These comp-ounds provide protection to the surface in absence of moisture. However, if ingress of moisture and other gases take place to the surface, these protective films are
damaged and an onset of deterioration of metal/coating takes place as a result of formation of zinc hydroxide, Zn(OH)2. This initial discolouration of the zinc surface is known as white rusting.