Abstract
Cosmetic corrosion can usually be distinguished from perforating corrosion by the fact that the later occurs from the inside of the car body whereas the former occurs from the outside. Both initiate from local defects in the paint generated by chips or mechanical damage caused to the paint. However, the thickness and nature of the paint film are completely different from one side to the other. Also, it has been shown that a difference in film thickness can generate two types of corrosion mechanisms. It is likely that the kinetics of the degradation process are depending upon the presence of water at the interface and the oxygen diffusion in the paint. We have shown that unexpected galvanic couplings could be generated at the surface of a zinc coated steel sheet provided there was enough oxygen in the confined environment that was created. In particular, a cathodic reaction is susceptible to occur, in such case, on a zinc electrode. Such phenomenon can be explained by the nature of the corrosion products and their electrochemical properties. We have attempted to monitor the galvanic currents between those corrosion products in a confined cell and to measure the changes in pH as a function of time and oxygen concentration.
Also, we have investigated the properties of the paint with respect to oxygen and water vapor diffusion/transportation. To this aim, free films of cataphoretic paint have been prepared. Water vapor as well as oxygen permeation have been measured in a specially designed cell. It appears from our measurements that oxygen and water vapor are able to diffuse at rates significantly higher than in conventional two components epoxy paints. AC impedance measurements have been carried out on free films of cataphoresis and have proved to be a very useful technique to study these type of systems.
