Corrosion Mechanisms under Organic Coatings - A study in relation to Next Generation’s Pretretments

The demands on the corrosion protection in the automotive industry are very extensive. A car is subjected to various environments and there are many factors affecting the corrosion. There are thus high demands also on the corrosion testing. Many studies have been made concerning atmospheric corrosion but corrosion under a paint coating system is fairly unexplored. Such studies are important in understanding the corrosion mechanism possible in automotive corrosion. Phosphating is the pretreatment used in the automotive industry today and offer an excellent corrosion protection to the painted metal. However due to environmental aspects and possible legislation a need for new pretreatments, next generation pretreatments (NGPT) have developed and are under evaluation. Two types of NGPT´s are of greatest interest, a silane based one and a zirconium based one. The purpose of this project is to investigate corrosion under organic coating in an attempt to understand the corrosion mechanisms. The influence of different pretreatments in this context is also studied. The most existent corrosion product found was a zinc hydroxy chloride called simonkolleite [Zn5Cl2(OH)8*H2O] despite different pretreatments and exposure methods. This compound was found in a growth ring pattern on all materials from the cyclic corrosion test. The growth rings are related to the cycles in corrosion test chambers. The growth ring pattern is more distinct on the NGPT panels. The corrosion is spread in a circular manner from a small cathodic spot in the scribe. The scribe is applied to act as an initiation point for the corrosion. The analyses were preformed mainly with SEM/EDS and to some extent with optical microscopy and FTIR. The study shows a clear difference in morphology between the different pretreatments, however bending test shows no differences in adhesiveness to paint coating. The bending test showed that the fracture occurred within the pretreatment layer. In the cyclic corrosion test, however, the pretreatment layer is present on the paint side of the fracture