Corrosion resistance of painted zinc alloy coated steels

Organic coating in combination with sacrificial metal coating is the most popular method of protecting steel strips against atmospheric corrosion. Experiences over the years have proven that such duplex coating systems are best suited for the coil industry for the long term corrosion protection of steel. The excellent corrosion resistance of such systems has been attributed to the synergy between the cathodic protection provided by the sacrificial coating of zinc alloys and the combined barrier resistance of the metal and organic coatings. Traditionally continuously hot dip zinc-coated steels are used for such applications. However, off late the quest for further extending the longevity of the coil coatings has led to the replacement of the zinc coating with a host of other hot dip zinc–aluminium alloy coatings such as Galvalume^®, Galfan^®, ZAM^®, SuperDyma^®, etc. Each of these metal coatings has its own unique metallurgical features in terms of flexibility, bonding, microstructure and electrochemical characteristics which may significantly influence the performance of the organic coatings applied over it. This paper looks into the various aspects of these features of the hot dip coatings on the corrosion performance of the pre-painted steel strips. For simplicity only polyester paint system, the work horse of the coil industry, is considered.