When compared to galvanized steel, galvannealed steel offers improved painting and welding characteristics. However, galvannealed tends to powder during the forming process, particularly in areas of compression. The tendency for powdering is known to increase with increasing coating weights. Conversely, powdering can be minimized by reducing the galvannealed coating weight. Even though lowering the coating weight reduces material costs and improves the manufacturing process, some are hesitant to reduce the specified coating weight due to corrosion concerns.
The corrosion performance of galvannealed is more difficult to determine than the corrosion performance of galvanized. The iron present in the galvannealed coating will cause a “red rust” discoloration on the surface of unpainted galvannealed, which is cosmetic in nature. Standard salt spray corrosion testing, however, is measured to the degree of red rust resulting from base metal corrosion. Discerning between the two is difficult. As most galvannealed is painted after fabricating, collecting data on painted sheet rather than bare may be more appropriate.
Corrosion testing on painted galvannealed is performed using a scribe-creep test. Using this test method, there is statistical evidence in cyclic corrosion testing that increasing coating weight from A40 to A60 has a marginal effect on the scribe-creep test (Reference: “Effects of Coating Weight and Pretreatment on the Painted Corrosion Performance of Coated Steel Sheet”, SAE Paper Number 970732, Theresa C. Simpson and Jay D. Hoffman of Bethlehem Steel Corporation, 1997.)
Reducing the coating weight of galvannealed offers many benefits including lowering material cost, lowering the tendency to powder, and reduces related issues in the manufacturing process. It also appears that reducing the coating weight may only have minor implications on the corrosion performance of painted galvannealed. For all of these reasons, reducing the coating weight on painted galvannealed could be a viable option. Factors such as performance requirements and applicable industry standards should be considered, and appropriate testing performed, when determining if this is an appropriate change.
