Galvanized steel paint adhesion

This specification covers an alkyd painting system for new, unrusted, untreated, galvanized steel. This system is suitable for use on parts or structures exposed in Environmental Zones lA (interior, normally dry) and IB (exterior, normally dry). The primer has good adhesion to clean galvanized steel but does not adhere properly to rusted galvanized steel. Painting System No. 1.04 should be specified for this condition. The finish paint allows for a choice of durable, fade-resistant colors. 

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

The benefits of a duplex coating are achieved only if there is good, long-lasting adhesion of the paint to the zinc coating. The NMP (A(-methyl-pyrrolidone) test has been described by Van Ooij et al. (1982). The test panels are immersed in the heated solution at 60 C (Fig. 1.16) and the time before the paint lifts is measured. Figure 1.17 shows the use of this test to demonstrate how silanes increase adhesion of an automotive epoxy-polyurethane primer to galvanized steel. 

Van Eijnsbergen, J. F. H. (1981). Hydrogen blistering and its influence on the adhesion of paint films on galvanized steel. Report to ZDA, London, 2 pp. 

The film thickness of the paint on each substrate was not so important as its adhesion to the substrate, which was greater for aluminum, galvanized steel, and steel than pine and particle board. 

The steel under the paint is frequently a form of galvanize for corrosion resistance. The metal is chemically treated to help adhesion and corrosion resistance. 

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