Although paints offer a significant enhancement to the life of metallic-coated steel, they do eventually “fail” in some fashion. This can take the form of chalking or fading to a color that is no longer acceptable to the user. It can also take the form of blistering or flaking. Both blistering and flaking can occur by separation along the paint primer bond line, the primer pretreatment bond line or the pretreatment metallic coating bond line. The specific nature of blistering or flaking depends on many factors associated with the specific combination of paint, primer, pretreatment, metallic coating, and the environmental conditions. Loss of paint adhesion can take several forms. The most common ways are (reference):
Lateral undercutting corrosion at a scratch in the paint, or at a sheared edge (where the paint/primer/metallic coating/steel are all exposed to potential corrosion). The net effect of this lateral undercutting corrosion is the loss of adhesion between the paint and the metal substrate. The corrosion can occur by (a) chemical reaction along the paint/ metallic coating interface which can cause the chemical adhesion bond to be degraded, or (b) bulk corrosion of the metallic coating leaving the paint totally “unconnected” to the steel sheet.
Blistering beneath the paint caused by corrosion reactions beneath the paint film. Paints are not impervious and water can penetrate through the paint to the substrate surface during times of wetness. If the initial bond strength is not real good, if the pollutants in the environment are particularly insidious for the type of paint system used, etc., blisters can develop beneath the paint even though there are no discontinuities in the paint. As the blisters grow larger and begin to combine, the net effect can be gross flaking of the paint in large areas.
To minimize the tendency for loss of paint adhesion through undercutting corrosion or blistering, one needs to take into account very specific recommendations from the steel supplier and paint manufacturers. The “best” coated-product design requires that the user pay attention to the type and thickness of the metallic coating, the type of pretreatment, the type and thickness of the primer, and the type and thickness of the topcoat. The recommendations from the suppliers will take into account issues such as: