Corrosion Corner W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC Spray package metal, internal coating and laminate film anomalies and their relationship to aluminum spray package corrosion. H ello everyone. Aluminum spray package metals, internal coatings and laminate films are far from perfect (I will hereafter refer to coatings and laminate films as polymers). Indeed, the polymers are not always continuous (i.e., they have holes) and the metal surface is a complex mixture of different crystal structures and non-metallic materials. In addition, a variety of aluminum alloys are used to fabricate aluminum spray packages. The different alloys often have different surface morphologies and thus polymers form different types of bonds with each type of morphology. The wide variety of polymer-metal bonds produces opportunities for spray package metal and polymer corrosion. Anomalies on metal surfaces and anomalies in polymers could theoretically cause package corrosion. However, corrosion does not always occur when metal or polymer anomalies are present. Let’s take a look at a few examples of the spray package metal and polymer anomalies, and discuss their relationship to spray package corrosion. Figure 1. Hole in a polyacrylamide (PAM) coating The anomaly in Figure 1 is a hole commonly found in internal coatings for aluminum aerosol containers. The hole directly exposes the aluminum metal to your formula. Corrosion in aluminum aerosol containers does not occur at the same frequency as the holes. Consequently, holes in coatings do not necessarily lead to container corrosion. 20 Spray July 2013 Figure 2. Divot in aluminum filled with coating (~0.2 mm wide along the minor axis) The anomaly in Figure 2 is an example of a divot on the metal surface that was created when a small piece of aluminum metal was torn from the metal during container fabrication. The coating flowed into the hole during coating application inside the container. Corrosion with this type of anomaly is rare. Figure 3. Solvent pop Figure 3 has an example of a solvent pop. Solvent pops are very common with lacquer coatings, such as polyacrylamide (PAM). Small bubbles are created during heating to remove the solvent, and the bubbles produce solvent pops like that in Figure 3. Corrosion caused by solvent pops is also rare. Continued on page 22
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