Coating spits like the one in Figure 5 are also common in aluminum aerosol containers. Spits occur when entrained gas escapes while the coating solution is applied to the container metal. Coating spits rarely cause or contribute to container corrosion. Corrosion Corner Figure 1. Inclusions 42 Spray December 2013 W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC Surface anomalies in spray packaging materials and their relationship to corrosion. H ello, everyone. The surfaces of spray package materials are far from perfect. The metal-making process, the processes used to fabricate metal containers, coating application processes and the foil lamination formation process often produce surface anomalies that could be mistaken for corrosion, or thought to be the starting-point for corrosion. Let’s take a look at the more common surface anomalies that are found in various types of spray packages. Spray package metals are actually mixtures of metal and small amounts of non-metal materials. Non-metal materials often precipitate as small particles (inclusions) as the molten metal cools. The shape of these inclusion particles are often distorted when the metal is rolled into sheets or extruded into the final package shape. Distorted inclusions like the one in Figure 1 typically don’t cause or contribute to container corrosion. However, there are rare instances where distorted inclusions caused container pitting corrosion with specific formulas. Coating drools like the one in Figure 2 are common in aluminum aerosol containers. However, drools typically do not cause or contribute to container corrosion. Scratches in polymer coatings are also common for both tinplate and aluminum aerosol containers. The scratch shown in Figure 3 is also covered with coating. A coated scratch like the one in Figure 3 typically does not cause or contribute to aluminum container pitting corrosion. Scratches in polymer coatings on tinplate could cause rusting and de-tinning. Holes in polymer coatings on steel are rare. However, holes in polymer coatings on aluminum are small and similar to the one shown in Figure 4. Coating-holes typically do not cause or contribute to aluminum container pitting corrosion. Figure 3. Scratches in coating Figure 4. Holes in coating Figure 2. Coating drool Figure 5. Coating spit ~1 cm ~0.65 mm
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