Figure 7. Variations in weld color Stretch marks like those in Figure 8 are common in formed components of tinplated steel containers, such as tops and bottoms. Stretch marks rarely cause or contribute to container corrosion. ~.3 mm Figure 10 has an example of a bubble in the laminated aluminum foil used for internal bags. Bubbles rarely cause or contribute to bag corrosion. ~.23 mm Occasionally we’ve observed cracks in the aluminum foil used for laminated bags like the crack shown in Figure 11. So far we’ve not encountered instances where cracks caused aluminum foil corrosion or bag leaking. There are many more types of surface anomalies in the various spray packages. The ones presented and discussed in this Corrosion Corner are the anomalies I’ve encountered most often when doing corrosion testing consulting and failure analysis on spray packaging. ~100 microns ~70 microns December 2013 Spray 43 Coatings are called lacquers when the coating material is dissolved in a solvent. The solvent is evaporated from the coating-solution using heat after the coating-solution is deposited on the container metal. In some instances, small bubbles form and solidify as the solvent evaporates. These bubbles are called solvent pops and typically look like the photomicrograph in Figure 6. Solvent pops rarely cause or contribute to container corrosion. The steel sheet used for tinplate is rolled from ingots into thin sheets. The rolling process causes streaks in tinplate as shown in Figure 9 (the double arrow indicates the streak direction). Roller marks rarely cause or contribute to tinplate corrosion. Steel sheet metal is coated with tin metal to form tinplate. The steel surface is typically not completely wetted by the tin metal. Indeed, approximately 16% of a 20# tinplate surface is not wetted by tin. Consequently, holes that expose either iron-tin alloy or the base steel are very common with tinplated steel aerosol containers. Figure 12 shows two types of tin-hole morphologies (shapes). The holemorphology depends on the plating process used to deposit the tin on the steel. At this time, we have no evidence that one type of hole morphology is more corrosion resistant than the other. Holes in tinplate might or might not cause or contribute to container corrosion. The chemical composition of your formula determines when holes in the tin coating do or don’t cause container corrosion. Figure 7 has an example of the weld color variations that are common in three piece welded tinplate aerosol containers. Color variation rarely causes or contributes to container corrosion. Figure 8. Stretch marks Figure 10. Bubbles Figure 12a. Holes in tinplate Figure 12b. Holes in tinplate Figure 6. Coating solvent pop Figure 9. Tinplate roller marks Please send your questions/comments/suggestions to rustdr@pairodocspro. com. Back issues of Corrosion Corner are available on CD from ST&M. Thanks for your interest and I’ll see you in January. Dark color Light color Figure 11. Cracks in aluminum foil
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