June 2015 Spray 31 Corrosion Corner Coating spit Figure 4: Coating spits and variations in coating color (noted by arrows and captions) Entrained air in the bulk coating material causes a coating nozzle to instantaneously eject excess coating—referred to as a spit. Figure 4 contains an example of a coating spit at the top of an aluminum aerosol container and bottom of another container. Figure 4 also shows variations in coating color. Coating color variations could be caused by variations in the thickness of the coating. Variations in coating thickness are well known in the container industry. Spits are very common in aluminum aerosol containers. However, I have not observed an instance where a spit contributed to or caused container corrosion. I have observed rare instances where variations in color appeared to cause random container failures (leaking). Figure 5: Solvent pop Figure 6: Area not wetted by coating (the hole is inside the dashed line) Variations in coating color Coating spit High temperatures are used to cure aerosol container coatings. Coatings and coating components are dissolved in solvents that evaporate during the curing process and small bubbles can form during solvent evaporation. Sometimes these bubbles harden, producing solvent pops like the one in Figure 5. Solvent pops rarely contribute to or cause corrosion. Pitting corrosion inside solvent pops can only occur when there is also extensive coating corrosion in a large area around a solvent pop. Holes in coatings are very common. Figure 6 shows an example of a small area where a coating did not wet (cover) the container metal, resulting in a hole that exposes metal. This type of defect only causes pitting corrosion when there is also extensive coating corrosion in a large area around the hole. One or several of the defects in aluminum and coatings are present in most aluminum aerosol containers. Corrosion testing is needed to determine when defects will contribute to or cause container corrosion that leads to failure. Next month I will continue this discussion on material defects in laminated foil bags used with traditional metal aerosol containers and traditional tinplated steel aerosol containers. We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Want a specific topic discussed in an issue of Corrosion Corner? Please send your suggestion/questions/ comments to rustdr@pairodocspro.com or visit www. pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in July. SPRAY
Spray June 2015
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