March 2016 Spray 41 on an aerosol container typically lead to both false negative and false positive corrosion results. Thus, applied electrical voltages should not be used to accelerate spray package corrosion. 4 Scratching coatings to mimic “worst case” coated metal corrosion It has been theorized that holes in coatings cause metal corrosion and that metal corrosion under coatings could be mimicked by manually scratching a coating inside an aerosol container. However, scratching coatings has a very low correlation with actual corrosion because a) the method used to generate the scratch (e.g., spinning while dragging vs. only dragging the scratching tool) determines if metal corrosion occurs under the coating; b) the type of scratching tool used to scratch the coating also determines if metal corrosion occurs and; c) holes in coatings are not the primary cause for metal corrosion under coatings. Thus, manually scratching container coatings for a storage test is not recommended. However, commercial spray packages with normal manufacturing scratches should be included in a storage test. 5Insufficient number of samples examined for corrosion The statistical confidence (or statistical risk) is a function of the number of packages examined. For example, examining less than three packages has approximately no statistical confidence or approximately 100% statistical risk. In addition, a low number of packages for each examination also risks missing how package variability affects spray package corrosion. Variability often leads to random unexpected corrosion in commercial packages. A minimum of 18 samples per corrosion examination is recommended if a high statistical confidence (low statistical risk) is desired. 6Not evaluating how ingredient concentration variability affects corrosion Variability in the concentration of formula ingredients and corrosion inhibitors could also produce random unexpected corrosion in commercial packages. Corrosion inhibitors in particular have effective concentration ranges and straying outside this range often causes corrosion. Multiple storage tests on the same formula-package variable using different ingredient and/or corrosion inhibitor concentrations are recommended for determining how variability affects corrosion. Understandably, R&D resources are often not available for conducting repetitive tests. Alternatively, narrow ingredient and corrosion inhibitor concentration ranges should be specified whenever multiple tests on the same formulapackage system are impractical. 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 suggestions/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 April. Spray
Spray March 2016
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