Corrosion Corner 24 Spray May 2014 W. Stephen Tait, Ph.D. Chief Science Officer & Principal Consultant, Pair O Docs Profesionals, LLC Why doesn’t corrosion occur in all spray packages for an individual storage test? Why does “surprise corrosion” occur with commercial products? ello everyone. I used the two most difficult questions to answer about spray package corrosion as the title for this edition of Corrosion Corner. An example for the first “Why” challenge is: you have a static storage test on a new formula or line extension, and you open twelve unlined tinplated steel aerosol containers after 12 months, why do you find 11 clean containers and one container with deep pitting corrosion on the bottom? The lack of corrosion in all storage test containers is perplexing because a) formula in the containers is from the same pilot plant/ lab/production batch, b) the containers were manufactured on the same day and c) the containers were all filled on the same day. The second “Why” question arises whenever a product-package system suddenly has a corrosion issue after being produced for years without a problem. In the March 2013 edition of Corrosion Corner, I discussed the microscopic causes for the inconsistency of corrosion found in packages from storage stability tests. This month, we expand the March 2013 discussion to include “surprise” corrosion in commercial products and apparent contradictions among two or more separate storage tests on the same product-package system. Variability is the culprit that causes the apparent corrosion inconsistency between multiple storage tests on the same productpackage system and the sudden appearance of corrosion in a previously corrosion-free commercial product. There are three types of spray package variability: variability within each individual package—referred to as within-package variability; variability within an individual production batch—referred to as within-lot variability; and lot-to-lot variability between different production batches (lots) and repeat individual storage tests on the same package-formula system. The main contributing factors to the three types of variability are: • Product chemical composition • Package and valve laminate/coating morphology (e.g., thickness and adhesion) • Laminate/coating chemical composition • Crevice depth to diameter/width ratios a. Valve crimps b. Container double seams c. Welds in laminated foil bags In my experience, variability of package metal chemistry does not usually have a significant effect on spray package corrosion. Consequently, variations in metal chemical composition are not included in the above list. These four factors could cause corrosion either alone or H
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