Written on: May 1, 2014 by W. Stephen Tait
Hello everyone. I used the two most difficult questions to answer about spray package corrosion as the title for this 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 and 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 product-package 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:
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 in combination with one another. The number of combined factors could be very large. For example, there are 24 possible combinations of the four factors for traditional metal aerosol containers.
The relationship between variability factors and the apparent inconsistency of container corrosion may include the following:
Product chemical composition
Small changes in chemical composition, such as pH, fragrance concentration or amount of water, could transform a benign formula into a package-eater. Variability is exacerbated by lot-to-lot variations in raw materials. There are also chemistry differences and variations in raw materials from different sources, even when the specifications for the different sources is nominally the same.
Package & valve laminate/coating morphology (e.g., thickness & adhesion)
Laminate films and coatings have variable thickness on individual packages, on packages within the same lot and on packages from different lots. Variability in the metal cleaning process and the coating application process could also produce small, susceptible areas on individual package surfaces that lead to delamination and corrosion under the film or coating. Coatings from different sources could also produce a narrower or wider range of thicknesses, even when the specifications for the two sources are identical.
Laminate/coating chemical composition
Variations in the chemistry of laminate films and coatings could cause variability in how the coating adheres to the substrate metal. Variability in adherence could cause laminate/coating delamination with subsequent corrosion under the coating.
Crevice depth to diameter/width ratios
Spray package crevices formed by valve-crimps, double seams and laminated foil welds will be filled with product via capillary action and osmosis. However, not all crevices are created equal. Some have a large opening with a small length while; some have a small opening with a large length and so on.
The ratio of the crevice opening to its length determines whether corrosion will occur inside the crevice and if the corrosion will be general or pitting corrosion. The chemical compositions of formulas also determine how a given crevice ratio and a given formula composition interact to make a crevice more or less vulnerable to corrosion. In other words, there are two variable quantities interacting—composition and ratio—that determine when crevice corrosion will occur.
There are a range of crevice ratios within individual production lots and between different production lots. Hence, crevice corrosion—with or without pitting—does not occur in all spray packages unless the product is extremely corrosive.
There are two ways to account for variability when conducting corrosion tests:
The corrosion test could be a storage test, an electrochemical test or a combination of both.
Want to learn more about spray package corrosion? We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Please contact firstname.lastname@example.org or visit www.pairodocspro.com. Please send your questions/comments/suggestions to email@example.com. Back issues of Corrosion Corner are available on CD from Spray. Thanks for your interest and I’ll see you in June.