W. Stephen Tait, Ph.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC Corrosion Corner What are the different types of corrosion tests? Figure 1: The empirical relationship between test length and the risk of pitting corrosion in traditional aerosol containers. 38 Spray July 2016 tests have a correlation that exceeds 99% (or lower than 1% risk). Other types of corrosion tests, such as the scratch test and (electrical) driven-can test, have no or very low correlations with actual spray package corrosion. Therefore, I will focus my discussion on the more reliable storage tests and electrochemical tests. I will provide an overview for each test type and briefly discuss the most common mistakes associated with each. Constant temperature storage stability tests Storage testing is the oldest type of spray package corrosion testing. Filled packages are stored at one or more constant temperatures and a small number of package samples are periodically removed for destructive examination of corrosion. The information obtained from storage tests includes: • Corrosion compatibility between the formula and the spray packaging components • Product ingredient stability with age and at a higher temperature • Product efficacy with age and at a higher temperature • Product weight loss with container age • Spray package component estimated service lifetimes • The estimated magnitude of failures for the various spray package components The most common mistakes made with storage stability tests are: • Conducting abbreviated testing—less than one year • Insufficient numbers of replicate package samples for each examination • Lack of training or experience of personnel examining containers for corrosion • Attempting to accelerate corrosion—and abbreviate test time—with higher temperatures • Not examining all spray package components for corrosion • Not estimating spray package component corrosion rates • Not estimating component service lifetimes using corrosion rates • Not estimating component failure magnitudes as a function of time • Not determining how package component and ingredient concentration variability affects spray package corrosion Hello, everyone. In May and June, I discussed how the cost of spray package failures (leaking) is significantly higher than conducting corrosion tests and that derivative products also need to be tested for corrosion. In other words, it is costly to skip corrosion testing and risky to assume a derivative product will not be corrosive when the original product is not. In the May and June Corrosion Corners I provided an empirical risk curve. The June version of this curve is reproduced in Figure 1. The graph for traditional storage testing in Figure 1 is an empirical curve that was generated for a wide range of spray products in traditional metal aerosol containers. The two data points for electrochemical testing are empirical relationships derived from over 700 direct comparisons with actual spray package corrosion. The electrochemical data include comparisons with all types of spray packages. Figure 1 can be used to approximate the risk when no internal company data is available to estimate risk. For example, Figure 1 could be used to approximate risk if one is considering truncating a storage test. The May and June articles along with Figure 1 are useful to justify corrosion testing during budget discussions and for discussions with R&D business partners as to why corrosion testing is cost effective and necessary. Reliable corrosion tests are constant temperature storage stability tests and electrochemical tests. We have found that storage tests have an approximately 93% correlation (or 7% risk) with actual spray package corrosion and electrochemical
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