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Spray May 2016

W. Stephen Tait, Ph.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC Corrosion Corner Which is cheaper—corrosion testing or investigating a corrosion failure? 28 Spray May 2016 that pitting corrosion will occur. These risks are plotted as a function of percent test completion on the X-axis to illustrate that risk decreases as the amount of data increases. The final risk for a one-year storage test is around 7% (100% of the data collected). Intermediate risks range from approximately 30% after 25% of the data have been collected, to a 12% risk after 75% of the storage test data have been collected—three and nine months of storage testing, respectively. Therefore, the risk of pitting corrosion remains high until one year of storage testing. In other words, the probability of surprise corrosion in commercial units remains high when decisions are made to commercialize a spray product with abbreviated storage corrosion tests. Figure 1 also illustrates that less time is needed to decrease risk with electrochemical corrosion tests—30 to 90 days versus one year. The figure also illustrates that the risk with a completed electrochemical test is less than 1%—significantly lower than the corresponding 7% for a completed storage test. Which is cheaper: corrosion testing or investigating a corrosion failure? Spray package failures typically trigger initiation of an internal investigation into the failures. The objectives for an investigation include determining the root cause of the failures and developing recommendations to avoid more failures. A failure investigation typically lasts from around three months to one year, depending on the number of manufacturing batches involved. The investigation team typically includes people from all R&D disciplines (including management), plus personnel from manufacturing, quality assurance, legal, marketing, sales and customer service. Internal investigations typically involve a cumulative total of 10 to 30 employees during the investigation lifetime (usually not all are involved at the same time). Consequently, the cumulative hours for an investigation typically adds up to a significant amount of time and cost. For example, the cost for an internal investigation would be $90,000 when the average internal personnel cost is $150 per hour and a cumulative total of Hello everyone. Sometimes it’s tempting to either skip or abbreviate corrosion testing. Skipping corrosion testing is often rationalized by budget restraints and/ or lack of resources, the belief that a formula has no corrosive ingredients or that small changes in formula chemical composition or package component materials will not affect spray package corrosion. Abbreviated corrosion tests are often rationalized by short development timetables, using inappropriate corrosion test parameters or both. The March 2016 Corrosion Corner discussed the six common mistakes made with storage tests (inappropriate storage test parameters). To follow up, skipping corrosion tests and abbreviated storage tests causes unexpected corrosion failures in commercial spray products. In other words, the risks associated with a lack of corrosion testing or abbreviated corrosion tests are higher than most realize. Risks associated with skipping and abbreviating corrosion tests Figure 1 provides an empirical relationship between risk and the percent completion for storage and electrochemical corrosion tests and was developed from actual spray product corrosion data. The Y-axis has the empirical probability Figure 1: The estimated risk of pitting corrosion as a function of the total percent data collected from a given corrosion test.


Spray May 2016
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