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SprayDec12

Corner W. Stephen tait, ph.D. Chief Science Officer and principal Consultant, pair O Docs professionals, LLC Why should corrosion stability storage tests be conducted for at least one year? Htest) provides the lowest risk decision on commercializing size often takes a short time; the mature pit growth rate (steady state(pit birth) might not occur until the filled packages are six to ninemonths old. To make matters worse, the growth of pits to criticalello everyone. Many of you have heard me say that datafrom a one year corrosion stability test (or static storage new spray products or line extensions of existing spray products. growth rate) could be fast enough to cause perforation in less than One of the main reasons for this statement is the risk is higher two years—even when the pits initiated as late as nine months after with data collected at times less than one year. For example, the filling. probability of spray package corrosion (or risk) is approximately Storage test temperatures are often raised in an effort to decrease 31% for results from three months of testing at room temperature. the amount of storage time (i.e., accelerate the rate of corrosion). The risk decreases to approximately 16% after six months of I’ve seen numerous instances where raising temperature produced testing, and ranges from approximately 2% to 7% after one year false results. For example, in some instances the corrosion was severe of storage testing. In other words, it takes one year of storage at the higher storage temperature and no corrosion was observed testing to reduce the risk to less than 10%. These percentages with the corresponding room temperature data. In contrast, I’ve were developed from over 7,000 storage test data and actual spray also seen numerous instances where the higher storage temperature package corrosion. prevented corrosion. There are several other underlying reasons for why I believe There are two options for reducing the amount of time needed to storage tests should be conducted for at least one year before complete corrosion testing without increasing your risk. The first making decisions on commercialization: option is to develop and use a large company corrosion data base. For example, your data base might demonstrate that new line extensions • Pitting corrosion that causes spray package perforation for product-X can be introduced into the market after only three does not typically start when the spray package is filled with the to six months of storage testing at room temperature; and the product corresponding risk of corrosion is very low for the line extensions. • Raising temperature does not typically Electrochemical corrosion testing can also be substituted for a —accelerate metal corrosion or coated metal corrosion many storage tests while reducing the statistical risk. However, the —accelerate polymer corrosion appropriate test parameters; plus data analysis and interpretation protocols must be used for electrochemical corrosion testing to Pitting corrosion proceeds in three distinct stages; 1) pits are born produce accurate predictions of actual long-term spray package (initiate), 2) selected pits grow to a critical size, and 3) selected pits corrosion. that reach the critical size continue to grow until they perforate Please send your questions/comments/suggestions to rustdr@ the spray package material. Each stage requires a finite time to pairodocspro.com. Back issues of Corrosion Corner are available complete. on CD from ST&M. Thanks for your interest and I’ll see you in The insidious thing about pitting corrosion is the first stage January. December 2012 Spray 45


SprayDec12
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