Written on: February 1, 2020 by W. Stephen Tait
Hello everyone. Sometimes it’s tempting to take a shortcut by either skipping or abbreviating corrosion testing during new product or derivative product development. Such shortcuts are sometimes justified by budget constraints and/or lack of resources, plus the belief that a formula has no corrosive ingredients or that small changes in formula chemistry and/or package materials won’t affect spray package corrosion.
Last month’s Corrosion Corner discussed when corrosion testing should be conducted on new products and derivative products (line extensions). It provides the background for this two-part series on the cost effectiveness of corrosion tests. Let’s start the new discussion with a few definitions.
Package service lifetime
Package material corrosion has the potential to reduce spray package service lifetime. Package service lifetime is defined as the filled-package-age when:
• Packages leak product or propellant
• Valves leak propellant; or
• Partially full packages cease to spray
In other words, service lifetime is the length of time during which spray packages and valves function properly.
Statistical confidence & statistical risk
Statistical confidence is based on the theoretical normal distribution. Statistical confidence increases as the number of replicate samples increases and statistical risk is 100% minus the percent statistical confidence.
Correlation/empirical probability
I use the terms “correlation” and “empirical probability” interchangeably. Correlation is the percentage of times a test prediction/estimation is the same as the actual corrosion. Think of correlation as a grade on how well a corrosion test predicts the actual corrosion—higher correlations mean more accurate predictions.
Corrosion test objectives
Corrosion tests have four basic objectives to:
1. Determine if there will be spray package corrosion
2. Determine what types of corrosion will occur
3. Determine the rates for each type of corrosion
4. Use the rates to estimate the package service lifetime with a given formula
Typical spray package service lifetimes range from 2–5 years, depending on the type of product.
Corrosion test statistical confidence & statistical risk
High statistical confidence means a low risk. Figure 1 provides an empirical relationship between risk as a function of the corrosion test percent completion. It was developed from many decades of spray product corrosion test data and its correspondence to actual package corrosion.
The Y-axis for Figure 1 is the % correlation (as risk) between predictions from corrosion testing and the actual corrosion in commercial spray packages. The X-axis has the test percent completion for storage stability tests at constant temperature and electrochemical corrosion tests.
Electrochemical tests are significantly shorter than storage tests. Hence, the percent-completion on the X-axis in Figure 1 allows direct comparison between the risk for completed storage corrosion tests and electrochemical corrosion tests.
Notice in Figure 1 that the empirical no-corrosion-testing risk is approximately 62% that corrosion will occur. This risk is for all types of corrosion, however not all types of corrosion cause package or product failure. In other words, Figure 1 is not the risk for package failures.
Figure 1: The estimated risk of corrosion versus the percent test
completion.
The risk for (100%) completed storage testing is around 7% and the corresponding risk for completed electrochemical testing is less than 1%. Hence, the risk associated with electrochemical tests is significantly lower with a significantly shorter test length than the corresponding one-year storage test.
The risk for abbreviated storage testing ranges from approximately 30% after 25% completion, to an approximately 12% risk after 75% completion—three and nine months of storage testing, respectively.
In other words, the probability of surprise corrosion in commercial spray packages is high with no corrosion testing (~62%) and remains high with abbreviated storage test data. Indeed, a 7% risk for a completed storage test is considered by some to also be too high.
Storage tests and electrochemical tests have several parameters in common:
• The statistical confidence increases with the number of replicate samples for each measurement
• Both tests must be conducted for the appropriate length of time
• Appropriate data collection, analysis and interpretation procedures
• Temperature does not accelerate the corrosion; and
• Inappropriate parameters and procedures invalidate test results
In the next issue, I’ll complete the discussion with some details on electrochemical corrosion testing and an example of how much an unexpected corrosion incidence could cost.
Please visit www.pairodocspro.com for more information. Thanks for reading and I’ll see you in March. SPRAY
