Written on: October 1, 2015 by W. Stephen Tait
Hello, everyone. Time is typically short for commercial development of new products and derivatives of existing products (e.g., an existing product with a new fragrance). However, corrosion takes time a) to initiate, b) to grow to a critical size that will sustain steady state growth, c) for steady state growth to either perforate the package metal or completely delaminate the coating/laminate film, d) to cause container leakage of product or propellant and e) for corroded product or coating/laminate film pieces to clog aerosol valves and prevent package spraying.
Metal corrosion and coating/laminate film corrosion both have growth rates that are determined by the chemical composition of a formula and the type of metal and coating/laminate film in a spray package.
Figure 1 provides an empirical graph of corrosion risk as a function of storage test time. This graph was generated with an aggregate of data from approximately 7,500 storage test containers for a wide range of aerosol products. The associated risks with electrochemical methods are also provided in Figure 1.
Notice in Figure 1 that the risk of corrosion decreases with storage test time. Notice also that the corrosion risk is around 7% after one year of storage testing. The corrosion risk decreases more rapidly with electrochemical testing and is significantly lower than the corresponding risk after one year of storage testing.
Thus, electrochemical testing provides corrosion test results in a significantly shorter time with a lower level of risk. However, not all electrochemical tests are the same. The appropriate test parameters (including test time) should be used to obtain the high correlation (low risk) shown in Figure 1.
Why is time important with corrosion tests? It takes time for metal and polymer corrosion to generate sufficient corrosion products to be seen with the unaided eye or with a light microscope (e.g., rusting or blisters). It also takes time for steady state corrosion to be developed to a level at which it can be detected with electronic instruments.
Metal pitting corrosion can start after many months, but still be rapid enough to cause package perforation in less time than your target package service lifetime. Figure 2 illustrates this concept with two different corrosion initiation times and similar corrosion rates. The Y-axis is the percent penetration through container metal or metal foil, or the amount of the coating/laminate film delaminated from the package metal surface. The X-axis indicates the ages of the spray packages in months.
Figure 2 illustrates that early detection of metal or polymer corrosion after very short storage times typically indicates a very corrosive formula. Figure 2 also illustrates that the absence of corrosion after very short storage times does not ensure that package corrosion will not occur and will not cause package metal perforation or complete coating/laminate delamination within 26 months.
Therefore, I recommend conducting storage corrosion tests for at least one year before concluding if the formula and package are compatible (with a 7% risk). Electrochemical tests provide results in a shorter time because this method uses very sensitive electronic instruments to detect and measure corrosion long before it can be seen with the unaided eye.
Why not raise the storage test temperature to accelerate the coating or metal corrosion? Increasing the storage test temperature does not normally increase the rate of detinning corrosion, aluminum or steel pitting corrosion or polymer laminate and coating corrosion (e.g., blistering).
Consequently, raising the storage test temperature does not normally increase the rate of corrosion. In other words, storing test packages at 100ºF (37ºC) does not reduce the test time needed for a room temperature storage test.
There are instances where thermally unstable products cause package corrosion above room temperature (i.e., 70ºF). Thus, higher temperature testing is necessary to determine the thermal stability of your spray product in warmer climates, and if thermal instability contributes to or causes package corrosion.
We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Want a specific topic discussed in an issue of Corrosion Corner? Please send your suggestion/questions/comments to email@example.com or visit www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in November.