Written on: June 1, 2025 by W. Stephen Tait
Hello, everyone. This month I’ll complete the two-part series about the science of higher temperatures for spray package corrosion testing. Last month, we discussed the Arrhenius equation—the basis for using higher temperature to accelerate corrosion. It was demonstrated that the Arrhenius equation is usually invalid for spray package materials. Consequently, higher temperatures do not accelerate metal and polymer corrosion rates, nor do they accurately predict package service compatibility with a formula.
4. Why don’t higher temperatures above room temperature accelerate corrosion?
Typical corrosion storage test temperature parameters range from around 20°C (68°F, which is room temperature [RT]) to 30°C (86°F). Sometimes corrosion at a higher temperature appears to be more severe, leading to the impression that a higher temperature accelerates corrosion.
Figure 1 is graph of corrosion rates as a function of temperatures for tinplated steel with an epoxy coating. This graph includes the temperature range typically used for storage testing. Notice that the corrosion rate actually decreases between RT and 60°C (140°F) instead of increasing as predicted with the Arrhenius equation. Notice also that the corrosion rate increases with increasing temperature only when the test temperature is >60°C (140°F).
The corrosion rate of package materials (metals and polymers) does not follow the Arrhenius equation because material corrosion is a complex, multi-step process that includes absorption of water, polar molecules and ions on the metal or polymer-coated metal surfaces. Polymer coated-metal corrosion also involves subsequent absorption and diffusion through the polymer to the underlying package metal causing both polymer and metal corrosion.
In addition, both absorption and diffusion change polymer physical properties, such the glass transition temperature (Tg) causing a polymer coating to lose its ability to be a barrier between the underlying metal and the environment (e.g., your formula).
Notice in Figure 1 that the wet-epoxy Tg is approximately 40°C (104°F) lower than the corresponding dry-epoxy Tg . Both polymer and metal corrosion initiated around the wet-Tg for this situation, thus, the wet-coating Tg caused coated tinplate corrosion for this situation and did not accelerate the natural corrosion.
The complexity of uncoated metal and coated metal corrosion makes the underlying assumptions for the Arrhenius equation—first order kinetics plus energy of activation controls kinetics—invalid for package materials.
5. Should storage tests above room temperature be conducted?
Spray products are probably stored at higher temperatures for a time in most regions. Consequently, RT and higher temperatures should be part of a storage test. I’m not actually contradicting myself—a higher temperature should be included to determine if formula temperature instability might cause package corrosion.
Higher temperatures could degrade formula ingredients and might subsequently make your formula corrosive. Higher temperature could also destabilize the physical properties of a formula. For example, higher temperatures could break emulsions, producing an unexpected new corrosive phase inside the package.
6. Recommended practices for higher storage temperatures.
The amount of time that packages are stored/exposed to higher temperatures depends on local climates. Figure 2 depicts the hourly temperatures for Middleton, WI, during a hot Summer day (Source: National Oceanic & Atmospheric Administration). Indoor temperatures are typically 5–10 degrees lower than the outside temperatures; Figure 2 includes a simulated graph for indoor temperatures without air conditioning.
Notice in Figure 2 that the outside air temperature is >30°C (86°F) for approximately 7–9 hours during this particular day. Consequently, the actual length of time when package temperatures are >30°C (86°F) is shorter than a full day. Indeed, the maximum time for >30°C (86°F) package temperatures in Figure 2 is approximately 29% to 38% of the day.
If there are 40 days each year where the temperature is >30°C (86°F), then there are cumulatively around 12–15 days each year where package temperatures are >30°C (86°F) for the particular location depicted in Figure 2. In other words, the filled aerosol spray packages stored in this location spend most of their year at or near RT (assuming no long exposures to direct sunlight).
Note that the temperature profiles in Figure 2 are for a specific location and the profiles for other locations/regions might be different. Please keep in mind that storage tests are typically continuous and actual diurnal temperatures are cyclic.
I recommend the following guidelines for the higher temperature storage test parameters:
1. Don’t use a higher storage temperature to accelerate package material corrosion rates in order to reduce test times
2. Use higher temperature storage to determine if there are possible formula thermal instability issues that could cause package material corrosion
3. Conduct tests at higher temperatures that reflect the actual length during which packages are stored at high temperatures for the regions in which your products are marketed
4. Use higher temperature test lengths that reflect the actual number of hours per year when your products are exposed to higher temperatures
In summary, there is no reliable way to accelerate spray package corrosion rates with higher storage test temperatures. However, electrochemical corrosion testing can provide accelerated results because sensitive electronic instruments detect corrosion much sooner than it can be observed either with the unaided eye or with a microscope.
Thanks for your interest and I’ll see you in July. Contact me at 608-831-2076; [email protected] or from our two websites: pairodocspro.com and aristartec.com. SPRAY
