Hello, everyone. Every year, we receive multiple requests to determine the cause of spray package failure from corrosion (leaking packages). In many of these instances the spray product was commercialized after a few months (e.g., three months) of storage testing at a high temperature, such as 100°F.
It is well known in corrosion and materials science that temperature typically does not accelerate metal and polymer corrosion. Figure 1 provides an example of the corrosion rate for polymer coated tinplate as a function of temperature. Notice that the corrosion rate actually decreases between 20°C (approximately room temperature) and 60°C. Thus, increasing temperature in this range does not accelerate coated tinplate corrosion.
It is well known that the glass transition temperature of a wet polymer is significantly lower than the glass transition temperature of the dry polymer. Polymer coatings lose their properties above their glass transition temperature, including their ability to provide corrosion protection.
Consequently, it is reasonable to conclude that above 60°C, the increasing corrosion rate in Figure 1 is caused by temperatures above the wet-polymer glass transition temperature. In other words, exceeding the coating glass transition temperature could cause coated tinplate corrosion. Figure 1 also illustrates why coated metals sometimes have significantly more corrosion at higher storage temperatures.
There are four conditions needed for higher temperatures to accelerate material corrosion:
- Material corrosion is present at all storage temperatures
- Metal corrosion
- Polymer coating corrosion or laminate film corrosion
- The corrosion rate for each higher temperature is larger than the corrosion rate for each previous lower temperature
- A temperature-corrosion rate graph is linear for the entire storage test temperature range (you will need to measure the corrosion rates at each temperature)
- The energy of activation for the corrosion reaction has a magnitude between 18,000 to 70,000 calories per mole per degrees Kelvin (°K)
The activation energy for a corrosion reaction is calculated using the Arrhenius equation:
Ea = R (T2T1)(Ln(CR2/ CR1))
- Ea is the activation energy of a corrosion reaction in calories per mole per °K
- R is the universal gas constant in calories per mole °K
- T1 is the lower storage temperature in °K
- T2 is the higher storage temperature in °K
- CR1 is the corrosion rate (in moles/second) at the lower temperature
- CR2 is the corrosion rate (in moles/second) at the higher temperature in
Temperature is not accelerating coating, laminate film or metal corrosion when all four of the criteria are not fulfilled. In other words, product thermal instability is causing corrosion when the four criteria are not fulfilled.
Should testing at a high temperature be included in a storage stability test? Yes, but not to accelerate spray package and aerosol valve corrosion. Higher temperature testing is needed to determine if:
- The product is thermally stable in the package at higher temperatures
- Product thermal instability contributes to or causes package corrosion
The exposure time for higher temperatures should be determined by the actual weather in the areas where commercial spray packages are stored. For example, if the hottest weather to which your products are exposed is 90°F (32°C) and above for approximately five hours each day and there are 90 days of this weather per year.
These weather conditions translate to a total of 19 days per year where temperatures are at or above 90°F (32°C). Consequently, in this example, storage testing at 100°F for 19 days should simulate approximately one year and approximately three years when tested for 60 days.
In summary, we recommend using higher temperatures to evaluate product thermal stability in a spray package and not to accelerate coating, laminate film and metal corrosion. The four temperature-acceleration criteria can be used to verify if higher temperature is accelerating package material corrosion. Product instability is most likely causing higher temperature package corrosion when all four of the criteria are not fulfilled.
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 protected] or visit www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in June.