Written on: May 1, 2025 by W. Stephen Tait
Hello, everyone. Limited budgets and the pressure to quickly commercialize new products and derivative products (line extensions) often force the taking of short-cuts in corrosion tests. In some instances, higher storage stability test temperatures are used to accelerate spray package corrosion to shorten the time needed for corrosion test lengths. However, there are major pitfalls to this approach.
In this issue and the next, I’m going to discuss the science behind higher temperatures to accelerate chemical and corrosion reaction kinetics/rates. To accomplish this objective, I’ll address the following questions:
1. Why consider higher temperature storage testing?
2. The science behind increasing temperature to accelerate chemical reactions.
3. Do higher temperatures actually accelerate the corrosion of package materials?
4. Why don’t temperatures above 20°C to 25°C (68°F to 77°F) (room temperature) accelerate package materials corrosion?
5. Should higher temperatures be part of storage tests?
6. What are the recommended practices for using higher temperatures in storage stability tests?
This issue’s discussion focuses on the first three questions and next issue’s discussion will focus on the final three.
1. Why consider using higher storage temperatures to accelerate the corrosion of spray package materials?
Spray package materials can be polymer films on aluminum foil; tinplated steel (ETP); tin-free steel (TFS); aluminum; polymer coatings on aluminum and both types of steel; and polymer films on both ETP and TFS.
Storage stability testing (SST) is the most common form of testing for spray package material corrosion and product stability. The most often used shortcuts for a storage tests is to increase the storage temperature above room temperature, thereby decreasing test length.
However, higher temperatures typically don’t accelerate the corrosion of package materials because metal and polymer corrosion typically do not follow the Arrhenius Equation.
2. The science of increasing temperature to accelerate chemical reactions
Chemical reaction rates were found to be a function of temperature by Svante Arrhenius in 1889. The mathematical expression for his equation in corrosion rate terms is:
Equation (1) states that the corrosion rate of a material at room temperature (CR1) exponentially increases as temperature increases (∆Τ). Equation (1) also indicates that chemical reaction kinetics/rates double for every 10° Kelvin (°K) increase in temperature. The various terms in Equation (1) represent:
According to Equation (1), all types of metal corrosion, polymer film and polymer coating corrosion—such as delamination and blistering—should be accelerated by increasing temperature above room temperature. Indeed, increasing the temperature by 10°K should halve the SST length for corrosion evaluations—hence, the alleged scientific validity of raising SST temperature to decrease its length.
However, the validity of the Arrhenius Equation is contingent on two conditions:
In other words, the Arrhenius Equation is only valid for a situation where the chemical reaction is both a first order reaction kinetics and the reaction is controlled by its activation energy. However, in most situations, neither of these conditions occur with the corrosion of package materials.
3. Do higher temperatures actually accelerate the corrosion of package materials?
The Arrhenius Equation does not apply to metal corrosion because corrosion is not a chemical reaction. Metal corrosion does involve a change of chemical state—metal atoms become ions. However, the change occurs because metal valence electrons are physically removed by ingredients in a formula. In other words, metal corrosion is a hybrid electrical/chemical reaction, or electrochemical reaction.
In addition, both polymer and metal corrosion reactions involve a complex interaction of:
As a result of this complexity, the overall corrosion rates are typically not first order for both metal and polymer package materials. Consequently, one cannot reliably accelerate localized and general metal corrosion, detinning, coating delamination or coating blistering by raising the storage stability test temperatures.
Should you stop using higher temperatures for your storage stability testing? The short answer is “No!” Higher temperature can destabilize emulsions that might cause package corrosion and also degrade active ingredients, which might also cause package corrosion. Consequently, a higher temperature variable should be included in an SST—just don’t use it to predict corrosion rates with data from a short test.
In the next issue, we’ll focus on the final three questions, as well as recommendations for when higher temperatures should be used in a storage stability test.
Thanks for your interest and I’ll see you in June. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
