Written on: March 1, 2017 by W. Stephen Tait
Hello, everyone. Storage temperatures above room temperature are often used to accelerate corrosion and thus reduce the length of time needed to determine the corrosion compatibility of products with packaging materials. Are there technical origins for this practice?
S.A. Arrhenius was a Swedish physical chemist who, in 1889, proposed a relationship between temperature and chemical reaction rates. This relationship—commonly referred to as the Arrhenius equation—states that the chemical reaction rate doubles for every 10°C (50°F) increase in temperature when the reaction has first order kinetics that are controlled by the
chemical activation energy. Thus, the Arrhenius equation is valid for increasing (or accelerating) the rate of a chemical reaction when these two conditions are satisfied.
Hence, the Arrhenius equation is the technical basis for using higher storage test temperatures to accelerate package material corrosion, and therefore reduce the time needed for a storage test. However, the Arrhenius equation is seldom valid for package material corrosion because material corrosion kinetics do not meet the two conditions required for the equation’s valid use.
Let’s examine material corrosion more closely in relation to the Arrhenius equation.
It is known that polymer materials deviate significantly from the equation at the polymer’s glass transition temperature (Tg). Indeed, when the temperature is at or above a polymer’s Tg, it loses its properties, such as corrosion resistance and the ability to be a barrier.
Aerosol valve dip tubes, plastic components, laminates on aluminum foil and internal container coatings all have a Tg. For example, the Tg for epoxy coatings range from 60°C (140°F) to 110°C (230°F) when the epoxy is dry.
However, a dry-polymer has a higher Tg than when it is wet. For example, several researchers have demonstrated that a wet-polymer Tg could be approximately 30°C (86°F) lower than the corresponding dry-polymer Tg.
Consequently, higher storage test temperatures might subject package polymer materials to temperatures above the wet-polymer Tg with the result that the polymers no longer have their dry-corrosion resistance property or the ability to be a protective barrier. In this instance, the corrosion at a higher temperature is probably not indicative of the corrosion at room temperature (20°C) (68°F).
In terms of the Arrhenius equation, the different Tgs for wet and dry polymers indicates that the corrosion reaction kinetics are not first order kinetics—one of the requirements for valid application of the Arrhenius equation to storage test parameters.
The change of state that occurs during metal corrosion—metal atoms become ions—is a chemical reaction. However, this chemical reaction is caused by electrons being transferred from the metal to ions or molecules in your formula. Thus, metal corrosion is a hybrid reaction and is referred to as an electrochemical reaction.
Hence, metal corrosion does not have first order chemical reaction kinetics—one of the requirements for valid application of the Arrhenius equation to your storage test parameters.
In addition, increasing the temperature of a metal increases its electrical resistance. Increasing resistance decreases electrical conduction (corrosion rate), in contradiction to the Arrhenius equation that states rates increase with temperature. Consequently, metal corrosion is not controlled by the activation energy, because raising temperature is actually impeding the transfer of metal electrons from the metal to ions and molecules in your formula.
In other words, the Arrhenius equation is not valid for metal electrochemical corrosion reactions and the rate of the reaction is not doubled for every 10°C (50°F) increase in the storage temperature above room temperature. Consequently, increasing temperature does not reduce the time needed for a corrosion stability test.
I’ve observed many situations where:
It’s tempting to say that the second situation is actually a worse-case-scenario and thus a valid test technique. However, in the second example—corrosion-free at room temperature and perforated at the higher temperature—a commercially viable formula would be rejected because of the higher temperature results. In other words, worse-case testing is not recommended for today’s highly competitive markets, particularly when every successful new formula and formula derivative contributes to competitiveness and market leadership.
Am I recommending not conducting tests at higher temperatures? No. Higher temperature testing is needed, particularly when your products are marketed in regions with high summer temperatures. However:
Pair O Docs has a state-of-the art corrosion testing laboratory, and in 2016 we introduced a Corrosion Partnership Program that provides a cost effective mix of all Pair O Docs services under a single purchase order. Contact: rustdr@pairodocspro.com; 608-831-2076; www.pairodocspro.com if you would like to know more or want to have our Elements course taught at your R&D facility. Thanks for your interest and I’ll see you in April.
