Written on: February 1, 2021 by W. Stephen Tait
Hello, everyone. In some instances, the COVID-19 pandemic has caused delays of new product and derivative product R&D programs. Consequently, it might be tempting to take shortcuts with corrosion tests to meet compressed timetables when programs are restarted. However, taking shortcuts with corrosion/stability testing could have unexpected and unwanted results for both completely new and derivative commercial products, e.g. a new fragrance, a raw material, containers, etc.
The most common shortcuts for corrosion tests are:
• Skipping testing
• Abbreviating testing
• Using a higher temperature to accelerate the material corrosion and abbreviate the test
Corrosion testing includes both long-term traditional storage stability tests and electrochemical corrosion tests. Table 1 summarizes risks associated with abbreviated corrosion/stability storage tests on spray containers using internal laminated-foil bags with an attached aerosol valve, traditional metal spray containers and aerosol valves.
Table 1 demonstrates that the no-testing risks are very high for both types of spray packaging and the lowest risks occur after one year of testing. The risks for shorter test lengths are also higher than what most would consider acceptable. In other words, the risks of corrosion are unacceptably high when corrosion testing is skipped, abbreviated or both.
The estimated risks in Table 1 are only valid when the appropriate protocols are used for:
• Measurement of corrosion
• Sampling intervals
• The number of replicate samples for each interval
• Data analysis
• Data interpretation
Electrochemical corrosion tests have less than 1% risk within three months. This very low risk is also only valid when the appropriate measurement, data-analysis and data-interpretation protocols are used.
Appropriate corrosion test protocols for both storage and electrochemical tests are typically complex, lengthy and usually proprietary. Hence a comprehensive discussion of the appropriate protocols for both types of tests is beyond the scope of this column.
Testing spray packages at multiple storage temperatures is a common practice to determine product stability at higher and lower temperatures, and to assess product efficacy for the range of temperatures to which your spray products will be subjected during everyday storage and usage. Higher temperatures are often also used in an attempt to accelerate material corrosion.
The origins for this practice are from the Arrhenius Law, which states that a chemical reaction rate doubles for each 20-degree temperature increase. However, the Arrhenius Law is only valid when the chemical reaction is a first order reaction or a pseudo first order reaction, and when the kinetics of the chemical reaction is controlled by the energy of activation for the reaction.
Metallic corrosion is not a chemical process. Corrosion involves a chemical change of state from metal atoms to ions; however, this change of state is in response to the transfer of electrons from the metal to chemicals in the environment (i.e., your formula ingredients). In other words, metal corrosion is a hybrid, electrochemical reaction. Hence, metal corrosion is not a first order chemical reaction and the corrosion rate is not controlled by the energy of activation for the chemical part of the corrosion reaction.
Polymer corrosion is also not a first order chemical reaction and use of the Arrhenius Law for polymer corrosion is invalid. In other words, raising temperature does not accelerate either the rate of metal corrosion, or the rate of polymer corrosion.
Some products will most likely be exposed to temperatures higher and lower than room temperature (20–25°C). Consequently, temperature is often a necessary part of a storage test to determine:
• Product stability at temperatures higher and lower than room temperature
• If higher temperatures degrade active ingredients
• If higher or lower temperatures cause physical instability, such as emulsion breakage
• If higher or lower temperatures reduce product efficacy
Storage stability and electrochemical corrosion tests with the appropriate parameters are recommended for:
• All types of new spray products—personal care, air care, laundry care, surface cleaning and pharmaceutical
• All new insecticide formulas and derivative insecticide formulas
• All derivative products of existing products, such as a new fragrance or substitutions for other ingredients
• All anhydrous formulas and derivative anhydrous formulas
• Changes to an existing formula’s chemical composition, including changes in ingredient concentrations
• Changing suppliers of formula ingredients
• Changing package materials for existing formulas
• When developing a corrosion inhibitor
In summary, please resist the temptation to skip and/or shorten corrosion tests to meet tight timetables. Please also resist the temptation to shorten corrosion tests by using temperature to accelerate material corrosion. Taking these types of shortcuts often results in unexpected and very expensive surprise corrosion in a commercial product.
Thanks for reading Corrosion Corner and I’ll see you in March. SPRAY