W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Corrosion testing
Dos & Don’ts…
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.
No testing 3 months 6 months 9 months 12 months
Traditional metal 62% 30% 19% 12% 7%
Internal laminated foil bags 22% 11% 7% 5% 3%
Table 1: Approximate corrosion risk as a function of corrosion test length.
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
February 2021 Spray 25