Two things to remember when conducting corrosion tests to qualify substitute
ingredients: conduct tests for the appropriate length of time to reduce the
corrosion risk, and don’t use higher temperatures to accelerate corrosion
Figure 1: Risk versus corrosion test time
May 2019 SPRAY 41
The shape of this curve also applies
to laminated metal foil bags with
attached aerosol valves.
Notice in Figure 2 that the
measured corrosion rate is decreasing
with increasing temperature—instead of
increasing with increasing temperature—
until around 60°C (140°F)
and the corrosion rate subsequently
increases with increasing temperatures
above 60°C. In other words,
corrosion rates are not linear functions
of temperature and increasing
temperature does not increase the
corrosion rate in a linear fashion.
The minimum inflection at
approximately 60°C in Figure 2
occurs because the storage temperature
equals or exceeds the coating’s
wet glass transition temperature (Tg).
Polymer coatings lose their physical properties,
in order to reduce test length.
such as barrier properties, when the
storage temperature equals or exceeds the
wet glass transition temperature.
The dry polymer glass transition
temperature for this example is
100°C (212°F) as noted in Figure
2. In other words, the Tg for the
wet coating is approximately 40°C
(104°F) lower than the Tg for the
dry coating.
Corrosion testing is needed to
have low-risk decisions when modifying
the chemical compositions of
formulas—such as finding substitute
sunscreen ingredients. It takes at
least one year to achieve low risk
with a storage test (approximately
3% or 7% for laminated foils and
aerosol containers, respectively) and
raising the storage temperature does
not shorten test time while lowering
risk. Electrochemical testing can
both lower risk and significantly
shorten test time when the appropriate
technology is used.
Please visit www.pairodocspro.
com for more information. Thanks
for reading and I’ll see you June.
SPRAY Figure 2: Corrosion rate for a coated metal as a function of temperature