W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Why conduct spray package
corrosion tests? Part 3
Hello, Everyone. I started this three-part series in June on
why spray package corrosion testing is necessary. Part 1
covered the materials used to fabricate spray packages—
how metals, coated metals and laminated metals corrode and
how polymer coatings and films corrode. Part 2 in July discussed
two theoretical equations to estimate the probability of corrosion
and how fast corrosion penetrates and/or degrades packaging materials.
The complexity of these two equations—plus the unavailability
of the numerous parameters in each equation—illustrated
how it’s not practical to substitute mathematical equations for
corrosion tests.
This final part of the series provides overviews of the most
common corrosion causing/contributing formula ingredients,
common factors that produce surprise corrosion and those that
provide corrosion control and prevention.
Common corrosion-causing formula ingredients
Water
Liquid water is the most common contributor to, and cause of,
package corrosion. Only 90 water molecules are needed to form
liquid water and thus initiate metal corrosion. Water molecules
above the initial 90 are needed to subsequently sustain and
propagate the corrosion.
Contaminant water in anhydrous formulas is typically high
enough in volume to at least initiate corrosion. In other words,
anhydrous formulas are not immune to corrosion.
Liquid water is electrochemically-active. For example, the corrosion
of iron (tinplated steel) by water has the following corrosion
reaction:
Fe0 + 2H2O → Fe(OH)2 + H 2
Water molecules are small and easily diffuse through polymers,
subsequently forming microscopic rivers. These rivers allow water
and other formula ingredients essentially free access to the underlying
metal or metal foil, creating the conditions needed for metal
corrosion. The microscopic rivers also cause polymer coatings
and films to delaminate from their metal substrates and degrade
a coating’s or film’s ability to be a protective barrier between the
metal and a formula.
Formula pH
Water molecules dissociate into hydroxyl and hydrogen ions. The
hydrogen ions are electrochemically-active and will remove electrons
from metals, causing corrosion. For example, the corrosion
of tinplated steel by hydrogen ions is:
H2O → OH- + H + (water dissociates)
2H+ + Fe0 → Fe+2 + H2
(hydrogen ions corrode steel)
24 Spray August 2020
The concentration of hydrogen ions in a formula (water or contaminant
water in an anhydrous formula) is referred to as pH, and
pH magnitude often determines the rate of package corrosion.
Aluminum is not always more corrosion-resistant at a given pH
than tinplated steel and vice versa. The corrosively of each pH is
also determined by the other ingredients in a formula, such as
surfactants or ingredient concentrations.
Fragrance
Fragrances are complex mixtures of both numerous natural
extracts and manufactured ingredients. Many decades ago, it was
believed that fragrances were corrosive, some more than others.
There are some fragrances that actually cause corrosion, such as
vanilla. However, it was discovered that the “fragrance corrosivity”
belief was only partially true and that most fragrances actually have
some ability to inhibit corrosion.
Surfactants
Surfactants adsorb on material surfaces and make them more or
less susceptible to adsorption. They also make metal corrosion
and polymer corrosion easier with electrochemically-active ions
and molecules.
There is little or no public-domain corrosion research on
how/which surfactants cause or contribute to material corrosion.
Hence, extensive tables of corrosive and non-corrosive
surfactants are unavailable. However, sodium lauryl sulfate has
caused the corrosion of spray packaging, consumer packaging
and manufacturing equipment. Surfactants derived from phenols
and nonyl phenols also cause or contribute to corrosion in
many situations.
Active ingredients
Formula ingredients that have unsaturated bonds in their molecules
are potentially electrochemically-active. Insecticides typically
cause metal corrosion and require an inhibitor to either control
or prevent spray package corrosion.
Surprise corrosion
Spray package surprise corrosion is likely whenever corrosion tests
are not conducted, are too short or are conducted with inappropriate
parameters. Hence, complete and appropriate corrosion
tests should be conducted when:
• Derivative formulas are developed from a base formula—no
matter how small the chemical composition differences
between the derivative and the base
• Changing a fragrance or its concentration (no matter how
small the concentration change)
• Changing one or more surfactants in a base formula or their
concentration of one or more surfactants (no matter how
small the concentration changes)