There is no such thing as a one-size-fits-all corrosion inhibitor. There are also
many types of formula ingredients, such as fragrances, that in some instances
act as corrosion inhibitors.
February 2019 SPRAY 29
The formula-spray package surface tension determines how effectively
various formula ingredients wet the internal package materials.
Wetting determines how easy or difficult it is for formula
ingredients to absorb into and diffuse through polymer coatings
and laminate films and onto uncoated or substrate metals under
laminated films and coatings. The exponent f(i) is a complex function
that generates a single number for each component surface
The fourth factor accounts for how electrochemically active
(ECA) ions and molecules affect corrosion rates. The ECAγj symbol
represents the electrochemical activity for individual ions and
molecules. The exponent f(j) is a complex function that generates
a single number for each specific electrochemically active ion or
molecule in a formula.
The fifth factor accounts for how the metal substrate surface
treatment affects the corrosion rate. Surface treatments include
polymer coatings or laminate films, tin coatings on steel and
chromium/chromium oxide coatings on steel. Note that tin
coatings are typically a complex multilayer surface treatment. The
exponent is a complex function that generates a single number
for each type of surface treatment.
Metal surfaces—both coated and uncoated—are composed of cathodic
areas where valence electrons are transferred from surface
atoms to electrochemically active (ECA) formula ingredients and
anodic areas where the atoms are ejected from the bulk metal as
ions. The cathode/anode ratio determines pitting corrosion rates.
The exponent f(m) is a function whose value is determined by
the specific chemical composition of a formula and the type of
Emulsions break after a certain time (or age) and can also be
broken with either high or low temperatures. Water and cream
phases are typically generated when an emulsion breaks, and one
or more of these phases could be very corrosive. Consequently,
the exponent for this factor is an equation that is a function of
temperature and emulsion age. This particular factor is zero for
non-emulsion products. In other words, this factor is equal to one
for non-emulsion products.
Steady state corrosion rates are used to estimate spray package
service lifetime. Steady state rates are achieved after spray
packages are filled and the time-to-steady-state could range from
a few days to nine months after filling. The exponent “n” for
this factor is a single number that is determined by the specific
chemical composition of a formula and the type of package
For example, an uncoated tinplated container might rapidly
de-tin (tin coating corrosion) and the large number of tin
ions quickly break the emulsion. Conversely, a coated tinplate
container with the same formula would also most likely de-tin,
but the number of tin ions generated would be lower and the
emulsion would probably break more slowly.
There is no such thing as a one-size-fits-all corrosion inhibitor.
There are also many types of formula ingredients, such as fragrances,
that in some instances act as corrosion inhibitors. Consequently,
the exponent of this factor is a complex function that
accounts for specific formula chemical compositions, pH, synergy
between all ingredients that have the ability to inhibit corrosion
and the effective concentration range for each ingredient that can
The empirical corrosion rate equation presented in this Corrosion
Corner could be used to estimate package service lifetime
when every probability is known for a specific formula chemical
composition. However—as with last month’s equation—most of
the probabilities for each factor and individual factor-components
are unknown. In addition, the probabilities and factor components
are unique to each formula chemical composition and type
of package for the formula.
The majority of the components for each factor in the equation
are either unknown or not available in the public domain at this
time. Consequently, corrosion testing is the only reliable way to
• Will a formula or line extension corrode the chosen
spray package materials?
• What type or types of corrosion will occur?
• How fast will corrosion proceed through the materials?
In other words, what is the package service lifetime with
We plan to drive from Wisconsin to California and back for
two weeks from late April–early May 2019. Consequently, Pair
O Docs would be pleased to teach our Elements of Spray Package
Corrosion short course at your company if you are located west of
Wisconsin (north or south route). Please contact me at 608-831-
2076 or firstname.lastname@example.org if interested. Thanks for your
interest and I’ll see you March. Spray