W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Why conduct spray package
corrosion tests? Part 2
Hello, Everyone. By the time this edition of Corrosion
Corner reaches your mailbox I hope the pandemic
is over and everyone is healthy.
The June edition of Corrosion Corner began a series on
why spray package corrosion measurements are necessary.
I started with a discussion on the various materials used to
fabricate spray packaging; how metals, coated metals and
laminated metals corrode and how polymer coatings and films
corrode.
There are always market and resource pressures to either reduce
the time for corrosion testing or measurements and, in some
instances, to skip them altogether. Indeed, I’ve often been asked
if corrosion could be mathematically modeled as a substitute for
storage corrosion tests/measurements. The short answers are Yes
and No.
Two questions should be asked for every new formula (as well
as any formula derived from an existing formula-chassis), particularly
whenever the formula chemistry is altered and package
materials or vendors are changed:
1. Will the formula cause spray package corrosion?
2. How fast will corrosion by the formula degrade the package
materials?
Many years ago, I derived empirical, probability equations to
address these questions (and have been periodically improving
them ever since). An overview for each equation follows.
Will the formula cause spray package corrosion?
The Gibbs free energy determines whether or not a chemical reaction
is possible1. Corrosion is a chemical reaction associated with
an electrical current, so the Gibbs free energy can also be used to
estimate the probability for an electrochemical corrosion reaction
with a specific formula/package system.
Figure 1 provides my empirical equation for the probability of
spray package corrosion. This equation estimates the probability
that package corrosion will occur with a specific formula.
24 Spray July 2020
Figure 1: The probability equation to estimate if spray package material
corrosion is possible. Please see Corrosion Corner (SPRAY January 2019)
for more details on this equation.
There are approximately 15 parameters in the Figure 1 equation.
Consequently, there are approximately 15 possible combinations
of these parameters (i.e., 1,307,674,368,000)! I say “approximately”
because some of the exponents are also equations with
their own set of parameters.
Each formula-family and package material have unique parameters
for this equation. Thus, each estimation from this equation is
typically only for a specific formula/package system. The Figure 1
equation does not provide information as to how fast the corrosion
will degrade the packaging materials, thus reducing the package
service lifetime. Service lifetime is defined as the filled package age
when:
• Packages leak product or propellant
• Valves leak propellant
• Partially full packages cease to spray, or
• Corrosion degrades product performance and/or efficacy.
In other words, service lifetime is the amount of time during
which spray packages function properly and do not degrade the
product.
How fast will corrosion by the
formula degrade the package materials?
Figure 2 provides the empirical probability equation to estimate
how fast corrosion by a specific formula degrades spray package
materials.
There are approximately 25 parameters in the Figure 2 equation.
In other words, there are 25 possible parameter combinations
to estimate package corrosion rates (i.e., 15,511,210,043,3
30,985,984,000,000)! I again say “approximately” because some
of the exponents are actually equations with their own set of
parameters.
Each formula-family and package material also has unique parameters
for this equation. Thus, each estimation from this equation
is typically also only for a specific formula/package system.
Extensive knowledge of how each parameter affects corrosion
enables complex equations to be simplified—e.g. use fewer groups
in an equation—for estimations. However, most of the parameters
in the Figure 1 and Figure 2 equations are either unknown or
not available in the public domain at this time. In addition, a sim-
Photo: The Organic Chemistry Tutor