Thus, an empirical version of the theoretical Gibbs equation that
accounts for these five factors is:
The “n” and “F” symbols are the same as those in the theoretical
Gibbs free energy equation and the “K” is a conversion
constant. The Ψ symbol represents the probability for individual
corrosion factors. The Γ symbol indicates a combination of several
sub factors multiplied together.
Lower-case superscript letters are exponents for individual
terms in the equation. For example, in the term Ψ1(pH)a the “a”
is an exponent that could be from zero or greater than zero. The
exponent magnitude determines how much a factor influences
whether or not corrosion will occur (pH in this example).
Subscript lower-case letters in each term of the equation are numbers
ranging from one to the actual number of subfactors that determine
if corrosion will or will not occur. For example, in the term:
k
Γ(Ψh (type of metal) h) bh
h =1
The “h=1” and “k” below and above the Γ, respectively,
indicate there are from one to k subfactors that are multiplied
together to determine whether or not corrosion will occur
32 Spray January 2019
An empirical Gibbs equation could be used to determine if
corrosion will occur and how much influence each factor/subfactor
has in causing corrosion. However, all the exponents and
parameters are needed for the empirical equation to be accurate.
Unfortunately, most of the exponents and probabilities are either
unknown at this time or not available in the public domain. In
addition, the exponents and probabilities are specific for each
different formula chemical composition.
Consequently, it is actually easier to measure the corrosion
potential. However, it is essential that corrosion potentials are
measured at steady state, because measuring non-steady state
potentials will lead to erroneous conclusions about product corrosivity.
Corrosion potentials should also be measured with a high
impedance electrometer and not a voltmeter.
Measured corrosion potentials are composites and thus do not
provide information about how many specific factors are contributing
to or causing corrosion. Please remember that the corrosion
potential only indicates if corrosion will occur and does not provide
any information on how fast corrosion will occur.
Hopefully the empirical Gibbs equation provides an appreciation
of the complexity of corrosion and why reviewing formula
ingredients is not always a reliable way to determine if a formula
will or will not cause spray package corrosion. Hopefully, the
complexity of the empirical Gibbs equation also demonstrates why
measurement is preferred over a review of formula ingredients.
The February edition of Corrosion Corner will discuss an
empirical equation for predicting “how fast” corrosion will occur
using first principles. Thanks for reading and I’ll see you in
February. Spray