W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Spray Packaging
Corrosion Quiz — Part 1
Hello, everyone. This month I’m starting a six-question
spray package corrosion quiz. A quiz?
Hold on, don’t panic! There’s no grade and the quiz
won’t be part of your next performance review—it’s merely intended
to be educational.
The questions are all of the true/false variety and we’ll cover
half of them this month.
The tin coating on tinplated steel containers
always protects the container steel from
corroding—true or false?
Mostly False (Hey, what happened to True/False
answers?)
The concept of “tin-always-protecting-steel”
originated from the time when tin was applied
to steel by dipping a steel sheet into molten tin,
resulting in a very thick tin layer. However,
the tin coating on modern tinplate is
electroplated on the steel substrate and
the tin layer is very thin with many holes
that expose either an iron-tin alloy or
steel. These holes can easily be seen under a
microscope.
I’ve observed instances where the interior
surface of a container was as shiny as the day it was
purchased, but there was a very small pinhole through the container
wall. I’ve also seen several instances where pits are located
in the center of circular or oval-shaped patches of detinning on a
container wall.
Tin does indeed prevent unfilled containers from atmospheric
corrosion and, in some cases, protects a filled container from corrosion.
However, corrosion testing is needed to determine if the
tin coating will protect the substrate steel.
You can stop corrosion by eliminating oxygen from the
container—true or false?
Sometimes true
Oxygen is electrochemically active and it dissolves in formula
water. However, it is difficult to remove all oxygen from a spray
package.
For example, a 22-inch vacuum on an aerosol container leaves
enough oxygen in the container headspace to result in approximately
50 parts per million dissolved oxygen in the formula. Oxygen
solubility in water increases as pressure increases, thus most
of the residual oxygen remaining after applying a vacuum most
34 Spray March 2018
likely dissolves in formula water. Approximately 6–10 parts per
million of dissolved oxygen is enough to cause uncoated tinplated
steel to corrode.
In addition, there are many other types of electrochemically active
ions and molecules that can cause or contribute to corrosion.
For example, water molecules are electrochemically active. Thus,
even so-called “oxygen-free” water can cause tinplated steel corrosion.
Consequently, it requires more than removing
oxygen to prevent container corrosion.
You can avoid corrosion testing when
you only tweak a formula—true or false?
Emphatically False!
Very small changes in the chemical composition
of a formula could cause very large
changes in formula corrosivity. If a tweak
causes the formula’s performance to
change, then it is could also cause the
corrosivity to change. Changing a surfactant
or a fragrance, for example, is not
really a formula tweak from a corrosion
perspective.
This piece of folklore reminds me of one of
the written examinations that I took over four
decades ago as the first step toward my certification
as a Corrosion Specialist. One question asked what concentration
of copper ions causes steel pitting. The answer was one
part per billion. That’s not a lot of copper ions! The point is that a
small contamination or small changes in formula chemistry (such
as a new surfactant or fragrance) could cause a non-corrosive
formula to become a spray package-eater. The flip side of this
phenomenon is that even a small amount of corrosion inhibitor
often transforms a corrosive formula into a non-corrosive
formula.
I hope you’ve enjoyed the quiz so far. Next month we’ll continue
with questions 4–6.
Pair O Docs has a state-of-the art electrochemical corrosion
testing laboratory; please contact me if you would like to know
more about our rapid, predictive corrosion testing. You can also
visit our new website which has a short Vision Video that discusses
all our corrosion prevention and control services. Please also
contact me if you would like to a have our Elements of Spray Package
Corrosion short course taught at your R&D facility. Back issues
of Corrosion Corner are available on CD-Rom. Thanks for reading
and we’ll continue the quiz in April. Spray
I’ve observed instances where the interior surface of a container
was as shiny as the day it was purchased, but there was a very
small pinhole through the container wall...