Corrosion Corner
Spray Packaging
Corrosion Quiz — Part 2
Hello, everyone. Last month, we discussed the first three
of six common questions about spray package corrosion;
we’ll tackle the final three questions this month.
Anhydrous formulas are immune from corrosion—
true or false?
Kind of True
A completely anhydrous formula is not
expected to corrode spray packages. However,
does a completely anhydrous formula exist? For
example, humid factory air introduces small
amounts of water (parts per million) into a
formula during manufacturing and a contaminated
anhydrous formula could be a voracious
package-eater.
Strangely, completely anhydrous ethanol
aggressively corrodes aluminum, but a
few parts per million of water in ethanol
prevents aluminum corrosion. In other
words, there are no simple guidelines for
when and how much contaminant water will or
will not cause spray package corrosion.
You can always rely on sodium nitrite to inhibit
corrosion—true or false?
Cautiously true
Sodium nitrite inhibits a wide range of corrosive liquids but it
doesn’t work for all formulas. Indeed, no corrosion inhibitor
does.
Sodium nitrite is consumed while inhibiting corrosion. It
also oxidizes formula ingredients, causing the formula to yellow.
Sodium nitrite consumption depletes its concentration and thus
W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
sodium nitrite is often referred to as an unreliable inhibitor.
Figure 1 illustrates why it’s considered unreliable.
Notice the corrosion rate in Figure 1 is approximately zero
at a concentration around 0.1% (for this specific metal-formula
system). Notice also that the corrosion rate increases when the
concentration of nitrite is below 0.1% and above
0.1%. In other words, too little or too much
sodium nitrite actually causes corrosion instead
of inhibiting it.
Therefore, a sodium nitrite inhibited formula
may initially be non-corrosive but could
become very corrosive as the formula-package
system ages and the nitrite is consumed. Sodium
nitrite could also form nitrosamines or
nitrous oxide gas in low pH formulas.
Corrosion can concentrate in
holes in polymer coatings and
thus cause pitting corrosion—
true or false?
Emphatically False!
Holes in coatings that expose the package metal
are very common. In one instance, I found approximately
25% of the test containers had at least one very small
hole in the internal polymer coating.
Figure 2 provides examples of holes in a polymer coating on
aluminum and tin coating on tinplated steel. The lighter colored
“circle” in the center of the tinplate example is detinning and the
edge of the hole is next to the dashed oval.
Coating holes like those in Figure 2 typically occur because
metal surfaces are complex and random mixtures of crystal
defects, pure metals, metal alloys, different types of metal crystals,
grain boundaries and non-metal particles called inclusions.
Polymer and tin coatings will not bond with many of these defects
and surface materials, causing non-wetted areas that become a
hole in the coating like those in Figure 2.
Pitting corrosion only occurs in a polymer coating hole when
there is a surrounding area of coating that is simultaneously
corroding. A corroding polymer loses its barrier properties and
subsequently becomes a semi-permeable membrane. Liquid moving
through the semi-permeable membrane removes surface metal
electrons and supports the pitting corrosion in the hole. In other
words, pitting corrosion in a polymer coating hole is not possible
without a large area of coating corrosion surrounding the hole.
Polymer coatings also have voids in the bulk coating and at
the coating-metal interface. These voids are randomly distributed
throughout the coating and at the coating-metal interface. Liquids
accumulate in voids creating microscopic lakes.
Microscopic lakes could lead to failure of the coating barrier
properties, coating delamination (such as blistering) and differ-
Figure 1: Corrosion rate as a function of sodium nitrite concentration
26 Spray April 2018