Corrosion Corner
The limitations of coatings
for corrosion prevention
Hello, everyone. It’s commonly believed that coatings
and laminate films are barriers between a spray package
metal or metal foil and your formula and that a barrier is
needed to prevent corrosion of the metal or metal foil. However,
coatings do not always prevent corrosion and are not always
needed to prevent corrosion.
For expediency, I’m going to collectively refer to coatings and
laminate films as coatings.
Why don’t coatings always prevent corrosion of the coating and
the substrate metal? Metal surfaces are a complex mixture of:
• Different agglomerates of metal crystals referred to as grains
• Thin non-metallic boundaries between different grains
• Non-metallic alloying precipitate particles in grains
• Molecular structural defects within grains
Consequently, complete metal surface coverage requires that
a coating bonds to all materials in the list. However, coatings do
not always bond with all materials causing macroscopic coating
defects such as the hole shown in Figure 1.
Figure 1: The hole in this polymer coating exposes the package
metal to the formula
It’s tempting to conclude that the hole in Figure 1 will cause
pitting corrosion. However, coating-holes are very common and
I’ve observed numerous instances where no pitting occurs in
the holes or pitting corrosion occurs where there are no holes.
Indeed, pitting corrosion only occurs in a hole when a large area
of coating around the pit is also corroding.
Another type of macroscopic coating defect is a solvent pop like
the one in Figure 2. Notice that the solvent pop in Figure 2 is
approximately five times larger than the hole in Figure 1.
W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Figure 2: Solvent pop in polymer coatings
Pair O Docs Professionals, LLC
Solvent pops are common. For example, we observed at least one
solvent pop in approximately 25% of the containers from a single
manufacturing lot.
The thin cap over a solvent pop is usually quickly removed after
a package is filled. However, metal pitting corrosion or coating
delamination in a solvent pop also needs to be surrounded by a
large area of corroding coating.
Coatings also have numerous internal microscopic defects that
allow liquids to absorb into and accumulate in the bulk coating.
Absorbed liquid could saturate the coating, degrade coating
properties and cause the metal under the coating to corrode. The
two most common defects in spray package coatings are pores and
voids.
Pores and voids are microscopic and difficult to observe.
Indeed, coating voids are indirectly detected with analytical
methods, such as positron annihilation spectroscopy. Figure 3
illustrates voids and pores in coatings.
The dark blue lines in Figure 3 depict pores and the dark blue
ovals depict voids. The three numbers in Figure 3 are labels for
different types of internal defects.
The first defect depicts a pore that eventually ends at a coating
void. Liquid fills the void by capillary action and the chemical
composition of the liquid could be either the same as, or significantly
different from, your bulk formula composition.
The second defect in Figure 3 illustrates how pores can meander
throughout the coating without ending at a void. Both “defect
one” and “defect two” typically cause coating corrosion with
subsequent loss of barrier properties.
The third defect illustrates that pores could also end at the coating
metal interface. Liquid accumulating at the interface could
lead to metal corrosion and coating delamination, such as blisters.
38 Spray May 2018
20 microns
100 microns