W. Stephen tait, ph.D.
Chief Science Officer & principal Consultant,
pair O Docs professionals, LLC
Corrosion Corner
A plethora of spray package
corrosion opportunities—
Part 2
Hello everyone. In last month’s Corrosion Corner, I identified
and discussed the multiple microenvironments
that occur inside spray packaging and spray packaging
components.
Microenvironments are why spray package corrosion is so
complex and unpredictable without corrosion test data. Aluminum
aerosol containers have four microenvironments, steel
aerosol containers have 10 microenvironments, bag-on-valve
packages have four microenvironments and aerosol valves have
three microenvironments.
There are two overarching types of metal, polymer coating
and laminated metal foil corrosion and they can occur separately
or together in any of the various spray package microenvironments:
• General corrosion
• Localized corrosion
General corrosion typically does not cause packages to leak.
However, general metal corrosion produces metal ions that
could degrade product efficacy. Localized metal corrosion could
also cause packages to leak before their desired product service
life.
Let’s discuss general and localized corrosion while looking at
a few examples.
General Corrosion
General corrosion occurs in large areas of the spray package
interior surface and removes package metal, coating or laminate
film at a nominally uniform rate. Figure 3 and
Figure 4 provide examples of general metal corrosion and general
polymer coating corrosion, respectively.
Figure 1: General metal corrosion (detinning of tinplated steel)
22 Spray November 2019
General metal corrosion, such as that in Figure 1, typically
does not reduce package service lifetime. However, general
corrosion contaminates the product with metal ions that could
adversely affect product efficacy, such as smell and color.
Figure 2 has an example of extensive coating delamination
without metal corrosion under the coating. Pieces of free (delaminated)
coating could clog spray valves, thus preventing the
package from spraying and reducing package service lifetime.
Figure 2: General coating delamination corrosion
General corrosion could occur in any type of microenvironment
discussed in last month’s Corrosion Corner. General metal
corrosion under a coating could also cause large areas of coating
or laminate film delamination from the package base metal.
Localized corrosion
Localized corrosion occurs in very small areas and in occluded
areas where diffusion into and out of an area is restricted.
Occluded areas in spray packaging occur when two
components are seamed or crimped together, such as tops and
bottoms seamed to container bodies and valves crimped to
container tops, respectively. Localized corrosion is different from
general corrosion in the following ways:
• Localized corrosion occurs in very small areas of metals
and coatings
• It is significantly faster than general corrosion
• It occurs in occluded areas such as container seams and
laminated bag welds
• It often needs a large amount of surrounding surface area
to support the high rate of localized corrosion