W. Stephen tait, ph.D.
Chief Science Officer & principal Consultant,
pair O Docs professionals, LLC
Corrosion Corner
What is package
service lifetime?
Hello, everyone. Those of you who have used Pair O Docs
corrosion consulting, failure analysis, inhibitor development
or Aristartec corrosion testing services have heard
the terms “package service lifetime” (PSL) and “container service
lifetime” (CSL). CSL is typically used for aerosol containers—also
spray packaging—thus CSL is actually a subset of PSL. Hence, I’ll
use PSL for both traditional aerosol containers and other types of
spray packages in this edition of Corrosion Corner.
Propellant or product leaking occurs when the package wall is
perforated by corrosion. Packages stop spraying when they lose
propellant or the spray valve orifices are plugged by metal corrosion
or delaminated pieces of coating/laminate film. PSL is the
age of a filled package before it leaks or stops spraying; please
note that PSL is not shelf life. Shelf life is considered to be
the time when a specified percentage of the product’s active
ingredients have lost their potency.
Metal, coating and laminate film corrosion might or
might not lead to package failure with a resulting shorter
PSL than specified. For example, general corrosion occurs
over large areas of a package surface. Typical examples of spray
package general corrosion are de-tinning of tinplated steel aerosol
containers (tin corrosion), spots of corrosion on aluminum or
large coating blisters. The tin coating on steel (i.e., tinplated steel)
is not a structural component of a tinplate container and large or
small patches of de-tinning do not reduce tinplate container PSL.
Spots of corrosion on aluminum or steel that have very shallow
or immeasurable depths are also examples of general corrosion
that don’t significantly reduce PSL. Thus, general corrosion
typically does not reduce PSL—unless the amount of corrosion
product (e.g., rust) and/or free pieces of delaminated coating or
laminate film from blisters clog the spray valve orifices.
In some instances, general corrosion might degrade product
efficacy or package functionality. For example, de-tinning and
rusting might not reduce the PSL but they might degrade the efficacy
of the fragrance or discolor the product. In these instances,
corrosion would cause product rejection, even though the
package did not fail (leak or stop spraying). Product degradation
28 Spray December 2019
without package failure is an example of the product shelf life that
is reduced because of general corrosion.
Localized corrosion occurs in small areas on spray package
materials. Metal localized corrosion is referred to as pitting corrosion
and small spots of metal corrosion might be the beginnings
of pitting corrosion, particularly when observed early in a storage
test (e.g., at 3–6 months). Localized coating and laminate film
corrosion is referred to as blistering. Localized metal corrosion is
often very fast; it causes product or propellant leakage and significantly
reduces PSL. However, there are instances where localized
corrosion is slow enough that it does not reduce PSL below the
target or specified PSL.
Metal container and laminated foil PSL can be estimated from
storage tests by using the pitting corrosion rate (localized corrosion
rate). The pitting rate is the pit depth divided by the package
age when the depth is measured.
For example, when a 2Q steel aerosol container has an estimated
pitting depth of 0.006" after one year, the estimated PSL
is 0.008./0.006 = 1.3 years (2Q steel aerosol containers have a
nominal 0.008" thickness). The PSL for the same 2Q container
is four years when the estimated pitting depth is 0.002" after one
year. PSL lengths typically range from 2–5 years for different types
of spray packages.
Pits are “born” at different times, tempting one to conclude
that there are a range of pitting rates. However, the actual pitting
rate is a narrow range of magnitudes for specific formula-package
combinations.
Consequently, the deepest pit depths found during each examination
interval for a storage test provide the best estimations
for the actual pitting rate range. Hence, one needs to examine
Packages stop spraying when they lose
propellant or the spray valve orifices
are plugged by metal corrosion or
delaminated pieces of coating/
laminate film…