W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Variability & Spray
Package Corrosion
Hello, everyone. Variability is a fact of life that complicates
corrosion testing. It occurs both within a given manufacturing
batch (within-batch) and between different manufacturing
batches (batch-to-batch) with both formulas and packages.
Within-batch and batch-to-batch variables that affect spray package
corrosion susceptibility and formula corrosivity could include
one or more of the following:
1. Formula chemical composition:
• Active ingredient concentration
• Corrosion inhibitor concentration
• Surfactant concentration
• Fragrance concentration
• Formula or contaminant water pH
• Downward active ingredient assay drift with increasing time
• Formula or contaminant water pH drift with increasing time
• Loss of product efficacy with increasing time
2. Package materials and components:
• Coating thickness inside the package
• The number of holes and defects in polymer coatings and
laminate films
• Cracks in the aluminum foil used for laminated foil bags
• Tin porosity in tin coatings (holes) on steel (electrodeposited
tin plate ETP aerosol
containers)
• Aerosol valve crimp
dimensions
The different types of variability
could determine when spray
package corrosion occurs and
the corresponding corrosion
severity. Severe corrosion causes
a significant reduction in the
package service lifetime and
less severe corrosion causes a
smaller reduction in the service
lifetime. Figure 1 provides
three graphs for the corrosion
risk as a function of time:
1. Storage stability testing on
traditional metal aerosol
containers
2. Storage stability testing on
bag-on-valve packages
3. Electrochemical corrosion
tests on all types of spray
packages
32 Spray November 2020
These graphs were generated with data from numerous corrosion
tests and their associated actual, commercial corrosion.
The data in Figure 1 are cumulative data from all types of
package corrosion (e.g., general corrosion, pitting, coating blisters,
etc.) collected from decades of test data on a wide variety of spray
formulas. Notice that the corrosion risk decreases as the test time
increases. Notice also that the time for low risk with electrochemical
corrosion tests is significantly lower than the associated storage
stability tests (30–90 days versus one year, respectively).
Please note that the data in Figure 1 for electrochemical
corrosion tests are only valid for the Aristartec technology. The
correlation (risk) for other types of electrochemical test protocols
either have not been established, or are very low (e.g., 84% risk for
the driven can test). Please also note that the storage test data in
Figure 1 are also only valid for a specific protocol.
Suggested best evaluation practices
Therefore, how does one determine how much the within-batch
variability and the batch-to-batch variability affect spray package
corrosion susceptibility and formula corrosivity? The answer:
By conducting corrosion tests with the appropriate number of replicate
samples as a function of time.
Figure 1: Corrosion risk as percent occurrence versus test time.