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 while
less severe corrosion causes a smaller service lifetime reduction…
November 2020 SPRAY 33
The number of replicate samples that need to be examined
or measured for corrosion decreases as the test time increases, as
indicated in Figure 1 (risk decreases with time). In other words,
the number of samples to examine/measure is inversely proportional
to test time—which in equation form is:
Number of replicate samples α (1/test time)
Examining one or two replicates at short storage times, such as
three months, has essentially no statistical confidence (essentially
100% statistical risk). My favorite number is 18 replicate samples
for package examinations when the time is < 9 months, then a
smaller number of replicates after nine months.
Please treat these replicate sample numbers only as guidelines.
The appropriate number of replicates for your formulas is determined
by the type of formula chemistry, plus the type of spray
package materials and components used with your formulas.
Spray packages from multiple replicate commercial batches
should also be evaluated to determine if scaling up from the
laboratory to commercial manufacturing causes variability that
also affects spray package corrosion.
Admittedly there is often not enough time or resources to
conduct the comprehensive corrosion testing program outlined
in this month’s Corrosion Corner. However, not doing so increases
the risk of surprise corrosion with a corresponding possibility of
a very expensive product recall and loss of customer good will.
Please keep in mind that Corrosion Corner only provides an
overview of corrosion science, engineering, control and prevention
and is not intended to teach how to establish and conduct
a comprehensive corrosion testing, control and prevention
program.
If you have any questions, call 608-831-2076 or email
rustdr@pairodocspro.com. Thanks for reading and I’ll see you in
December. Spray