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November 2019 SPRAY 23
The most common forms of localized metal corrosion are pitting
and corrosion inside the crevices formed by aerosol container
double seams and the container curl-aerosol valve crimps.
Figure 3 is an example of metal pitting corrosion that causes
aerosol container perforation. The top diameter of the pit in
Figure 3 is approximately 1–2mm and the bottom of the pit is
approximately 1/4 to 1/5 the size of the top diameter.
Figure 3: Metal pitting corrosion
Figure 4 has an example of blistering—the most common
form of localized coating and laminate film corrosion. The diameter
of the blister in Figure 4 is approximately 1–2mm. Sometimes
blisters can also have metal corrosion under the blister.
Figure 4: Polymer coating blistering corrosion
Localized coating/laminate film corrosion could occur in any
type of microenvironment discussed in last month’s Corrosion
Corner.
There is no theoretical way to predict if general and localized
corrosion will occur. An extensive corrosion database is useful for
modeling potential corrosion for derivatives of existing formulas
(line extensions), but corrosion testing is necessary for both new
formulas and derivatives of existing formulas to confirm if corrosion
will or will not occur and if failure is expected prior to the
target package service lifetime. Corrosion testing could be either
a long-term storage stability test (1-year minimum) or electrochemical
corrosion testing with the appropriate parameters
(3-month maximum with the appropriate test parameters).
Please visit www.pairodocspro.com for more information.
Thanks for reading and I’ll see you in December. SPRAY
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