Not all types of corrosion cause spray package failure. Failure in the context
of this article is leaking packages or packages that do not spray because
October 2019 SPRAY 29
of clogged valves...
Aluminum aerosol containers have four microenvironments:
1. Vapor phase crevice where the valve is crimped to the
container curl
2. Vapor phase above the propellant or product (when the
propellant is not a separate phase)
3. Interfacial areas between separate propellant-product and
propellant-head space layers
4. Liquid phase (bulk product area)
Steel aerosol containers have 10 microenvironments:
1. Vapor phase crevice where the valve is crimped to the
container curl
2. Vapor phase crevice where the top is seamed onto the body
3. Welds in the vapor area of the container
4. Vapor phase above the propellant or product (when the
propellant is not a separate phase)
5. Interfacial areas between separate propellant-product and
propellant-head space layers
6. Welds in the interfacial areas of the container
7. Liquid phase (bulk product area)
8. Weld along the container body in the liquid phase
9. Liquid phase crevice where the bottom is seamed onto the
body
10. Weld in the bottom crevice
Vapor phase corrosion is different from liquid phase corrosion.
It occurs under a very thin film of liquid as opposed to the
liquid phase, where the package surface is completely submerged.
Estimations of the vapor phase liquid film thicknesses are around
30 microns. The mechanism for initiating and propagating vapor
phase corrosion includes diffusion of corrosive species, such as
water, through the thin liquid film to the spray package surface.
Figure 2 illustrates the four microenvironments inside a BOV
package:
1. The adhesive/weld between the bag and the aerosol plastic
valve body
2. Bag welds
3. Laminated film
4. Metal foil under the laminate film
Bags are typically liquid-fill so there is essentially no vapor area in
this type of package.
Aerosol valves have three microenvironments as illustrated in
Figure 3:
1. The non-crevice area—identified as polymer coating corrosion
and metal corrosion
2. The crevice formed where the plastic valve body is crimped
to the metal valve cup
3. Liquid area inside the valve body—identified as the stainless
steel spring corrosion
Not all types of corrosion cause spray package failure. Failure in
the context of this article is leaking packages or packages that do
not spray because of clogged valves.
General and localized corrosion could occur in any of the
microenvironments shown in Figures 1–3. In the next issue, we’ll
continue the discussion on both of these forms of corrosion.
Please visit www.pairodocspro.com for more information. Thanks
for reading and I’ll see you in November. SPRAY
Figure 2: A diagram of BOV and aerosol valve microenvironments
Figure 3: Various types of valve corrosion