Written on: October 1, 2019 by W. Stephen Tait
Hello everyone. How a spray package is constructed, its materials of construction and the chemical composition of a formula all interact to determine if corrosion will occur and how fast the corrosion will proceed to degrade the package materials. The rate of corrosion, in turn, determines if corrosion will cause package failure and the length of spray package lifetimes.
Spray packages are typically multi-component systems that have multiple microenvironments inside each type of package. The type of corrosion in each microenvironment is often significantly different from the corrosion in the other microenvironments because the:
• chemical composition of the microenvironment is often different from the chemical composition of your formula
• different types of microenvironments often have different chemical compositions from other microenvironments
• probability of corrosion in a microenvironment is different for each different type of microenvironment
• corrosion rate in each microenvironment is governed by the chemical composition of the microenvironment
Figures 1–3 illustrate the different microenvironments in aerosol containers, bag-on-valve (BOV) packages and aerosol valves, respectively.
Figure 1 illustrates the type of microenvironments that are inside traditional aerosol containers. A cut-away for aluminum containers is on the left photograph and a cut-away for steel containers is on the right.
Figure 1: A diagram of aerosol container microenvironments
Aluminum refillable, multiple use (RMu) packages are similar to the aluminum aerosol container on the left of Figure 1 (sans the aerosol valve) and steel RMu packaging is similar to the steel aerosol container on the right (also sans the aerosol valve, plus the top and bottom double seams are sometimes replaced with welds). 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:
Figure 2: A diagram of BOV and aerosol valve microenvironments
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:
Figure 3: Various types of valve corrosion
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
