By W. Stephen Tait, Ph.D., Pair O Docs Professionals, LLC
Hello, everyone. There are essentially up to two phases in a spray package: a liquid phase (could be your product plus the propellant) and a gas phase. Spray packages that use liquid propellants have both phases , while spray packages with internal bags typically only have a liquid phase.
Spray package corrosion could be one of three general types: general corrosion (sometimes referred to as uniform corrosion), localized corrosion and vapor phase corrosion. The different types of corrosion are often referred to as different corrosion mechanisms. I’m going to provide an overview for each of the three mechanisms.
General corrosion could occur in either the liquid area of the spray package or in the vapor area. In some instances, general corrosion could be in both areas.
General corrosion covers a large area of the spray package interior and removes package metal, coating or laminate film at more or less a uniform rate. General corrosion is typically very slow and does not significantly reduce container and container component service lifetime.
However, general corrosion does contaminate the product with metal ions and these ions could adversely affect product efficacy, such as smell and color.
General corrosion could also cause large areas of coating or laminate delamination from the package base metal. Pieces of free coating could clog spray valves, thus preventing the package from spraying and reducing package service lifetime.
Service lifetimes are defined as the length of time before spray package leaks product or propellant, valves leak propellant or partially full containers no longer spray. In other words, service lifetime is the amount of time during which spray packages and valves function properly.
Localized corrosion occurs in very small areas and in occluded areas that restrict diffusion into and out of the area. Localized corrosion is different from general corrosion in the following ways:
- Localized corrosion is significantly faster than general corrosion and often reduces service lifetime
- Localized corrosion is in a very small area
- Localized corrosion occurs in occluded areas such as container seams and laminated bag welds—diffusion into occluded areas is restricted and significantly lower than bulk diffusion
- Localized corrosion often needs a large amount of surrounding surface area to support the high rate of this type of corrosion
Localized corrosion could be pitting, crevicing and delamination of coatings and laminate films from the package metal. The most common forms of metal localized corrosion is pitting and corrosion inside the crevices formed by aerosol container double seams and the container curl-aerosol valve crimp area. The most common form of coating and laminate film localized corrosion is blistering.
Vapor phase corrosion
Vapor phase corrosion is typically not found in spray packages with internal bags. It is different from liquid phase corrosion in that it is corrosion under a very thin film of liquid. Estimations of the liquid film thickness are around 30 microns thick.
General and localized corrosion could occur in the vapor area of a spray package. The mechanism for initiating and propagating includes diffusion of corrosive species, such as water, through the thin liquid film to the spray package surface. In liquid phase corrosion, bulk diffusion is part of the mechanism for initiating and propagating the corrosion.
We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Please contact [email protected] or visit www.pairodocspro.com. Please send your questions/comments/suggestions to [email protected] Back issues of Corrosion Corner are available on CD from Spray. Thanks for your interest and I’ll see you in November.