Written on: April 1, 2025 by W. Stephen Tait
Hello, everyone. I thought it would be interesting to discuss the top six frequently asked questions (FAQs) I’ve encountered while writing Corrosion Corner over the last 24 years; here we go!
FAQ 1: Is one type of spray package more corrosion resistant than the others?
Corrosion is everywhere and no type of package material is immune. We’ve observed corrosion in all types of spray packaging, including:
• Traditional coated and uncoated steel aerosol containers, as well as coated aluminum aerosol containers
• Internal laminated foil packages attached to aerosol valves
• Aerosol valves with laminate coatings
• Trigger sprayers for plastic bottles
• Pump sprayers
• Polymer coatings
• Plastic bottles
There is a wide variety of spray product formulations and spray packages/package components. Consequently, I’ve not attempted to rank the relative corrosion resistance for all the different types of spray packages and their associated spray products.
There are situations where a specific type of package materials and/or package type is more corrosion resistant with a specific corrosive environment (i.e. spray product formula).
However, corrosion testing is needed to determine the most corrosion-resistant package materials and their service lifetime with specific formulas. Anything less than comprehensive corrosion testing is guesswork with an approximate 62% risk. Consequently, the corrosion resistance and service lifetime for all spray packages should be evaluated with corrosion testing before commercializing a formula/package system (i.e. the commercial product).
FAQ 2: What is the spray package corrosion mechanism?
There are many different types of spray package corrosion mechanisms. In very general terms, to just name a few, there is:
• Liquid phase corrosion
• Vapor phase corrosion in the package headspace
• Weld corrosion in three-piece welded steel aerosol containers
• Interfacial corrosion at the propellant-product interface (aerosol containers)
• Interfacial corrosion at the liquid-gas interface (all package types)
• Crevice corrosion where two package components are joined together:
– aerosol valve-container body crimps
– package seams (all package types)
More specifically, liquid phase corrosion occurs in the bulk liquid (a formula) and could be general metal corrosion—sometimes referred to as uniform corrosion. It could also be metal pitting corrosion, metal crevice corrosion, weld corrosion (welded packages only) and coating delamination, such as blistering.
Interfacial corrosion could be general metal corrosion, metal pitting corrosion, weld corrosion (welded packaging only) and coating delamination, such as blistering.
Vapor phase corrosion could also be the same types of corrosion possible in the liquid phase. However, vapor phase corrosion occurs under a thin layer of water—approximately 30 microns thick—so its mechanism for initiation, propagation and failure are different from the corresponding mechanism for liquid phase corrosion.
Any spray package can also simultaneously have multiple types of corrosion mechanisms with any given formula inside the package.
FAQ 3: How much water is needed for corrosion?
Metal and polymer corrosion initiates in liquid water. Thermodynamically, liquid water occurs when there is an agglomerate of 90 water molecules. However, the actual number of water molecules needed to form liquid water depends on the chemical composition of a formula and the type of spray package materials used for packages.
Additional liquid water is needed to propagate (sustain) corrosion after it initiates. The amount of water needed to sustain corrosion is determined by the type(s) of corrosion, as well as the type(s) of packaging material(s) and package components exposed to a formula.
FAQ 4: What corrosion inhibitor works for most, if not all, spray package corrosion?
There is no one-size-fits-all corrosion inhibitor and effective corrosion inhibitor concentration range. The chemical composition of a formula and types of materials used in a spray package determine if an individual inhibitor or an inhibitor blend inhibits package corrosion caused by the formula.
FAQ 5: How long should a corrosion test be conducted for low-risk commercial decisions?
It has been my experience that a storage stability test needs to be conducted for at least one year to reduce the corrosion risk to approximately 7%. Shorter storage test times have larger risks.
Electrochemical corrosion testing is shorter than the minimum one-year storage test because its sensitive electronic instruments detect corrosion long before it can be seen with the unaided eye or with a light microscope. The length of an electrochemical test can be typically completed in 90 days or fewer when the appropriate parameters are used.
FAQ 6: Can I use higher temperatures to shorten the length of a storage test?
Corrosion of metals and polymers does not follow the Arrhenius equation. Consequently, raising the storage test temperature does not accelerate corrosion of spray package materials and therefore does not shorten the length of a storage test.
I’ll discuss how higher temperatures do and don’t affect spray package corrosion in the next edition of Corrosion Corner.
Thanks for your interest and I’ll see you in May. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
