Corrosion by formula & package factors, Part 1

Written on: November 1, 2025 by W. Stephen Tait

I’m sure everyone has heard about the EU 2025/24 legislation on packaging materials, made law in January of this year. This game-changing legislation will probably require a significant amount of work to either find alternate plastic (polymer) materials for packaging, and/or migrate products from plastic packaging to metal packaging.

This two-part series of Corrosion Corner is written to help guide projects that either transition to new plastic materials, or transition from plastic packages to metal containers. I’m also in the process of writing a White Paper on the EU 2025/24 legislation from the perspective of a materials engineer/scientist.

Package corrosion is a complex interaction (synergy) between a large number of formula and package factors. It has been my experience that there are at least seven major formula factors and five major package factors that can cause or contribute to spray package corrosion.

Factors that can cause/contribute to corrosion
The seven major formula factors that cause or contribute to spray package corrosion are:

1) Formula water or contaminant water
2) Type of aerosol propellant (e.g., HFC 152a, DME, CO2)
3) Formulas containing ingredients with unsaturated molecules
4) Surfactants/Detergents
5) Corrosion inhibitors
6) Fragrances
7) Emulsions

Package factors that can cause/contribute to corrosion
Plastics, coatings and films are all polymers and all three will be collectively referred to as polymers in both parts of this column.

The five major packaging factors are (labeled 8–12 for continuity):

8) Type of package (e.g., aerosol containers, bag-on-valve, plastic bottles, etc.)
9) Type of internal package surface treatment (e.g., tinplate and polymers)
10) Polymer chemistry and micro-morphology
11) Microenvironments inside a package (e.g., head space, welds, seams, etc.)
12) Internal package metallurgy (e.g., aluminum or steel cans, aluminum foil, polymer-laminated aluminum foil and internal metal coatings)

There are 479,001,600 possible combinations of these 12 factors that could cause synergy that produces significantly more aggressive corrosion than that caused by a single factor. In other words, the high probability of synergy further increases the likelihood of aggressive and unexpected corrosion that causes or contributes to very short package service lifetimes.

In this issue, I’ll discuss formula factors 1–4; in the next issue, we’ll finish it with formula factors 5–7, along with all the package factors.

1) Water
Water could be in a formula either as an ingredient or as an unexpected contaminant in an anhydrous formula. Water molecules are small and easily absorb into the holes and empty channels in polymers; they then diffuse through the polymer to cause blistering, paneling or leaking. Water is also a solvent for other formula ingredients and often brings them to the metal surface under polymers. The chemical compositions of liquids diffusing into and through polymers are typically different from their bulk formula compositions.

Water molecules dissociates into a hydrogen ion and a hydroxyl ion as follows:

Equation 1
H2O → H+ + OH-

The hydrogen ion concentration in water is referred to as pH. For example, water with a neutral pH of seven has 0.0000007 moles/liter hydrogen ions and a pH of two has 0.02 moles/liter hydrogen ions. In other words, lower pH means more hydrogen ions are available for corrosion.
Hydrogen ions from Equation 1 are electrochemically active and remove metal electrons, as shown in Equation 2.

Equation 2
2H+ + 2e- (from a metal) → H2 (gas)

2) Propellants
Water is slightly soluble in HFC152A propellant; hence it can bring contaminant water into anhydrous formulas. Contaminant water might or might not contribute to or cause spray package corrosion, depending on its concentration in the formula and the chemical composition of a formula.

DME propellant is a strong solvent for many polymers. Hence, there is a higher probability that DME could cause polymer corrosion that leads to metal corrosion under polymers.

Carbon dioxide (CO2) propellant dissolves in water to form carbonic acid. Carbonic acid dissociates into hydrogen ions and bicarbonate ions as shown in Equation 3:

Equation 3
H2CO3 (carbonic acid) → HCO3- (bicarbonate ion) + H+

The hydrogen ion in Equation 3 is electrochemically active (see Equation 2) and the bicarbonate ion in Equation 3 is also electrochemically active, as shown in Equation 4.

Equation 4
2HCO3- (bicarbonate ion) + 2e- (from a metal) → H2 + 2CO3=

Not all propellants contribute to or cause spray package corrosion. For example, neither nitrogen nor LPG (hydrocarbon propellants) are electrochemically active and LPGs are typically weak solvents. Hence, nitrogen and LPG propellants do not typically contribute to or cause spray package corrosion.

3) Formula ingredients having unsaturated molecules
Formula ingredient molecules with unsaturated chemical bonds could remove electrons from metals. However, when unsaturated bonds have weak electro-negativity, they often adsorb on a metal’s surface and share the electrons instead of removing them. This situation sometimes causes corrosion inhibition instead of corrosion.

4) Surfactants/Detergents
Surfactants make metal surfaces more or less susceptible to water adsorption. Consequently, surfactants could contribute to metal corrosion when they wet the metal surface (hydrophilic). Hydrophilic surfactants also enhance absorption and diffusion through polymers.

Hydrophobic surfactants repel water—decreasing absorption and diffusion through polymers, and thus either impeding, or inhibiting, polymer corrosion.

Some surfactants, such as sodium lauryl sulfate, cause corrosion in a wide variety of environments, such as consumer packaged goods formulas.

Corrosion testing is needed to determine if a given formula, or its ingredients, will either cause corrosion of the chosen spray package components or inhibit corrosion.

In the next issue, we’ll finish discussion on formula factors 5–7, along with package factors that cause or contribute to package corrosion.

Thanks for your interest and I’ll see you in December. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY