Corrosion by formula & package factors, Part 2

Written on: January 1, 2026 by W. Stephen Tait

Hello everyone and Happy New Year! In this month’s column, we’ll continue on from Part 1, found in the November 2025 issue. Spray package failures occur when:

• Metal and plastic packages leak liquid product;
• An aerosol container or aerosol valve leaks propellant;
• A partially full package no longer sprays;
• The product is discolored or has a malodor caused by corrosion contamination and active ingredients degraded from corrosion contamination.

In other words, spray package failure occurs when the package and/or its product no longer performs to specification.

Let’s pick up where we left off in the November issue and discuss formula corrosion factors 5–7, plus all the package corrosion factors. Plastic sheets and films, polymer coatings and laminated polymer films will be collectively called polymers.

5) Corrosion inhibitors
Corrosion inhibitors are typically used to inhibit or prevent spray package metal corrosion. Inhibitors typically have an effective concentration range determined with corrosion tests. Corrosion inhibitors can also slow or stop permeation of formula ingredients into (absorption) and through (diffusion) polymers, helping to control and prevent contamination from leaching of polymer chemicals into formulas.

6) Fragrances
Very few fragrances—such as vanilla-based fragrances—contribute to or cause spray package corrosion. Most fragrances provide some degree of corrosion inhibition instead. It is unknown why and when fragrances act as corrosion inhibitors, mainly because fragrances have complex chemical compositions that are closely guarded trade secrets.

7) Emulsions
Products composed of insoluble ingredients are often packaged as emulsions. Emulsions can either be water-in-oil (water droplets suspended in oil), oil-in-water (oil droplets suspended in water) and micro emulsions (particles and droplets with sizes ranging from < 10 to 100 nanometers, suspended in liquids and gasses).

All types of emulsions are metastable and eventually break into two or more phases. In other words, it’s a matter of time before an emulsion breaks into more than one phase, and not if the emulsion will break. One of these phases is often a very corrosive water phase.

Package factors that can cause or contribute to corrosion
All package materials corrode. There are five major package factors that cause or contribute to package corrosion (labeled here as 8–12).

8) Types of packages
Spray packages include:

• Traditional aerosol containers
• Polymer-laminated aerosol valves and aluminum foil bags
• Polymer bags and pistons inserted into traditional aerosol containers
• Plastic bottles and with trigger sprayers and pumps

No one type of package is corrosion-resistant with all types of formulas.

9) Type of internal package surface treatment
Surface treatments inside metal packaging could include other metals such as thin tin coatings (referred to as ETP) or thin chromium coatings (referred to as TFS). Surface treatments can also include a variety of thin polymer coatings such as epoxy, PAM, PET, vinyl and nylon.
Surface treatments are intended to improve package corrosion resistance and increase package service lifetime. However, many environments (formulas) also corrode surface treatments and diminish or negate their corrosion resistance.

10) Polymer chemistry & micro-morphology
Polymers are a network of long-chain, intertwined molecules. Microscopically, polymers resemble layers of cooked spaghetti. Like spaghetti layers, the spaces between molecule-strands allow contaminant water, formula water and select formula ingredients to absorb into and diffuse through polymer networks.

Polymer coatings on metals typically have multiple microscopic holes that expose the underlying substrate metal. Whether or not the exposed substrate metal will corrode, and how fast it corrodes, is determined by the chemical composition of a formula and the extent of adjacent, failed coating around each hole.

Absorption into and diffusion through polymers:

• Extracts polymer chemicals (e.g., plasticizers) into a formula
• Degrades polymer properties (e.g., mechanical strength, glass transition temperature, etc.)
• Causes or contributes to distortion (e.g., paneling ), cracking and leaking

Most polymers, such as PET, can be recycled a limited number of times for use with virgin polymers (circularity). Recycling and reprocessing polymers breaks them down and they ultimately lose the requisite mechanical strength and barrier properties needed for packaging.

11) Microenvironments inside a package
Metal and polymer packages have multiple, internal microenvironments. The chemical compositions inside each micro environment are often significantly different from the bulk formula.

Aerosol containers have the largest number of internal microenvironments, typically:

• Vapor and liquid phases
• Crevices in both the vapor and liquid phases
• Liquid-liquid and liquid-gas interfacial areas
• Welded metal in the bulk formula, the various interfacial areas, the liquid and gas crevices

Polymer bottles have fewer microenvironments—liquid and gas phases and the liquid-gas interface. These three microenvironments are also typically inside the polymer housing for pump and trigger-sprayer springs after the pump/sprayer is initially activated.

12) Internal package metallurgy
The various components for steel aerosol containers typically have different metal specifications with corresponding different corrosion resistances. In other words, steel containers are fabricated with multiple types of steels—particularly when the container body is seamed onto a top and/or a bottom. Steel container body welds also have a different type of metallurgy.

The number of alloys for aluminum aerosol containers is smaller than those for steel. Steel aerosol valves on aluminum containers increase the number of different metals inside aluminum containers.

Bag-on-valve (BOV) packages inside aluminum and steel aerosol containers are fabricated from an aluminum foil whose metallurgy is different from aluminum containers. Hence, an aerosol container with a BOV inside has an additional metal type inside.

Further thoughts
EU 2025/24 caused three paradigm shifts on plastic packaging, from the traditional single-use packaging paradigm to:

1. Polymers with higher recycle content (circularity paradigm)
2. The permanent packaging paradigm; and
3. The reusable packaging paradigm

I suspect that incompatibility between some formulas and the new EU 2025/24-compliant polymer packaging materials will necessitate the transfer of some products from plastic packaging to metal packaging to continue sales of these products.

Tomorrow’s challenges can’t be solved with yesterday’s technology
Current package compatibility tests are often ASTM standards (or derivatives) that have no correlation between their test results and actual corrosion (per Section 3 warnings by the ASTM for their standards) and are not designed to statistically predict package service lifetime.

Corrosion testing is necessary to determine both package formula (environment) compatibility and the length of package service lifetime. The three new paradigms necessitate a new generation of corrosion tests—having a high correlation between test results and actual corrosion—that can determine if:

• Spray packages will leak or crack and leak—and when
• Polymer delamination will cause or contribute to packages no longer spraying
• Formulas are compatible with all types of metal and polymer packages
• Product discoloration or malodor by metal and polymer corrosion is expected
• Products cause or contribute to polymer distortion, cracking and loss of mechanical properties

In the next issue, we’ll discuss the highlights of my soon-to-be-published white paper on the impact of EU 2025/24 on package corrosion testing.

Thanks for your interest and I’ll see you in February. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com if you would like a copy of the White Paper concerning EU 2025/24. SPRAY