Written on: August 1, 2020 by W. Stephen Tait
Hello, Everyone. I started this three-part series in June on why spray package corrosion testing is necessary. Part 1 covered the materials used to fabricate spray packages—how metals, coated metals and laminated metals corrode and how polymer coatings and films corrode. Part 2 in July discussed two theoretical equations to estimate the probability of corrosion and how fast corrosion penetrates and/or degrades packaging materials. The complexity of these two equations—plus the unavailability of the numerous parameters in each equation—illustrated how it’s not practical to substitute mathematical equations for corrosion tests.
This final part of the series provides overviews of the most common corrosion causing/contributing formula ingredients, common factors that produce surprise corrosion and those that provide corrosion control and prevention.
Common corrosion-causing formula ingredients
Liquid water is the most common contributor to, and cause of, package corrosion. Only 90 water molecules are needed to form liquid water and thus initiate metal corrosion. Water molecules above the initial 90 are needed to subsequently sustain and propagate the corrosion.
Contaminant water in anhydrous formulas is typically high enough in volume to at least initiate corrosion. In other words, anhydrous formulas are not immune to corrosion.
Liquid water is electrochemically-active. For example, the corrosion of iron (tinplated steel) by water has the following corrosion reaction:
Water molecules are small and easily diffuse through polymers, subsequently forming microscopic rivers. These rivers allow water and other formula ingredients essentially free access to the underlying metal or metal foil, creating the conditions needed for metal corrosion. The microscopic rivers also cause polymer coatings and films to delaminate from their metal substrates and degrade a coating’s or film’s ability to be a protective barrier between the metal and a formula.
Water molecules dissociate into hydroxyl and hydrogen ions. The
hydrogen ions are electrochemically-active and will remove electrons
from metals, causing corrosion. For example, the corrosion
of tinplated steel by hydrogen ions is:
The concentration of hydrogen ions in a formula (water or contaminant water in an anhydrous formula) is referred to as pH, and pH magnitude often determines the rate of package corrosion.
Aluminum is not always more corrosion-resistant at a given pH than tinplated steel and vice versa. The corrosively of each pH is also determined by the other ingredients in a formula, such as surfactants or ingredient concentrations.
Fragrances are complex mixtures of both numerous natural extracts and manufactured ingredients. Many decades ago, it was believed that fragrances were corrosive, some more than others.
There are some fragrances that actually cause corrosion, such as vanilla. However, it was discovered that the “fragrance corrosivity” belief was only partially true and that most fragrances actually have some ability to inhibit corrosion.
Surfactants adsorb on material surfaces and make them more or less susceptible to adsorption. They also make metal corrosion and polymer corrosion easier with electrochemically-active ions and molecules.
There is little or no public-domain corrosion research on how/which surfactants cause or contribute to material corrosion. Hence, extensive tables of corrosive and non-corrosive surfactants are unavailable. However, sodium lauryl sulfate has caused the corrosion of spray packaging, consumer packaging and manufacturing equipment. Surfactants derived from phenols and nonyl phenols also cause or contribute to corrosion in many situations.
Formula ingredients that have unsaturated bonds in their molecules are potentially electrochemically-active. Insecticides typically cause metal corrosion and require an inhibitor to either control or prevent spray package corrosion.
Spray package surprise corrosion is likely whenever corrosion tests are not conducted, are too short or are conducted with inappropriate parameters. Hence, complete and appropriate corrosion tests should be conducted when:
• Derivative formulas are developed from a base formula—no matter how small the chemical composition differences between the derivative and the base
• Changing a fragrance or its concentration (no matter how small the concentration change)
• Changing one or more surfactants in a base formula or their concentration of one or more surfactants (no matter how small the concentration changes)
• Changing ingredient concentrations (no matter how small)
• Changing package vendors
• Changing package materials
• Changing the type of propellant or propellant concentration
(no matter how small the change)
Corrosion Control & Prevention
A corrosion control program incorporates a corrosion inhibitor in a formula to reduce the corrosion rate. The package then does not perforate and lose product and/or propellant before the product is exhausted, nor does corrosion reduce product efficacy during the package service life.
A corrosion prevention program incorporates a corrosion inhibitor that prevents package corrosion during its service life. A company’s internal corrosion database is also part of a comprehensive corrosion control and prevention program. An extensive database helps guide formulators while developing new and derivative formulas, thus minimizing surprise spray package corrosion and corrosion failures.
To wrap up…
This series provides an overview of why corrosion is unpredictable and why formula testing is essential. Visit pairodocspro.com for more information. Thanks for reading and I’ll see you in September. SPRAY