Guidelines for packaging engineers & formulators

Written on: February 1, 2022 by W. Stephen Tait

Hello, everyone. I’d like to share my current perspective on corrosion guidelines for both those who formulate new and derivative spray products and those who specify and qualify packaging for spray products.

The various guidelines can be placed into one of the five following categories:

1. Formula chemistry
2. Corrosion inhibitors (a subset of formula chemistry)
3. Spray packaging components
4. Corrosion test practices
5. Your company corrosion knowledge

Let’s review the guidelines in each category:

1. Formula chemistry
• Don’t assume that small changes in your formula’s chemical composition won’t affect spray package corrosion
• Don’t assume raw materials from different vendors have the same corrosivity or non-corrosivity
• Raising pH is not a sure-fire way to reduce spray package corrosion
• Propellant might also cause or contribute to spray package corrosion
• The amount of contaminant water in an anhydrous formula determines if it is or is not corrosive
• Don’t assume a given formula is not corrosive just because individual ingredients by themselves are typically not corrosive
• Corrosion is a synergistic relationship between all of your formula ingredients, the type of spray package and the physical form of the formula (e.g., a water-out or oil-out emulsion, single phase product, etc.)

2. Corrosion inhibitors
• Corrosion inhibitors are often the most effective and cost-effective way to control and prevent spray package corrosion
• Corrosion inhibitors have effective concentration ranges
• Concentrations outside of the effective range often cause severe package corrosion
• No single type of corrosion inhibitor is effective with all formula chemical compositions

3. Spray packaging components
• Aluminum containers are not universally more corrosion-resistant than steel containers and vice versa
• No individual type of spray package will resist corrosion by all possible types of formulas
• Package components from different vendors might not have the same corrosion resistance to a given formula
• Changing or adding polymer coatings inside aerosol containers typically do not always solve/prevent corrosion
• Thicker internal coatings also typically don’t prevent corrosion by a corrosive formula

4. Corrosion test practices
• Commercializing a new and derivative formulas (line extensions) without corrosion testing has an approximately 62% risk that a formula will cause spray package corrosion
• There are two generic types of corrosion tests: constant temperature storage tests and electrochemical corrosion tests
• Don’t rely on temperature to accelerate spray package corrosion
• Commercializing a spray product with low time storage test data is also typically high risk (e.g., three months of testing has an approximate 31% risk; six months has an approximate 16% risk)
• One year of constant temperature storage corrosion testing provides the lowest range of risk (2%–7%)
• The risk associated with constant temperature corrosion storage is also directly proportional to the number of samples examined and the length of the test at each examination
• My favorite number of containers for constant temperature corrosion storage tests ranges from 18–26 samples, depending on the examination age
• The length of a corrosion test can be significantly reduced from one year to 90 days or fewer with the appropriate electrochemical test protocols, data analysis methods and models for package service lifetime
• Electrochemical corrosion tests can also be used to significantly reduce corrosion test times
• Electrochemical corrosion tests with appropriate protocols have a higher correlation with actual spray package corrosion than a corresponding constant temperature corrosion storage test

5. Your company corrosion knowledge
• A company corrosion database could significantly reduce corrosion test times, particularly for derivative formulas
• The more extensive your company corrosion database, the less the risk associated with decisions made using the database
• Constant temperature corrosion storage test data can be integrated with electrochemical corrosion test data to provide a very high level of confidence with a corresponding low risk

These guidelines are current and state-of-the-art, hence will evolve with research that increases our knowledge of what causes container corrosion, produces better methods to control and prevent spray package corrosion and improves corrosion test methods.

Thanks for your interest and I’ll see you in March. Contact me at 608-831-2076, [email protected] or from our two websites: pairodocspro.com and aristartec.com. SPRAY