Written on: June 1, 2024 by W. Stephen Tait
Hello, everyone. Which chemical most often contributes to or causes spray package material corrosion? Water! Water can be in a formula, either as an ingredient or as a contaminant. There are four reasons why I am so quick to implicate water as the most frequent contributor to or cause of spray package corrosion.
First: Water and metals are thermodynamically unstable when in contact with each other. The simple chemical equations for corrosion of the steel and aluminum used in spray packages by water are:
These two equations show that iron (steel) and aluminum corrosion by water is possible, but do not tell us the corrosion reaction rates of aluminum and iron with water. Corrosion rates are important because they are directly proportional to the spray package service lifetime. Thus, corrosion reaction rates should be measured.
Second: A variety of corrosive ions and molecules dissolve in water. Consequently, water is also a carrier that delivers corrosive ions and molecules to spray package metals and metal foils.
Third: Water easily diffuses through materials, such as polymer coatings and laminate polymer films. Water diffusion degrades and disables coatings and film barrier properties, causing them to lose the ability to protect the underlying metal from corrosion. Water diffusion through polymer coatings/films brings corrosive water, molecules and ions to the metal under the polymer coating/film.
Fourth: Metals and metal-polymer interfaces have a molecular cloud of negative electrons on the surface and at the interface, respectively. Thus, water molecules are drawn to uncoated metal surfaces, and through polymer coatings and films, to these negative clouds. Water molecules cause corrosion when they adsorb onto metals and subsequently remove electrons from the water atoms:
What about anhydrous formulas?
It is extremely difficult to keep anhydrous formulas from being contaminated by small amounts of water. Thus, anhydrous formulas are not immune to corrosion by water.
Water is corrosive as a liquid and a cluster of only 90 water molecules is needed to form liquid water that initiates corrosion (based on thermodynamic calculations). Of course, additional water is needed to sustain corrosion after it initiates. The amount of additional water needed depends on the treatment of the metal or metal-foil surface (such as uncoated or coated/type of coating/etc.) plus the chemical composition of the formula inside the spray package.
Consequently, corrosion testing is needed to determine if an anhydrous formula is corrosive and what is the safe concentration of contaminant water. Corrosion testing can be either an electrochemical corrosion test with the appropriate measurement parameters or a minimum one-year constant temperature storage-stability test.
Please note that raising the storage temperature does not accelerate corrosion of spray package materials, such as aluminum, steel, polymer coatings and films. In other words, raising storage temperature does not shorten the length of the test.
Controlling & preventing corrosion
Spray package corrosion by both formula water and contaminant water can be controlled or prevented. For anhydrous formulas, there often is a non-corrosive concentration range for contaminant water. In other words, corrosion of anhydrous formulas might be controlled or prevented by keeping the concentration of contaminant water inside the non-corrosive range.
Corrosion by formula water can often be prevented with corrosion inhibitors. Indeed, in my almost five decades of corrosion research, testing and consulting, I’ve not found a corrosive formula that could not be inhibited! In some instances, slowing down the corrosion is required; in other instances, corrosion prevention is needed.
A corrosion inhibitor is often only effective for a very specific environment or formula. There are literally thousands of chemicals that might inhibit corrosion, hence finding the most effective corrosion inhibitor often takes time.
Ironically, corrosion inhibitors can actually cause corrosion when their concentration is above or below the effective concentration range. Hence, the effective corrosion inhibitor concentration range should also be determined when developing an inhibitor for a corrosive formula.
Storage testing can be used to screen potential corrosion inhibitors; however, a storage test requires at least one year to complete. Electrochemical corrosion testing is more precise and the quickest way to screen a large number of potential corrosion inhibitors—assuming the appropriate measurement parameters and data analysis protocols are used.
Changing package materials also can, in some situations, control and prevent corrosion. However, the options for changing package materials are more limited than the options for corrosion inhibitors.
In summary, water contributes to, or causes, spray package corrosive because water is:
• Thermodynamically unstable when in contact with metals
• Electrochemically active and causes corrosion
• Moves (diffuses) easily in solutions towards metal surfaces
• Attracted to charged surfaces—indeed the electrical field on charged surfaces often pull water towards the surface, as well as through polymer coatings and polymer films
• A carrier of corrosive ions and molecules
• Degrades polymers so they are not barriers between a formula and the package metal under the polymer coatings and films
Anhydrous formulas are not immune to corrosion. However, in some instances, there is a non-corrosive concentration range for contaminant water.
Corrosion inhibitors are an effective way to control or prevent spray package corrosion by formula water and anhydrous formulas. However, finding an inhibitor that works for individual formula-package systems takes time and the effective concentration for the inhibitor must also be determined.
Corrosion testing is needed to determine:
• If corrosion will occur
• Where in the package corrosion will occur
• The spay package service life with a specific formula
• The non-corrosive concentration range for contaminant water—if one exists
• An effective corrosion inhibitor and its effective concentration range
Thanks for your interest and I’ll see you in July. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
