Written on: April 1, 2020 by W. Stephen Tait
Hello everyone. I’ve often been asked which chemical most often contributes to or causes spray package material corrosion and water always comes to mind. So why am I so quick to implicate water as the most frequent contributor to or cause of spray package corrosion?
1. 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:
Fe0 + 2H2O → Fe(OH)2 + H2
2Al0 + 6H2O → 2Al(OH)3 + 3H2
These two equations only show that iron (steel) and aluminum corrosion are possible and do not tell us the reaction rates. Thus, corrosion reaction rates have to be measured. Corrosion rates are important because they are directly proportional to the spray package service lifetime.
2. 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.
3. Water easily diffuses through materials, such as polymer coatings and laminate polymer films. Water diffusion through polymers degrades or disables their barrier properties, causing them to lose the ability to protect the underlying metal from corrosion.
4. Metal surfaces and metal-polymer interfaces have a molecular cloud of negative electrons on the surface or at the interface. Thus, the polar water molecules are drawn to metal surfaces—and through polymer coatings and films—to the negative cloud on a metal surface. The water molecules cause corrosion when they adsorb on the metal surface and remove the surface electrons.
What about anhydrous formulas?
It is extremely difficult to keep anhydrous formulas (formulas containing no water) from being contaminated by small amounts of water. Thus, anhydrous formulas are not immune to corrosion by water.
Water becomes corrosive when it is liquid and only 90 water molecules are needed to form liquid water (based on thermodynamics). 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 (uncoated or coated) 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 has a safe concentration of contaminant water. Corrosion testing can be either the appropriate electrochemical corrosion test, or a one-year (minimum) constant temperature 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.
Formula water & contaminant water:
Controlling spray package corrosion
Corrosion by water can be controlled or prevented. There is often a non-corrosive concentration range for contaminant water in anhydrous formulas. 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 rarely found a corrosive environment or formula that could not be inhibited from corrosion. In many instances, slowing-down the corrosion was all that was required, and in other instances, preventing corrosion was 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.
Corrosion inhibitors can actually cause corrosion when the 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 to change package materials are more limited than the options for corrosion inhibitors.
In a nutshell…
In summary, water contributes to or causes spray package corrosion
because it is:
• Thermodynamically unstable when in contact with metals and water that is electrochemically active
• Moves freely in solutions towards metal surfaces
• Attracted to charged surfaces—indeed the electrical field on charged surfaces often pull water towards the surface, even when the surface is covered with a polymer coating or a polymer film
• A carrier of corrosive ions and molecules
• Diffuses easily through polymer coatings and films
• Degrades polymers so they are not barriers between your formula and metals under 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
• Determine where in the package corrosion will occur
• Estimate the spray package service life with a specific formula
• Find the non-corrosive concentration range for contaminant water—if one exists; and
• Find an effective corrosion inhibitor and its effective concentration range.
Please visit www.pairodocspro.com for more information. Thanks for reading and I’ll see you in May. SPRAY