Hello, everyone. Modern spray packages are often fabricated with two or more different metals, making galvanic corrosion possible. Galvanic corrosion is a form of localized corrosion where one metal causes the other different metal to corrode. Galvanic corrosion could be either general corrosion or pitting corrosion. However, both forms of galvanic corrosion are restricted to the immediate area where the two different metals are in physical (electrical) contact with each other.
There are three specific conditions that must all be present for galvanic corrosion to occur:
- The two different metals are in electrical (physical) contact with each other
- The corrosion potentials of the two metals have significantly different magnitudes
- The two metals are submerged in an electrolyte (i.e., your formula) that is corrosive to one or both of the metals.
The most common types of spray packages fabricated with two or more different metals that are:
- Tinplated steel—tin metal deposited on a base steel
- Steel valves with aluminum aerosol containers
- Laminated tin-free-steel container tops or bottoms with tinplate steel bodies
- Aluminum tops or bottoms with tinplate steel bodies
Let’s briefly discuss galvanic corrosion in each type of multiple-metal packages.
Tin can galvanically cause steel corrosion or prevent steel corrosion. The metal with the smallest corrosion potential is corroded by the one with the larger corrosion potential.
There are holes through the tin metal that are deposited on steel. However, galvanic corrosion does not typically occur in the holes until the difference between the tin and the steel corrosion potentials is above 0.100 Volts. For example, we encountered one situation where the tin metal galvanically caused pitting corrosion of the base steel when the tin potential was 0.198 Volts larger than the steel potential.
There are also formulas that do not corrode either the tin or the steel. Whether or not tin-steel galvanic corrosion occurs is determined by the chemical composition of your formula.
Steel valves with aluminum aerosol containers
Aluminum valves in contact with steel aerosol containers might or might not cause one of the two metals to galvanically corrode. Whether or not galvanic corrosion occurs is determined by the chemical composition of your formula and the types of other metals in contact with the aluminum. The chemical composition of your formula determines the magnitude of the differences in corrosion potentials in the liquid product area, propellant area or vapor area of the spray package.
Laminated tin-free-steel bottoms or tops seamed onto tinplated steel bodies
Laminated metals also have measurable corrosion potentials when exposed to an electrolyte, such as your formula. The magnitude of the corrosion potential is determined by the permeability of the film to your formula ingredients; which ingredients permeate the film; the chemical composition and physical morphology of the film and the chemical composition of your formula.
Consequently, galvanic corrosion between a laminated tin-free-steel and tinplate is a possibility.
Aluminum tops or bottoms are used with tinplated steel bodies
The corrosion potential of aluminum is often significantly different from the corrosion potential for steel, making galvanic corrosion between these two metals a higher probability. However, the chemical composition of your formula could make the corrosion potentials differences insignificant, cause galvanic corrosion of either the aluminum or the steel or inhibit corrosion of both metals such that package service life is not significantly reduced by galvanic corrosion.
One might get the impression that galvanic corrosion can be predicted by simply measuring the corrosion potential of the different metals. However, measuring the corrosion potential of the two metals is not sufficient to determine if galvanic corrosion will reduce package service life below the target length. The rate of galvanic corrosion is determined by the chemical composition of the formula and the ratio of the areas for the different metals. Consequently, you need to do corrosion testing to determine if galvanic will occur and the rate of galvanic corrosion is large enough to reduce package service life below the desired target length.
We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. We have also introduced the Corrosion Partnership Program, which makes our electrochemical corrosion laboratory, corrosion consulting and anti-corrosion technology conveniently available for your R&D program. Contact: [email protected]; 608-831-2076; www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in November.