November 2016

The relationship between spray packaging corrosion

Batteries, Electrolysis, Electrolytic Corrosion, Electro galvanic Corrosion, Galvanic Corrosion and Electrochemical Corrosion.

Hello, everyone. A number of terms are used to describe or explain spray package corrosion. The most common terms are:

  • Batteries
  • Electrolysis
  • Electro galvanic corrosion
  • Galvanic corrosion
  • Electrolytic corrosion
  • Electrochemical corrosion

Many of the above terms do not apply to spray package corrosion. Let’s see which terms actually do and don’t:

Batteries
Corrosion of spray packages is not a battery. A battery has an isolated anode and an isolated cathode. In contrast, the anodic and cathodic corrosion sites on spray package material surfaces are in contact with each other.

Battery electrical current flows between its isolated anode and cathode when they are connected together with an external electrical wire or an external electrical circuit. The flow of electrons through the wire or circuit powers various appliances, such as cell phones.

The anode in a battery is usually carbon or a metal and the metal corrodes when current is drawn from the battery. The corrosion of the anode can be reversible, such as for re-chargeable batteries like the automobile lead-acid and the lithium ion batteries. The corrosion of the anode could also be irreversible, such as with carbon and alkaline batteries.

The surface of uncoated and coated spray package metals are covered with a very large number of randomly distributed microscopic anode and cathode sites. In other words, the anode and cathode sites on spray package metals are not the large anodes and cathodes that are isolated from each other like in a battery.

Electrolysis
Electrolysis is not a form of spray package material corrosion. Indeed, electrolysis is actually not metal or polymer corrosion. Electrolysis is the splitting of water molecules into oxygen gas and hydrogen gas using an electrical current that passes between two physically separate vessels.

Electro galvanic corrosion
Electro galvanic corrosion is actually galvanic corrosion.

Galvanic corrosion
Galvanic corrosion is possible in spray packages that use different metals, such as an aluminum aerosol container with steel aerosol valves. Galvanic corrosion is possible when two different types of metals are in electrical contact with each other and submerged in an electrolyte.

Galvanic corrosion of spray packaging was discussed in last month’s Corrosion Corner (October 2016).

Electrolytic corrosion
Electrolytic corrosion does not occur in spray packages. This term is often mistakenly used for electrochemical corrosion.

Electrolytic corrosion is a type of metal corrosion caused by an external electromagnetic field or an external electrical current. Buried metal pipelines can experience this type of corrosion when an electrical power line is buried next to the pipeline.

Electrochemical corrosion
Metals also corrode when valence electrons are drawn from the bulk metal by electrochemically active ions and molecules, such as hydrogen ions and oxygen atoms, respectively. Removing electrons from a metal causes metal ions (formerly metal atoms) to be ejected from the bulk metal.

Consequently, electrochemical corrosion involves a transfer of electrons from the metal to its environment, such as your formula, and change in the chemical state of the metal—an atom becomes an ion. Thus, this type of corrosion process is referred to as electrochemical corrosion because the mechanism involves both an electrical current—valence electrons removed from the metal—and a change in the metal’s chemical state—metal atoms become metal ions.

The electrochemical nature of spray package metal corrosion enables electrochemical corrosion measurements with sensitive electronic instruments. These instruments detect and measure the electrical and ionic currents associated with metal and polymer corrosion, respectively. The measurements are used with the appropriate experience and mathematical models to predict long term spray package corrosion.

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 be back in December.