What minimum materials are needed for pitting corrosion?

Written on: November 1, 2014 by W. Stephen Tait


Hello, everyone. Metal pitting corrosion causes perforation and leaking of spray packages. What is the minimum number of elements needed to initiate corrosion and sustain pitting corrosion of the metals used for spray packaging?

Only two materials are needed for metal corrosion: water and metal. Metals are thermodynamically unstable and prefer to be in the form of more stable metal oxide/hydroxides. I refer to the corrosion product as a mixture of metal oxides and metal hydroxides because water is always present in our environment and water will transform an oxide into a hydroxide.

In addition, water is thermally unstable when in contact with a metal. Water is reduced by electrons from the package metal, which is oxidized (corroded) when electrons from the metal are transferred to water. Hence, water enhances the thermodynamic instability of the spray package metal.

There are two types of structural metals used for spray packages: aluminum and iron. Aluminum is used for traditional aerosol containers and laminated foil packages and iron is used for traditional steel aerosol containers.

The aluminum used for spray packages is actually an aluminum-alloy composed of multiple elements, much like your formula is composed of multiple chemicals. The steel used for spray packages is also an iron-alloy formulated with multiple different elements.

The corrosion reactions between water, aluminum and steel spray package materials are:

2Al + 6H2O    →        2Al(OH3) + 3H2

Fe + 2H2O      →        Fe(OH)2 + H2

Notice in these two reactions that:

  • Water is electrochemically active
  • Water is consumed by iron (steel) and aluminum corrosion
  • You don’t need dissolved oxygen for spray package metal corrosion
  • You don’t need chloride ions for spray package metal corrosion

It’s natural to ask how much water is needed when examining the two corrosion equations for aluminum and steel (iron). Let’s use an actual perforation to estimate the amount of water needed to sustain pitting corrosion.

The perforation I have in mind is approximately shaped like a truncated cone and has the following dimensions:

  • 0.23 mm deep (thickness of the steel container metal at the perforation site)
  • An inside pit diameter of approximately 227 micrometers
  • An outside pit (perforation) diameter of approximately 57 micrometers

I’ll spare you the algebra. A pit (perforation) with these dimensions would consume approximately:

  • 0.112 milligrams of water with aluminum
  • 0.324 milligrams of water with steel

These amounts of water would correspond to approximately 224 parts per billion (ppb) of water in 500mL of product (formula without propellant) in an aluminum spray package and approximately 648 ppb of water in a steel spray package. In other words, the example perforation needed 224 ppb of water to perforate an aluminum spray package and the same perforation needed 648 ppb of water to perforate a steel spray package (both examples are based on a 500mL product-fill). This is not a lot of water for either metal.

Liquid water is also needed to initiate corrosion. The number of water molecules needed to form liquid water is approximately 90, based solely on thermodynamic considerations.

Other ingredients in your formula could raise or lower the number of water molecules needed to form the liquid water needed to initiate corrosion. The amounts of water calculated above would be needed to sustain the pitting corrosion until it perforated the spray package with the (pit) perforation dimensions used in this calculation.

In other words, a very small amount of water is needed to initiate and sustain corrosion until the spray package is perforated. In addition, the chemical composition, physical form of your spray formula and types of spray package materials determine if corrosion will be initiated, the amount of water needed for corrosion initiation and the amount of water needed to sustain corrosion until perforation occurs.

We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Contact rustdr@pairodocspro.com or visit www.pairodocspro.com. Please send your questions/comments/suggestions to rustdr@pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in December.