Corrosion Corner 30 Spray November 2014 W. Stephen Tait, Ph.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC What minimum for pitting cmoarrteorsiiaoln?s are needed H ello, 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 continued on page 36
Spray November 2014
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