continued from page 30 Corrosion Corner 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. SPRAY 36 Spray November 2014
Spray November 2014
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