December 2015 Spray 31 The chemical composition of the liquid around the corroding area is typically significantly different from the chemical composition of your formula. The chemical composition of the liquid moving through and accumulating in the coating determines if metal substrate corrosion will occur, what type of metal substrate corrosion occurs and if coating delamination from the metal substrate will occur. Figure 3 illustrates several other important polymer properties: • The wet polymer is chemically different from the original dry polymer. • The wet polymer could act as a semi-permeable membrane that allows only select formula ingredients through to the metal substrate. • The liquid accumulating at the polymer-metal interface could have a significantly different composition from the composition of your formula. • The liquid accumulating at the polymer-metal interface could cause polymer corrosion and metal corrosion. In summary, the key corrosion properties for polymer coatings and laminate films are: • Polymers are not perfect barriers between the environment (your formula) and the substrate package metal. • Formula ingredients could diffuse into and through microscopic pores in a polymer coating/laminate. • Water and other ingredients in your formula can create microscopic rivers through a polymer causing it to swell. • The wet polymer has a different composition than the original dry polymer. • The chemical composition of the liquid diffusing into and through the polymer could be significantly different from the composition of your formula, particularly if the polymer acts as a semi-permeable membrane. • The liquid accumulating at the polymer-metal interface could cause polymer corrosion and metal corrosion. We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Want a specific topic discussed in an issue of Corrosion Corner? Please send your suggestion/questions/comments to rustdr@ pairodocspro.com or visit www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in January. Spray Figure 3. Spray package coating/laminate and metal corrosion.
Spray December 2015
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