Corrosion Corner 34 SPRAY June 2014 W. STEPHEN TAIT, PH.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC When spray package internal coatings and laminate films corrode... ello, everyone. Internal coatings in spray packages are not barriers between the package metal and your formula. Laminate films on metals are typically much thicker than coatings. However, laminate films are also not true barriers between metal and your formula. The typical ranges of thicknesses for spray package coatings and laminate films are: • 2 to 10 microns for traditional aluminum and steel spray packaging (1 micron = 1x10-6 meters) • On the order of 300 microns for laminated tinplated steel aerosol valves • On the order of 100 microns for polymer films on the laminated foils for internal bags in spray packaging • On the order of 20 microns for the laminate films on tin-free-steel containers Coatings are applied to spray package metals and metal foils as: • A separate polymer film bonded to the metal or the metal foil • Various modified versions of epoxy that are thermally cured • Powder spray coatings that are heat-treated after application • Polymer dissolved in a solvent that is subsequently heated to remove the solvent (referred to as lacquers) There are a variety of types of coating and laminate film corrosion: • Wet adhesion loss • Pitting corrosion that causes blisters • General corrosion that causes blisters • Filliform corrosion Let’s briefly discuss each form of coating/film corrosion. Wet adhesion loss causes the coating to delaminate from the substrate metal Figure 1 provides an example of wet adhesion loss of a coating. Water and formula ingredients diffuse into and through the coating. Accumulation of liquid at the coating-metal interface breaks the coating-bonds, resulting in delamination of the coating from the metal. Typically, no corrosion is observed with this type of coating corrosion. The orifices in spray package valves are small and loose of pieces coating could prevent a package from spraying if the pieces clog valve orifices or dip tubes. H Figure 1: Wet adhesion loss Figure 2: Blistering from pitting corrosion
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