Corrosion Corner 22 Spray January 2014 W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC Pitting corrosion in spray packaging is caused by a variety of factors H ello, everyone. “What causes pitting corrosion?” is a very common question for spray packaging. There are multiple factors that could cause pitting corrosion in spray packaging. Let’s start the discussion by defining pitting corrosion. Corrosion Basics: Definition of pitting corrosion Pitting corrosion is a specific type of localized corrosion, and localized corrosion is a type of corrosion that occurs in small occluded areas where diffusion of your bulk formula into the occluded area is restricted. Occluded areas include the a) bottom and top double seams in three piece welded containers and two piece drawn and ironed containers, b) the crimp area between the valve cup and container curl and c) a growing pit. Notice that the weld in threepiece welded aerosol containers is not included in the list. The overlap formed by the weld is too shallow to create an occluded area. Pitting corrosion forms an occluded area after the pit has reached a certain depth, referred to as the critical depth. There is a wide range of critical depths, and the magnitude of the depth is determined by a) your formula chemical composition and b) the type of spray package metal, and c) the type of surface treatment on the spray package metal (e.g., uncoated tinplate or a polymer coating). The most common causes for spray package pitting corrosion are: • Inclusions in the spray package metal • Crystal defects in the spray package metal • General corrosion of the spray package metal • Holes in tin coatings • Transforming an internal coating into a semi-permeable membrane Notice that holes in internal coatings are not on the list. A hole in a coating is insufficient to cause pitting corrosion. A large area of the coating surrounding the hole must become a semi-permeable membrane or completely fail as a barrier in order for a hole in a coating to cause pitting corrosion. Inclusions in the spray package metal The metals used for spray packaging are mixtures of different metals and non-metals. The various ingredients are added to the molten base metal (iron or aluminum). The various ingredients are added to give the package material its properties, such as strength. When the molten metal cools some of the ingredients become insoluble and precipitate out of the metal as particles. These particles are called inclusions, and inclusions on the surface of the package metal are possible sources for pitting corrosion. Crystal defects in the spray package metal Metal atoms are arranged in one or more repeating three-dimensional patterns. However, the atom patterns are not perfect and defects, such as missing metal atoms occur in the crystal structure of the package metal. There are millions of metal crystalline defects in a square centimeter of metal surface. Under the appropriate conditions some types of crystal defects are possible sites for pitting corrosion. General corrosion of the spray package metal General corrosion produces a layer of corrosion product on the surface of spray package metals. This layer is porous and has a very non-uniform
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