Written on: January 7, 2014 by W. Stephen Tait
“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 three-piece 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:
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 thickness. The porosity and non-uniform thickness causes non-uniform diffusion of materials through the corrosion product layer. The non-uniform diffusion over the surface of the package metal could also cause pitting corrosion under the general corrosion product layer.
Holes in tin coatings
Tinplated steel is steel sheet metal with a thin layer of metallic tin on both sides of the sheet. The tin layer is not perfect and has holes that expose either the iron-tin alloy layer between the tin and the steel, or the holes expose the base steel.
Certain formula compositions will cause the tin layer to pit the steel base wherever steel is exposed by a hole in the tin layer. My experience has been that this type of pitting corrosion is not as common as other types of pitting corrosion.
Transforming of an internal coating into a semi-permeable membrane
Coatings are actually not barriers between your formula and the spray package metal under the coating. Water, ions and other formula ingredients can absorb into the coating, thereby changing the properties of the coating.
A saturated coating could become a semi-permeable membrane that subsequently allows only specific formula ingredients to pass freely through the coating to the package metal under the coating. In some instances, the material passing through the coating could be significantly more corrosive than your formula and cause pitting corrosion.
The chemical composition of your formula ultimately will determine whether or not spray package metal pitting corrosion is caused by an inclusion, crystal defect, general corrosion, holes in tin or coatings that become semi-permeable membranes.
Please send your questions/comments/suggestions to rustdr@pairodocspro.com. Back issues of Corrosion Corner are available on CD from ST&M. Thanks for your interest and I’ll see you in February. SPRAY
