What is localized corrosion?

Written on: February 4, 2013 by W. Stephen Tait

Hello everyone. There are many ways that a spray package could fail (e.g., malodorous product, leaking or no longer sprays). However, the most likely mode of spray package failure is from localized corrosion. Localized corrosion could perforate the spray package material and cause polymer coatings or laminate films to detach from their substrate metal. Small clumps of detached, free-floating coating or laminate film could clog spray package valve orifices.

In generic terms, localized corrosion is metal or polymer corrosion that occurs in small areas, typically in occluded areas such as the double seams in steel aerosol containers; the valve crimp area in all types of spray packages and the welds in bag-type spray packaging

Localized corrosion of polymer coatings and laminate films is typically in the form of blisters. Localized corrosion for metals is crevice corrosion, pitting corrosion or both.

Crevice corrosion is a form of general corrosion that occurs inside an occluded area of the container, such as aerosol container seams and valve crimps. However, crevice corrosion is distinguished from general corrosion in open areas of your packages because crevice corrosion is much faster than general corrosion.

In addition, microenvironments form inside corroding crevices. The chemical composition of a microenvironment is significantly different from the chemical composition of your formula, even though the solution that forms the microenvironment originates from your formula.

Pitting corrosion is very small pin-point corrosion—typically ranging from less than one millimeter to several millimeters. Pitting corrosion is very rapid and eventually perforates the container metal of traditional steel or aluminum aerosol containers; aerosol valves and the aluminum foil in laminated foil packaging (e.g., bag-on-valve or bag-in-can). Pitting corrosion occurs a) under polymer coatings and laminate films, b) in occluded areas and c) in open areas of the spray package. Pitting corrosion is also often associated with crevice corrosion.

An interesting aspect of metal pitting corrosion is that individual pits need a very large area surrounding them to support their very high corrosion rates. Ironically, the pit protects the surrounding (supporting) area from corroding. Hence, it might look like corrosion is concentrated at a pit, but the supporting area is actually protected from corrosion by the growing pit.

Another interesting aspect of both crevice and pitting corrosion is the microenvironment chemistry inside the pit cavity and inside the crevice. The corrosion reaction and transport of materials into and out of the crevice or pit cavity buffers the microenvironment solution to a pH of four. The solution outside the crevice or pit cavity—your formula—could be lower than four or very basic (above pH 10), but the microenvironment solution pH s maintained at four.

A solution with a pH of four is very aggressive toward the steel and the aluminum used for all types of spray packaging. Hence, the corrosion rates inside crevices and pits are very high, often leading to package service lifetimes that are less than one year.

Polymer blistering is also a complex form of localized corrosion. A persistent spray package corrosion myth is that pitting corrosion under polymer coatings is caused by a hole in the coating. However, this myth does not explain why blisters with pits are often filled with gas or liquid under pressure. The gas and liquid under pressure could not form if the coating had a hole in it (much like trying to inflate a balloon with a hole in it).

Much like uncoated metals, individual pits under coatings need a very large area surrounding them to support their very high corrosion rates. Consequently, a large area of coating around the pit also needs to fail—lose its barrier properties—to support the pit growth under a blister or under a coating. Hence, it looks like corrosion is concentrated at the pit under the blister, but a large area of coating must also fail if the pit is to continue growing.

The substrate metal atoms are oxidized around the center of a blister and subsequently leave the metal as ions. This area is referred to as an anode (metal atoms are oxidized). Free electrons produced by metal oxidation move through the metal to the outer edges of the blister to reduce formula water and formula ingredients. The outer edges of the blister are referred to as cathodes (reduction occurs).

The anode area under a blister often has a low pH and the pH for the cathode area is very high—on the order of 12. This high pH breaks the polymer-metal bonds causing the coating to separate (disbond) from the substrate metal.

Formula water and ingredients continue to diffuse through the blister to fill the cavity in response to metal corrosion under the blister, and thus support further metal-polymer disbonding at the edge of the blister. In other words, diffusion of solution into the blister causes the blister to continue growing.

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 March.