Written on: October 1, 2013 by W. Stephen Tait
Crevice corrosion is a type of localized corrosion that occurs in the small gap between two pieces of overlapping metal. Crevice corrosion could be general corrosion, pitting corrosion or both.
Crevices can be found in several areas of traditional aerosol containers. These areas are where a) valves and valves attached to bags are crimped to an aerosol container and b) container tops and bottoms are seamed onto the body. Container welds do not typically form a crevice where the original edges are overlapped for welding because a complete weld is a single piece of metal.
The illustration shows the location for a) valve-crimp crevices, b)
top double seam crevices and c) bottom double seam crevices.
Click image for full size.
Notice the top and bottom seam crevices have similar morphologies. The crevice area is also similar for a traditional valve crimp and a crimp between valves with attached bags.
What makes crevice corrosion different from other types of corrosion? The gap between the two pieces of metal form a microscopic capillary that initially draws liquid from your formula into the gap (crevice) via capillary action. The crevices at the top of a container and the valve crimp area are submerged under formula when the propellant is pumped into the container at high pressure, allowing capillary action to draw liquid into these two top (vapor area) crevices.
The liquid drawn into the crevice often has a chemical composition that is different from the chemical composition of your formula. Corrosion might start in the crevice after it is filled with liquid; depending on the a) chemical composition of the liquid, b) the type of container metal and c) the type of surface treatment on the container metal (e.g., coated or uncoated).
The fun begins after corrosion starts. Metal ions from corrosion diffuse into the crevice liquid, further changing the chemical composition of the liquid inside the crevice. Liquid from the container diffuses into the crevice in an attempt to dilute the crevice liquid.
However, electrochemically active ions and molecules in your formula can be reduced by electrons from the container metal, and electrons move through metal many times faster than liquid diffuses into the crevice.
Consequently, metal corrosion inside a corroding crevice is accelerated in an effort by nature to make the chemical composition of the liquid inside the crevice the same as the chemical composition outside the crevice. In other words, electrons from the metal inside the crevice are sent to metal outside the crevice in an effort to reduce electrochemically active ions and molecules in the formula outside the crevice area. This attempt to reduce electrochemically active ions and molecules is an effort to make the chemical composition of the liquid inside the crevice the same as the chemical composition of your formula outside the crevice.
Metal ions produced by corrosion draw water (or contaminant water) from your formula into the crevice to hydrolyze the metal ions. The corresponding hydrolysis reaction acts as a buffer that drives the pH of the crevice liquid to four. A pH of four is very corrosive toward the steel and the aluminum alloys used to fabricate aerosol containers.
The cycle of corrosion, diffusion of water into the crevice and buffering the crevice to a pH of four continues until the container is perforated. A portion of the hydrated metal ions in the crevice also diffuse out of the crevice and precipitate as hydroxides in the area around the crevice opening. These precipitates are the reddish-brown rust (with steel) and white (with aluminum) material that is present in the area next to a pit.
Our Elements of Spray Package (Aerosol) Corrosion short course provides an introduction to all aspects of spray package corrosion, corrosion testing and corrosion prevention.
More info: www.pairodocspro.com. Please send 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 November.
