Written on: July 1, 2012 by W. Stephen Tait
Hello everyone. One of the most persistent myths in corrosion is that chloride ions cause pitting corrosion for all metals and coated metals.
The myth about chloride ions and corrosion most likely originated with the pitting corrosion behavior of some stainless steel alloys in sea water. Stainless steel alloys form
a thin oxide layer that protects the alloy from a large number of corrosive environments (but not all). Chloride ions can irreparably damage this film in small spots, resulting in the
pitting corrosion of uncoated stainless steel.
However, the tin-free steel, tinplated steel and aluminum alloys used for spray packaging do not naturally form the same type of thin protective oxide layer found on stainless steel
alloys. Hence, chloride ions do not interact with spray packaging metals in the same way that chloride ions interact with stainless steel.
What are Chlorides?
Chlorides are negatively charged free chlorine ions in aqueous solutions. Some typical chlorides are table salt (sodium chloride), potassium chloride, and ferric chloride (used to
catalyze the reaction for making ethoxylated and propoxylated surfactants). Chlorine atoms bonded to organic molecules, such as chloroform, carbon tetrachloride or 1,1,1
trichloroethane, are not chlorides or sources of free chlorine ions in most situations.
The role of chlorides in spray package corrosion
Electrochemically active ions or molecules in your formula remove electrons from spray
package metals, thereby causing metal corrosion. Chlorine ions (chlorides) have saturated
electron valence shells— identical to the valence shell for the noble gas argon—so chlorine (chloride) ions have no room in their valence shells for more electrons. Hence, chlorine ions have no need to or capability to remove electrons from spray package metals.
In other words, chloride ions are not electrochemically active and thus do not cause corrosion by removing electrons from the metals used to fabricate your spray packaging. However, general and pitting corrosion generates positive metal ions and the resulting ionic charge on the metal surface at the corrosion site must be balanced in the metal’s environment (your formula in this instance) and on the metal surface.
Negative ions diffuse toward the corroding area to balance charge, and often accumulate at the site of corrosion and are found during physical chemical analyses. However, the negative ions diffuse to the corrosion site in response to the corrosion and are not the direct cause of the corrosion.
To summarize: the necessary amount of negative ions—such as chloride ions—move to a
corrosion site to balance the positive metal ionic charge generated by corrosion. However, any negative ion and molecule, such as sulfite and sulfate ions, could also balance the metal ionic charge, so chlorides are not necessary to support on-going corrosion. A corrosion site could be either general corrosion, pitting corrosion or both.
There are numerous examples of steel packaging that demonstrate chloride ions do not cause corrosion of the materials used to fabricate spray packages. For example, a large number of salted food products are safely packaged in both coated and uncoated tinplated steel containers, demonstrating that chlorides do not cause tinplate corrosion. The tinplate used to fabricate food containers is the same as that used to fabricate steel aerosol containers.
Several years ago, I conducted a study on tinplated steel with 18-mega ohm deionized-reverse osmosis water. The tinplate samples were all severely rusted and pitted in this chloride-free water, indicating that chlorides are not necessary for both tinplated steel general and pitting corrosion.
I’ve seen instances where aluminum containers need a nearly defect-free coating to prevent corrosion by chloride-containing formulas (e.g., certain food products), and other instances where containers with a high level of defects were not corroded by chloride-containing formulas (e.g., contact lens cleaners). These types of corrosion resistance differences with different chloride-containing formulas demonstrate that chlorides are not the primary cause of aluminum container corrosion.
It has also been determined by me and several other researchers that chloride ions do not cause coated metal container corrosion. Indeed, these studies indicated that the positive ions associated with the chloride ions were the principal cause for the coated metal corrosion.
I’m often asked how much chloride is needed to support corrosion. Theoretically it only
requires a very small amount of chlorides—approximately 3 ppm—to support a pit that
perforates a tinplated steel container after three months (electrically balance the ionic
charge generated by corrosion).
However, as previously mentioned, other negative ions and molecules can also balance the electrical charge of the corrosion sites. Hence, chlorides are not really necessary to support on-going corrosion. Consequently, there is actually no intrinsic minimum amount of chlorides needed to support corrosion.
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