Written on: September 1, 2020 by W. Stephen Tait
Hello everyone. One of the most persistent corrosion myths is that chloride ions cause corrosion of all metals and coated metals.
This myth most likely originated with the pitting corrosion behavior of stainless steel alloys in seawater. Stainless steel alloys typically form a unique, thin oxide layer that protects the alloy from a large number of corrosive environments (but not all). Chloride ions, such as sodium chloride in seawater, can irreparably damage this protective film in small spots, resulting in stainless steel pitting corrosion.
However, the tin-free steel alloy, the tinplated steel alloy and the aluminum alloys used to fabricate spray packages do not naturally form the same type of thin protective oxide layer found on stainless steels. Hence, chloride ions do not interact with spray package metals the same way they interact with stainless steel. In addition, chlorides typically don’t cause polymer corrosion.
What are Chloride ions?
Chlorides are the negative chlorine ions (Cl-) of salts that dissolve in water, such as table salt (sodium chloride), potassium chloride and ferric chloride (used to manufacture surfactants). The chlorine ions are commonly referred to collectively as chlorides. Organic chlorides—such as chloroform, carbon tetrachloride or 1,1,1 trichloroethane—are not chlorides because the chlorine atoms typically do not dissociate from these molecules when dissolved in water.
The role of chlorides in spray package corrosion
Electrochemically-active ions and molecules in a formula remove electrons from spray package metals, thereby causing metal corrosion. However, Cl- ions (chlorides) have saturated electron valence shells—identical to that for the noble gas argon—so chlorides don’t need additional valence electrons. Hence, chlorides are not electrochemically- active. In other words, Cl- ions cannot cause corrosion because they are not electrochemically-active.
However, metal corrosion generates positive metal ions and the resulting ionic charge must be balanced in both the micro-environment surrounding the corrosion site and at the corrosion site. A corrosion site could mean either general corrosion, pitting corrosion or both.
Any type of negative ion diffuses toward an unbalanced electrical charge and in doing so supports corrosion. Negative ions often accumulate at a corrosion site and are thus found during physical chemical analyses of a corroded area after the package has failed. However, the accumulation of Cl- ions at the corrosion site is a response to corrosion and not the cause.
To summarize; negative ions, such as Cl- diffuse to a corrosion site to balance the positive electrical charge generated by corrosion. However, any negative ion and molecule, such as sulfite and sulfate ions, could also balance the electronic charge. Therefore, chlorides are not necessary to support ongoing corrosion. There are numerous corrosion studies that demonstrate chloride ions do not cause corrosion of the materials used to fabricate spray packages. For example:
1. Numerous salted foods are safely packaged in both coated and uncoated tinplated steel containers. The tinplate used to fabricate food containers is the same as that used to fabricate tinplate aerosol containers.
2. Corrosion measurements on tinplated steel exposed to 18-mega ohm deionized-reverse- osmosis water demonstrated that purified water is significantly more corrosive than water with dissolved chlorides.
3. There are also numerous instances where aluminum aerosol 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 contradictory instances illustrate that chlorides are not the primary cause of aluminum container corrosion.
It has also been shown by positron annihilation measurements that Cl- ions do not damage polymers. Instead, the positive counterion of a chloride salt (e.g., Na+, K+ or Ca+2) is the actual cause of damage to a polymer coating or film. The damage reduces a polymer’s ability to be a barrier and leads to metal corrosion under the polymer. My subsequent research also indicated that the type of positive counterion determines the extent of damage and subsequent metal package corrosion.
I’ve often been asked how much chloride is needed to support corrosion. Theoretically, it only requires a very small amount of chloride ions—approximately three parts per million (3ppm)—to support a pit that perforates a tinplated steel container after three months. The 3ppm amount was estimated by calculating the weight of metal removed by the pitting corrosion to estimate the weight of negative Cl- ions needed for electrical balance and converting this weight to ppm in the formula.
However, as previously mentioned, other types of negative ions and molecules will also balance the electrical charge at a corrosion site. Hence, chlorides are not really necessary to support ongoing corrosion. In other words, there is actually no intrinsic minimum amount of chlorides needed to support corrosion.
Pair O Docs has developed a web version of its Elements of Spray Package Corrosion course in response to the COVID-19 pandemic. The course is offered for 5–30 participants, enables team members to participate from any location and costs less than the on-site version. Call 608-831-2076 or email rustdr@pairodocspro.com for more information. Thanks for reading and I’ll see you in October. SPRAY
