Written on: February 1, 2025 by W. Stephen Tait
Hello, everyone. Corrosion is everywhere and no type of metal, coated metal or laminated metal foil package is immune from both metal and/or polymer corrosion.
It is often erroneously believed that chloride ions are the principal cause of spray package corrosion, particularly when chloride ions are found around or inside corrosion sites. In this issue, I’d like to discuss how chloride ions are actually involved in spray package corrosion.
Water is electrochemically active, so it does cause metal corrosion…
Let’s start by defining two terms commonly used in corrosion engineering and science: corrosion site and corrosion product. A corrosion site is any area on a coated or uncoated metal surface where electrons are removed from the metal and the metal ions are ejected from the metal. Corrosion sites can be:
• Very large (referred to as general corrosion) and/or
• Small localized areas, such as pits.
A corrosion product is a complex mixture of metal hydroxides and oxides with various polar molecules (e.g., water) plus both positive and negative ions. Corrosion product colors typically range from black to red for steel and white to black for aluminum. The actual color of corrosion products depends on a number of chemical factors, such as the pH of the corrosion product and the amount of oxygen in the environment around the corrosion products (e.g., the oxygen dissolved in formula water).
How chloride ions are detected & measured
Scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) are the most common types of analyses techniques used to examine corrosion sites and determine the chemical composition of corrosion products for both general corrosion and inside pit cavities.
EDS is often erroneously referred to as a surface analysis method. However, the SEM electrons bombarding the corroded metal penetrate into the bulk metal and the resulting emitted x-rays are from both the corrosion product and the bulk metal.
The role chloride ions play
Electrochemically active ions and molecules cause metallic corrosion by removing electrons from the metal. Chloride ions have a saturated valence shell—much like noble gases. Hence, chloride ions have no room in their valence shells for more electrons and are not electrochemically active. In other words, chloride ions can’t remove electrons from metals to cause corrosion.
However, chloride ions are negatively charged and corrosion products have a concentrated mix of positive metal ions, water and formula ingredients. The metal ions produce a large electro-osmotic pressure between the corrosion site and the bulk of a formula. This pressure enhances diffusion of polar molecules, such as water, along with ions and ionic molecules toward and into the corrosion site. This both decreases the concentration of the metal ions generated by corrosion and balances the electrical charge in the corrosion product.
Thus, the corrosion product chemistry is diluted by water, and the positive charge in the corrosion product is balanced by negative ions. Consequently, the chemical composition in a corrosion product is significantly different from that of a bulk formula; it could also be significantly more corrosive than the bulk formula.
It should be noted that most negative ions and molecules can fulfill the need to balance electrical charge, so chloride ions are not exclusively needed to support spray package metal corrosion. Also of note is that water is electrochemically active and thus causes corrosion.
Spray packages with an internal coating or laminate polymer film
Corrosion of polymer coatings and polymer films (laminated onto the package metal) is actually caused or enhanced when positively charged ions—such as cations like Na+ from sodium salts—diffuse through a coating/film to its substrate metal. Accumulation of water and cations in a coating damages it, reduces and/or eliminates its ability to be a barrier between the metal and a formula, and creates microscopic liquid-rivers through the coating that allow metal corrosion to initiate and continue growing through the substrate metal.
Cations are also surrounded by a water sheath that moves with its cation through coatings. Thus, cations have the ability to also deliver additional water to the metal substrate under a coating. To reiterate, water is electrochemically active, so it does cause metal corrosion.
Concept origins: Chloride ions cause spray package corrosion
Metals and alloys that spontaneously passivate—such as stainless-steel and nickel alloys—form very thin, transparent metal oxide film barriers between the metal and the environment (e.g., a formula). These passive films are approximately 20 Angstroms thick (2e-6 mm) and their compositions are very close to the theoretical oxygen-metal ratio for a metal oxide.
Chloride ions damage passive films in small spots by displacing the oxygen ion in the metal oxide via ion exchange. The resultant metal chlorides are water-soluble and dissolve in water leaving a hole in the passive film. Damaged areas of a passive film (holes) can quickly regenerate; however, pitting corrosion occurs whenever the chloride ions damage a passive film faster than it can regenerate.
The aluminum, steel and tin metals used for spray packages do not spontaneously form passive films. Hence, chloride ions do not cause corrosion of packaging metals because they do not spontaneously passivate.
The actual role of chloride ions in corrosion
The supporting role of chloride ions for pitting corrosion of non-passivating steel was confirmed several decades ago with auger electron spectroscopy studies on the chemical composition inside corroding pits. This research chronicled development of the chemistry inside pit cavities at various stages ranging from initiation to mature growth. Chloride ions were found inside pits after pitting corrosion was fully developed but not during the initial stages of pitting corrosion. Thus, this research demonstrated that the chloride ions corrosion role was supportive and not causative.
At my request, a similar separate study was conducted on a sample of tinplated steel that I provided. Chloride ions were found inside tinplate corrosion pits, but also only detected after the pitting process was fully developed.
In summary, chloride ions do not cause pitting corrosion in the types of metals used for fabricating spray packaging. Chloride ions can support corrosion by balancing the positive charge of the corrosion product at corrosion sites. However, any negative ion or molecule (anion) can balance charge, so chloride ions are not strictly necessary for spray package pitting corrosion.
Thanks for your interest and I’ll see you in March. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
