W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Do chloride ions cause spray
package corrosion?
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.
34 Spray September 2020
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