Corrosion Corner
Ingredients that cause
spray package corrosion
Hello, everyone. Wood rots, plastics become brittle and
crack, fabrics fade and metals rust. In other words, all
materials corrode.
Metal corrosion involves both a change in the chemical state
(atoms become ions) of the surface atoms caused by the transfer of
metal atom valence electrons to electrochemically active ions, and
molecules in the environment contacting the metal surface—such
as your formula. Thus, metal corrosion is a hybrid reaction and is
called electrochemical corrosion.
Corrosion is also a complex surface phenomenon. Hills and valleys
covered with close-hanging fog provide a good analogy for metal
surfaces. The uneven terrain is analogous to the molecular topology
of a metal surface and the fog is analogous to the valence electrons
covering the surface metal atoms.
Valence electrons roam freely between the metal surface atoms
making them thermodynamically unstable. Consequently, surface
atoms are ejected from the bulk metal when electrochemically active
ions and molecules in your formula remove valence electrons.
One iron atom becomes an ion for every two electrons removed
from steel spray package components, such as tinplated steel or tinfree
steel. One aluminum metal atom becomes an ion for every three
electrons removed from aluminum spray package components, such
as containers, valves and laminated aluminum foil.
The metal surface does not have to be bare for metal corrosion to
occur. Water and formula ingredients can diffuse through laminate
films and polymer coatings to the metal below.
Some of the more common formula ingredients are electrochemically
active and might cause spray package corrosion:
Hydrogen ions
Hydrogen ions are electrochemically active and can remove valence
electrons: 2H+ + 2e- (from package metal) → H2 (gas)
Water
Water could be in a formula as an ingredient or as a contaminant.
Water removes metal valance ions as follows: 2H2O + 2e- (from package
metal) → H2 (gas) + 2OHPH
is a measure of hydrogen ion concentrations. For example, a
neutral pH has 10-7 moles per liter of hydrogen ions in the contaminant
or formula water. Water molecules are small and also absorb
into polymer coatings and laminate films and often lead to diffusion
to the metal substrate underneath. Water is also often a solvent for
other formula ingredients and thus could also carry them to the
substrate metal surface.
Dissolved oxygen
Oxygen dissolves in water and could also cause package metal corrosion
in some instances: O2 + 2H2O + 4e- (from package metal) →
4OHCarbon
Dioxide (CO2)
As a propellant, carbon dioxide dissolves in formula or contaminant
W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
water to form carbonic acid. The carbonic acid dissociates into
hydrogen ions and bicarbonate ions, both of which are electrochemically
active: H2CO3 (carbonic acid) → HCO3- (bicarbonate ion) +
H+ (dissociation of carbonic acid). The bicarbonate corrosion reaction
is: 2HCO3- + 2e- (from package metal) → H2 + 2CO3=
Unsaturated bonds in organic molecules
Organic molecules with unsaturated bonds in some instances
could also cause package metal corrosion. However, typically the
unsaturated bonds are not electronegative enough to remove valance
electrons. In some instances, unsaturated bonds adsorb to the metal
and subsequently share the metal’s valence electrons. This situation
sometimes causes corrosion inhibition.
Carboxylic acids
Carboxylic acids, such as benzoic acid, also dissociate to produce
hydrogen ions that could cause spray package corrosion. However,
salts of carboxylic acids, such as sodium benzoate, in some instances
adsorb onto the package metal surface and inhibit corrosion.
Copper ions Cu+2
Copper ions are not electrochemically active. However, in some instances
they deposit on the package metal surface and cause pitting
corrosion. This phenomenon it typically observed in traditional steel
aerosol containers that do not have an internal polymer coating.
Please note that electrochemically active ions and molecules do
not always cause spray package corrosion. The chemical composition
of your formula determines when electrochemically active ions and
molecules will cause spray package corrosion and if the corrosion
rate is high enough to reduce spray package service life.
There are other formula ingredients that might contribute to spray
package corrosion such as surfactants and fragrances.
Surfactants
Surfactants make metal surfaces more or less susceptible to water
adsorption. Consequently, surfactants could enhance metal corrosion
when they wet the metal surface (hydrophilic surfactant). A
hydrophilic surfactant could also increase absorption and diffusion
through coatings and laminates—thus contributing to corrosion.
A hydrophobic surfactant repels water, thus decreasing absorption
and diffusion through coatings and laminates—thereby inhibiting or
impeding corrosion.
Fragrances
A small number of fragrances are known to contribute to or cause
spray package corrosion and some act as corrosion inhibitors. It
is unknown why and when fragrances act as corrosion inhibitors.
Perhaps the presence of unsaturated organic molecules in certain
fragrances give them the ability to be corrosion inhibitors.
Ultimately corrosion testing is needed to determine if a given formula
will cause corrosion of the chosen spray package components.
Thanks for your interest and I’ll see you next year. Spray
46 Spray December 2018