Corrosion Corner
Water and spray
package corrosion
Hello everyone. Water is often referred to as being the closest
thing to a universal solvent. It dissolves a wide range
of formula ingredients and dissolves into a wide range of
formula chemicals. For example, inorganic salts and organic salts,
such as sodium chloride or sodium benzoate, dissolve in water.
Most gases dissolve in water and some gases—such as carbon dioxide—
react with water to form corrosive ions and molecules. Dimethyl
ether (DME) propellant is slightly water soluble and small
amounts of water will dissolve in HFC152a propellant; liquified
petroleum gas (LPG) propellants and most organic solvents could
also contain small amounts of water.
Water-insoluble organic chemicals can be mixed with water and
surfactants to form emulsions. Emulsions can be either water- inoil
(oil-out or w/o emulsions) or oil-in-water (water-out or o/w
emulsions).
Corrosion Basics: Water chemistry & properties
relevant to spray packaging
Water molecules are composed of one oxygen and two hydrogen
atoms. The valence electrons in water molecules are not equally
shared between the hydrogen and oxygen atoms. Thus, one
area of the water molecule is positively charged and the other
area is negatively charged. This charge-polarization of water
molecules causes them to migrate to and adsorb on metal and
polymer surfaces. Water’s small size also allows it to absorb into
and migrate through the internal coatings in aerosol containers
W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
and the laminate films on metal foils. The polarized water
molecules swell the polymer as water absorbs into and diffuses
through a polymer, thereby creating microscopic rivers through
the polymer.
Corrosion Basics: Water & spray package corrosion
Water could be in your formula either as an ingredient or as
a contaminant. Water disassociates into hydrogen ions and
hydroxyl ions and the hydrogen ions are electrochemically active.
In other words, water is potentially also a corrosive ingredient in
your formula.
Water plays many different roles in spray package material corrosion.
For example, water molecules:
• Could be a corrosive formula ingredient
• Transport corrosive formula ingredients to the package metal
• Move ionic charges to and from corroded areas to balance
the electrical charge at both the corrosion site and the area
surrounding the corrosion site
• Hydrate metal ions formed by corrosion, thereby
producing more electrochemically active hydrogen ions
• Provide hydroxyl ions that react with metal ions to form
visible corrosion (i.e., the rust observed on steel aerosol
containers or the black corrosion on aluminum)
• Migrate through package polymer coatings and laminate
films to the package metal to cause metal corrosion and/or
the coating or the laminate film to separate from the package
metal (delaminate)
• Transform container coatings into semi-permeable
membranes that only allow specific formula ingredients to
migrate through the coating to the container metal
• Form microscopic rivers flowing through container coatings
or laminate films that could subsequently cause coating
delamination and/or metal corrosion—your formula might
not be corrosive but the microscopic rivers could have a
different chemical composition from your formula and also
be corrosive
• Are a solvent for the various corrosion inhibitors used to
prevent and control spray package corrosion
• Could transform non-corrosive anhydrous formulas into
package-eaters
Water could simultaneously be involved in several of the items
in the above list. For example, corrosion of uncoated tinplated
steel involves water as follows:
• Water reacts with aluminum oxide in the container steel to
form an aluminum hydroxide gel layer on top of the
container surface (aluminum is used in the steel-making
process)
26 Spray September 2019