W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Why conduct spray package
corrosion tests? Part 1
Corrosion Testing now known as Corrosion Measurement…
Hello, I hope everyone is in good health. As we go to
press, our family is feeling well and the Wisconsin
safer-at-home order is working to stem the tide of the
pandemic.
In this issue, I’ll begin a multiple-part series on why spray
package corrosion is so complex and why long-term spray package
corrosion cannot be accurately predicted without corrosion testing.
First, I’m renaming corrosion testing to corrosion measurement.
Corrosion measurement more accurately describes electrochemical
corrosion testing and the fact that corrosion should also be
measured during storage stability test package corrosion examinations.
Metal and polymer corrosion are complex, multi-step processes.
The variety of spray package materials also contributes to this
complexity. Spray packages and spray valves are fabricated from a
variety of materials, such as:
• Polymer films and bilayer films laminated on aluminum foil
• Aluminum metal
• Tinplated steel (ETP)
• Tin-free steel (TFS)
• Polymer coated aluminum, ETP and TFS
• Stainless steel springs in aerosol valves and pumps (pumps
also often use stainless steel check balls)
Corrosion is always possible with any of the spray package materials
mentioned above. Whether or not they will corrode is different
for each material with each specific formula. In addition,
the type of corrosion and the rate of corrosion are also different
with a given formula for each of these materials.
22 Spray June 2020
In other words, no single type of spray package material is more
corrosion-resistant with all types of formulas. Instead, one type of
spray package material might have more corrosion resistance to a
specific formula chemistry.
The corrosion rate determines the length of the package service
lifetime (PSL). PSL is the age of the filled-package when it perforates
and leaks; corrosion degrades product efficacy, producing a
malodor or an off-color; or the package is partially full but does
not spray. In other words, PSL is the length of time when the
package performs to specification and dispenses its product with
the specified efficacy.
Metal corrosion is an electrochemical process, not a chemical
reaction. An electrochemical process includes a change in the
chemical state of a material (chemical reaction) and transfer of
electrical charge from the metal to formula ingredients (electrical).
Ingredients that receive electrons are electrochemically-active
ions or molecules. For example, hydrogen ions (low pH) are
electrochemically-active ions and water is an electrochemicallyactive
molecule.
The simplified electrochemical equation for the corrosion of
the iron in ETP, TFS and stainless steel is shown in Equation 1.
The equation has two parts: 1) the corrosion reaction for iron that
produces free electrons and 2) the reaction of hydrogen ions with
the free electrons from the iron.
Equation 1: A simplified corrosion reaction for iron with hydrogen ions
Equation 1 illustrates that the chemical state changes for both
the iron metal and the hydrogen ions (chemical reaction) as the
result of the electrical (electron) transfer of charge from the metal
atoms to the hydrogen ions. A similar equation can be written for
the aluminum and the aluminum foil used for spray packages.
Equation 1 is simplistic. The actual process for the iron corrosion
reaction proceeds in multiple distinct steps that include
diffusion, displacement of adsorbed water molecules from the
metal surface, hydrogen ion adsorption, electrical charge transfer,
combination of hydrogen ions to form hydrogen gas, de-adsorption
of the hydrogen gas and ejection of metal ions from the
metal lattice.
Polymer coatings and films are a complex structure formed
from molecular intertwining of a large number of polymer backbones
and their side chains. This complex structure provides a
narrow, torturous path for diffusion through the polymer—much
like walking through a dense forest that has no path. The tortur-
Polymer coatings and films are a complex structure formed from molecular
intertwining of a large number of polymer backbones and their side chains.
The structure provides a narrow, torturous path for diffusion through the
polymer—much like walking through a dense forest that has no path.