Written on: May 1, 2020 by W. Stephen Tait
Hello, everyone. I hope this May 2020 edition of Corrosion Corner finds everyone safe and healthy. As we go to press, Pair O Docs was designated by the state of Wisconsin as an essential business, so we have been able to continue operating our corrosion testing laboratory during the pandemic. However, we spend as much time as possible working at home.
Spray packages and spray valves are fabricated from a variety of materials, such as:
* Uncoated aluminum
* Polymer laminate film(s) on aluminum foil
* Polymer coating on aluminum
* Uncoated tinplated steel
* Polymer-coated tinplated steel
* Polymer-coated tin free steel
* Polymer laminate films on tin free steel
* Polymers (plastic bottles and cans)
Water-either as a formula ingredient or a contaminant-plus other electrochemically active formula ingredients cause or contribute to the corrosion of spray package materials. Consequently, corrosion of spray package materials is always a high probability.
Corrosion is a complex surface phenomenon that causes degradation of spray package materials. Spray package corrosion requires materials (such as metals and polymers), a surface and a corrosive environment (water and other ingredients in a formula).
Material degradation could produce unwanted effects such as leaking packages, packages that stop spraying, product malodor or discoloration. Polymer coating or laminate film corrosion could produce pieces of polymer that clog spray package valves. Corrosion of plastic bottles and cans could cause packages to become brittle or lose their strength and burst under pressure.
All types of spray package materials are susceptible to corrosion. For example, electrochemical corrosion causes package metal loss that could lead to:
* Broken emulsions with a corrosive water phase
* Product discoloration and malodor
* Product leaking
* Propellant leaking
* Blistering or complete delamination of polymer coatings and laminate films inside the package
The occurrence of corrosion is difficult to predict because there are numerous factors that contribute to and/or cause corrosion. The situation is further complicated when several factors interact to cause or speed up corrosion. The probability of corrosion occurring and the rate of corrosion are determined by:
* The type of package material(s)
* The chemical composition of your formula
* The physical form of your formula, e.g., emulsion or anhydrous
* The package surface properties (note that dry, unexposed surfaces typically have significantly different properties from surfaces wetted by your formula)
How your formula interacts with the internal package surfaces
Water and package materials are thermodynamically unstable when exposed to each other. Almost all formulas have some water in them, even if only very small amounts from contamination. Consequently, corrosion of spray package materials is always a possibility.
Various formula ingredients-besides water-cause or contribute to spray package corrosion:
* Electrochemically-active ingredients cause metal corrosion-for example, a low pH formula has more hydrogen ions that are electrochemically active
* Surfactants make the package surface more-or-less susceptible to corrosion
* Surfactants could also make diffusion of corrosive ingredients through polymer coatings and laminate films more-or-less possible
* Fragrances could either contribute to, or cause, corrosion
* Fragrance could also inhibit corrosion
Extremely low corrosion rates don’t produce enough corrosion to reduce the service lifetime of the package materials. Very high corrosion rates produce corrosion that significantly reduce package service lifetime. Thus, the corrosion rate determines if spray package corrosion will lower spray package service life to an unacceptable length.
The variety of corrosion-causing factors, their number and possible combinations make predicting spray package corrosion virtually impossible without corrosion data. Consequently, corrosion testing is recommended for all new formulas and derivatives formulas (i.e. line extensions) and alternate packaging for existing formulas.
A few corrosion-guidelines to consider when developing formulas or derivative formulas:
* Don’t assume that small changes in the chemical composition of your formula won’t affect spray package corrosion
* Don’t assume raw materials from different vendors have the same corrosivity or non-corrosivity
* Raising pH is not a sure-fire way to reduce spray package corrosion
* Propellant might also cause or contribute to spray package corrosion by partitioning formula ingredients between the bulk formula and propellant phases
* Don’t assume an anhydrous formula is water-free
* The amount of contaminant water determines if an anhydrous formula is or is not corrosive-sometimes there is a safe concentration range
* Don’t assume a given formula is not corrosive just because individual ingredients by themselves are not typically corrosive (i.e., watch out for synergy)
* Don’t assume one type of spray package or spray package materials is more corrosion-resistant than others
* Higher temperatures rarely accelerate corrosion
Please visit www.pairodocspro.com for more information. Thanks for reading and I’ll see you in June. SPRAY
