Why is spray package corrosion unpredictable without corrosion data?

Written on: September 1, 2016 by W. Stephen Tait

Hello, everyone. 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)

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 (ingredients in your 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. Plastic bottle and plastic can corrosion could cause packages to become brittle, panel 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:

• Emulsions broken by metal contamination
• Product discoloration and malodor
• Product leaking
• Propellant leaking
• Blistering or complete delamination of polymer coatings and laminate films

The occurrence of corrosion is difficult to predict because there are numerous factors that contribute to and/or cause corrosion. In addition, several of these factors often interact synergistically with others to cause corrosion or make corrosion go faster. 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., and emulsion or anhydrous
• The package surface properties—the dry, unexposed surface typically has significantly different properties from the surface that is exposed to your formula
• How your formula interacts with the internal package surfaces

Water and package materials are thermodynamically unstable when exposed to each other, and almost all formulas have some water in them, even if only as 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 reduces package service lifetime and degrade product efficacy. 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 difficult without corrosion data. Predicting package material corrosion rates without data is also not possible at this time. Consequently, corrosion testing is recommended for all new formulas and derivatives formulas 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
• The amount of contaminant water determines if an anhydrous formula is or is not corrosive
• Don’t assume a given formula is not corrosive just because individual ingredients by themselves are not typically corrosive (synergy)
• Don’t assume one type of spray package or spray package materials is more corrosion resistant than other types of packages and materials
• Most of the time higher temperatures don’t accelerate corrosion

We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. We have also introduced the Corrosion Partnership Program, which makes our electrochemical corrosion laboratory, corrosion consulting and anti-corrosion technology conveniently available for your R&D program. Contact: rustdr@pairodocspro.com; 608-831-2076; www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in October.