October 2013

Why isn’t spray package corrosion easier to predict, so we can skip corrosion testing?

 

There is often push-back on the need for corrosion testing, particularly when market timetables are short.

The no-corrosion-data risk is approximately 62% that some type of corrosion will occur in traditional aerosol containers. In other words, there’s only a 38% probability that corrosion will not occur in traditional aerosol containers. A similar risk has not yet been developed for spray packages with internal laminated aluminum foil bags.

The complexity of corrosion makes it impractical to use first principles to predict a) if corrosion will occur, b) how fast corrosion will degrade packaging materials and c) the package service lifetime. Consequently, corrosion testing is necessary to reduce the risk from approximately 62% to a more acceptable level of risk, such as 5% to 10%.

Let’s review the most common factors that make spray package corrosion complicated and thus make corrosion testing necessary to reduce the corrosion-risk to an acceptable level.

Microenvironments inside spray packages
Spray packages have numerous microenvironments formed by the internal geometry of the package and the physical form of the formula inside the spray package. The internal geometry of the package includes the open area inside the package plus seams formed when the container body is joined with the container top, bottom and valve. Tinplated steel aerosol containers and containers with laminated bags both have internal welds.
Gas-propellant formulas typically have at least a product phase (liquid phase) and a gas phase. Liquefied-propellant formulas typically have the liquid phase, the propellant phase and the gas phase. Formulas inside laminated bags typically have a single phase. Emulsion formulas could also have a product area with multiple cream phases.

Different types of corrosion
There are two major categories of corrosion referred to as general and localized corrosion. General corrosion typically does not affect package service lifetime, but could degrade product efficacy.

Localized corrosion is confined to small areas, typically on the order of microns to several millimeters. Localized corrosion is faster than general corrosion and typically reduces service lifetime by causing leaking of product or propellant.

Both general and localized corrosion could occur in any microenvironment inside the container. The corrosion in the different container microenvironments:

  • Requires different conditions to initiate general and pitting corrosion
  • Requires different conditions to sustain corrosion after initiation
  • Has different penetration rates and growth rates

Very small changes can significantly alter corrosivity
Typically only parts per million of corrosive ingredients are needed to initiate and sustain corrosion. However, there are some instances were only a few parts per billion of a contaminant is needed to initiate and sustain metal corrosion.

Corrosivity is often a function of concentration
The occurrence of corrosion and corrosion rates are often determined and influenced by concentrations of formula ingredients. Some of the more common concentration-factors are:

  • Ph (hydrogen ion concentration)
  • Concentration of formula ingredients
  • Concentration of contaminant water
  • Concentration of a corrosion inhibitor

Variability in manufacturing processes could alter corrosivity or corrosion resistance

Variability of your formula compositions and the materials used for your spray packages always occurs. Some of the more common sources of formula and package variability are:

  • Formula ingredient concentrations
  • Sporadic contamination
  • Laminate film and internal coating variability
  • Container and valve metal chemical composition variability (rare)
  • Sporadic physical defects in container and valves (rare)

Different metals have significantly different corrosion resistances with the same environment

Spray package metals could be tinplated steel, tin-free steel and aluminum. The number of polymer materials for spray packages is larger and many of these package materials are changing to address environment and regulatory issues.

None of these materials is corrosion resistant to all types of spray formulas. For example, aluminum generally is more corrosion resistant to corrosion by low pH formulas than steel. The opposite is generally true for high pH formulas.

In summary, spray package corrosion complexity is caused by a large number of factors and makes it difficult to predict a) if corrosion will occur, b) where corrosion will occur and c) how fast corrosion penetrates the package metals and grows through a laminate film or internal coating.

Consequently, corrosion testing is needed to determine when corrosion will be an issue for a given spray package-formula system. SPRAY

Please send your questions/comments/suggestions to [email protected] Back issues of Corrosion Corner are available on CD from ST&M. Thanks for your interest and I’ll see you in December.