March 2021 Spray 27
Worst-case corrosion testing by breaking
an emulsion and conducting corrosion
tests on only the free-water phase is not
recommended...
• Emulsion type
• Stability of the emulsion; and
• Corrosivity of the various phases formed from
a broken emulsion
Smaller droplets coalesce into larger ones when an emulsion
is unstable or destabilized. For example, a surfactant can
become partitioned between the propellant and droplets at the
propellant-bulk formula interface when a surfactant is soluble
or partially soluble in a propellant. Partitioning decreases the
surfactant concentration on the droplets, allowing them to
coalesce. The larger droplets could subsequently contribute to
or cause corrosion:
• In the interfacial area
• Where larger droplets stick to the interior package surfaces
• At the bottom of the package when larger droplets sink
to the bottom
How fast an emulsion corrodes the package materials
(i.e., the corrosion rate) determines the package service lifetime.
Whether or not corrosion will occur, as well as the package
corrosion rate, are both determined by:
• Emulsion type
• Chemical composition of the matrix and the dispersed
droplets
• Corrosivity of the various phases resulting from a broken
emulsion—corrosive phases could be free-water and/or
cream-phases
• Type of surfactant used to stabilize the emulsion
Microemulsions and nano-emulsions in some instances are
significantly more corrosive than emulsions having droplet sizes
greater than 100nm. Increased corrosivity by micro/nano-emulsions
is believed to be caused by the significantly higher surface
area of the smaller droplets for these emulsion types.
Some surfactants—such as sodium lauryl sulfate, nonylphenol
and di-nonylphenol—contribute to or cause spray package material
corrosion.
Corrosion testing of the complete product (formula plus
propellant) is recommended for emulsion spray products.
Corrosion testing can be accomplished with either a one-year
storage stability test or a shorter electrochemical corrosion test
using appropriate test and measurement parameters. Worstcase
corrosion testing by breaking the emulsion and conducting
the corrosion tests on only the free-water phase is not
recommended.
Thanks for your interest and I’ll see you in April. Contact me
at 608-831-2076, rustdr@pairodocspro.com or from one of our two
websites pairodocspro.com or aristartec.com. Spray