December 2017 Spray 29
Corrosion basics:
Chlorides and stainless steel corrosion
Chloride ions in your formula breakdown the protective chromium
nickel oxide layer on stainless steel springs. Film breakdown
is referred to as depassivation. In a wide range of instances, the
protective layer is repaired faster than it is depassivated.
However, there are chemical compositions that depassivate the
protective layer faster than it is repaired. Formula pH, the concentration
of chloride ions in your formula and the particular type of
valve spring stainless steel are a few of the factors that determine if
the oxide depassivation rate will exceed the repair rate.
Pitting corrosion results from depassivation that exceeds repair
and pitting corrosion often causes spring cracking with subsequent
breakage. A broken spring could cause the container to
stop spraying or cause product leakage through an incompletely
sealed valve orifice.
There are no rules for what combinations of these factors
cause pitting corrosion or what concentration of chloride ions
cause corrosion. Consequently, corrosion testing or a large valve
spring corrosion database are the only ways to determine if a
particular formula will cause aerosol valve springs to pit and
possibly crack.
The aerosol container chloride myth
Stainless steel pitting corrosion by chloride ions gave rise to the
spray package corrosion myth that chloride ions cause aluminum and
steel aerosol container corrosion. Chromium and nickel are not in
aluminum packaging metals and the amount of chromium and
nickel in both tinplate and tin-free-steel is too low to passivate
tinplate and tin-free-steel.
Thus, aluminum and steel aerosol containers do not have the
passive oxide film that is susceptible to chloride depassivation.
Consequently, chlorides do not typically cause aluminum or steel
aerosol spray package corrosion.
In January, we’ll complete the series with a discussion of laminated
metal packaging.
Pair O Docs has a state-of-the art electrochemical corrosion testing
laboratory; please contact me if you would like to know more
about our faster and predictive corrosion testing. You can also
visit our new website which has a short Vision Video that discusses
all our corrosion prevention and control services. Please also contact
me if you would like to a have our Elements of Spray Package
Corrosion short course taught at your R&D facility. Back issues of
Corrosion Corner are available on CD-Rom. Thanks for reading and
I’ll see you in 2018. Spray