It’s commonly believed that coatings and laminate films are barriers
between a spray package metal and your formula and that a barrier is
needed to prevent corrosion…however, coatings do not always
May 2018 Spray 39
prevent corrosion and are not always needed.
Figure 3: Diagram of pores and voids in polymer coatings
Liquid moving into and through pores also enlarge pores and
create what I refer to as microscopic rivers. Microscopic rivers could
lead to more extensive coating corrosion, more extensive loss
of coating properties and corrosion of the metal substrate. The
chemical composition of the microscopic rivers could be significantly
different from the composition of your formula.
Metal corrosion under the coating occurs when the liquid
moving through the coating is corrosive toward the metal, sufficient
liquid accumulates at the metal-coating interface or liquid
continuously moves through the rivers in the volume needed to
sustain metal corrosion.
I’ve seen instances where coating and metal corrosion started
and subsequently stopped because either metal corrosion products
plugged the rivers or coating swelling by other pores closed
off the rivers (thus preventing the volume of liquid needed to
support metal corrosion).
The chemical composition of a formula, type of package metal
and type of coating interact to determine how much liquid
absorbs into the coating through pores, if the coating will corrode
and lose its barrier properties and if the coating will protect the
substrate metal from corrosion.
Sensitive electrochemical measurement instruments detect
and measure both the coating corrosion and the metal corrosion
under the coating.
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 rapid, 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. Thanks for reading
and I’ll see you in June. Spray
info@condensa.cl • www.condensa.com • +56 225 195 759 • Presidente Errazuriz 2999 • office #502 • Santiago • Chile • 7559357
Presidente Errazuriz 2999, office #502, Santiago, Chile, ZIP: 7559357