April 2018 Spray 27
~20 Microns ~70 Microns
Hole in polymer coating on aluminum Hole in the tin coating on tinplated steel
ent types of metal corrosion, such as detinning and pitting. The
rates of coating and metal corrosion in microscopic lakes are
determined by the chemical composition of a formula, the type of
coating, the type of metal under the coating, the morphology of
the coating and the topology of the metal and the coating.
Polymer coatings also have meandering microscopic pores,
some of which form winding paths through the coating to the
metal surface. Liquids diffusing into pores by osmosis could create
microscopic rivers through the coating and the microscopic
rivers could cause the intertwined coating molecules to unravel,
creating a hole in the coating and
eventually leading to metal corrosion
under the coating.
In summary, holes in polymer
coatings don’t lead to concentrated
corrosion in the hole. Coated
metal (concentrated) pitting corrosion
occurs:
• only when the polymer coating
around a hole also corrodes
• from microscopic pores in polymer
coatings
• from microscopic voids in the
bulk polymer coating
• from microscopic voids at the coating-metal interface
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 May. Spray
Figure 2: Examples of holes in internal container coatings