This type of corrosion occurs
under painted or plated surfaces when moisture permeates the coating.
Lacquers and "quick-dry" paints are most susceptible to the
problem. Their use should be avoided unless absence of an adverse effect
has been proven by field experience. Where a coating is required, it
should exhibit low water vapor transmission characteristics and
excellent adhesion. Zinc-rich coatings should also be considered for
coating carbon steel because of their cathodic protection quality.
(Courtesy of www.cp.umist.ac.uk)
corrosion normally starts at small, sometimes microscopic, defects in
The picture on
the left shows filiform corrosion causing bleed-through on a welded
tank. The picture on the right shows "worm-like" filiform
corrosion tunnels forming under a coating at the Atmospheric
corrosion is minimized by careful surface preparation prior to coating,
by the use of coatings that are resistant to this form of corrosion (see
above), and by careful inspection of coatings to insure that holidays,
or holes, in the coating are minimized.