What causes filiform corrosion? Filiform corrosion is a
special case of crevice corrosion. During propagation, water
is supplied to the head of the filament from the surrounding
atmosphere by osmotic action due to the high concentration
of dissolved ferrous ions on the surface of steel substrate.
Osmosis tends to remove water from the inactive tail,
because of the low concentration of soluble salts (iron has
precipitated as ferric hydroxide).
