Environmental Engineering Reference
In-Depth Information
3.5.2
Remedial Measures
1.
Crevice corrosion can be controlled by proper design to avoid crevices.
Welded butt joints should be preferred to riveted or bolted joints. Crevices
in existing lap joints may be closed by welding or caulking.
2.
Tanks and vessels should be designed to ensure complete drainage, thus pre-
venting solid deposits from forming on the bottom of the vessel. If the tank
is placed on a masonry platform, closing of the gap along the periphery with
tar or bitumen is advocated to avoid seepage of rainwater.
3.
Regular inspection and removal of deposits should be emphasized.
4.
Impervious gasket materials are preferable to porous ones. Also, the removal
of wet packing materials during long shutdown periods is necessary.
5.
Solution aggressiveness may be reduced, where possible, by decreasing the
chloride content, acidity, and temperature.
6.
Alloys resistant to pitting are usually also resistant to crevice corrosion.
Where practicable, a vulnerable material may be substituted by a more resis-
tant material. Increased chromium, nickel, molybdenum, and nitrogen in-
crease the resistance to crevice corrosion of stainless steels. Type 316 stain-
less steel containing 2-3% Mo is fairly resistant. Nickel alloys are more
resistant than stainless steels. Titanium alloys, however, are prone to crevice
corrosion in halide solutions above 70
°
C.
3.5.3 Filiform Corrosion
Filiform corrosion is a special type of crevice corrosion sometimes encountered
under protective coatings on metals. The attack manifests itself in the form of
thread-like filaments spreading in a zigzag manner. The filaments are 0.1 to 0.5
mm wide, grow steadily, but, interestingly, do not cross each other. Filaments
show an active head and an inactive tail. If an advancing head meets another
filament, it gets reflected from there and starts growing in another direction. The
reflected filaments at times enter into a ''death trap'' for not being able to cross
the inactive tail and with the decrease of the available space for growth. An
example of filiform corrosion is shown in Fig. 3.14.
Filiform corrosion has been observed on steel, aluminum, zinc, and magne-
sium, usually under organic coatings like paints and lacquers. Attack under tin,
enamel, and phosphate coatings is also experienced. The attack does not damage
the metal to any great extent, but the coated surface loses its appearance.
On steel, the head of the filament is usually blue and the tail is red-brown,
indicating the presence of Fe
2
ions in the head and Fe
2
O
3
or Fe
2
O
3
⋅
n
H
2
O at the tail
as corrosion product. The growth mechanism of filiform corrosion is explained by
the formation of a differential aeration cell and is illustrated in Fig. 3.15. The
head absorbs water from the atmosphere because of the presence of a relatively
