Efficient Plant January 2018 - 30

feature | reliability issues

Fig. 3. A common example of galvanic corrosion involves a joint between steel and
copper pipe where the steel will always be attacked. This joint was submerged in
water for only nine months before damage occurred.

description of corrosion have been driven largely by work initiated by Dr. Mars
Fontana of The Ohio State University (osu.
edu, Columbus, OH). We usually describe
corrosion by how it appears. The common
categories (or types) are:
 Uniform corrosion causes about 80% of
all corrosion. It occurs where anode and
cathode sites relatively uniformly swap
position. Examples include the railroad-bridge-support column shown in Fig. 1,
buried steel water lines, nooks and crannies
on vehicles where deposits build up, and
machine frames and bases in damp areas.
 Pitting corrosion manifests as isolated
areas of attack. With carbon steel, it may
take years before leakage occurs while stainless-steel pitting might progress at a rate of
0.001 in. (0.025 mm)/day. Steel examples frequently include water and wastewater tanks.
Stainless-steel examples include external
areas with dirt deposits on them.
 Galvanic corrosion occurs when two
chemically different metals are joined.
One is always the anode and continuously
attacked, protecting the other piece. A common example involves a joint between steel
and copper pipe, where the steel will always
be attacked.
Figure 3, at the top of this page, shows
a bronze fitting and a steel pipe that had
been submerged in water. Perforation of the

30

| EFFICIENTPLANTMAG.COM

freshly cut pipe threads happened in only
nine months.
 Selective leaching is essentially galvanic
corrosion within a metal. The common industrial application involves buried cast-iron
water or waste lines where the graphite in
the iron acts as a cathode, and the iron is eaten away, leaving a weak and brittle graphite
pipe. When initially excavated, the pipe may
appear almost undamaged, but sandblasting
will rapidly remove the graphite leaving
proof of the mechanism. (A frequent problem with buried-pipe replacement is that the
new piece is always anodic to the older sections. The new one will rapidly corrode and
leak, and personnel will blame the material,
not knowing that the actual problem is their
lack of corrosion knowledge.)
 Crevice corrosion occurs in a small gap
between two pieces of metal. It allows a
corrosion mechanism to act in a way that's
similar to pitting corrosion. Although it's
not a common industrial mechanism, it can
happen with poor joint control on welded
assemblies.
 Intergranular corrosion involves galvanic
attack at the grain boundaries within a
metal. It's usually associated with a poor
choice in materials of construction for
chemical processes.
 Erosion corrosion is a combination of
actions. Corrosion results in an oxide on a

metal's surface. The oxide, though, slows
the attack because it prevents fresh corrodent from reaching the surface. If there's a
fast fluid flow that scrubs the oxide off the
surface, corrosion continues at a very rapid
rate. A common site for erosion corrosion
is the outer radius of piping elbows in steel
lines with untreated waters and flow rates
exceeding approximately 10 ft./sec. (3 m/sec).
It's also been seen in pumps as a result of
poor choices of construction materials.
The previous seven categories/types
are basically different-looking versions of
galvanic corrosion. Two other corrosion
types-stress-corrosion cracking and
hydrogen damage-result in metallurgical
damage leading to often hard-to-detect
catastrophic failures.
 Stress corrosion cracking (SCC) can
occur with almost any metal and is the
result of a combination of stress, a chemistry that attacks the metal's structure, and a
susceptible metal. Industrially, although it is
sometimes seen with nitrates and steel, the
most common situation involves 300 series
(austenitic) stainless steels and chlorides.
One interesting example of the interaction of stress, a sensitive material, and
attacking chemicals involved a series of
large stainless-steel vessels. The problem
stemmed from two critical design flaws: The
tanks were downwind of several cooling
towers, and their roofs were supported by
ASTM A 36 steel beams. Thermal expansion caused stresses where the beams were
welded to the roof panels, and drying of
the mist off the cooling towers increased
chloride content in the air. After about 10
years of operation, hundreds of small stress
corrosion cracks in the vessels were noted.
Consultants monitored this collection of
cracks for approximately two years and saw
little change in them. When operators increased the pressure in the tanks, however,
the number of cracks suddenly increased.
The following real-world examples are

JANUARY 2018


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