Efficient Plant January 2018 - 31

feature | reliability issues
representative of more common SCC
problems in plants:
+ A 500-gal. (1.9 m3) tank where
careless filling led to overflow: The
tank held a 160 F (70 C ) caustic
cleaning solution. The overflow
ran down the tank's side and
evaporated, eventually reaching a
concentration at which cracking
occurred.
+ A series of stainless-steel lines
that carried oil for process
Fig. 4. Viewed through a low-power microscope, jagged,
machinery: Surface temperature
irregular stress corrosion cracking (SCC) is readily visible
in this section of a stainless-steel oil line.
of the oil lines exceeded 140 F
(60 C). In a scenario similar to
Fig. 5. A victim of hydrogen cracking, this coil spring with a
the above example, process water
ground plain end had been supported on a cup-like frame
dripping on the hot lines caused
that was frequently wet.
cracking. Figure 4 (right) shows
a view through a low-power
microscope of one 1/2-in.-dia.
(12-mm-dia.) line. A jagged and
irregular stress corrosion crack is
readily visible, as are a number of
pitted areas.
 Hydrogen damage is a particularly diabolical form of attack because
it is often impossible to see prior to
catastrophic failure. It starts with the
corrosion reaction reflected in the
image of the railroad-bridge-support
column in Fig. 1, and can go on to cause
hydrogen embrittlement in metals such
as titanium, blisters in low-carbon steel,
and cracking in hardened steels.
Hydrogen blisters and cracking
appear to be similar mechanisms in
that the hydrogen ions are so incredibly small they can float through the atomic
components and cracking in hardened ones.
structure of steel until they find an irreguNACE advises that hydrogen cracking can
larity. Because of the void, the ion tends to
occur in steel with hardness values as low as
rest there until another one comes along
HRC 23, if sulfur compounds are present.
and the two combine to form a hydroIt's even been seen in leaking steam-line
gen-gas molecule, which has a much larger
joints with cracks in flange bolts that were
volume than the single ion. The result is
HRC 27. Rockwell hardness [HRC] readan internal pressure that, when an external
ings are one of the ways of measuring the
stress is applied, causes blisters in mild-steel
hardness and strength of a bolt. SAE Grade

JANUARY 2018

5 and ASTM A 325 bolts are commonly
in the range of HRC 25 to HRC 34. SAE
Grade 8, U.S. socket-head cap screws
and ASTM A 490 bolts typically range
from HRC 33 to HRC 39. Because of
their lower ductility, these high-strength
components are much more susceptible to hydrogen cracking in corrosive
environments. A cardinal rule of good
reliability is to not use high-strength bolts
in such areas.
Although hardened bolts are among
the most common examples of hydrogen
cracking, leaf and coil springs are other
frequent victims. The coil spring shown
in Fig. 5 (left) had a ground plain end
supported on a cup-like frame that was
frequently wet. The corrosion is obvious.

KEEP IN MIND
The battle against corrosion is never
ending. In summary, if an area is wet and
metal isn't protected, there will be corrosion. What's worse, the seriousness of the
damage caused by this scourge may not be
recognized for years. EP
Neville Sachs has spent many years working
in the field of machinery reliability and lubrication for a wide range of industries. The
author of two books on failure analysis and
a contributor to others, he has written more
than 40 articles on these topics. A Registered
Professional Engineer, he holds STLE's CLS
certification, among others. Contact him
directly at nevsachseng@gmail.com.

LEARN MORE
To learn more about preventing/managing
corrosion, visit:
nace.org/resources
corrosion-doctors.org
(This resource operates out of Cana-

da's Royal Military College, Kingston,
Ontario.)

EFFICIENTPLANTMAG.COM |

31
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Table of Contents for the Digital Edition of Efficient Plant January 2018
