BASICS OF CORROSION - STRESS
CORROSION CRACKING (SCC)
What is SCC?
SCC is the conjoint action of stress and a corrosive environment
which leads to the formation
of a crack which would not have developed by the action of the
stress or environment alone.
Why is it a problem?
Because, it can happen ‘unexpectedly’
and rapidly after a period of satisfactory service leading to catastrophic
failure of structures or leaks in pipework.
Where does it occur typically?
Typical SCC failures are seen in pressure vessels, pipework, highly stressed components and
in systems when an excursion from normal operating conditions or
the environment occurs.
Where do the stresses come from?
The stresses that cause SCC are either produced
as a result of the use of the component in
service or residual stresses introduced during
manufacturing.
Where does the corrosive
environment come from?
The environment is either the permanent service environment i.e.
sea water or a temporary
one caused by operations such as cleaning of the system which can
leave a residue, or if the
stress is applied during the operation initiate cracking.
How is this different from ‘normal’
corrosion?
SCC is a corrosion mechanism that requires the pairing of a
material with a very particular environment and the application of a tensile stress above a critical value. Corrosion can occur
in other environments without SCC.
Examples of well-known material/environment pairs are:
MATERIAL ENVIRONMENT
Brass, Ammonia
Stainless steel, Chlorides
High strength steels, Hydrogen
How can SCC be controlled?
By selecting a material that is not
susceptible to the service environment and by ensuring that any changes to the
environment caused by cleaning etc are not detrimental.
By controlling the service stresses through careful design and minimising stress concentrations to keep them below the
critical value. Residual stresses can be reduced by heat treatments and careful
design for manufacturing.
By using corrosion inhibitors during cleaning operations or to
control the environment in a closed system
By coating the material and effectively isolating the material
from the environment.
For more detailed information read our on-line guide to SCC
NW
The wedge plate and strands had
previously been removed from the trumpet and most of the grout
had been removed from the trumpet except
for a layer about ½ inch thick which was attached to the
lower half of the inner trumpet
circumference. The upper half of the trumpet circumference was
heavily corroded throughout its length with
heavy scaling and moderate pitting. The galvanizing in
the upper circumference was completely
consumed. The overall appearance of the upper circumference
suggests that it had never been in contact with
grout and had instead been occupied by grout
bleed water, (See Fig. 2 and 3). Examination
of the wedge plate shows heavy rust scaling and pitting
on the interior surface (See Figs. 4 and
5). As shown in Figure 5, the majority of the strands failed at
a distance approximately 6 to 8 inches
from the wedge plate. Most of the strand fracture
points
displayed the classical “neck-down”
appearance consistent with tensile failure as either a result of
section reduction or by increased
stresses from adjacent failed strands. Although the heaviest corrosion
on the strands was within the trumpet
region, the strands were generally corroded to a distance
approximately five feet from the bulkhead. In all
instances, the fracture faces were heavily corroded
indicating that the actual failure had taken place
quite some time ago. Dr. Hartt
examined the failed
strands and concluded that hydrogen embrittlement played no role in the failure since the
individual
strand wires did not display
longitudinal cracks characteristic of hydrogen induced embrittlement.
The exterior face of the wedge plate
shows only superficial corrosion. The wedge bores show
evidence of corrosion pitting as shown in Fig. 6
(See wedge bore at 2 and 3 o’clock positions). The
9 o’clock to 3 o’clock
position is believed to have been the upper half of the wedge plate as
installed
since the more heavily corroded half
circumference corresponds to the corroded trumpet interior.
Note that the bores are relatively clean
from the 4 o’clock to 9 o’clock position. Referring to Fig. 4,
grout residue is still visible within the
bores from 5 o’clock to 7 o’clock. This observation further
supports the belief of how the wedge plate was
oriented, especially since the grout was still intact in
the lower circumference of the trumpet
where these three wedge bores would have resided.
BOOK:
Ferritic: Immediately after solidification, iron forms a BCC structure called d-ferrite. On further cooling, the iron transforms to a FCC structure called g, or austenite. Finally, iron transforms back to BCC structure at lower temperatures, this structure is called a or ferrite.
Martensite is a phase that forms as the result of a diffusionless solid-state transformation (athermal transformation). In steels with less than about 0.2% C, the FCC austenite transforms to a supersaturated BCC martensite structure. In higher carbon steels, the martensite reaction occurs as FCC austenite transforms to BCT(body centered tetragonal) martensite.