A case study of Stress Corrosion Cracking (SCC) of pipeline steels has been discussed.

1. Stress Corrosion Cracking of
Pipeline Steels
Term paper for the course of
CORROSION AND ENVIRONMENTAL DEGRADATION OF MATERIALS
IIT KHARAGPUR
METALLURGICAL AND MATERIALS ENGINEERING

2. Introduction
Stress corrosion cracking has been attributed to many engineering failures, some of which
have resulted in loss of life and others in significant economic losses. Corrosion and stress
corrosion mechanisms are the most frequent causes of pipelines disasters, they cause from
15% to 20% of failures of gas pipelines. Stress corrosion cracking is a dangerous, often
discussed mechanism, probably the most complicated from the point of view of prevention
and safety.
Especially complex mechanisms including combinations of mechanical fatigue and corrosion
are the main features of SCC. Numerous factors affect the process: fracture resistance of a
material, its composition, microstructure and inhomogeneity, quality of the surface
affecting the initiation period, service conditions (global and local stresses), quality of
insulation etc. Natural barriers against the fracture process are reduced or destroyed by the
corrosion. The process significantly differs from those of mechanical fatigue in the air as well
as surface corrosion.The main reasons why stress corrosion cracking is complicated can be
summarized as follows:
SCC damage cannot be detected by usual internal inspection methods indicating
changes of walls thickness.
Cracks, creating usually networks, are very thin.
Pipelines pressure fluctuations, destroying surface oxide films, accelerate the SCC
process, particularly in basic environments.
Various SCC mechanisms can occur in environments of different acidity including
diluted ground solutions (preferably transgranual SCCTGSCC), in a big range of
potentials including cathodic polarization.
Mechanism
There are two forms of SCC penetrating from the external surface of buried pipelines. One is
intergranular SCC (IGSCC) and is usually called the “high pH SCC” or “classical SCC”. The
other is transgranular SCC (TGSCC), and is designated “near-neutral pH SCC” or “low pH
SCC” or “non-classical SCC”.
Intergranular stress corrosion cracking of high pressure gas pipelines occurs most commonly
as a result of hoop (circumferential) stresses due to internal operating pressures and results
in longitudinally orientated cracks. Stress corrosion testing of pipelines, usually on long test
pieces, is most commonly performed in the axial direction of the pipe. The primary
corrosion mitigation of the external surface of high pressure steel gas pipelines is protective
coatings with secondary protection usually by cathodic protection. Adhesion and resistance
to cathodicdisbondment of the coating is critical for its integrity and grit blasting is an
important process in achieving this adhesion.

3. High pH SCC and near-neutral pH SCC of pipelines
There are many similarities between the two forms of pipeline SCC. Cracks of both forms
usually occur on the outside surface in colonies, mostly oriented longitudinally along the
pipe, primarily at the bottom of the pipeline. These cracks coalescence to form long shallow
flaws, that can lead to ruptures. The fracture surfaces are usually covered with black
magnetite film or an iron carbonate film. However, there are many differences between the
two forms of pipeline SCC.
High pH SCC, engendered by concentrated bicarbonate or carbonate-bicarbonate solutions
associated with pH of 9, has usually an intergranular morphology, and the cracks are sharp,
with little lateral corrosion. Near-neutral pH SCC, engendered by dilute ground water with a
relatively low pH of around 6.5, has a trangranular, qua quasi-cleavage crack morphology
with very little branching. The transgranular cracks are generally wide with appreciable
lateral corrosion of the crack sides. Moreover, the near-neutral pH SCC occurs over a wider
potential range than high pH SCC which has only narrow width of no more than 100 mV.
Figure 1:Example of SCC on gas pipeline in acid
environment.
Figure 2:Potential-pH diagram showing the regimes for
IGSCC and TGSCC at 24◦C in solutions containing
different amounts of CO32−,HCO−3and CO2to achieve
different pH values
SCC requires the simultaneous action of the following three factors:
potent environment at the pipe surface
susceptible pipe material
stress
If any of these can be eliminated or reduced, then SCC can be prevented.
Environmental conditions
SCC failures have been mostly associated with high electrical resistivity tape coatings. The
composition of the groundwater solution depends on the amount of cathodic protection
current reaching the pipe surface. The ground water is not be changed if the coatingdoes
not allow the cathodic protection current to pass through, or if there is high electrical
resistances within the soil or the solution in the crevice between the pipeline surface and
the coating, or if there is no significant cathodic protection current reaching the

4. exposedsurfaces. The natural ground water solution has a pH from 6 to 7 resulting from the
equilibrium between HCO3-and CO32−. This solution can cause TGSCC.
However, a substantial cathodic current at thepipeline surface causes hydroxyl ions to be
generatedand accumulated, and the pH increases according toreaction:
O2 + 2H2O + 4e = 4OH−
The solution chemistry also relates to the conversionof bicarbonate to carbonate ions. With
time, the solutionbecomes concentrated and the concentration of carbonate is high, which
leads to the tendency for thesolution to passivate the steel surface, and IGSCC canoccur.
There is a difference of the polarization curve measuredin the high pH solution and the
near-neutral solution.In the high pH solution, the curve exhibits anactive-passive transition
over a certain potential rangeas illustrated in Fig. 3 from the work of Parkins.This transition
has been shown to be associated withIGSCCof ferritic steels in various environments.
Incontrast, the near-neutral pH environments do not promotepassivation and do not exhibit
an active-passivetransition (Fig. 4).
Figure 3:Potentiodynamic polarization curves showing
the potential range for IGSCC in concentrated
carbonate bicarbonate solution at 90◦C.
Figure 4:Fast and slow sweep rate polarization curves at
24◦C for a linepipe steel in simulated ground water
saturated with CO2, pH=5.8.
Metallurgical conditions
Asahi showed that, for a range of pipeline
steels from X52 to X80 grades,
thermomechanical controlled processing or
quenched and tempered steels with finegrained bainitic structures, or acicular
ferrite, uniform microstructures, were
more resistant to IGSCC than controlled
rolled steels with ferrite-pearlite structures.
It is well established that the mill scaled
Figure 5:SCC in zones with different microstructure in heat
affected zone of X60 steel pre-strained to 1% plastic
deformation.

5. surfaces on pipeline steels are more susceptible to SCCthan polished surfaces.
Moreover, if appropriately applied, grit blastingleaves the pipe surface in a state of
compression thatis beneficial in at least delaying, if not preventing, theincidence of SCC in a
variety of system.
Mechanical conditions
In order to propagate for SCC cracks, there must be an appropriate stress at the crack tip.
Beavers found that pressure fluctuations may be necessary for cracks to occur not only with
near-neutral pH SCC but also with high pH SCC. Parkins showed that cyclic loading
significantly decreased the threshold stress for IGSCC below that associated with a static
load.
The effect of surface roughness, from grit blasting, on the intergranular stress corrosion
cracking resistance of X70 gas pipelines was investigated using slow strain rate testing in
carbonate/bicarbonate solution at 75 °C.
Time to failure ratios decreased
with increasing surface roughness
indicating reduced stress corrosion
cracking resistance. The reduced
resistance
to
cracking
with
increasing roughness would be
predominantly associated with
stress concentration effects related
to the surface roughness resulting
from the grit blasting. Crack
concentration
decreased
with
increasing roughness, which is likely
to be associated with the
concentration of surface damage from the grit blasting using varying sized grit. The stress
concentration factors associated with the roughened surfaces may be similarto corrosion
pits, where Beavers et al. stated that 0.25 mm wide pits, 0.65 μm deep, had a stress
concentration of approximately 2.1.
Figure 6:Effect of roughness, Ra, on time to failure for all grit blasted
samples. Note the surface roughness is not relevant to the as formed
samples.
Discussion
The reduced resistance to stress corrosion cracking with increasing surface roughness was
likely to be associated with the stress concentration effect of the grit blasted
surfaces.Surface structural inhomogeneities, either different microstructure zones in the
steel or sulphide and other inclusions were priority initiation sites of microcracks.

6. Both stress concentration and compressive residual stresses from grit blasting are likely to
contribute to stress corrosion cracking behaviour. As formed pipe surfaces, with no grit
blasting, resulted in some of the lowest time to failure ratios and hence some of the lowest
resistances to stress corrosion cracking. These also showed some of the deepest cracks.
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