Lyons W.C. (ed.). Standard handbook of petroleum and natural gas engineering.2001- Volume 1
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Corrosion and Scaling
1275
Grain boundaries
corroded
Figure
4-428.
Intergranular corrosion
of
sensitized (improperly annealed)
stainless steel.
(From
Ref.
[185].)
-rc---
Woter
flow
Oriqinol
metal
Figure
4-429.
Erosion corrosion.
(From
Ref.
[183].)
Use proper coatings.
Alter environment-add inhibitors, remove abrasives as soon as possible.
Use cathodic protection wherever possible. This does not affect the erosion
part of the attack, but it may reduce the corrosive attack.
Cavitation Corrosion
Cavitation is a special form of erosion corrosion results from formation and
collapse of vapor bubbles in a liquid near a metal surface. Collapsing bubbles
destroy the protective surface film, thereby exposing the metal to increased
corrosion attack. Cavitation corrosion is shown schematically in Figure
4-430.
Cavitation damage
is
often seen in high-velocity, turbulent, liquid-flow areas such
as drilling-mud pumps. To reduce problems associated with this kind of corro-
sion, methods described for erosion corrosion should be considered. Also
materials with high hardness and strength with tenacious passive films should
be used. Titanium and corrosion-resistant, cobalt-base alloys appear
to
be suitable
for a wide range of environments causing cavitation erosion
[184].
Ringworm Corrosion
During hot-forming or upsetting operations the steel is subjected to large
temperature gradients. These thermal gradients cause variations in metallurgical
composition along the pipe.
In
the sections exposed to intermediate tem-
peratures, the iron carbides condense, forming spheroids. During exposure
to
corrosive environment this narrow section corrodes preferentially, as shown in
1276
Drilling and Well Completions
A
bubble
collapses
and
a
liquid
jet
results
Figure
4-430.
Cavitation
corrosion.
(From
Ref.
[185].)
Figure
4-431.
This type of attack can be completely eliminated by annealing the
entire length of the pipe after forging. The annealing results in a uniform
metallurgical structure throughout the length
of
the pipe
[
184,1861.
Concentration Cells
Concentration cells have two similar electrodes in contact with a solution of
differing composition. The two kinds of concentration cells are salt concen-
tration cells and differential aeration cells
[
1861.
Salt Concentration Cells.
In this type of cell the two electrodes are of the same
metal (Le., copper). These electrodes are immersed completely in electrolytes
of the same salt solution (Le., copper sulfate) but of different concentrations.
When the cell is short circuited, the electrodes (anode) exposed to the dilute
solution will dissolve into the solution and plate the electrode (cathode) exposed
to the more concen-trated solution. These reactions will continue until the
solutions are of the same concentration. Figure
4-432
shows a schematic of a
salt concentration cell.
Differential Aeration Cells.
This type of concentration cell is more important
in practice than is the salt concentration cell. The cell may be made from two
electrodes of the same metal (i.e., iron), immersed completely in dilute sodium
chloride solution (Figure
4-433).
The electrolyte around one electrode (cathode)
is thoroughly aerated by bubbling air. Simultaneously the electrolyte around the
other electrode is deaerated by bubbling nitrogen. The difference in oxygen
concentration causes a difference in potential. This, in turn, initiates the
flow of current. This type of cell exists in several forms. Some of them are as
follows
[
1881.
Crevice Corrosion.
Crevices are formed at the interface of two coupled pipes,
near the ends of drillpipe protectors, at threaded connections and where surface
Corrosion and Scaling
1277
Figure
4-431.
“Ringworm” corrosion-at the inside
of
the tubing where grain
structure near edge
of
upset portion makes metal susceptible to rapid
corrosion.
(From
Ref.
[219].)
I
Dil.
CuSO4
I
Conc.
CuSO4
I
Figure
4-432.
Salt concentration cell.
(From
Ref.
[186].)
1278
Drilling and Well Completions
deposits create stagnant conditions. Oxygen may be consumed at
a
much faster
rate inside the crevice than it can diffuse into the crevice. This lowers the pH
in the crevice and creates acidic conditions, which, in turn, increases the
corrosion rate. Figure
4-434
illustrates the concept schematically.
Oxygen
Tubercles.
Similar to crevice attack, it is encouraged by the deposit
of a layer, semipermeable to oxygen (porous layer of iron oxide or hydroxide).
Air
I
I
Dil.
NaCl
I
Dit.
NaCI
Figure
4-433.
Differential aeration cell.
(From
Ref.
[186].)
Figure
4-434.
Differential aeration cell illustrated
by
waterline corrosion.
(From
Ref.
[l85].)
Corrosion and Scaling
1279
The deposit partially shields the steel surface, creating a differential aeration
cell (Figure
4-435).
Air-Water Interface.
This is another good example of a differential aeration
cell (Figure
4-436).
Here the water at the surface contains more oxygen than
the water slightly below the surface. This difference in concentration can cause
preferential attack just below the waterline.
Scale Deposits
Scale deposits create conditions for concentration-cell corrosion as they do
not form uniformly over the metal surface. Sulfate-reducing bacteria thrive under
these deposits, producing hydrogen sulfide and, consequently, increasing the rate
of
corrosion. Due to the following factors, the drilling fluid environment is ideal
for
scale deposition
[189].
These factors are as follows:
high pH levels
turbulent flow
pressure variations
temperature changes
Figure
4-435. Differential aeration cell formed
by rust
on iron.
(From
Ref.
[184].)
Dilute concentration
High
concentration
and
high oxygen
con tent depletion
of
oxygen
of
metal
ions and
Figure
4-436. Crevice corrosion.
(From
Ref.
[186].)
1280
Drilling and Well Completions
dissolution of formation salts
influx of carbon dioxide
influx of low-pH, high-total-dissolved-solid content and formation fluids
evaporation
Calcium carbonate (CaCO,) calcium sulfate or gypsum (CaSO,) and iron(I1)
carbonate (FeCO,) are the most common types of scales formed in drilling.
If hydrogen sulfide is present, then there is a possibility of iron sulfide (FeS)
scale depositing.
From the corrosion point of view, it is very important to control the deposition
of scale. Removal of deposited scale by mechanical means is the first step. Standard,
industrial water-treating techniques can be used to control scale deposition in
general. In deep, hot wells or geothermal wells it is best to avoid untreated makeup
water (i.e., geothermal brines).
To prevent or minimize problems associated with concentration cells, the
following methods should be considered in general:
Welded butt joints should be used instead of bolted or riveted ones.
Existing crevices should be closed by soldering, welding
or
caulking.
Stagnant conditions should be avoided.
“Solid” nonabsorbent gaskets such as Teflon should be used when necessary.
Proper use of appropriate inhibitors.
Maintain regular inspections of equipment and treatment status.
Drillpipe protectors should be moved to different locations at each trip.
Hydrogen Damage
In general corrosion, hydrogen ions
(H)
are reduced to atomic hydrogen
(HO).
These hydrogen atoms combine with each other and form molecular hydrogen
[
186,190,191].
2H’
+
2e-
+
2H0
+
H,
t
However, certain substances such as sulfide ions, phosphorus and arsenic
compounds reduce the rate at which hydrogen combines to form molecules.
Atomic hydrogen can diffuse through metal matrix and cause mechanical
damage. Therefore, hydrogen damage is a mechanical damage of metal caused
by the presence of atomic hydrogen. Some of the forms of hydrogen damage
are as follows.
Hydrogen Blistering.
Penetration of hydrogen in low-strength steel with
any discontinuities in the steel such as laminations, inclusions or voids may
result in hydrogen blistering. Hydrogen produced on the surface of the metal
evolves in part as gas bubbles and the rest diffuses through the metal. The
different hydrogen atoms collect in the void, and combine to form hydrogen
gas molecules. Hydrogen gas molecules are too large to diffuse back through
the metal. Thus, the gas concentration and pressure continue to increase within
the void. In time, the gas pressure increases to sufficient levels to burst out to
the metal surface. Figure
4-437
schematically illustrates the mechanism of
hydrogen blistering
[
1831.
Corrosion and Scaling
1281
Electrolyte
H+
H+
H
H
$.
c
H+H,+H
Air
Figure
4-437.
The simplified mechanism
of
hydrogen damage.
(From
Ref.
[797].)
Hydrogen Embrittiement.
Hydrogen embrittlement is a form of mechanical
damage of a high-strength steel in a brittle failure. Limited to high-strength steels,
the mechanism and cause are similar to those which cause hydrogen blistering.
Hydrogen gas is trapped within the metal lattice, causing
loss
in ductility of
the metal. Once ductility is reduced or lost, and the metal is subjected to high
tensile stress, the metal will fail in a brittle manner. The path of the failure
will be transgranular when the process takes place at normal temperatures when
hydrogen diffuses through the grain, and gathers at inclusions, voids, or any
other lattice defects. The failure is said to be integranular when the temperatures
are higher and the absorbed hydrogen gathers at the grain boundaries [183,184].
To prevent or minimize hydrogen blistering and embrittlement, the following
methods should be considered:
Proper use of appropriate corrosion inhibitors.
Using suitable, more-resistant alloys for the environment.
Practice proper welding techniques.
Apply suitable coatings in a proper manner.
Select steels with minimum voids or discontinuities.
Remove “poisons” such as sulfide ions from the environment.
Hydrogen Sulflde Cracklng.
Known also as sulfide stress cracking, the exact
mechanism of this form of hydrogen damage is not yet fully understood. Thus,
some experts classify it as a form of hydrogen embrittlement. Nevertheless, for
hydrogen sulfide cracking of steel, the following conditions should be present
[
186,1921:
1282 Drilling and Well Completions
Hydrogen sulfide gas, as low as one ppm.
Even traces of water are sufficient.
High-strength steel (i.e., yield strength above
90,000
psi or Rc
20
to
22).
The steel must be under residual or applied tensile stress.
With the above conditions present, hydrogen sulfide stress cracking may take
anywhere from a couple of hours to years to occur. Figure 4-438 illustrates some
examples of typical hydrogen sulfide cracking failure, with a characteristic brittle
fracture and final ductile-shear lip. Hydrogen sulfide stress cracking occurs on
most nonferrous materials, for example aluminum alloys. Hydrogen sulfide stress
cracking can occur if appropriate conditions such as high residual
or
applied
tensile stress, corrosive environment and susceptible metallurgy exist simultaneously.
To prevent
or
reduce the possibilities of problems associated with hydrogen
sulfide cracking the following measures should be considered
[
1841:
Strength: steels with yield strength of
90,000
psi or less and Rc
22
hardness
or less should be used.
As
the strength goes up, the time to failure goes
down. This can be seen in Figure
4-439.
Reduce applied or residual stress levels.
Reduce hydrogen sulfide concentration levels.
Increase pH levels. Figure 4-40 shows that steel fails much less rapidly when
the pH is above
7
or
8.
As
the temperature goes above 150°F
(66’C),
the susceptibility to cracking
decreases.
Stress
Corrosion
Stress-corrosion cracking occurs when a metal is under constant tensile stress
and exposed simultaneously to a corrosive environment. The source of this stress
can be external (caused by slip
or
tong notch, etc., on drillpipes, collars and
tool joints, and the weight of the drill stem) or it can be residual in the metal
from heat treatment or cold working. The area damaged by the slips becomes
stressed and undergoes accelerated corrosion as it becomes anodic to the rest
of the unstressed area of the pipe. The appearance of damage is of a brittle
mechanical fracture, but it results from local corrosion attack. The cracks are
intergranular
or
transgranular depending on the metal structure and the
corrosive environment. Most structural metals and alloys are susceptible to this
form of attack in specific environments. Cracks generally develop perpendicular
to the applied stresses. They are randomly oriented and can vary in degree of
branching depending on the metal structure and composition, the nature
of
stress and the corrosive environment. Cracks vary from being totally branchless
to extremely branched like the “river delta.”
As
the stress increases, the time
before failure decreases. It is speculated that there is a minimum stress required
to prevent cracking, depending on the alloy composition, environmental tem-
perature and composition. The minimum stress may vary from 10% of the yield
stress to
70%
of the yield stress. Thus, for each alloy-environment combination
there is minimum or threshold stress [185].
It is very important to have some idea of the service life of a component.
As
stress corrosion proceeds, the cross-sectional area is reduced, resulting in a
cracking failure due to mechanical action. Figure 4-441 shows the rate of
cracking as a function of crack depth for a specimen subjected to constant
tensile load. At first, the crack propagation rate is roughly constant; but as
cracking progresses, the cross-sectional area decreases. Eventually, the rate of
Corrosion and Scaling
1283
..
..,
.
I
..
.
,
..
.
I
(a)
Figure
4-438.
SSC
failure, drillpipe
(a);
tubing failure (b); coupling
(c).
(From
Refs.
[I
84,2
181.)
I-
O
I
2
C
V
5
-0
1
8
(I
OI