1. Finished pipes coated with Modumetal’s nanolayer coating technology. The Seattle-based company’s coatings have been shown to slow
corrosion rates for metal components, including those used in offshore applications. Photo courtesy of Modumetal.
Stephen Whitfield, Oil and Gas Facilities Staff Writer
Improving
CORROSION INHIBITION
Fluids andTechniques
in Offshore Projects

2. June 2015 • Oil and Gas Facilities 29
F
or offshore exploration and production projects,
corrosion is an accepted occupational hazard
because there is no way to stop it completely. Over
time, it will negatively affect the life of an asset,
sometimes to a significant degree.
Inhibitor fluids are widely accepted as an optimal
method for slowing corrosion rates and mitigating its
damaging effects on a system. Nevertheless, operators and
service companies are continuing to find new ways to better
the process.
This feature takes a look at the work being done by
companies to improve corrosion inhibitor fluids and
inhibition techniques for offshore projects.
Clariant: Phosphate Ester-Based
Inhibitor
As there is no technological answer to why oxygen, which
plays an important role in the corrosion process, is present
in a system, most oilfield corrosion inhibitors are developed
under the presumption that no oxygen will be present. This
means that most inhibitors perform poorly in solutions
containing oxygen such as water (Wang and Wylde 2011).
Typically, additional chemicals must be used to handle the
increase in corrosion that comes from injected oxygen, with
phosophate esters being among the most popular
(Yepez et al. 2015).
Last year, the oil service unit of Clariant, a specialty
chemical company, tested the performance of a phosphate
ester-based inhibitor against a commercially available
imidazoline-based oxygen corrosion inhibitor at its
laboratory in The Woodlands, Texas. The company wanted
to see if the phosphate ester could limit the amount of
additional chemicals typically used with traditional corrosion
inhibitors. It used a potentiodynamic polarization technique
in the presence of oxygen and carbon dioxide and conducted
open-circuit corrosion experiments to examine the
electrochemical behavior of each inhibitor.
The experiments used a brine containing 3.5% sodium
chloride (NaCl), and the corroding gas was a mixture of 3%
oxygen (O2) and carbon dioxide (CO2). Another experiment
using 100% CO2 gas was also performed. The metal used in
the experiments was a carbon steel.
Clariant conducted rotating cylinder electrode tests
under the following conditions:
• Condition 1: 3% O2 balanced with CO2 gas in
3.5% NaCl
• Condition 2: 100% CO2 gas in 35% NaCl
• Condition 3: Inhibitor B (imidazoline-based
commercial oxygen corrosion inhibitor) in 3.5% NaCl
plus 3% O2 balanced with CO2 gas
• Condition 4: Inhibitor A (phosphate ester-based
corrosion inhibitor plus sulfiding agent 2) in 3.5% NaCl
plus 3% O2 balanced with CO2 gas
• Condition 5: Sulfiding agent in 3.5% NaCl plus 3% O2
balanced with CO2 gas
• Condition 6: Phosphate ester plus sulfiding agent 0 in
3.5% NaCl plus 3% O2 balanced with CO2 gas
• Condition 7: Phosphate ester plus sulfiding agent 4 in
3.5% NaCl plus 3% O2 balanced with CO2 gas
The sulfiding agent was systematically varied in
the phosphate ester for some of the given conditions.
The injection of a different inhibitor into the solution
from condition 1 produced conditions 3 through 7. The
corrosion rate for each condition was measured from
the intersection of the anodic and cathodic Tafel slopes
(Table 1). A Tafel equation determines the rate of an
electrode’s electrochemical reaction to its overpotential.
The degree of corrosion inhibition in the experiments
was related more to the inhibitors’ anodic protection, where
the chemical converts the electrons of the metal surface
into the anodes of electrochemical cells, vs. their cathodic
performance, where the electrons of the metal surface
become the cathodes of electrochemical cells. Omar Yepez,
a senior integrity management chemist at Clariant, said that
this relationship was because of the phosphate ester-based
inhibitor's effect on the the anodic current, as shown
in Fig. 1.
“In the cathodic branch of the curve, the imidazoline-
based inhibitor is more or less similar to the phosphate
ester-based inhibitor, whereas in the anodic branch of the
curve, the phosphate ester-base inhibitor reduced the anodic
current at least 10 times more than the imidazoline-based
one,” he said.
Table 1—Measured corrosion rates under select conditions in Clariant’s rotating cylinder electrode test.
Condition Corrosion Potential (mV [SCE]) Corrosion Rate (mil/yr) Inhibition (%)
3% O2+CO2 –636 169 0.0
Inhibitor B –579 59 65.2
Sulfiding agent –557 80 52.6
Phosphate ester –533 13 92.6
Inhibitor A –532 11 93.2
Phosphate ester +
sulfiding agent 4
–531 11 93.7
Source: Yepez et al. 2015

3. 30 Oil and Gas Facilities • June 2015
Yepez also said he was surprised to see similar corrosion
potentials for conditions 1 and 2, even though the cathodic
and anodic currents for condition 1 were higher than those
for condition 2.
“I was expecting something near 120 mV (SCE), but
I measured –636 mV instead,” he said. “This is near the
corrosion potential of carbonic acid corrosion, which is
–638 mV. This happened because the steel surface is not iron,
in which case I would have measured 120 mV (SCE). The
steel surface was iron oxide.”
A saturated calomel electrode (SCE) is a
reference electrode.
The measured corrosion potential of carbonic acid
corrosion was close to the standard potential of reactions in
conditions 3 to 5, which was approximately -689 mV (SCE),
but the measured corrosion potential for oxygen corrosion
(–636 mV) differed significantly from the standard potential
of reactions in conditions 2 to 4 (120 mV). As the presence of
oxygen changes the steel surface potential, it was difficult to
measure oxygen corrosion potentials (Yepez et al. 2015).
Imidazoline adsorbs under carbonic acid corrosion,
which means no lateral reaction between adsorbed molecules
occurs. It will be protonated at the pH of a given solution,
meaning that a proton will be added to each molecule to
form the conjugate acid in the solution. The protonated
imidazoline heads will be attracted by the steel surface
because the cathodic reaction makes them negatively
charged. This also prevents water from reaching the steel
surface, a process known as geometric blocking.
Because phosphate strongly adsorbs on oxidized iron
surfaces, the phosphate ester-based inhibitor presented a
higher inhibition of the cathodic current compared with
Inhibitor A
–550
–600
–650
–700
–750
–800
–850
–2.0 –1.5 –1.0
ElectrodePotentialvs.SaturatedCalomelElectrode(mV)
Corrosion Resistance From Intersect (log[l/mAcm–2
])
–0.5 0.5 1.00.0
Inhibitor B
3% O2
+ CO2
Source: Yepez et al. 2015
Fig. 1—The polarization curves of the carbon steel rotating
cylinder electrode test at 6,000 rev/min show the degree to which
a phosphate ester-based inhibitor affects the anodic current.

4. June 2015 • Oil and Gas Facilities 31
imidazoline in the experiment, Yepez said. This produces a
film on the steel surface that prevents water from reaching it,
a process known as geometric coverage.
The study concluded that imidazoline acts as a good
anodic inhibitor because the amidoamine compounds,
a byproduct of imidazoline industrial production, act as
a tridentate ligand that anchors ferric cations. However,
Yepez said that phosphate esters are better inhibitors than
imidazoline-based agents because the sulfiding agent
prepares the steel surface for a significantly stronger
interaction with the inhibitor, which allows the phosphate
esters to perform well in anodic and cathodic conditions.
“The phosphate ester needs an oxidized surface, and the
presence of oxygen does not guarantee that the entire surface
will be oxidized,” he said. “The sulfiding agent helps this to
happen, thus the phosphate ester made for a better coverage
of the (steel) surface.”
Baker Hughes:
Antiagglomerant Chemistry
In deepwater systems, operators often use low-dosage hydrate
inhibitors (LDHIs) as an alternative to thermodynamic
inhibitors, such as methanol or monoethylene glycol, to
manage hydrates because they control agglomeration at
lower dosages, thus requiring smaller storage capacities
and creating fewer logistical challenges (Panchalingam
et al. 2015). Antiagglomerants (AAs) are a popular type of
LDHI since they allow hydrates to form while still limiting
the growth and buildup of hydrate crystals. However, some
AAs present a risk of pitting corrosion for offshore umbilical
chemical delivery systems.
Baker Hughes tested these pitting tendencies on stainless
and duplex steels in its attempt to develop an inhibitor
that would address the problem. From 2013 to 2015, the
company ran the electrochemical test method of cyclic
potentiodynamic polarization (CPP) and an immersion test
of stainless and duplex steels in solutions containing various
AA formulations.
LDHIs must meet stringent criteria before they are
injected into the umbilical lines of deepwater applications.
Two of the most important criteria are the viscosity of
the formulated products and the compatibility with the
umbilical’s material of construction.
Vaithilingam Panchalingam, a research scientist
of products and technology at Baker Hughes, said the
specifications are often set by customers because meeting
the deliverability requirements is particularly important in
long-reach subsea tiebacks. Chemical compatibility with
the umbilical’s material of construction is important to the
overall integrity of the system.
A third crucial element of subsea chemical injection is
the overall stability of the chemical, said Panchalingam, a
coauthor of the paper detailing the study.
“If the product precipitates at a low temperature, it can
deposit along the umbilical or settle to the bottom, which can
lead to plugging,” he said. “If the material decomposes at high
injection temperatures and forms materials, it can again plug
the umbilical.”
AAs are typically injected at a high concentration range
over a long distance, which makes the pitting corrosion
risk in the umbilical a significant challenge for operators.
To eliminate corrosion risk entirely, a possible solution
is to switch from a methanol-based solvent package to
a nonalcohol-based package, However, nonmethanol
solvents create a high viscosity in the AA formulation,
which negatively affects the applicability for a deepwater
system (Panchalingam et al. 2015).
Table 2 lists the AAs used in the experiments. The
researcher said that AA-B was more valuable to the
experiments because it is based on improved technology,
thus leading to better treatment efficiency than AA-A.
The corrosion inhibitor used in the test is an additive that
is commercially available from Baker Hughes. The study
referred to this additive as additive C.
The CPP test showed that additive C changed the
open circuit potential on the stainless steel to a more
cathodic value, and the protection margin increased (Fig. 2),
which suggests that the additive reduced the tendency
of pitting for the antiagglomerant formulation. Positive
hysteresis happened in CPP tests with and without the
corrosion inhibitor.
For the duplex steel, the CPP tests also showed positive
hysteresis with and without the inhibitor. However, the
current increased in the passivation region up to the pitting
potential, while the stainless steel experienced no change
in current. This meant that the duplex steel was a more
active material.
The immersion tests showed no signs of pitting on the
stainless steel after 30 days, 60 days, and 90 days. However,
the duplex steel showed pitting, indicating that corrosion
was not stopped under the given test conditions. Increasing
the inhibitor volume from 1.5% to 3% produced some
Inhibitor and
Antiagglomerant
Solvents
A1 Methanol
A2 Toluene
A3 Isopropyl alcohol/toluene
B1 Methanol/toluene/water
B2 Isopropyl alcohol/toluene/water
B3 Isopropyl alcohol/toluene/water
Corrosion inhibitor Water
Source: Panchalingam et al. 2015
Table 2—Descriptions of the antiagglomerants, corrosion
inhibitors, and solvents used in Baker Hughes’ cyclic
potentiodynamic polarization tests.

5. 32 Oil and Gas Facilities • June 2015
improvements. The duplex steel experienced no pitting after
30 days, but pitting occurred after 60 days.
To fix the problem, a new solvent package was created
with toluene and isopropyl alcohol as the solvents. The blend
protected the duplex steel against pitting for AA-A, but it did
not show any protection for AA-B.
Panchalingam said the water in AA-B was the
primary reason for the lack of corrosion protection on
the duplex steel because water solvates the chloride,
which tends to promote corrosive behavior. The
solvent switch from methanol to toluene confirmed the
observation. Results of a CPP test of AA-B made with
the least amount of water showed pitting protection for
the duplex steel with no additional corrosion inhibitor,
while a test on the stainless steel showed that a lower
volume of 0.5% corrosion inhibitor was needed to protect
against pitting.
The tests showed that adding the corrosion inhibitor
and switching the solvent caused no adverse effect on the
performance of the AA.
Modumetal: Nanolaminated Alloys
Used in Coatings
The search for better corrosion protection does not only
involve developing fluids. A company, Modumetal, is
working on nanolaminated alloys that could potentially
lessen the need for operators to use inhibitor fluid in
offshore facilities.
The company uses an electrochemical process to coat
metal components. Similar to electroplating, this process
involves the immersion of metals in a bath containing
different types of metal ions to create a thin layer of coating.
The process is repeated several times by varying the electrical
current in the bath to control the way the ions are deposited,
with each layer building on top of another and up to a desired
overall thickness (Bullis 2015).
Christina Lomasney, president and CEO of Modumetal,
compared the nanolayers to the rings found in the cross
section of a tree, which are formed by changes in the tree’s
surrounding environment. Putting two dissimilar metals
together leads to a greater control of the nature of the
interface between them, and this control improves in the
performance of an alloy, she said.
“We’re leveraging that phenomenon,” Lomasney
said. “We’re controlling it. We’re putting dissimilar
metals together with a little bit of potential difference.
It’s not quite as dramatic a difference as, say, steel versus
zinc, but if we create a little potential difference, we
find that we can create zinc that performs better than a
homogeneous zinc.”
Nanolayer coatings may help with extending the life
of an asset by strengthening the metal components used in
facilities. Another benefit of the technology is the protection
against corrosion, as the nanoscale layers delay the exchange
of electrons in electrochemical reactions that leads to
corrosion. While nanoscale coating is not a direct alternative
to the injection of corrosion inhibitors, the protection it
provides could allow operators to use less inhibitor fluid over
time, potentially saving costs.
“You use a corrosion inhibitor to prevent that kind of
electron exchange,” Lomasney said. “Now, if you don’t have
that in the first place, or if you have a coating that’s providing
that kind of protection, then you don’t need as much
chemical to provide that protection as well, and so the whole
idea is not only to reduce the cost of corrosion inhibition, but
also to reduce the cost of the asset itself.”
The company tested one of its latest nanolaminated
materials, the NanoGalv zinc-based coating, for corrosion
in several settings. Modumetal performed a salt-fog
performance test comparing its technology with hot
dipped galvanized steel panels. The tests were carried
out until more than 5% of the surface of the panels was
covered in red rust or 4,300 hours had passed, whichever
came first.
The hot dipped galvanized steel panels showed rust after
744 hours of testing, while no rust formed on the NanoGalv
panels. Another salt-fog test of studs and nuts stressed to
10% yield strength produced similar results. The NanoGalv-
coated metals showed no signs of corrosion after 240 hours of
exposure (Lomasney et al. 2015).
These coatings were also tested in offshore field trials
conducted with ConocoPhillips and the United States Coast
Guard. Lomasney said the technology is suited for harsh
offshore environments, particularly in warmer climates.
With inhibitor
Without inhibitor
–175
–200
–225
–250
–275
–300
–325
0 2,000 4,000
Time (seconds)
ElectrodePotential(mV)
6,000
Source: Panchalingam et al. 2015
Fig. 2—Baker Hughes’ cyclic potentiodynamic polarization
test showed that additive C increased the protection margin
for a given stainless steel surface by decreasing the open
circuit potential.

6. June 2015 • Oil and Gas Facilities 33
“It has more to do with the environment the
facility is in than anything else,” Lomasney said. “So if
it’s a particularly aggressive rig located in a hot, tropical
environment like a lot of them are, and if there’s a lot
of seawater around like a lot of them tend to have,
those are the types of environments where we see
an opportunity.”
Last year, Modumetal expanded its prototype production
facility and commissioned the construction of a new facility
that will support the production of coatings and claddings
for production and casing tubulars. Lomasney said the
company can produce components as long as 15 ft, and
that it hopes to scale that capability up to 45 ft by the end
of the year.
Although it has been in existence for a long time,
the technology that produces nanoscale coatings had
not been widely used because the manufacturing
processes were impractical, Lomasney said. She
described previous production attempts of the material
as better suited for small samples of model system alloy.
However, she said her company seeks to be a viable
commercial alternative.
“We can do it on a large scale,” she said. “We’re building
up the infrastructure.” OGF
For Further Reading
OTC 25830 Developments in Nanolaminaated Materials
to Enhance the Performance and Longevity of Metal
Components in Offshore Applications by C. Lomasney,
L. Collinson, and T. Burnett, Modumetal.
SPE 173721 Development of Novel Anti-Agglomerant
Chemistries with Reduced Localized Corrosion Potential
to Stainless and Duplex Steels by V. Panchalingam, Z. Liu,
G. Rivers, et al., Baker Hughes.
SPE 173723 Development of Novel Phosphate Based
Inhibitors Effective for Oxygen Corrosion by O. Yepez,
N. Obeyesekere, and J. Wylde, Clariant Oil Services.
Bullis, K. 2015. Nano-Manufacturing Makes Steel 10
Times Stronger. MIT Technology Review. http://
www.technologyreview.com/news/534796/nano-
manufacturing-makes-steel-10-times-stronger (accessed
13 May 2015).
Wang, H. and Wylde, J. 2011. Corrosion Inhibitor
Development for a Slightly Sour Environment with an
Oxygen Intrusion Issue. Journal of Materials Science and
Engineering 5: 41–55.