Volume 6 Preprint 30
Localised Corrosion and Inhibitor Selection
W.Y. Mok, A.E. Jenkins and C.G. Gamble
Keywords: hydrogen sulphide, inhibition, localised corrosion, pitting and electrochemical noise
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Volume 6 Paper C072
Localised Corrosion and Inhibitor Selection
W.Y. Mok, A.E. Jenkins and C.G. Gamble
Baker Petrolite, Kirkby Bank Road, Knowsley Industrial Park, Liverpool L33 7SY, UK,
Corrosion control of oil and gas production systems is an essential element in the
overall asset integrity programme. It is common industrial practice to inject corrosion
inhibitor into the produced fluid to provide corrosion protection of the internal of
carbon steel structures. In the sour environment, corrosion of carbon steel is
typically in the form of localised attack, e.g. pitting. It is recognised that there are
limitations in the selection of chemical by simply basing the decision on the results of
linear polarisation resistance (LPR) measurements obtained in standard laboratory
bubble test set-up and field side stream testing. This is particularly the case in sour
environments where the general (or uniform) corrosion rate is at such low value that
the evaluation of the performance of candidate inhibitors can be difficult.
This paper considers the merits of conducting supplementary laboratory tests to
complement the LPR results in the evaluation process. Of particular interest is the
data obtained in autoclave tests, from which the localised corrosion behaviour of
carbon steel can be characterised and the effectiveness of chemical in controlling
localised corrosion qualified. In addition, the use of electrochemical noise monitoring
technique in the assessment of the performance of corrosion inhibitor in localised
corrosion environment was explored. The overall approach in the laboratory
chemical selection process and field assessment are discussed.
In oil and gas production and processing systems, carbon steel is widely used as the
construction material for vessels, pipelines and ancillary equipment. This is primarily
due to its mechanical properties as well as the cost effectiveness of material.
However, carbon steel is susceptible to corrosion attack in the oil and gas production
environment, in which acidic gas such as carbon dioxide (CO2) and/or hydrogen
sulphide (H2S) may be present. The dissolution of these gases in the produced brine
water can result in the lowering of the pH of the brine, and thereby promoting the
The injection of corrosion inhibitor is a standard practice in oil and gas production
systems to control internal corrosion of carbon steel structures. This strategy has
shown to be very successful and cost effective. The corrosion inhibitors can be
broadly classified into amides/imidazolines, salts of nitrogenous molecules with
carboxylic acids, quaternaries, polyoxyalklated amines/amide/imadazolines, nitrogen
phosphorus/sulphur/oxygen atoms, etc. Typically, the chemistry of a commercial
corrosion inhibitor is a blend of one or a combination of the above compounds
together with the incorporation of other components, e.g. surfactants, solvents and
demulsifier, etc. [1, 2].
The selection of a corrosion inhibitor for a specific field application would typically
undergo a rigorous laboratory evaluation programme followed by field assessment.
The laboratory evaluation process may involve series of performance and secondary
tests, e.g. bubble tests, partitioning tests, rotating cylinder electrode tests, flow loop
tests and compatibility tests, etc. [3, 4]. The best performing candidate inhibitors in
the laboratory tests would then be forwarded for further field evaluations, typically by
conducting side stream tests. Overall, this approach in the selection of corrosion
inhibitors has shown to be successful and is adopted by the oil and gas producers as
well as the chemical vendors.
In general, the assessment of the corrosion behaviour in the above evaluation tests
is usually by linear polarisation resistance (LPR) measurements over relatively short
period of time, typically within a 24-hours period.
In sweet systems, i.e.
environments that contain CO2 only and possibly very small amounts of H2S, LPR
monitoring is recognised to be very effective in measuring the corrosion rate as the
main form of corrosion is normally general (or uniform) . However, the technique
is considered to be unsuitable in systems deviated from the linear relationship, e.g.
in localised corrosion systems [6, 7]. It has been reported that corrosion failures are
normally associated with localised corrosion , thus the use of LPR monitoring only
in the assessment, especially in systems susceptible to localised corrosion, should
be treated with caution.
In systems where H2S is present in appreciable quantity, i.e. sour environment, the
general corrosion rate is usually low. This can be attributed to the formation of iron
sulphide film on the surface of the carbon steel material. However, if there are any
local defects in the iron sulphide film, localised pitting type of attack can result. In
the present work, the tests undertaken to select a corrosion inhibitor for a sour
system are described. The conventional approach of corrosion inhibitor evaluation
described above was employed, but additional tests to qualify the localised corrosion
behaviour were also undertaken to assist the selection process.
A number of candidate corrosion inhibitors were evaluated for a specific Far East
application by a sequence of laboratory and field tests. The incumbent chemical
(Inhibitor G) was used as benchmark in the tests. All the candidates were initially
assessed for their brine solubility, emulsion and foaming tendency.
candidates that failed these secondary property tests were precluded from the
corrosion inhibition performance tests.
Preliminary Laboratory Bubble Cell Tests
Preliminary screening tests based on the bubble cell test set-up were first conducted.
The test fluids consisted of 800ml of field brine and 200ml of hydrocarbon. Prior to
corrosion rate measurement using LPR the fluids were dearated by sparging with
CO2 gas for approximately 1 hour. The sour environment was simulated by the
addition of sodium sulphide solution, which gave an equivalent concentration of 28
ppm sulphur in the brine phase. The test temperature (70°C) was maintained at the
set point temperature by using a stirrer / heater unit via a temperature controller.
Field Side Stream Evaluations
A visit to the field location was made to test the best performing products identified in
the preliminary laboratory tests. The side stream unit consisted of a series of test
sections, into which corrosion inhibitor free produced water from a separator was
allowed to flow though the unit via a sampling line. Corrosion probes were inserted
into the test sections via access ports. This set-up enabled the exposure of the
sensor elements to the produced water to facilitate the monitoring of the corrosivity
of the brine. The design of the unit also allowed the injection of corrosion inhibitor
into a mixing section upstream of the corrosion probes, and its performance was
again monitored by LPR corrosion monitoring technique.
Laboratory Autoclave Tests
The inhibition performance of the corrosion inhibitors was further assessed in the
laboratory using autoclave equipment. The autoclave test work was designed to
investigate the pitting inhibition capability of the inhibitors. In this series of tests,
corrosion coupons were exposed in the test environment and the corrosion inhibition
performance was assessed with respect to the inhibitor concentration. In addition to
the corrosion coupon exposure tests, electrochemical noise (ECN) measurements
were also made to characterise the corrosion behaviour [9 - 11]. On completion of
the tests, the coupons were cleaned and a calibrated microscope was employed to
examine the surface conditions as well as for the measurements of the pit depth.
The test conditions of this series of tests are summarised below.
Brine Oil Ratio
0.86 bara CO2 / 0.14 bara H2S
Bubble Cell Tests
The first set of laboratory tests was conducted to evaluate the relative corrosion
inhibition performance of the candidate inhibitors at a concentration of 10 ppm/v. A
blank (i.e. solution with no inhibitor) was also included as control. The corrosion
inhibition given by inhibitors C, D and E was relatively poor in this test.
Consequently these inhibitors were eliminated from the testing process. Inhibitors A
and B were carried forward to the next stage of the evaluation process. The results
are summarised in Table 1.
Stabilised Corrosion Rate
Table 1. LPR corrosion rates recorded at an inhibitor concentration of 10 ppm/v
For the second stage of the evaluation, products were tested at 20ppm/v. For this
stage, the incumbent product (Inhibitor G) was also included in the testing along with
another chemical (Inhibitor F) that had performed well in previous field tests. The
results summarised in Table 2 suggested that Inhibitors B, F and G gave comparable
corrosion performance and all three offered marginally higher protection than
Inhibitor A. The increase in inhibitor concentrations from 10 to 20 ppm/v also had an
obvious impact in improving the inhibition performance.
Stabilised Corrosion Rate
Table 2. LPR corrosion rates recorded at an inhibitor concentration of 20 ppm/v
Field Side Stream Tests
On the basis of the laboratory test work, the candidates Inhibitors A and B were
selected for further assessment under actual operating conditions. The incumbent
Inhibitor G was also included along with Inhibitor F for comparison benchmark
purposes. The test set-up incorporated attaching a side stream unit to a water
sampling line of a test separator at the production and processing facility. Corrosion
probes inserted in the side stream unit were exposed to the flowing fluid throughout
the duration of the tests. The corrosion behaviour of carbon steel was continuously
monitored by LPR technique.
The results obtained from the field tests are graphically presented in Figure 1. The
data suggested that very low corrosion rates were obtained with the LPR
measurements. The baseline corrosion rate was only ~0.02 mm/year, even when
corrosion inhibitor injection was turned off. It was noted during the field tests that
because of operational problems with the separator that some amount of oil was also
allowed into the separated water stream. The presence of the oil as well as the
presence of an iron sulphide film on the surfaces of the electrodes may have
contributed to the low corrosion rate recorded.
When inhibitors were injected into the fluid, there were clear indications that a
reduction in corrosion rate was achieved. Overall, all the inhibitors may be
considered to give similar performance in this set of tests as the corrosion rate was
at such low level that the variations was insignificant. It was interesting to note from
the historical corrosion data of the production facility that localised corrosion was
observed when using Inhibitors F & G. The inhibition of localised corrosion was
therefore an important requirement for the inhibitors. On the basis of the LPR data,
no useful information could be obtained regarding the localised corrosion inhibition
properties of the inhibitor tested.
Corrosion Inhibitor Side Stream Tests
LPR Corrosion Rate (mm/year)
Figure 1. Corrosion rate profiles recorded during the side stream tests.
Laboratory Autoclave Tests
Coupon Weight Loss Tests
The pitting corrosion behaviour of the chemicals was evaluated in the laboratory
using the autoclave test. Corrosion coupons were exposed in the test environment
over a range of inhibitor concentrations, which varied from 25 – 400 ppm/v. In
addition, a limited number of ECN measurements were made to assess the localised
corrosion behaviour. On completion of the tests, the average corrosion rates (based
on weight loss) and the maximum pit depths (based on microscope examinations) of
the coupons were measured. The results are tabulated in Table 3.
Max Pit Depth (µm) / Average Corrosion Rate (mm/year)
Table 3. Recorded maximum pit depth and average corrosion rates in autoclave
The coupon results illustrate that at the lower range of inhibitor concentrations, up to
150 ppm/v, localised corrosion was recorded with all the inhibitor tested. This was
especially the case with Inhibitor G, which showed severe localised pitting damage
at low inhibitor concentrations. In the case of average general (or uniform) corrosion
rate, the data suggested that the rate decreased with increasing inhibitor
concentration, which was generally expected in inhibited systems. However, the
maximum pit depth data were more variable and there did not appear to have a clear
correlation between pit depth and inhibitor concentration with some of the inhibitor
tested. Some examples of the surface appearances of the coupons after tests are
shown in Figure 2.
When the inhibitor concentration was increased to 180 ppm/v, Inhibitor A gave no
observable localised damage.
When the inhibitor concentration was further
increased to 200 ppm/v, Inhibitor B also exhibited negligible localised loss. Inhibitor
F was further tested at a maximum concentration of 400 ppm/v but pitting attack was
Figure 2. Surface appearance of the coupons after tests at 180 ppm/v inhibitor
Electrochemical Noise Tests
Electrochemical noise measurements were made during the test to assist the
evaluation of the localised corrosion behaviour. A typical example of the ECN time
record obtained shortly after the start of the tests is shown in Figure 3. The
potentials recorded during this scan were relatively small, between 2.5 – 4.5 mV, and
were typical of a 3-identical electrode set-up. The coupled current measured was
approximately 45 µA, which was indicative of low activity on the test electrodes.
There were no characteristic potential and the corresponding current transients in
the data which could be associated with the occurrence of localised (pitting initiation
or propagation) events on the electrodes.
Blank - Run A
Figure 3. Electrochemical noise time records obtained near the start of the test.
An example of the ECN time record obtained some 14 hours later is presented in
Figure 4. The features of both potential and current were generally similar to those
of Figure 3. There may be one small potential transient at ~160 second, which may
be associated as a localised event. However the size of the transient (1.1 mV) was
not significant. In addition, an episode of localised event which gives rise to a
negative potential transient, would normally be expected to have a corresponding
current transient. However, this was not found. Therefore, this suggested that the
appearance of that transient was probably not related to localised activity on the test
Blank - Run B
Figure 4. Electrochemical noise time records obtained at ~14 hours into the test.
Comparison of the two sets of data suggested that the potential difference measured
had increased to approximately 47 mV, whilst the current had decreased to circa 20
µA between the period of measurements. The higher potential difference suggested
that a larger difference existed between the reference and the working electrodes
since the start of the tests, whilst the variations in activities between the two working
electrodes were marginally lower. Statistical parameters, such as localisation index
(LI) , pitting factor (PF)  and characteristic charge (q) in combination with
characteristic frequency (fn) , were analysed in order to provide a guide to the
corrosion behaviour, i.e. localised or general, (Table 4).
Even though localised attack was observed on the electrode, the LI and PF
parameters suggested that the corrosion was more general rather than localised.
With the q and fn parameters, no firm conclusions could be made regarding the
localised corrosion behaviour based on the values derived. As a results of the
uncertainty of the value of ECN measurements, it was decided that the determination
of corrosion behaviour in this series of tests would be best based on the coupon data
only, and no further electrochemical measurement was taken.
Localised Corrosion Parameter
Table 4. Localised corrosion parameters derived from ECN data files.
The general approach in the selection of corrosion inhibitor in oil and gas production
and processing systems is initially to screen candidate inhibitors using standard
bubble cell test equipment. This may be followed by additional tests under more
severe test conditions, e.g. high shear, high temperature and high pressure
The assessment of performance is normally based on
electrochemical measurements, e.g. LPR or impedance, which provides
instantaneous corrosion rate information, and/or average corrosion rate based on
weight loss data. On the basis of the recorded corrosion rates, the best performing
chemicals in the laboratory tests would then be tested in the field using a side stream
unit set-up for a relatively short period of time. (In some cases, a longer term
evaluation, e.g 3-months period, may be adopted in a field trial.) In almost all short
duration tests, the assessment is based on the conventional electrochemical
monitoring data, and/or the uses of more sensitive weight loss measurements such
as Ceion or Microcorr. The limitations of these types of measurements are that they
are more suited to general corrosion.
In systems where localised corrosion predominates, the above approach may not be
able to identify if an inhibitor is appropriate for controlling localised corrosion. It is
important that the capability of the inhibitor is qualified in localised corrosion
applications as most reported field corrosion failures are due to localised attack. In
the present work, the localised corrosion performance of a number of inhibitors was
evaluated in autoclave units which allowed an assessment of the morphology of the
test coupons and/or electrodes. The LPR corrosion rate data obtained in laboratory
and field side stream tests suggested that there was little difference in performance
between Inhibitors A, B, F and G. However, the results of autoclave tests revealed a
differentiation in the capability of minimising the risk of localised attack. Inhibitors A
and B exhibited much better inhibition performance by preventing localised attack on
the coupons; whereas Inhibitors F and G were not able to fulfil this requirement,
though both may be considered to be perfectly adequate in controlling general
Electrochemical noise monitoring was employed in this study to assess the localised
behaviour. The time records were examined, and a number of localised corrosion
parameters were employed to assist the interpretation of localised behaviour. The
data obtained did not appear to suggest localised attack, though it must be stated
here that only a limited amount of data were recorded. The uses of electrochemical
noise corrosion techniques, ECN and P-ECN  are currently under further
evaluations to investigate localised corrosion.
One interesting aspect of this study is that when only LPR data were used as the
main criterion to select an inhibitor, there was no significant difference between
some of the inhibitors tested, and hence, other criteria may gain a more significant
emphasis in the selection process, e.g. secondary properties, costs and
environmental properties, etc., even though these criteria are important and are
normally considered in the selection process.
An important finding of this study is that the corrosion tests to be undertaken in the
selection process must be relevant to the corrosion behaviour in the field, and the
tests should be able to reveal the limitations or the capabilities of the chemicals.
This is very important as the selection of an inappropriate chemical can be costly
and damaging because of pre-mature corrosion failures, which can result in
unscheduled downtime, loss production, increased maintenance costs, undesirable
environmental impacts, health and safety risks to operating personnel.
LPR is a quick and simple measurement to provide general corrosion rate
information. It is widely adopted as the preferred technique in screening tests in
oil and gas production systems.
LPR can be used to differentiate the performance of inhibitors in general
corrosion systems, though there are limitations in applying LPR in localised
Inhibitors which work well in preventing general corrosion, may not perform
similarly well in preventing pitting corrosion as was illustrated by Inhibitors F and
Short duration side stream tests using LPR measurements may have limitations
in differentiating corrosion inhibition performance of different chemicals. This is
especially in sour systems where the general corrosion rate is low.
Autoclave test is a relatively better tool in evaluating the localised corrosion
behaviour. The extent of localised damage can be more accurately quantified.
ECN measurements carried out in this study were shown to be insensitive in
revealing localised corrosion events. The use of a number of localised corrosion
parameters to qualify the localised corrosion behaviour did not correlate with the
surface feature of the test electrodes.
The authors express their thanks to Baker Petrolite for permission to present this
paper. The authors also wish to record their thanks to K.A. Bartrip and colleagues
for their technical and experimental contributions.
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