Online Corrosion Monitoring for Dummies

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ONLINE CORROSION MONITORING
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ONLINE CORROSION MONITORING
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TABLE OF CONTENTS
TABLE OF CONTENTS 2
1.0 INTRODUCTION 3
2.0 REFERENCES 4
3.0 CORROSION MONITORING TECHNIQUES 4
3.1 DIRECT TECHNIQUES 4
3.1.1 Corrosion Coupon 5
3.1.2 Electrical Resistance (ER) 6
3.1.3 Linear Polarization Resistance (LPR) 11
3.1.4 Ultrasonics 13
3.1.5 Radiography 14
3.2 INDIRECT TECHNIQUES 14
3.2.1 Corrosion Potential (ECorr) 14
3.2.2 Chemical Analyses 15
4.0 ONLINE CORROSION MONITORING 16
4.1 INTRUSIVE ONLINE MONITORING METHOD 16
4.2 NON-INTRUSIVE ONLINE MONITORING METHOD 17
5.0 CONCLUSION 18
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1.0 INTRODUCTION
Corrosion process naturally and readily occurs at metal surface, the backbone material of
almost all operating equipment in oil and gas industry. Various methods and mechanisms
are put operational to control and monitor corrosion process in order to maintain
operational continuity by having provided latest update information about metal based
equipment. Simulation model is also applied in order to predict corrosivity of the system by
using operating parameter combined with natural existing parameter. Actual and predicted
corrosion rate are valuable output expected from these methods, and with correct
understanding of these methods, proper interpretation and specific data significantly can
be used as decision bases.
To deal with the threat of corrosion, the corrosion monitoring is generally performed.
Corrosion monitoring is the practice carried out to assess and predict the corrosion
behaviour in operational plant and equipment. Some of the objectives of corrosion
monitoring are:
(a) To provide information on the state of operational equipment with the intention of
avoiding unplanned shut-downs, occurring due to unforeseen deterioration of the
plant.
(b) To provide information on the interrelation between corrosion processes and
operating variables to allow more efficient use of the plant. Eg, chemical injection.
(c) To provide information that plant inspection departments may use to prevent safety
failures and potential disasters.
(d) To assess levels of contamination of process fluids
The current technologies to monitor corrosion in the industry are based on intrusive
methods and non-intrusive measurement of the remaining wall thickness of the pipes.
Online monitoring technology is being widely discussed today because of its integration
capabilities. With the integration of field data directly to the computer at the office, it will
save the cost of inspections in the field, especially for submerged and underground
structure; and to determine cause of high corrosion rate in real time. Therefore the online
monitoring can provide data quickly and accurately enough so that we can take swift
action to maintain the continuity of the process industry.
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2.0 REFERENCES
The following documents are used as a reference for online corrosion monitoring:
[1] ASTM C 876 “Standard Test Method for Half-Cell Potentials of Uncoated
Reinforcing Steel in Concrete”
[2] ASTM G 96 “Standard Guide for Online Monitoring of Corrosion in Plant Equipment
(Electrical and Electrochemical Methods).”
[3] ASTM 908 “Corrosion Monitoring in Industrial Plant Using Non-Destructive Testing
and Electrochemical Methods”
[4] NACE RP0497 “Field Corrosion Evaluation Using Metallic Test Speciments”
[5] NACE RP0775 “Preparation and Installation of Corrosion Coupons and
Interpretation of Test Data in Oil Field Operations”
[6] NACE SP0206 “Internal Corrosion Direct Assessment Methodology for Pipeline
Carrying Normally Dry Natural Gas”
[7] NACE SP0106 “Control of Internal Corrosion in Steel Pipeline and Piping System"
[8] NACE Publication 3T199 : 1999 “Techniques for Monitoring Corrosion and Related
Parameters in Field Applications”
3.0 CORROSION MONITORING TECHNIQUES
Assessment of corrosion in the field is complex due to the wide variety of applications,
process conditions, and fluid phases that exist in industrial plants where corrosion occurs.
A wide range of direct and indirect measurement techniques is available, but each
technique has its strengths and weaknesses. In some applications certain techniques
cannot be used at all. Some techniques can be used online, while others are done off-line.
Commonly more than one technique is used so that the weaknesses of one are
compensated for by the strengths of another.
Basically there are two types of corrosion monitoring techniques, namely :
• Direct Techniques
• Indirect Techniques
3.1 DIRECT TECHNIQUES
Direct techniques describe measurement of metal loss or corrosion rate. Some examples of
direct technique are widely used for corrosion monitoring, are :
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3.1.1 Corrosion Coupon
The simplest, and longest-established, method of estimating corrosion losses in plant and
equipment is weight loss analysis. A weighed sample (coupon) of the metal or alloy under
consideration is introduced into the process, and later removed after a reasonable time
interval. The coupon is then cleaned of all corrosion product and is reweighed. The weight
loss is converted to a total thickness loss, or average corrosion rate using proper
conversion equations.
Mass-loss coupons are small test specimens of metal that are exposed to an environment
of interest for a period of time to determine the reaction of the metal to the environment.
The mass-loss coupon is removed at the end of the test period and any remaining
corrosion products mechanically and/or chemically removed.
The environment of interest can be the full process flow at a location where the conditions
are deemed to be suitably severe to give a meaningful representation. The design of the
coupon usually matches the objective of the test—simple flat sheets for general corrosion
or pitting, welded coupons for local corrosion in weldments, stressed or precracked test
specimens for stress corrosion cracking. Coupons can be complex and consist of metal
couples, or incorporate connectors or crevices. The average corrosion rate over that period
can be determined from the mass loss of metal over the period of exposure. The technique
is an in-line or side-stream monitoring method but does not provide real-time
measurements.
Figure 1. Corrosion Coupon With Coupon Holder
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3.1.2 Electrical Resistance (ER)
The electrical resistance technique operates on the principle that the electrical resistance
of a measuring element (wire, strip, or tube of metal) increases as its conductive cross-
sectional area decreases as the result of corrosion, erosion, or a combination of both. In
practice, the electrical resistance ratio between a measuring element exposed to the test
environment and a reference element protected from the environment is made to
compensate for resistance changes due to temperature. Because the resistance of the
measurement element is very small, very sensitive measurement electronics are used. The
general assumption that the cross-sectional area of the measurement element reduces
uniformly as metal loss occurs is made in this method. The technique is an online, or side-
stream, method that provides real-time measurements when sufficiently sensitive probes
are used.
Although universally applicable, the ER method is uniquely suited to corrosive
environments having either poor or non-continuous electrolytes such as vapors, gases,
soils, “wet” hydro-carbons, and non-aqueous liquids. Examples of situations where the ER
approach is useful are:
• Oil/gas production and transmission systems
• Refinery/petrochemical process streams
• External surfaces of buried pipelines
• Feedwater systems
• Flue gas stacks
• Architectural structures
An ER monitoring system consists of an instrument connected to a probe. The instrument
may be permanently installed to provide continuous information, or may be portable to
gather periodic data from a number of locations. The probe is equipped with a sensing
element that has a composition similar to the process equipment.
3.1.2.1 Principle of Operation
The electrical resistance of a metal or alloy element is given by:
= . / where : L = Element length
A = Cross sectional area
r = Specific resistance
Reduction (metal loss) in the element’s cross section due to corrosion will be accompanied
by a proportionate increase in the element’s electrical resistance
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In this diagram, a standard ER instrument is connected
to a 40mil wire loop element which has a useful life of 10 mils.
The instrument still reads close to zero because the
element is new.
Here the instrument reads around half-scale, indicating
that the element has experienced about 5 mils of metal
loss or about half of its useful life. The instrument’s
reading is increasing proportionally with the resistance
of the element, which increases as a result of metal
loss.
Here the instrument reads almost full scale, indicating
that the element has experienced 10 mils of metal loss
and requires replacement
Practical measurement is achieved using ER probes equipped with an element that is freely
“exposed” to the corrosive fluid, and a “reference” element sealed within the probe body.
Measurement of the resistance ratio of the exposed to reference element is made as
shown in Figure 2.
Measurement of the ER probe may either be taken periodically using a portable instrument,
or on a continuous basis using a permanently installed unit. In either case, Corrosion
Monitoring
Figure 2. Probe Instrument
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Systems ER instruments will produce a linearized signal which is proportional to the metal
loss of the exposed element. The rate of change in the instrument output is a measure of
the corrosion rate. Continuously monitored data is usually transmitted to a computer/data-
logger and treated to give direct corrosion rate information. Manual graphing techniques
are usually used to derive corrosion rate from periodically obtained data as illustrated in
Figure 3.
3.1.2.2 ER Sensing Elements
The probe is equipped with a sensing element having a composition similar to that of the
process equipment of interest. The sensing element itself can be manufactured in one of
many geometries:
• Wire loop elements are the most common elements available. This type of element
has high sensitivity and low susceptibility to system noise, making it a good choice
for most monitoring installations. Wire loops are generally glass-sealed into an end
cap which is then welded to the probe body.
• Tube loop elements are recommended where high sensitivity is required to rapidly
detect low corrosion rates. Tube loop elements are manufactured from a small
bore, hollow tube formed into the above loop configuration. Carbon Steel is the
alloy most commonly used.
• Strip loop elements are similar to the wire and tube loop configurations. The strip
loop is a flat element formed in a loop geometry. The strip loop may be glass or
epoxy sealed into the end cap depending on the required application. The strip loop
is a very sensitive element. Strip loops are very fragile and should only be
considered for very low flow applications.
• Cylindrical elements are made by welding a hollow tube inside of another hollow
tube. The element has an all welded construction which is then welded to the
Figure 3. Graph Corrosion Rate vs Time
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probe body. Because of this element's all welded construction, exotic alloy
elements can be produced relatively easily. This probe is ideally suited to harsh
environments including high velocity and high temperature systems, or anywhere a
glass-sealed element is not an option.
• Spiral loop elements consist of a thin strip of metal formed on an inert base. The
element is particularly rugged and ideal for high-flow regimens. Its comparatively
high resistance produces a high signal-to-noise ratio, which makes the element very
sensitive.
• Flush mount elements are designed to be mounted flush with the vessel wall. This
element is very effective at simulating the true corrosion condition along the interior
surfaces of the vessel wall. Being flush, this element is not prone to damage in high
velocity systems and can be used in pipeline systems that are subject to pigging
operations.
• Surface strip elements are thin rectangular elements with a comparatively large
surface area to allow more representative results in non-homogeneous corrosive
environments. Strip elements are commonly used in underground probes to monitor
the effectiveness of cathodic protection currents applied to the external surfaces of
buried structures.
Figure 4. ER Sensing Element
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3.1.2.3 Corrosion Rate Calculation
When measuring the ER probe, the instrument produces a linearized signal (S) that is
proportional to the exposed element’s total metal loss (M). The true numerical value being
a function of the element thickness and geometry. In calculating metal loss (M), these
geometric and dimensional factors are incorporated into the “probe life” (P) (see Table 1),
and the metal loss is given by:
= ( )/1000
Metal loss is expressed in mils (0.001 inch). Corrosion rate (C) is derived by :
ΔT being the lapse time in days between instrument readings S1 and S2.
Table 1 lists element types, thicknesses, probe life, and identification numbers. For
temperature and pressure ratings see respective probe data sheets. When selecting an
element type for a given application, the key parameters (apart from the fundamental
constraints of temperature and pressure) in obtaining optimum results are response time
and required probe life. Element thickness, geometry, and anticipated corrosion rate
determine both response time and probe life. Response time, defined as the minimum time
in which a measurable change takes place, governs the speed with which useful results
can be obtained. Probe life, or the time required for the effective thickness of the exposed
element to be consumed, governs the probe replacement schedule.
Table 1. Probe Life and Element ID
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3.1.3 Linear Polarization Resistance (LPR)
Polarization resistance is particularly useful as a method to rapidly identify corrosion
upsets and initiate remedial action, thereby prolonging plant life and minimizing
unscheduled downtime. The technique is utilized to maximum effect, when installed as a
continuous monitoring system. This technique has been used successfully for over thirty
years, in almost all types of water-based, corrosive environments. Some of the more
common applications are:
• Cooling water systems
• Secondary recovery system
• Potable water treatment and distribution systems
• Amine sweetening
• Waste water treatment systems
• Pickling and mineral extraction processes
• Pulp and paper manufacturing
• Hydrocarbon production with free water
3.1.3.1 Principle of Operation
When a metal or alloy electrode is immersed in an electrolytically conducting liquid of
sufficient oxidizing power, it will corrode by an electrochemical mechanism. This process
involves two simultaneous complementary reactions.
At anodic sites, metal will pass from the solid surface into the adjacent solution and, in so
doing, leave a surplus of electrons at the metal surface. The excess electrons will flow to
nearby sites, designated cathodic sites, at which they will be consumed by oxidizing
species from the corrosive liquid. A simple example of iron dissolving in acidic solution is
illustrated in Figure 5.
Figure 5. Corrosion Electrochemical Process
ICORR
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3.1.3.2 Probe System
LPR probes are typically a two- or three-electrode configuration with either flush or
projecting electrodes.
With a three-electrode system, the corrosion measurement is made on the test electrode.
Because the measurement takes only a few minutes, a stable reference electrode is not
necessary; the potential of a half electrode is normally sufficiently stable. The reference
electrode typically is stainless steel or even the same alloy as that being monitored on the
test electrode. The auxiliary electrode is normally also of the alloy being monitored. The
proximity of the reference electrode to the test electrode governs the degree to which
compensation for solution resistance is effective.
With a two-electrode system, the corrosion measurement is an average of the rate for
both electrodes. Both electrodes are of the alloy being monitored.
Figure 6. LPR Probe
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3.1.3.3 Corrosion Rate Calculation
The basic technique of LPR determines the corrosion rate of an electrode. The tendency of
the metal ions of the electrode (cation) to pass into solution, or corrode, is inferred from
the ratio between a small change in applied potential (typically 10 to 20 mV) around the
open-circuit potential of the electrode and the corresponding change in the current density.
The electrode is normally polarized both cathodically and anodically by reversal of the
impressed current and held at the polarized potential until a stable current density can be
measured. The ratio of the change of potential to the change of current density (ΔE/ΔIapp)
relates to corrosion rate through the Stern-Geary equation:
Where :
ba = measured Tafel slope for anodic reaction
bc = measured Tafel slope for cathodic reaction
E =applied potential charge
I = resultant current density charge
I
corr
= corrosion current density at free-corroding
potential
The corrosion current (ICORR), generated by the flow of electrons from anodic to cathodic
sites, could be used to compute the corrosion rate by the application of a modified version
of Faraday’s Law:
Where :
C = Corrosion Rate (MPY)
E = equivalent to weight of corroding metal (g)
A = Corroding electrode area (cm
3
)
d = Density of corroding metal (g/cm
3
)
3.1.4 Ultrasonics
Ultrasonic inspection has been used for decades to measure the thickness of solid objects.
A piezoelectric crystal referred to as a transducer is made to oscillate at high frequencies,
coupled directly or indirectly to one surface of the object whose thickness is to be
measured, and the time a wave of known velocity takes to travel through the material is
used to determine its thickness.
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With the more sophisticated systems, in which great numbers of thickness measurements
are possible over small areas, statistical comparisons of the areas scanned can allow rapid
comparison of selected spots used in a corrosion-monitoring program. The volume of
material in the area scanned can be calculated, and this information can then be used to
develop volumetric changes over time (or mass loss). The change in area of corrosion can
be compared, as can the remaining wall thickness and pit depth, which can be used to
calculate pitting rates.
The uses of ultrasonics as described above are primarily considered as inspection, because
they are usually concerned with vessel integrity, although in severe cases of metal loss,
measurements can be made sufficiently regularly to become more of an ongoing corrosion
monitor. Developments are now being made with individual transducers or transducer
arrays that are left in place to provide continuous monitoring. Permanently attached
transducers improve accuracy by removing errors in relocating a transducer to exactly the
same point with exactly the same couplant thickness, depending on the accuracy of the
transducer, its temperature compensation, and the measurement frequency. The technique
is capable of being used on-line, but its sensitivity generally excludes its use for real-time
measurements.
3.1.5 Radiography
The thickness of corroded piping and other equipment can be deduced from radiographic
images in several ways. One such technique has been reported in the literature and has
been used successfully for well over a decade in harsh oilfield environments. With this
technique, the difference in optical density of the film in a non-corroded area of the image
compared with the optical density in the pitted area can be correlated with the difference
in thickness of the two areas, and thereby the pit depth is determined. With repeated
surveys of specific areas on a frequency determined from the severity of the corrosion, the
changing depth and area of corrosion can be readily resolved and corrosion rates
calculated. The method can be used on-line but is too insensitive to provide real-time
measurements.
3.2 INDIRECT TECHNIQUES
Indirect techniques describes measurement of any parameters that may influence, or are
influenced by, metal loss or corrosion. Some examples of direct technique are widely used
for corrosion monitoring, are :
3.2.1 Corrosion Potential (ECorr)
The corrosion potential (Ecorr) is the potential of a corroding surface in an electrolyte
relative to a reference electrode under open-circuit conditions (also known as rest
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potential, open-circuit potential, or freely corroding potential). The potential is normally
measured relative to a reference electrode such as saturated calomel (SCE), silver/silver
chloride (SSE), or copper/copper sulfate (CSE).
The value obtained is useful only if it is related to other measurements of the same
phenomenon. The value is used to assist prediction of corrosion behavior by comparison
with polarization data obtained from laboratory or site polarization scans. The corrosion
potential is also useful in the development of information for use in conjunction with
Pourbaix diagrams (E versus pH diagrams) of the environment and redox comparisons, etc.
The corrosion potential can determine whether stainless steel is in the active or passive
region.
3.2.2 Chemical Analyses
Different types of chemical analyses can provide valuable information in corrosion
monitoring programs. The measurement of hydrogen flux, pH, conductivity, dissolved
oxygen, metallic and other ion concentrations, water alkalinity, concentration of
suspended solids, inhibitor concentrations and scaling indices all fall within this domain.
Figure 7. E-pH Diagram of Iron with CP criterion at -0.53 V vs
SHE (-0.85 V vs CSE)
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Several of these measurements can be made on-line using appropriate sensors. In many
situations, process status and product quality are determined by using chemical methods
for which advanced and automated systems for chemical analysis are used. But its to
expensive and complicated to install all sensor and integrated its. For example, hydrogen
analysis have varies correlation to corrosion rate, because the amount of hydrogen passing
into the steel compared with that being released into the process stream varies. Beside
that, hydrogen evolution does not apply to oxygen reduction in neutral or base solutions,
so the technique is not considered suitable.
4.0 ONLINE CORROSION MONITORING
Definition of online corrosion monitoring is installation of monitoring equipment for
continuous measurement of metal loss, corrosion rate, or other parameters in an operating
system. Online monitoring is basically just adding features integration with a data logger in
the field of computer networks in a corporate office with the help of specific software.
Data communication can be carried out via acoustic modem, cable or optic link, radio link,
or satellite (e.g. cell phone GSM frequency).
There are two types of online monitoring is widely used in the oil and gas industry, which
is :
4.1 INTRUSIVE ONLINE MONITORING METHOD
Intrusive monitoring methods are widely applied in the aboveground structures. Corrosion
monitoring methods that are commonly used in the oil and gas industry is ER probe. The
principle of measurement of the corrosion rate the same as the conventional measurement
of ER probes, except that the data logger integrated with a transmitter that sends data to
a corporate computer network through a gateway. Explanation of the data communication
can be seen in Figure 8 and 9.
Figure 8. Wireless Data Communication System
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4.2 NON-INTRUSIVE ONLINE MONITORING METHOD
Non-intrusive monitoring methods can be used both in the aboveground, submerged, or
underground structures. Non-intrusive corrosion monitoring method that has been widely
used globally is the NDT technique by using ultrasonic pulse. Nowadays most UT
measurements made are still single-point thickness measurements, which do not provide
the capability of the more sophisticated systems. Rugged systems based on modern
microcomputers are now available from many sources. These systems, complete with
motor-driven robotic devices to manipulate the transducer(s), have created the ability to
measure wall thickness of corroded components at tens of thousands of points over 0.1
m
2
(1 ft
2
). This capability, coupled with increased precision of field measurements possible
with computer-controlled systems, has made these automated ultrasonic systems well
suited for online corrosion monitoring.
Data integration system in this method is same as intrusive method. UT measurement data
is stored in the data logger and transmitter will transmit the data to the corporate network
through a gateway. Examples of online monitoring application using the UT can be seen in
Figure 10.
Figure 9. ER Probe Wireless Integration System
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5.0 CONCLUSION
Basically, online corrosion monitoring have a same principle as a general monitoring
method, only comes with accessories that can transmit and integrate the data wirelessly.
Therefore the online monitoring can provide data quickly and accurately enough in real
time.
In short, online corrosion monitoring and technology provides a cost-effective method for
assessing the condition of plant, and provides a mechanism whereby life-cycle costs may
be minimized.
Figure 10. Online Corrosion Monitoring Using UT Wall Thickness Measurement