Online Corrosion Monitoring for Dummies

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PT. PETROCORDOCUMENT NO:

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ONLINE CORROSION MONITORING

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A 19 DEC 2013 ISSUED FOR REVIEW Dimas Aldiantono Akhmad Munthohar

Rev. Date Description Prepared Reviewed ApprovedPT PETROCOR


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TABLE OF CONTENTS

TABLE OF CONTENTS 21.0 INTRODUCTION 32.0 REFERENCES 43.0 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 164.1 INTRUSIVE ONLINE MONITORING METHOD 16

4.2 NON-INTRUSIVE ONLINE MONITORING METHOD 17

5.0 CONCLUSION 18


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1.0 INTRODUCTIONCorrosion 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 onlinemonitoring 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 REFERENCESThe 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 TECHNIQUESDirect 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 CouponThe simplest, and longest-established, method of estimating corrosion losses in plant andequipment 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 testsimple flat sheets for general corrosion

or pitting, welded coupons for local corrosion in weldments, stressed or precracked testspecimens 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-timemeasurements.

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 elements cross section due to corrosion will be accompanied

by a proportionate increase in the elements 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 instruments

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 elementhas 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 striploop 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 hollowtube. 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. Thiselement 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 largesurface area to allow more representative results in non-homogeneous corrosiveenvironments. 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 elements 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 :

T 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 (E/Iapp)

relates to corrosion rate through the Stern-Geary equation:

Where :

ba = measured Tafel slope for anodic reactionbc = measured Tafel slope for cathodic reaction

E = applied potential charge

I = resultant current density charge

Icorr = 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 Faradays Law:

Where :

C = Corrosion Rate (MPY)

E = equivalent to weight of corroding metal (g)

A = Corroding electrode area (cm3)

d = Density of corroding metal (g/cm3)

3.1.4 UltrasonicsUltrasonic 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 RadiographyThe 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 TECHNIQUESIndirect 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 AnalysesDifferent 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 vsSHE -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 MONITORINGDefinition of online corrosion monitoring is installation of monitoring equipment for

continuous measurement of metal loss, corrosion rate, or other parameters in an operatingsystem. 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 METHODIntrusive 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 METHODNon-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

m2 (1 ft2). 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 CONCLUSIONBasically, 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

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Online Corrosion Monitoring for Dummies