Czichos H., Saito T., Smith L.E. (Eds.) Handbook of Metrology and Testing

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728 Part D Materials Performance Testing

Gas inlet

Bell jar

Semiconductor

pyrometer

Insulated

feedthrough

Base plate

Swagelock

10 mm bulkhead

Power

terminal

Vacuum

fitting

Four way

compensating

feedthrough

Testpiece

clamp

Testpiece

support

Fig. 12.55 Schematic of a test rig for thermal cycling oxidation

testing using ultrashort temperature cycles

recommended test cycle for ultrashort-dwell-time test-

ing consists of 5 min hot dwell and 2 min cold dwell.

For this speciﬁc type of test a special test rig is needed,

as described in detail in [12.112]. A schematic of this

rig is shown in Fig. 12.55.

Generally the test duration in thermal cycling test-

ing should be at least 300 h of accumulated hot dwell

time to allow signiﬁcant oxidation or corrosion of the

test pieces. For more reliable results it is however rec-

ommended to extend the accumulated hot dwell time to

at least 1000 h.

Evaluation of the Mass Change

After the tests, or during the tests at intervals when

the specimens are cold, the mass has to be determined

to quantify the corrosion kinetics. For this weigh-

ing procedure the specimens have to be taken out of

the equipment but they should never be touched with

hands, to eliminate any contamination (grease, salts).

Generally the use of tweezers is recommended. Af-

ter removing from the furnace the test-piece supporters

containing the test pieces should be settled in the weigh-

ing room for 15 min to allow them to acclimatize.

The test pieces should not be descaled unless speci-

ﬁed. For each mass-change determination the sum m

of the mass of the test-piece support (m

) contain-

ing one test piece (m

) and the spalled scales (m

i. e. m

,themassm

of the

test-piece support including the spalled scales, and the

mass m

of the test piece including the adherent scales

should be measured. The following values have to be

determined.

1. Gross mass change Δm

gross

, i. e. mass change of test

piece after cooling including collected spall

Δm

gross

−[m

) +m

)]; (12.91)

2. Mass of spalled oxide Δm

, i. e. scale ﬂaked from

the test piece

Δm

−m

) ; (12.92)

3. Net mass change Δm

net

, i. e. mass change of test

piece after cooling without spall

Δm

net

−m

) . (12.93)

The net mass-change results of the test pieces from

these measurements are plotted against time, as shown

in Fig. 12.56. To determine the quantitative key pa-

rameters characterizing the corrosion behavior of the

material a double logarithmic plot (log net mass change

versus log time) as shown in Fig. 12.57 is used. From

the y-axis intercept the oxidation rate constant k(T) can

be calculated as

a = log k(T)

1/n

→a =1/n log k(T) →k(T)

=10

. (12.94)

Δ m/A (mg/cm

)

0.3

0.25

0.2

0.15

0.1

0.05

0 15010050

Time t (h)

Fig. 12.56 Example of a plot of net mass change versus

time as a direct plot

Part D 12.8

Corrosion 12.8 High-Temperature Corrosion 729

If spalling or breakaway oxidation occur, the time at

the beginning of these mechanisms can also be of sig-

niﬁcant interest. In [12.109] a detailed procedure is

described that allows the quantitative determination of

these parameters.

12.8.4 Special High-Temperature Corrosion

Tests

Burner Rig Tests

In addition to the standard tests described so far sev-

eral special tests have also been developed that try

to take the speciﬁc loading situation under practical

conditions into account. Especially in the gas-turbine

manufacturing industries the so-called burner rig test

has become of some importance. In this test the test en-

vironment is produced by combustion of fuel with air

(Figs. 12.58 and 12.59). In Fig. 12.58 a schematic of

the so-called high-velocity burner rig is shown, where

the gas stream on the specimen surface can reach veloc-

ities of up to mach 0.3. In the case of a low-velocity

burner rig, combustion in a zone in the vicinity of

the specimen supporter produces the test gas, which

ﬂows at a relatively low rate through a large tube to the

specimens (Fig. 12.59). Especially in the case of a low-

velocity burner rig, impurities are often added to the

combustion environment, ﬁrst of all in the form of salt

water [12.87].

Thermocouple for

steady-state control

Thermocouple for recording

specimen temperature history

Compressed

inlet air 425 °C (800°F)

Fuel

Combustor

Blower

(thermal shock)

Rotating shaft

Thermal cycle

Specimen temperature measured

by pyrometer

50 mm

insulated

flame tunnel

Fig. 12.58 Schematic of a high-velocity burner rig (after [12.87])

log (Δ m/A)

log log

k(T)

log t

Δm

log log log t+

k(T)

a = log k

(1/n)

b = 1/n

Δm

Experimental data

Fit of the exp. data

0.97 limit

1.03 limit

Fig. 12.57 Double logarithmic plot of the data of Fig. 12.56 for the

evaluation of the parameters k(T)andn

Typical test specimens have the shape of a rod with

a rotating specimen carousel in the test rig carrying

a larger number of these rods. Using this carousel,

corrosion of rotating parts in engines is simulated. Fur-

thermore all specimens are subjected to the same test

conditions. In most cases with the burner rig test a ther-

Part D 12.8

730 Part D Materials Performance Testing

Atomizing air

Salt water

Atomizing air

Fuel

Secondary air

Thermocouple

Controlling

thermocouple

Specimen

temperature

by pyrometer

Thermal shock cycle

Rotating

shaft

Blower

(thermal shock)

Combustion zone

Fig. 12.59 Schematic of a low-velocity burner rig (after [12.87])

mal cycling test is performed, where the carousel with

the specimens is automatically moved out of the hot

zone once every 30 or 60 min and then cooled before

reinsertion.

Corrosion with Mechanical Loading

A parameter which is neglected in standard high-tempe-

rature corrosion tests is the role of mechanical stresses,

which are omnipresent in practical operation. Their role

has been investigated extensively over the past two

decades and it turned out that signiﬁcant effects on the

life time of high-temperature components can be possi-

ble [12.109,113–115]. For this reason different types of

tests have also been developed that combine mechanical

stresses with attack by high-temperature corrosion. In

most cases the equipment for such kinds of tests is basi-

cally the same as for the determination of the mechanical

properties of materials at high temperatures. In other

words, standard creep test [12.116] or constant-strain-

rate test [12.117] as well as low cycle fatigue (LCF)

and high cycle fatigue test (HCF) [12.118]rigshave

been used. In some cases even creep fracture mechanics

approaches have been combined with high-temperature

corrosion investigations [12.119, 120]. For these me-

chanical high-temperature corrosion tests the standard

mechanical test rigs were equipped with an environmen-

tal chamber that can hold different types of test gases.

Examples of the design of such environmental chambers

are shown in Figs. 12.60 and 12.61, where Fig. 12.61

shows the environmental equipment to be inserted into

a constant-load creep testing machine with two separate

specimens, where the strain can be monitored by ei-

ther mechanical or electronic strain gages. Figure 12.60

shows the schematic of a creep fracture mechanics envi-

ronmental equipment for CT specimens where ceramic

pull rods had been used. The information received from

these experiments is the mechanical properties under

the inﬂuence of corrosive attack, i. e. strain versus time,

strain rate versus time, or stress versus strain curves. In

the LCF and HCF test rigs it is usually the number of

cycles to failure and the cyclic deformation curve that

are measured. There are also examples in the literature

where crack growth rates at high temperatures have been

measured under corrosive conditions [12.119–121].

An important aspect of these tests is post-test eval-

uation by metallographic sectioning and microanalysis

of the corrosion products. The results from such in-

vestigations can reveal whether internal formation of

corrosion products at grain boundaries or at crack tips

can signiﬁcantly reduce the life time of components

Part D 12.8

Corrosion 12.8 High-Temperature Corrosion 731

by corrosion-assisted crack initiation and crack growth.

A number of examples have been studied in the litera-

ture [12.113].

12.8.5 Post-Test Evaluation of Test Pieces

Macroscopic Evaluation

The macroscopic appearance of the surface of the

test piece should be photographed if necessary using

a low magniﬁcation. Color photographs are preferred

over black and white photographs since coloring may

already give some information about the type of oxida-

tion/corrosion of the surface.

Metallographic Cross Section

For scale thickness measurements by metallographic

cross sections it is recommended that the recommen-

dations in [12.88] and the new draft ISO document

ISO/CD 26146 (Table 12.4) be followed. Care has to be

taken in mounting the specimen orthogonally to the pri-

mary axis of the test piece. The cross section of the test

pieces is analyzed using conventional light microscopy.

The measurements consist of

•

deposit thickness,

•

scale thickness,

•

depth of internal penetration inside the grains and at

grain boundaries,

•

depth of any depleted zone,

•

remaining cross section of unaffected material.

A minimum of eight measurements per test piece should

be taken. In addition the position of maximum attack is

measured.

Reporting

The report of each test should contain the following

information.

•

Test material: manufacturer, name of the mater-

ial (manufacturer designation, ASTM designation,

Deutsches Institut für Normung (DIN) designation,

etc.), grade or symbol, heat number/batch number,

chemical composition, processing conditions, heat-

treatment conditions.

•

Test piece: designation of test piece, dimensions and

surface area of test piece, surface ﬁnish condition of

test piece, degreasing method of test piece, method

of test-piece support, initial mass.

•

Temperature and testing environments: test temper-

ature (and maximum and minimum temperatures

Axial expansion joint

Water cooling

Furnace

CT specimen

Ceramic

pull rod

Metal pull rod

Axial expansion joint

Metal pull rod

Flange system

Quartz tube

Flange system

Fig. 12.60 Schematic of a closed system for crack growth measure-

ments in high-temperature corrosion environments (after [12.119])

during the test), test duration, hot dwell time, cold

dwell time, heating time, cooling time, dew point

temperature of humidiﬁed test gas or humidity of

laboratory air, ﬂow rate of the test gas, type of sys-

tem (open or closed).

•

Test results: plot of net mass change per area

Δm

net

/A in mg/cm

versus time t, results of any

metallographic investigations, amount of spalled

scale Δm

in mg, photograph of appearance af-

ter testing, photograph of the metallographic section

Part D 12.8

732 Part D Materials Performance Testing

of the test piece including the surface layer af-

ter testing, oxidation rate constant k(T ), exponent

of growth law n, the number of cycles N

and

Fig. 12.61 Two examples of closed systems for constant

load creep testing in high-temperature corrosion environ-

ments (after [12.122]) 

the accumulated hot dwell time t

that correspond

to the onset of spallation or breakaway oxidation.

If existing the report should also contain analyti-

cal results from scanning electron microscopy/EDS

(Energy Dispersive Spectrometer), electron probe

microanalysis/WDS (Wavelength Dispersive Spec-

trometrer) or x-ray diffraction.

12.8.6 Concluding Remarks

The aim of this section has been to provide a concise

and informative overview of the measurement meth-

ods used today in high-temperature corrosion testing.

Two of the more commonly used test methods were

described in some detail while the others were only dis-

cussed very brieﬂy. For more detailed information it

is recommended to read the literature given in the list

of references. It should, however, be pointed out that

there is still signiﬁcant development work going on in

these test methods. A certain settlement of this situ-

ation will be reached after ﬁnalization of the present

work items of work group 13 in ISO technical commit-

tee 156. In the meantime it is recommended to follow

the work of this work group closely when planning

the set up of new high-temperature corrosion testing

facilities.

12.9 Inhibitor Testing and Monitoring of Efﬁciency

The addition of small amounts of chemicals capable of

reducing corrosion of the exposed metal is a preventa-

tive measure known as corrosion inhibition. According

to the ISO 8044 deﬁnition corrosion inhibitors are

chemical substances that decrease the corrosion rate

when present in the corrosion system in sufﬁcient

concentration, without signiﬁcantly changing the con-

centration of any corrosive agent.

Inhibitors may act on both cathodic and anodic par-

tial reactions and are termed accordingly either cathodic

or anodic inhibitors. A cathodic inhibitor shifts the cor-

rosion potential in a negative direction, while the anodic

inhibitor causes a positive change; in both cases the

corrosion rate decreases.

Investigating corrosion inhibitor efﬁciency aims to

gather information on the proﬁle of the functional ad-

ditive, including its inhibition mechanism. Corrosion

experiments as well as electrochemical and surface

analytical methods are the major tools in such inves-

tigations. In this context it is of utmost importance

to note that electrochemical measurements are not

corrosion experiments and cannot replace real cor-

rosion experiments. On the other hand, testing tries

to ﬁnd out if and how well a functional substance

performs under speciﬁc experimental conditions. Per-

formance in a test may not correlate with performance

under service conditions. There are numerous exam-

ples where functional compounds have been developed

and designed to pass speciﬁc tests but have failed

in service application. Monitoring in this context in-

tends to create online information on the time-related

status of the corrosion intensity of an inhibited cor-

rosion system. Appropriate correlation between mon-

itoring data and the real corrosion status of a system

Part D 12.9

Corrosion 12.9 Inhibitor Testing and Monitoring of Efﬁciency 733

needs a detailed knowledge of the corrosion system

itself.

12.9.1 Investigation and Testing

of Inhibitors

As corrosion inhibitors are active at the electriﬁed

interface their performance can be investigated by elec-

trochemical means.

Various electrochemical measuring methods have

been developed and applied for investigations of inhib-

ited systems. The questions arise: which method gives

which information and what are the application limits of

the methods? Before applying any of the methods one

has to consider the actual task of the investigation. Is

it aimed mainly at scientiﬁc research or is information

needed on the performance of inhibitors under service

conditions.

In scientiﬁc research it is important, which of the

two partial reactions (Fig. 12.62), the anodic metal dis-

solution,

Me →Me

+n e

−

, (12.95)

or the cathodic reduction of an oxidizing agent,

Ox+n e

−

→Red , (12.96)

is inﬂuenced by the inhibitor. Is it an anodic, cathodic

or anodic–cathodic inhibitor (Fig. 12.63a–c)? Is the cor-

–

Ox Ox

Inhibitor

a) b) c)

Inhibitor

–

Red

Fig. 12.63a–c Schematic representa-

tion of inhibitor action on bare metal

surface: (a) anodic inhibition, (b) ca-

thodic inhibition, (c) anodic–cathodic

inhibition

–

Inhibitor film Inhibitor film Scale

Scale-

pore

a) b) c)

( = Inhibitor)

Red

–

Me Me

Red

–

Red

Fig. 12.64a–c Schematic representation of inhibitor action: (a) mass transport control at ﬁlms (b) charge-transfer control

at ﬁlms

–

Ox (e.g. O

, H

)

(e.g. OH

–

, H)

(e.g. Fe

, Al

)

Red

Fig. 12.62 Schematic representation of electrochemical

corrosion

rosion rate controlled by charge transfer or by mass

transport (Fig. 12.64a–c)? Is the inhibition effect due

to the formation of ﬁlms, adsorption on metal surfaces

(e.g. in corrosion product pores) or incorporation in cor-

rosion product layers? (Fig. 12.64c)?

For practical applications information is needed on

the effect of inhibitors on, e.g.,

•

the reduction of metal loss in corrosive environ-

ments,

•

the inﬂuence of hydrogen uptake of the metal (in

systems with hydrogen formation in the cathodic

partial reaction),

•

the likelihood of localized corrosion.

Before starting any investigation the corrosion system

must be carefully studied in terms of the appearance of

Part D 12.9

734 Part D Materials Performance Testing

corrosion (uniform, local, ability to passivation, charac-

ter of layers developed by corrosion, scaling etc.) and the

rate-determining step of the corrosion process. Particu-

larly in exposure testing the analysis of the electrolyte

over the period of testing can play an important role.

The methods of investigation were generally out-

lined in Sects. 12.1 and 12.2. All conclusions regarding

the applicability and interpretation of such tests also ap-

ply for testing of inhibitors. Special care has to be taken if

results from laboratory testing are transferred into prac-

tice. In any case it is advisable to carry out performance

monitoring during service. The aim of this section pri-

marily focuses on performance of systems affected by

corrosion. Therefore monitoring of efﬁciency will be the

main focus of this publication.

12.9.2 Monitoring of Inhibitor Efﬁciency

General Remarks

It is important to monitor the effectiveness of

a corrosion-inhibitor treatment. It is even more im-

portant to ensure that the conditions under which the

monitoring is carried out are representative and relevant

to the operational conditions. Monitoring should ide-

ally be carried out under the worst conditions that are

likely to be encountered (e.g. stagnant areas or very high

ﬂow rates etc.), otherwise failures may occur while the

monitoring process indicates that inhibition is effective.

Variables that need to be considered when monitor-

ing include

•

environment – both normal and extraordinary con-

ditions ﬂuid ﬂow conditions, including multiphase

ﬂow patterns,

•

temperature,

•

ﬂuid velocity chemical analysis – to cover seasonal

variations,

•

type of system – closed, open evaporative cooling,

once-through.

Application of Monitoring Data

The broad objective of carrying out monitoring is to

manage the corrosion problem, in this case to control

the corrosion-inhibitor treatment so that it works prop-

erly. The management process thus requires objectives

to be set, and measurements to be made that can be com-

pared with these objectives, so that adjustments to the

treatment can be made as necessary to achieve the re-

quired performance. The objectives may include one or

more of the following, depending upon the corrosion

management philosophy

•

to achieve a speciﬁed degree of corrosion protec-

tion (corrosion rates), to correlate with previous

experience or tests (corrosion rates, inhibitor con-

centrations),

•

to achieve a required inhibitor efﬁciency E

=100(w

−w

inh

) ;

where w

= weight loss or corrosion rate without

inhibitor, w

inh

= weight loss or corrosion rate with

inhibitor.

It is important that the type of measurement made pro-

vides the data required. Thus, if the objective is to

maintain a speciﬁed inhibitor concentration, corrosion

rate measurements will be of little value.

Measurement of Inhibition Parameters

to be Measured

A number of parameters relevant to corrosion inhibition

may be measured.

•

Actual inhibitor concentration,

•

Inhibitor efﬁciency by corrosion monitoring,

•

Operational parameters in the plant that may affect

inhibitor performance,

•

Chemical composition of the ﬂuid containing the

inhibitor.

Frequency of Measurement

The frequency of measurement may depend upon cor-

rosion monitoring practice. In general it is important

to monitor immediately after the ﬁrst addition of the

inhibitor, and then at least daily until the level has stabi-

lized, since some of the inhibitor will be removed from

the ﬂuid by adsorption on surfaces within the system.

Open systems are more likely to suffer unplanned

upsets than closed ones, and therefore should be mon-

itored more frequently. For critical systems online

measurements are useful. Online measurement systems

have other advantages.

•

They allow more rapid detection of changes or

trends in corrosion, and data can be continually an-

alyzed to reveal longer-term trends.

•

They eliminate variations associated with sampling

processes.

Chemical Analysis of Inhibitor Concentration

Sampling Techniques. It is essential to take as repre-

sentative a sample of the ﬂuid as possible. The location

of sampling points is important.

•

Avoid dead legs,

•

If possible, use isokinetic sampling techniques,

Part D 12.9

Corrosion 12.9 Inhibitor Testing and Monitoring of Efﬁciency 735

•

Avoid long sampling lines with high surface :

volume ratios.

When taking a sample

•

always run a volume of the ﬂuid to be sampled to

waste before taking a sample,

•

wash out the container with ﬂuid before ﬁlling with

the sample,

•

observe sample preservation requirements (e.g. air

exclusion, cool in ice).

Measurement of Inhibitor Concentration

The efﬁciency of an inhibitor is usually closely de-

pendent upon its concentration in solution, since it is

assumed that there is an equilibrium of the form: dis-

solved inhibitor +surface → inhibited surface.

Measurement of the concentration of the inhibitor

does not give direct information about its efﬁciency.

A minimum concentration is usually speciﬁed for the

inhibitor to provide the required performance, and this

depends upon the conditions. Changes in operating con-

ditions may therefore require adjustments to be made

in inhibitor concentration to maintain the same perfor-

mance.

Allowing the concentration to fall below the speci-

ﬁed level can be very dangerous with anodic inhibitors,

since partial failure of anodic inhibition can lead to

a high-cathode-area low-anode-area situation, and in-

duce rapid localized corrosion.

Characteristics of Commercial Inhibitors

Measuring the concentrations of commercial inhibitors

may not be straightforward as

•

commercial inhibitors are rarely pure analytical-

grade chemicals,

•

they may be formulated with other chemicals to im-

prove their performance (solvents, surfactants),

•

their speciﬁcation may allow a range of composi-

tions.

The variability in concentration and composition may

make it desirable to analyze the product before use,

Table 12.6 Field methods of analysis

Species Semiquantitative method Quantitative

CrO

−

Diphenylcarbazone complex Photometry of diphenylcarbazone complex

−

Magenta azo dye (a) Photometry of magenta azo dye; (b) Acidimetric titration with sulfamic acid

3−

Phosphomolybdenum blue Photometry of molybdenum-vanadate – phosphoric acid

SiO

/silicate Silicomolybdenum blue Photometry of silicomolybdenum blue

Zn Zn-thiocyanate-brilliant green Atomic absorption spectroscopy

unless a quality-assurance scheme is in operation. The

supplier’s recommendations for the concentrations to be

used should be adhered to; this avoids complications if

problems or failure occurs.

Laboratory Methods of Analysis

These have a number of advantages.

•

They are very versatile.

•

They can be very sensitive.

•

High selectivity is possible.

A wide range of techniques is available. Separation and

detection techniques are combined to increase sensitiv-

ity. Particularly useful are

•

ICPS – inductively coupled plasma spectrometry;

•

MS – mass spectrometry;

•

HPLC – high-performance liquid chromatography;

•

IC – ion chromatography.

The disadvantages are

•

they require special, usually expensive, equipment,

•

good performance requires well-trained operators,

•

samples have to be returned to the laboratory, result-

ing in a time delay.

Field Methods of Analysis

These have a number of common features.

•

They give almost instant results.

•

They employ preprepared and prepacked reagents

(the sell-by date should not be exceeded).

•

They usually involve color formation reactions.

•

They compare color to intensity scale or use

a battery-operated photometer.

Some common methods are speciﬁed in Table 12.6.

Some disadvantages are

•

individual kits cover limited concentration ranges;

choose the one you need,

•

look out for possible interferences (other colored

species present),

Part D 12.9

736 Part D Materials Performance Testing

Table 12.7 Summary of techniques

Technique type Information Time for Response Corrosion Application

measurement to change type interpretation

Optical Distribution of attack Fast, if, accessible Slow Localized Simple

Analytical methods Corrosion state,

total

Fairly fast Fairly fast General Relatively easy, needs

knowledge of corrosion in

system, inhibitor present

Corrosion coupons Average rate of

corrosion and form

Long exposure Poor General or localized Easy, simple

Polarisation

resistance

Corrosion rate Immediate Fast General, localized Simple to complex

Electrical resistance Integrated corrosion Immediate Moderate General Relatively simple

Corrosion potential Corrosion state Immediate Fast General or localized Easy to apply, needs

knowledge and experience

to interpret

ZRA = zero-

resistance ammetry

Corrosion state Immediate Fast Bimetallic galvanic

corrosion

Fairly easy, needs

corrosion knowledge

Ultrasonics Thickness of metal

left cracks, pits

Fast Fairly poor General or localized Easy to apply, care

with sensitivity

Eddy currents Cracks, pits Immediate Fairly poor Localized Easy

Acoustic emission Propagation of

cracks, leak detection,

cavitation

Immediate Fast Cracking, leaks Expensive, needs

specialist

Hydrogen probe Total corrosion Fast Poor General Easy, only in acid

Sentinel holes Go/nogoforslow

remaining thickness

Poor General Easy, relatively simple

Thin-layer

activation (TLA)

Integrated material

removal

Slow Va ria bl e General, localized Simple, special equipment

•

color intensity may vary with time, may bleach in

sunlight,

•

the number of species that can be detected is limited

(pH, nitrite, chromate).

Interpretation: Inhibitor Loss Processes

Lower than expected concentrations of inhibitor may be

the result of a number of processes.

•

Precipitation, which may depend on temperature,

solution composition,

•

Chemical reaction with other species, e.g. oxygen,

•

Adsorption on surfaces (metals, solids in suspen-

sion, corrosion products, precipitates),

•

Degradation by microorganisms,

•

Thermal degradation.

To assist in ﬁnding the reason for the loss of inhibitor,

measurements of other ﬂuid chemistry parameters may

be useful, as suggested below.

Additional Measurements

•

Water chemistry: including pH, dissolved species –

chloride, iron etc.,

•

Mass balance: gross check of concentration – chem-

icals used, make-up water volume,

•

System features

– concentration of atmospheric impurities in open

systems,

– process-side leaks,

– inefﬁcient blow-down microbiological activity,

– fouling of surfaces, including heat transfer sur-

faces.

•

Operational parameters

– ﬂuid temperature surface temperature, including

hot-spots,

– ﬂow velocities – periods of shut-down.

12.9.3 Monitoring Inhibition

from Corrosion Rates

General

The main objective of using inhibitors is to control cor-

rosion, therefore a measurement of the corrosion rate

can be the most direct way of assessing the effectiveness

of a corrosion control programme. However, a number

of points need to be borne in mind.

Part D 12.9

Corrosion 12.9 Inhibitor Testing and Monitoring of Efﬁciency 737

•

The technique must replicate the type of corrosion

(general, localized etc.).

•

The results must be presented simply and be easy to

interpret.

•

Some techniques are real-time online measure-

ments, others are not.

•

Most rates are measured with samples in the form

of a probe or coupon; the results will depend on the

geometry (ﬂush, cylindrical, crevice etc.).

Electrical Techniques

Polarization Resistance.

•

These are based on the rates of cathodic and anodic

electrochemical corrosion reactions.

•

They may be difﬁcult to interpret if protective layers

are formed.

•

Other redox reactions may occur at the same time as

the corrosion reaction.

•

Bimetallic and selective corrosion processes cannot

be detected.

Electrical Resistance.

•

This depends upon the change in resistance of a wire

or thin plate due to corrosion and/or erosion,

•

It can be very sensitive: 1–1000 mpy,

•

The metallurgical state of probe may be very un-

representative of the real material; geometry may be

important (e.g., ﬂush, intrusive).

FSM – Electrical Fingerprint.

•

This is similar to electrical resistance in principle.

•

It uses the actual vessel, pipe.

•

It provides nonintrusive, real-time, on-line wall

thickness data.

•

It allows multiple (96) measurements at different

locations e.g. in multiphase ﬂow.

•

Changes in thickness are measured from voltage

changes when current passed.

•

Computer display is provided.

•

It is relatively expensive.

Corrosion Coupons. These are the most widely used

means of monitoring corrosion. Their advantages are

•

cheap to buy,

•

available with standard ﬁnish, weighed, degreased,

•

can give information about localized corrosion,

•

bimetallic and welded coupons can be prepared,

•

effects of stress can be reproduced in C-rings or

four-point load probes can be inserted and retrieved

during operation through access ﬁttings.

The measurements that can be made with coupons in-

clude

•

weight determination after drying

(look out for spalling of deposits, corrosion prod-

uct),

•

weight loss after pickling,

•

percent area of localized corrosion,

•

pit depth by sectioning, grinding,

•

analysis of corrosion products,

•

optical, SEM metallography of corroded surface.

They have a number of limitations.

•

They must be electrically insulated from plant com-

ponents they only provide integrated measurements

of corrosion;

•

They must be of reasonable area (1500–2000 mm

)

to avoid cut-edge effects;

•

The extent of localized corrosion may depend on

area;

•

The surface ﬁnish and material should be the same

as for components to be tested;

•

There is no standard precorroded surface;

•

The location of the coupon is important (ﬂuid ﬂow,

temperature effects).

A special type of corrosion coupon is required

for measurements under heat-transfer conditions. Such

specimens usually have tubular geometries. A number

of features have to be considered in choosing and setting

up such specimens

•

choice of electrical or sensible ﬂuid heating,

•

electrical control is most convenient, and easiest,

•

most industrial heat transfer involves ﬂuids,

•

conditions at the surface depend on the heat-transfer

coefﬁcient,

•

the heat-transfer coefﬁcient depends on the ﬂuid and

the ﬂow conditions, surface temperature or heat ﬂux

control.

Table 12.7 provides information on techniques used in

the ﬁeld and their most important features.

Part D 12.9