In petroleum refineries, corrosion of equipment takes place all through its operating life. It is essential to monitor the corrosion damage so that timely corrective actions like maintenance / repairs / rehabilitation of equipment can be undertaken before it causes unsafe plant operations. The techniques employed for systematic corrosion monitoring of refinery equipment have been described in this presentation.

1. Corrosion Monitoring in
Petroleum Refineries
K.R. SONI,
SAGA GLOBAL CONSULTANTS

2. CORROSION MONITORINGCORROSION MONITORING

Corrosion monitoring may be defined as the
systematic measurement of corrosion rate of
equipment with the object of diagnosis and
controlling corrosion.
 It can also be used for monitoring efficiency of
implementation of corrosion control system.

3. CORROSION MONITORING
 Provides an early warning that damaging process or conditions exist
which may result in corrosion induced failures.
 Indicates the correlation of changes in process parameters and their
effect on system corrosion.
 Diagnoses a particular corrosion problem, identifying its cause and the
rate controlling parameters such as pressure, temperature, pH, flow
rates etc.
 Evaluates the effectiveness of a corrosion control prevention
technique such as chemical inhibition and the determination of
optimal applications.
 Provides management information relating to the maintenance
requirements and ongoing condition of plant.

4. OBJECTIVES OF CORROSION MONITORING
 Quantification of corrosion damage.
 Determining the corrosion rates.
 Understanding corrosion pattern / behaviour.
 Verifying effectiveness of corrosion control
measures.

5. OBJECTIVES OF CORROSION MONITORING
 Enabling implementation of corrective
actions to avoid equipment failures.
 Assessing control of corroding environment.
 Assisting in selection of materials for
withstanding corrosion

6. CORROSION MONITORING METHODOLOGY
 Actual metal loss due to corrosion can be assessed
by calipers, Ultrasonic thickness gauge, corrosion
coupons, corrosion probes or radiography.
 Corrosion rates can then be computed considering
duration of corrosion exposure.
 Corrosion Coupons are installed in parallel side
streams having isolation facility. Weight loss
exhibited by coupons is computed to work out
corrosion rate.
 Analysing for Iron ppm in overhead water.

7. Metal damage assessment Techniques
 Corrosion Monitoring : Direct, On-line
 Non Destructive Testing: Direct, Off-line
 Analytical Chemistry : Indirect, Off-line
 Operational Data : Indirect, On-line

8. CORROSION MONITORING TECHNIQUES
 Ultrasonic Thickness gauging
 Radiographic Examination
 Electrical Resistance Probes
 Polarization Resistance probes.
 Hydrogen Probes
 Corrosion Coupons

9. Ultrasonic thickness (UT) measurements:
• Measures time taken by a pulse of “sound” to travel from outside of a
piece of metal, bounce off the opposite wall (or crack, inclusion, etc.)
and return to the surface.
o Knowing the speed of sound through the metal, the thickness of the metal can
be calculated.
• Accuracy of measurements is ± 10 mils for handheld units.
• Skill of the operator very important for the accuracy of
measurements.

10. ULTRASONIC INSPECTION PRINCIPLEULTRASONIC INSPECTION PRINCIPLE

Ultrasonic inspection involves transmission of very
high frequency sound waves through the metal whose
thickness is required

Piezoelectric crystals are used for generation of
ultrasonics sound waves

Detection of reflected sound pulses from the front
and back faces of metal is done with equipment
electronic circuitary.

The time taken for the sound to travels the thickness
of metal and returned to the probe is given as digital
output in terms of thickness of metal

11. 
Widely used for thickness monitoring of equipment &
piping systems in Refineries.

Systematic thickness recording & comparison with
previous readings for any deterioration and
assessment of corrosion rate.

Advantage - Access to one side of object is sufficient
for thickness measurement.

Disadvantage - Not suitable for pitted or rough
surfaces.
High temperature can destroy piezoelectric properties
of the probe.
ULTRASONIC INSPECTIONULTRASONIC INSPECTION

12. Fig. 12 The priciple of time of flight measurement

13. Fig. 13 Block diagram: Pulse Echo Method

14. Fig. 18 Backwall echo at the 8th scale graduation

15. Fig. 58b
Top view with reflector for extension
CORROSION MAPPING

16. Radiography
• X-rays are beamed through equipment and expose a film on
opposite side of equipment.
• Darkness (density) of the developed film in any given area is
proportional to thinness of the metal.
• Cannot be used easily on large diameter or thick walled
vessels.

17. RADIOGRAPHY INSPECTIONRADIOGRAPHY INSPECTION

Radiography Technique depends on the opacity of
material to either gamma or X-rays and the passage of
radiation through the component reaching on the
photographic film.

The density of the image produced on the film is
related to the thickness and density of the material
under examination.

Used in wall thickness measurement / internal
corrosion / deposits assessment.

Disadvantages of this method is radiation hazards &
time required to complete exposure.

18. Electrical Resistance (ER) probes
 Metal loss measurements are based on the change in
OHMIC Resistance of test probe elements.
 With corrosion, the cross-sectional area of corroding
element reduces, resulting in increase in its electrical
resistance.
 The electrical resistance changes are measured using
the wheat stone bridge principle.
 ER changes are Time Dependant but results can be
assessed faster than corrosion coupons.
 ER Probe remains Installed for its Operational life
 Can be Used to Trigger an Alarm

19. • Gives integrated corrosion rate.
• Corrosion rate measurement in liquid & vapour phase.
• Corrosion measurement made without removal of test
sample.
• Suitable for measurement of uniform corrosion.
• Can detect low corrosion rates.
• Not suitable when pitting or stress corrosion attack are
likely.
• Errors introduced in measurement with metallurgical
condition of probe element & temperature changes.
ELECTRIC RESISTANCE METHODELECTRIC RESISTANCE METHOD

20. Electrical Resistance (ER) Probes
• Variety of probes available
• Probes measure change in electrical resistance of a metal specimen
and convert into a corrosion rate.
• The probes are available in a variety of metallurgies and corrosion
sensitivities.
• ER probes give essentially instantaneous corrosion rate
measurements.
• Meaningful data can be obtained in a few days if corrosion is severe
or the probe is sensitive to corrosion.
• Probes show corrosion at the location of the probe.

21. Presented by : Dr Mabruk Issa Suleiman
Electrical Resistance (ER) probes

22. Presented by : Dr Mabruk Issa Suleiman
Electrical Resistance (ER) probes – its elements

23. ER probes - wire loop probe
•Change in resistance of a thin wire is proportional to the corrosion
rate.
•Measurements are sensitive to rapid changes in temperature.
•Thin, sensitive elements can be damaged by high velocity streams or
particulate matter in streams unless properly shielded.
Electronics
for
measuring
resistance

24. ER probes - wire loop probes
• As corrosion occurs, diameter of the wire decreases and its resistance increases.
• Wire loop probes are very sensitive to pitting corrosion.
R = low R = high
R = low R = high

25. ER probes - cylindrical probe
• The same measurement method as wire loop except a thin metal
cylinder is the sensor.
• The cylinder is more mechanically robust than the wire loop.
• The cylinder is less sensitive to pitting corrosion.
Electronics
for
measuring
resistance
HAC INHIBITOR PROGRAM MONITORING

26. Probe Fittings
 Fixed (Threaded)
 Removable (Under Pressure)
• Special High Pressure Access System used

27. Corrosion Monitoring Probes

28. Monitoring with ER Probes

29. Linear Polarization Resistance (LPR) Probes
 Corrosion Rate indicated / measured
Instantaneously
 A Small Voltage is Applied to an Electrode
in Solution
 Measurement of Current indicates the
Corrosion Rate
 Can only be Used in relatively clean water /
liquids / eletrolytes.
 Will not Work in Gases or Water/Oil
Mixtures

30. LINEAR POLARIZATION RESISTANCE METHODLINEAR POLARIZATION RESISTANCE METHOD

This technique is based on the measurement of apparent
“resistance” of a test cell when it is polarized by a small
voltage of the order of 5 to 20 mV.

Icorr is the corrosion current (amp/m2
) which is readily
converted to metal loss (mm/year).

This method gives instantaneous corrosion rate.

More sensitive to small variation in corrosion rate.

Not suitable for low conducting media.

Not suitable for localised form of corrosion.
K
Rp =
Icorr

31. Hydrogen Probes
• Hydrogen is a by-product of the corrosion
reaction in acidic solutions.
• Hydrogen probes detect hydrogen
permeating through the steel – an
indication of corrosion rate.
• Hydrogen probes are surface mounted.
• Detects rate of hydrogen penetration /
diffusion through pipe wall.

32. CORROSION MONITORING INSTRUMENTATION
 Portable Meters
 Data Loggers
 Continuous Single Channel
 Continuous Multi-channel
 Remote Monitoring Units

33. Presented by : Dr Mabruk Issa Suleiman
Installation of probes

34. CORROSION COUPON METHODCORROSION COUPON METHOD

Weighed specimens are exposed to the environment for a
specified period and the weight loss of metal is measured
thereafter.

From the weight loss, corrosion rate is determined by the
following relationship.
22.3 x Wt loss (mg)Corrosion rate
Mils / year (mpy)
=
Sp. gr. of metal x exposed
area (in. sq) x time (days)

35. DETERMINING PITTING RATE
Max. pit depth x 365Pitting rate (mpy)
Mils/penetration Year
=
Time of test (days)
Types of coupons
• Strip Coupons
• Disc Coupons
• Rod Coupons
• Coupons with applied stress
• Coupons with residual stress
CORROSION COUPON METHODCORROSION COUPON METHOD

36. The advantages of coupons includes:

Visual interpretation

Deposits can be observed and analyzed and layer effects
studied.

Weight loss can be determined.

The degree of localization of corrosion can be observed &
measured.

Inhibitor film effects can be observed.

It gives average corrosion rate.
Used for detection of corrosion in gas / oil pipelines, vessels,
tanks & cooling water system.
CORROSION COUPON METHODCORROSION COUPON METHOD

37. CORROSION COUPONS

38. CORROSION COUPONS
Corrosion Coupon Racks At Inlet Header Of Cooling Towers

39. CORROSION MONITORING - Conclusions
 Corrosion monitoring offers an answer to the
question of whether more corrosion is
occurring today as compared to yesterday !
 It is possible to qualify the cause of corrosion
and quantify its effect.
 Corrosion monitoring remains a valuable
weapon in the fight against corrosion.

40. THANK YOU