1. Inspection and Monitoring
Chapter 7:
Inspection and Monitoring

2. Inspection and Monitoring
Upon completion of this chapter, students will be able to:
Describe the differences between inspection and monitoring
Identify common techniques for:
Inspection
Monitoring

3. Introduction and Definitions
Many systems are so complex that it is impossible to inspect or monitor every component before or during operation.
Organizations often identify the components that are either most likely to result in failure or those components whose failure is most likely to result in significant damage or losses.
This is the basis for risk-based inspection, a concept that has been used for many years in the refining industry and is now being applied to many other industries.
This chapter uses these definitions:
Inspection: Used to determine the condition of a system at the time of inspection.
Monitoring: Used either periodically or continuously as a tool for assessing the need for corrosion control or of the effectiveness of corrosion control methods.
Hydrostatic testing: Involves filling a system with liquid to determine if it has adequate strength to withstand the desired stresses, which often include code-mandated safety factors.
Other tests: Evaluate products to determine their suitability for use in a system, e.g., oil field compatibility testing to determine if scale, hydrate, and corrosion inhibitors will work effectively together.
This chapter does not discuss testing in detail, but you can apply the principles of inspection and monitoring to testing.
A number of NACE standards, technical manuals, and other publications discuss testing in detail.

4. Inspection
Inspections determine whether equipment or structures exposed to the environment still conform to the safe parameters of the original design.
The inspection must establish whether corrosion has consumed the “corrosion allowance.”
The “corrosion allowance” is added to the required wall thickness of equipment or structures on which defects, such as cracking or corrosion may occur.
Corrosion to a depth below the corrosion allowance will result in replacement of the component or a rigorous fitness-for-service evaluation in accordance with API/ASME 579.
Inspections can be scheduled or unscheduled.
Scheduled inspections are planned in advance and, when possible, conducted during scheduled downtime.
Unscheduled inspections usually occur because of a failure and may result in expensive shutdowns.

5. Inspection Inspection groups include a variety of skilled persons:
Reliability engineers
Chemists and microbiologists
Metallurgical, mechanical, or chemical engineers
Mechanical inspectors
Corrosion technicians
The inspection technique you select depends on the type of corrosion you can expect.
Inspection techniques include:
Visual (VI)
Radiography (RT)
Ultrasonic (UT)
Eddy-current (ET)
Dye (liquid) penetrant (DPI)
Magnetic particle (MT)

6. Inspection

7. Inspection Methods Visual
Visual examination is one of the oldest, simplest, and least expensive nondestructive test methods.
Inspectors examine the object visually or with the aid of a magnifying glass or discreet probing with a penknife.
While many visual inspections follow a programmed schedule and checklist, inspectors should look for any unexpected signs of deterioration.
Benefits of visual inspection include the ability to:2
Scan large areas quickly
Identify some forms of corrosion, pit depths, and pitting rates.
Use video techniques can be used in areas where personnel access is restricted, such as inside reactor cooling jackets
Limitations of visual inspection include:2
Must shutdown during internal inspection
Coatings or deposits may need to be removed
Can only identify surface defects

8. Inspection Methods Radiography (X-Ray and Radioactive Isotopes)
Radiography inspection uses penetrating radiation from either an X-ray tube or radioactive source to detect surface and subsurface flaws.
It measures the amounts and absorptive characteristics of the materials between the radiation source and the detector, usually a film or fluorescent screen.
Radiography inspection is useful for detecting voids, inclusions, and pit depths, but is less effective in locating cracks unless the orientation of the crack is known.
Radiography inspection works well for inspecting inaccessible areas, such as the insides of valves and pipes.

9. Inspection Methods Benefits of radiography inspection include:
Can use either electronic cameras or film
Creates permanent record of defects
Requires minimal surface preparation since coatings and thin surface deposits are transparent to x-rays
Works on most materials
Can show fabrication errors, such as incomplete weld penetration
Limitations of radiography inspection include:2
Allows inspection of local areas only
Does not provide depth of defect information with 2D images
Requires access to both sides of inspected equipment
Requires radiation safety measures
Needs free access for placement of radiation source
Misses crack-like defects if not oriented favorably
Expensive

10. Inspection Methods Ultrasonic
Ultrasonic equipment can gauge the nondestructive thickness of in service equipment subject to corrosive attack.
There are two main types of ultrasonic inspection: compression wave for thickness measurements (D-meter), and shear wave for flaw detection.
Ultrasonic techniques include pulse-echo, transmission, and resonance.
With the pulse-echo technique, inspectors only need access to one side of the part they are inspecting. A single transducer sends sound waves into the material and receives the returning echo.
Echoes are produced by internal discontinuities from the back of the part.
The echo from the back of the part reveals the material thickness.
The transmission method uses transducers on both sides of the material to detect internal discontinuities.
The resonance method uses a single transducer and is primarily used to measure thickness.

11. Inspection Methods Benefits of ultrasonic inspection include:2
Requires direct access to only one side of the inspected material
Can be used online
Provides accurate measurement of thickness and flaw depth
Can penetrate thick materials
Permits estimation of maximum allowable pressures based on measurements and ANSI/ASME B31G, API 653, API 510, API/ ASME 579, and similar codes
Limitations of ultrasonic inspection include:2
Usually requires direct access to material surface
Requires extensive training and experience
Many inspections are performed at easily-accessible locations, rather than likely areas of corrosion, e.g., pipe bottom
Less accurate on non-metals
Limited use on thin materials
May not be suitable for online inspection of hot equipment due to temperature limitations

12. Inspection Methods Eddy Current Inspection
Inspectors can perform eddy-current inspection (ET) on any electrically conductive material.
Defects such as cracks, bulges, or corrosion pits alter the flow of electrical current and produce signals that inspectors can analyze and correlate with flaws.
The basic equipment consists of an alternating electrical current source, a connected coil (probe) that inspectors pass near the part they are inspecting, and a voltmeter to measure the voltage across the coil.
Inspectors move the probe across the surface and note any current changes.

13. Inspection Methods Benefits of eddy-current inspection include:
Relatively simple and rapid method
Makes surface defects easier to detect
Works on all nonporous materials
Limitations of eddy-current inspection include:2
Requires extensive training
Complex analysis
Is limited to conductive materials
Has limited penetration depth

14. Inspection Methods Dye Penetrant Inspection (DPI)
Dye penetrant inspection is also called liquid penetrant inspection. Inspectors use DPI to locate crack-like surface defects on a variety of non-porous materials (metals, polymers, and even concrete).
Sometimes, flaws are open to the surface, but are impossible to find visually without aid. These include fine cracks caused by stress corrosion, fatigue, grinding, galling, etc.
Flaws of this type may be found more easily by applying a liquid dye penetrant that becomes visible when a thin layer of absorptive material called a developer, is applied and wicks the penetrant out of the flaws making them more visible.

15. Inspection Methods Benefits of DPI include:
Is a relatively simple and rapid method
Makes surface defects easier to see
Works on all nonporous materials
Limitations of DPI include:
Requires skilled inspectors
Is limited to surface defects
Requires direct access to surface being inspected
Requires chemical cleaning and disposal of dyes

16. Inspection Methods Magnetic Particle Inspection (MPI)
Magnetic particle inspection has two principle advantages over dye penetrant inspection since it can:
Detect near-surface flaws (e.g., hydrogen blisters or weld defects) that would be missed by penetrant inspection.
Sometimes detect smaller flaws than would be detected with penetrant inspection.
The process involves applying a magnetic field, typically with an AC coil or DC “prods,” to the area to be inspected.
Inspectors then spray fine iron powder (dry MPI) or iron powder suspended in a liquid (wet MPI) onto the surface.
In wet MPI, the solution can be non-fluorescing (visible to the naked eye) or fluorescing (visible only under black light). The particles “decorate” flaws due to residual magnetic fields in the structure’s surface.

17. Inspection Methods Benefits of magnetic particle inspection include:
Relatively simple and rapid method
May detect fine cracks missed by visual and dye penetrant inspection
May reveal shallow subsurface flaws
Limitations of magnetic particle inspection include:
Requires extensive training of inspectors
For ferromagnetic material inspection only
Requires clean, smooth surfaces

18. Significance of Inspections
Inspectors need guidance on how to collect and analyze surface deposits and biological samples, and protect them for shipment to analytical laboratories.
Inspections can become so routine that decision makers are not notified when important changes occur.

19. Corrosion Monitoring
Inspection determines the condition of equipment at the time of inspection, while monitoring allows operators to determine if corrosive conditions and corrosion rates are changing.
Both procedures are necessary.
Corrosion monitoring determines the effectiveness of corrosion control methods, such as chemical inhibitor injection.
As process conditions change, monitoring can be used to determine if environments are becoming more or less corrosive.
Corrosion monitoring cannot always accurately identify the extent of corrosion in the most corrosion susceptible parts of complicated systems.

20. Corrosion Probes
Most corrosion monitoring techniques require the insertion of intrusive metal samples of some type into the corrosive fluids.
The corrosion coupon on the Corrosion Probes Inserted into a Three- phase left is exposed into the process piping where it is exposed to produced water, oil, and natural gas.

21. Mass-Loss (Weight-Loss) Coupons
Mass-loss coupons, which are also know as weight-loss coupons, are the most commonly used devices for monitoring corrosion.
Some corrosive environments produce linear corrosion rates (linear rate law), because the reaction products in these environments are soluble and removed from the metal surface.
Most corrosion rates decrease with time (parabolic rate law).
This means that the analysis of weight-loss data from short-term exposures may overestimate the true corrosion rates of equipment.
Pitting and other corrosion mechanisms may not be detected from short-term exposures that are too short for corrosion initiation to be detected.

22. Mass-Loss (Weight-Loss) Coupons

23. Electrical Resistance Probes
Electrical resistance (ER) probes contain a sensing element which is exposed to the process stream.
The sensing element is made from the material of interest.
The electrical resistance of the sensing element is measured either continuously or periodically.
As the cross-sectional area of the sample is reduced by corrosion, the electrical resistance increases.

24. Electrochemical Methods
Electrochemical techniques measure the propensity of metal ions in a metal or alloy to pass into solution, by measurement of potentials and current density, on a corroding electrode system introduced into the process fluid to be monitored.
For electrochemical measurements, the process fluid needs to be sufficiently conductive.
Several electrochemical test methods were adapted to field-test procedures that are useful in corrosion monitoring.
These include:
Linear polarization resistance (LPR)
Tafel extrapolation
Galvanic monitoring
Electrochemical noise (EN)
Electrochemical impedance spectroscopy (EIS)

25. Electrochemical Methods
The first three techniques are suitable for field use in conductive fluids.
Electrochemical noise and AC impedance spectroscopy, while popular in research laboratories, currently cannot produce better results than LPR and Tafel extrapolation.
Electrochemical corrosion monitoring techniques have an almost instantaneous response (seconds or minutes) to changes in fluid corrosivity.
This may be useful in identifying what process changes have produced changes in fluid corrosivity.