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2A2-6: Corrosion of Metals > Objective 05: Corrosion Monitoring > Flashcards

Objective 05: Corrosion Monitoring Flashcards

1
Q

Corrosion Monitoring

A

A combination of continuous and intermittent activities for the purpose of obtaining comprehensive information on corrosion conditions over time

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2
Q

Why is the purpose of Corrosion Monitoring?

A
  1. Proactively quantify and control corrosion rates

2. Avoid conditions that could lead to corrosion-related failures

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3
Q

What does Real-Time Corrosion Monitoring require?

A

Simultaneous linking of process parameter changes with measured corrosion information, from sensitive instrumentation

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4
Q

When is Corrosion Monitoring useful?

A

When the information gathered is applied to an effective corrosion control program

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5
Q

What are 6 methods used to monitor either the actual corrosion of metal or the corrosive nature of the environment?

A
  1. Visual Inspection
  2. Corrosion Coupons
  3. Corrosion Probes and Meters
  4. Microbiological Testing
  5. Chemical Analysis
  6. Non-Destructive Examination
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6
Q

When is Visual Inspection carried out for Corrosion Monitoring?

A
  1. When a plant or piece of equipment is shut down and opened
  2. External surfaces of piping, vessels, supports, etc. where atmospheric corrosion is a concern
  3. Inside heaters, furnaces, and flue gas paths where products of combustion could create corrosive conditions
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7
Q

What are 6 ways to describe the corrosion condition of an internal surface?

A
  1. No Corrosion
  2. Definite Surface Corrosion
  3. Shallow Metal Attack
  4. Pitted or Grooved
  5. Blistering or Scaling
  6. Cracking
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8
Q

No Corrosion surface condition

A

The metal surface appears to be entirely unaffected by corrosion

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9
Q

Definite Surface Corrosion surface condition

A

Corrosive attack is deep enough to catch a knife blade

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10
Q

Shallow Metal Attach surface condition

A

The removal of a perceptible, although barely measurable, amount of metal from the surface

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11
Q

Pitted or Grooved surface condition

A

The metal is visibly removed to a measurable depth in the form of pits or grooves where their size, shape, and distribution are recognizable

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12
Q

Blistering or Scaling surface condition

A

Thin layers of metal appear to have detached or sloughed off from the metal surface to form blisters or scale

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13
Q

What Corrosion Mechanism is indicated by Blistering or Scaling?

A

Hydrogen Diffusion

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14
Q

Cracking surface condition

A

Cracks are visible to the naked eye, such that size, quantity and direction of cracks can be easily determined

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15
Q

Corrosion Coupons

A

Carefully machined pieces of metal that are exposed directly to the process stream (i.e. corrosive environment)

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16
Q

How are Corrosion Coupons evaluated?

A

They are periodically removed and either visually inspected or sent to a lab to test for mass loss, corrosion rate, corrosion type, and severity

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17
Q

What are 3 things that Corrosion Coupon evaluation gives insight to?

A
  1. System Corrosiveness
  2. Material Performance
  3. Inhibitor Performance
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18
Q

How are Corrosion Coupons measured?

A

They are made of the same metal as the pipe or vessel and weighed to an accuracy of 4 decimal places before being inserted into the system and again after removal, to determine the mass of metal lost to corrosion.

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19
Q

Why are Corrosion Coupons often installed in pairs?

A

For redundant, comparative measurement

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20
Q

What are different shapes and configurations that Corrosion Coupons may be found in?

A
  1. Flat Plates, either solid or with holes drilled through
  2. Round, metal rods, either singly or in groups of 6 or 8
  3. Banded coupons, in which a band is wrapped around a conventional coupon, creating a crevice
  4. Spools or nipples, threaded or flanged into the system in such a way that they can be removed for evaluation
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21
Q

What is the benefit of using a Banded Corrosion Coupon?

A

Its crevice creates a concentration cell so attacks occur under the band and therefore this can be useful in identifying oxygen corrosion

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22
Q

How must Corrosion Coupons be mounted?

A

In such a way that they are electrically isolated from contact with all other metals, thus preventing galvanic corrosion (except when testing for galvanic corrosion)

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23
Q

What is required of Corrosion Coupon Holders?

A
  1. Mounting materials should be fully resistant to the environment to avoid loss of data or loss of electrical isolation
  2. Holders must have isolation valves for positive isolation removal of the coupons
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24
Q

Can Corrosion Coupon Holders be retractable while system is under full pressure?

A

Yes

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25
Q

How should Corrosion Coupons be handled?

A
  1. They are kept in special treated envelopes before and after exposure
  2. They must not be touched with bare fingers when mounting them in the Coupon Holder to avoid finger print oil from interfering with corrosion
    3, After removing a coupon from the rack, the coupon must be air dried, but not cleaned, before being placed in a treated envelope for delivery to the lab
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26
Q

How can Corrosion Coupons be used to assess Corrosion Inhibitor performance?

A
  1. The uninhibited corrosion rate of a system is determined and used as a base corrosion rate
  2. Corrosion inhibitor feed is started
  3. Corrosion Coupons are installed in the system
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27
Q

What is the mathematical formula to determine the percent protection afforded by Chemical Corrosion Inhibitor?

A

(uninhibited rate - inhibited rate)/uninhibited rate x 100

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28
Q

Corrosion Probes and Meters

A

Devices/instruments that can be inserted into the path of a process flow or into a vessel for the purpose of measuring the corrosiveness of the fluid via the probe’s connection to external instruments which provide a reading relative to the corrosion

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29
Q

What are 4 common Corrosion Probes/Meters?

A
  1. Electrical Resistance Probes
  2. Galvanic Probe Meters
  3. Linear Polarization Meters
  4. Hydrogen Probes
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30
Q

What is the relationship between the cross-sectional area of a conductor, and its electrical resistance?

A

Inversely proportional

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31
Q

How do Electrical Resistance Corrosion Rate Meters work?

A
  1. They have a probe containing 2 similar elements: a corrosion rate element exposed to the corrosive environment, and a reference element protected inside the probe from the corrosive environment
  2. As the corrosion element corrodes, its cross-sectional area decreases, causing its electrical resistance to increase measureably
32
Q

What is the purpose of a reference element in Electrical Resistance Probes?

A

Since the resistance of the corrosion element is affected by temperature changes, the reference element provides compensation for any temperature variations

33
Q

In which temperature environment is it useful to have a reference element along with the corrosion element in Electrical Resistance Probes?

A

Lower temperature environments

34
Q

What kind of Electrical Resistance Probe is used for medium to high temperature environments?

A

Variable Temperature Electrical Resistance (VTER) probe with associated electronics which uses a single corrosion element without a reference element

35
Q

Why can Electrical Resistance Probes be used to measure corrosion in gas systems?

A

They don’t require an electrolyte to carry current

36
Q

What are cons of using Electrical Resistance Probes to measure corrosion in gas systems?

A
  1. Accumulation of corrosion products, e.g. iron sulphides, on the probe can cause resistance to decrease which falsely suggests a decrease in corrosion rate
  2. The probe does not reliably measure pitting so it must be removed regularly and visually checked for pitting
37
Q

How do Galvanic Probe Meters work?

A
  1. 2 dissimilar metals are immersed in the process fluid (which acts as the electrolyte) and connected electrically to a meter
  2. A current flows due to the potential differences of the metals and this current is proportional to the corrosiveness of the system
38
Q

What does a typical Galvanic Probe Meter Assembly consist of?

A

Brass and steel electrodes, connected to an ammeter to measure current flow

39
Q

What does Galvanic Probe Meters measure exactly?

A

Changes in the corrosiveness of the system (as opposed to a direct measurement of corrosiveness)

40
Q

What are Linear Polarization Meters useful for?

A

To indicate the corrosion rate of a coupon or electrode at the moment of measurement

41
Q

How are Linear Polarization Meters used?

A

To continuously monitor any fluctuations within a system (e.g. corrosion inhibitor dosage) and their effects on the corrosion

42
Q

What are 6 systems that Linear Polarization Meters are typically used in?

A
  1. HP water systems
  2. Cooling water systems
  3. Condensate systems
  4. Potable water treatment and distribution systems
  5. Pulp and paper manufacturing
  6. Amine systems
43
Q

How do Linear Polarization Meters work?

A
  1. A small current is caused to flow between a test electrode and a working electrode
  2. An electrolyte, e.g. water, must be present to conduct current between the probes
  3. The current flow alters the electrical potential of the “test” electrode
  4. The corrosion rate is proportional to the test current divided by the change in potential when the potential change is small (20 mV or less)
  5. Sufficient current is applied to the test electrode to change its potential by a specific amount (e.g. 10 mV)
  6. The current flow required to make the change is directly measured
44
Q

How many electrodes in are used in Linear Polarization Meters?

A

2 or 3

45
Q

How do 2-electrode Linear Polarization Meters differ from 3-electrode in how they work?

A

The reading from the 2-electrode probe must be corrected for the resistivity of the process fluid. The potential change of the test electrode is measured with reference to the second, working electrode.

46
Q

How do 3-electrode Linear Polarization Meters differ from 2-electrode in how they work?

A

It requires no correction for solution resistivity. The third electrode has no current applied to it and is allowed to corrode freely. It serves as a reference electrode.

47
Q

What do Hydrogen Probes measure?

A

The flow of atomic hydrogen that is generated from corrosion and which passes through steel, causing hydrogen-induced corrosion

48
Q

What areas are most susceptible to Hydrogen-Induced Corrosion?

A

Carbon steel pressure vessels and piping that contains process fluids with H2S, cyanide, or arsenic

49
Q

How do the 3 basic types of Hydrogen Probes work?

A
  1. A thin-walled carbon steel tube with a solid rod inside the tube to form a small annular space. This probe is inserted into the flow path. Any hydrogen atoms that escape from the carbon steel collect in the annular space and combine to form hydrogen molecules, which are too large to pass back into the process. As the hydrogen gas accumulates, the pressure in the annular space increases and registers on an external pressure gauge.
  2. Patch Probes are attached and sealed against the outside of the process piping wall and collects hydrogen atoms that penetrate to the outside of the pipe wall.
  3. Palladium Foil type electrochemical cell produces an electrical output proportional to the hydrogen evolution rate
50
Q

How is Microbiological Corrosion detected generally?

A

The same measurements used to detect chemical fouling and corrosion

51
Q

How is the presence of biofilm determined?

A

Counts of biological organisms cultured from water samples

52
Q

What are 2 cons of Microbiological Culturing Techniques?

A
  1. They require at least 24 hours for incubation

2. The data obtained does not necessarily correlate with surface fouling

53
Q

How do most facilities deal with the fact there is no definitive means of determining the onset of biofilm formation?

A

They apply biocide treatments on a preset schedule or as needed, corresponding to visual evidence of growth.

54
Q

How can Chemical Analysis be used to detect corrosion?

A

Since a primary indication of corrosion is the accumulation of iron in the process fluid, it is common practice to test for iron concentration

55
Q

What are the 2 tests that must be performed for Chemical Analysis?

A
  1. Filtered Iron Testing

2. Total Iron Testing

56
Q

What is the procedure for Filtered Iron Testing?

A
  1. A sample is filtered across a 0.2 micron filter pad
  2. The filtrate is tested for iron
  3. If the filtered sample is greater than 0.25 PPM iron, then corrosion control is inadequate
  4. The filter paper can be saved as a qualitative measure of system cleanliness
57
Q

Why does running a Filtered Iron Test give a snapshot of corrosion tendencies?

A

Any iron that does not pass through a filter paper can safely be assumed to be the result of corrosion

58
Q

What is the procedure of Total Iron Testing?

A
  1. A sample is boiled with hydrochloric acid and hydrogen peroxide
  2. It is then reconstituted with distilled water
  3. All iron present in water, whether soluble or insoluble, is identified
59
Q

What does the iron derived from the Filtered Iron Test represent?

A

Insoluble ferric iron which has usually flaked off a corrosion site and been transported in the water stream

60
Q

How is the amount of soluble iron present derived?

A

The total iron measurement minus the filtered iron reading

61
Q

What does the amount of insoluble iron in the fluid indicate?

A

That corrosion has occurred

62
Q

What does the amount of soluble iron in the fluid indicate?

A

The corrosive potential of the water and the likelihood that iron will deposit on downstream heat transfer surfaces

63
Q

What is detected by NDE techniques?

A

Surface and subsurface corrosion

64
Q

Can NDE techniques be used on equipment while it is in service?

A

Sometimes

65
Q

What is the procedure for Ultrasonic Testing?

A
  1. High frequency sound waves are transmitted into the material being inspected
  2. The waves travel through the material until they encounter the opposite surface or an internal discontinuity
  3. Material interfaces or discontinuities reflect the sound back to its origin, where it can be detected
  4. The time interval between transmission of the ultrasonic wave and the arrival of the reflected wave back at the point of transmission is directly proportional to metal thickness or the depth of discontinuity
66
Q

How are Ultrasonic Waves produced for Ultrasonic Testing?

A

They are generated by transducers constructed from piezoelectric materials, e.g. quartz

67
Q

Why are Piezoelectric materials used for Ultrasonic Testing?

A

They are capable of expanding and contracting when subjected to a changing electrical field and will also produce an electrical potential when the material is placed under mechanical stress. Therefore, ultrasonic transducers can be placed on a metal surface and used to both create and detect ultrasonic energy.

68
Q

How does Guided Wave Testing (GWT) differ from Ultrasonic Testing?

A

It uses much lower ultrasonic frequencies

69
Q

How does Guided Wave Testing work?

A

It propagates mechanical stress waves along an elongated structure while guided by its boundaries and with minimal energy loss to quickly identify any discontinuities

70
Q

What follow up is required when using Guided Wave Testing?

A

Where a possible defect is identified, follow-up inspection with conventional Ultrasonic Testing or other NDE method is used to obtain detailed information on the nature and extent of the defect

71
Q

How does Radiographic Inspection work?

A
  1. X-rays or gamma rays are emitted from a source and passed through the object being inspected, onto a piece of photographic film
  2. The film is developed into a radiograph
  3. Pits or corroded areas show up as dark spots on the radiograph
72
Q

What is Magnetic Particle Testing primarily used for?

A

To locate cracks that extend to the surface of a metal

73
Q

How does Magnetic Particle Testing work?

A
  1. The metal to be inspected is magnetized using an electromagnet or a large permanent magnet
  2. Cracks in the metal surface will break the magnetic lines of force and produce local leakage fields
  3. When finely divided particles, e.g. iron powder, are sprinkled on the magnetized surface, they are attracted to the cracks by the localized fields, making it possible to determine their size and exact location
74
Q

What is Dye Penetrant Inspection used for?

A

To detect flaws or cracks that extend to the surface of a metal

75
Q

How does Dye Penetrant Inspection work?

A
  1. A thin liquid with low surface tension is applied to the clean metal surface
  2. By capillary action, the liquid draws into any cracks or surface discontinuities
  3. Excess liquid is wiped from the surface and a developer is applied
  4. The developer draws the liquid back out of the defect, making it easily visible
76
Q

What are the 2 types of penetrants used in Dye Penetrant Inspection?

A
  1. Red Dye which provides good contrast with white developer

2. Dissolved substance that makes the penetrant fluorescent so the defects become easily visible under UV light

2A2-6: Corrosion of Metals (7 decks)

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