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2A2-6: Corrosion of Metals > Objective 06: Design and Operational Corrosion Control > Flashcards

Objective 06: Design and Operational Corrosion Control Flashcards

1
Q

When is the first opportunity to control and prevent corrosion within a facility?

A

Design Stage

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2
Q

What are 6 considerations in a facility that a designer must design for?

A
  1. External Environment
  2. Internal Environment
  3. Mechanical Conditions
  4. Operating Conditions
  5. Corrosion Protection
  6. Corrosion Monitoring
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3
Q

What are 3 aspects of the External Environment of a facility?

A
  1. Air
  2. Water
  3. Soils
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4
Q

What are 3 aspects to Mechanical Considerations that designers need to take into account?

A
  1. Stress
  2. Configurations
  3. Materials
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5
Q

What are 2 aspects to corrosion control in operations?

A
  1. Operational Strategies

2. Designed-In Components

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6
Q

What are 2 Operational Strategies used to control corrosion?

A
  1. Corrosion Management

2. Corrosion Inhibiting Chemicals

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7
Q

What are 3 Designed-In Components used to control corrosion?

A
  1. Cathodic Protection
  2. Deaeration
  3. Protective Coatings
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8
Q

What are 9 aspects to a good Corrosion Management Program?

A
  1. Clearly understood policies and procedures
  2. Corrosion prediction and risk assessment
  3. Continuous measuring, monitoring, and tracking
  4. Regular inspections and corrosion analysis
  5. Defined roles and responsibilities for the corrosion program
  6. Consultation with external corrosion “experts”
  7. Operational procedures that minimize corrosion
  8. Effective training of operating personnel
  9. Timely maintenance
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9
Q

Corrosion Inhibitor

A

Chemical substance that is added to a potentially corrosive environment where it reacts to decrease (or prevent) corrosion

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10
Q

Where are corrosion inhibitors added?

A

They are injected at specified locations in the system

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11
Q

What is the operator’s responsibility when it comes to using corrosion inhibitors?

A

They must test regularly and adjust chemical feed to ensure that a minimum chemical residual concentration is always present

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12
Q

What are corrosion inhibitors “inhibiting”?

A

Some inhibit anodic reactions while others inhibit cathodic reactions

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13
Q

What is another name for Anodic Inhibitors?

A

Passivating Inhibitors

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14
Q

How do Anodic Inhibitors work?

A

They react with the metal surface in an oxidizing reaction to produce a protective oxide coating. This coating makes the anode passive (nonreactive with its environment).

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15
Q

What are the 3 most common Anodic Inhibitors?

A
  1. Nitrite
  2. Molybdate
  3. Orthophosphate
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16
Q

What is another name for Cathodic Inhibitors?

A

Precipitating Inhibitors

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17
Q

How do Cathodic Inhibitors work?

A

They chemically precipitate a thin layer, which adheres directly to the metal surface, thus physically isolating the fluid (cathode) from the metal.

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18
Q

What are 2 common Cathodic Inhibitors?

A
  1. Zinc Carbonate

2. Calcium Carbonate

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19
Q

What are 6 different mechanisms that Corrosion Inhibitors can operate by?

A
  1. Adsorbing as a thin protective film on the metal surface
  2. Inducing the formation of a thick, unstable corrosion product
  3. Forming a passive film on the metal surface
  4. Changing the characteristics of the environment
  5. Producing a protective precipitate; or
  6. Removing a corrosion-enhancing constituent, e.g. oxygen scavenging
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20
Q

Why do Steam Condensate Systems require special attention to corrosion inhibition?

A

They are susceptible to pitting from oxygen and carbonic acid formation from carbon dioxide.

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21
Q

What is used in Condensate Systems to inhibit corrosion?

A
  1. Neutralizing Amines

2. Filming Amines

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22
Q

Cathodic Protection

A

Method of reducing or eliminating the corrosion of metallic structures located in corrosive environments, by causing them to act as non-corroding cathodes

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23
Q

How is Cathodic Protection achieved?

A

By ensuring that the electrical potential of the protected surface remains less than that of other metals in the same corrosive environment.

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24
Q

What are 2 methods of Cathodic Protection?

A
  1. Sacrificial Anodes

2. Impressed Current System

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25
Q

How do Sacrificial Anodes work to control corrosion?

A

A more active metal than the surface being protected is placed in a common electrolyte. It becomes the anode because it is significantly higher on the electromotive series than the protected structure and corrodes preferentially.

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26
Q

What are the most common (sacrificial) anodes used?

A

Magnesium and zinc

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27
Q

How can the efficacy of Zinc as a sacrificial anode be improved by 90%?

A

Alloying it with aluminum

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28
Q

Where are Magnesium Alloys primarily used as Sacrificial Anodes?

A

In soils and areas that have high resistance to current flow.

29
Q

Where are Zinc Alloys primarily used as Sacrificial Anodes?

A

Highly conductive electrolytes at ambient temperatures and in applications where sparking must be avoided.

30
Q

What are 5 advantages of Sacrificial Anode Systems for corrosion control?

A
  1. No external power requirements
  2. No voltage regulation requirements
  3. Easy to install and replace anodes
  4. Minimum maintenance requirements
  5. Inexpensive if installed at time of pipeline construction
31
Q

What are 5 disadvantages of Sacrificial Anode Systems for corrosion control?

A
  1. Differential potential depends on the electrolyte
  2. Small area of coverage per anode
  3. Poorly coated pipelines may require many anodes
  4. Protection reduced in highly-resistive soils
  5. Expensive if installed after pipeline construction
32
Q

How do Impressed Current Systems work to control corrosion?

A

A current is created by an external power source by using anodes connected to the positive terminal of a DC power supply and the protected structure connected to the negative side. The DC power supply maintains a flow of electrons to the protected structure, keeping it cathodic.

33
Q

What are 4 advantages of Impressed Current System for corrosion control?

A
  1. Large areas can be protected by a single installation
  2. Can be designed for a wide range of voltage and current
  3. High current flow is available from a single groundbed
  4. Effectively protects bare and poorly coated pipelines
34
Q

What are 4 disadvantages of Impressed Current System for corrosion control?

A
  1. Requires a power source which is subject to interruptions
  2. May cause cathodic interference problems
  3. Requires a higher level of maintenance
  4. Overprotection can damage protective coatings
35
Q

Groundbed

A

A system of many anodes spread in a pattern over a wide area

36
Q

What are 4 general types of Groundbeds?

A
  1. Convention/Remote Groundbed
  2. Distributed Anode Groundbed
  3. Deep Anode Groundbed
  4. Shallow Vertical Groundbed
37
Q

Conventional/Remote Groundbed

A

Group of anodes installed about 100m from the protected structure and spaced 5-10m apart

38
Q

Distributed Anode Groundbed

A

Group of anodes installed along the protected structure, less than 10m from the structure and spaced 10-200m apart

39
Q

What is Distributed Anode Groundbed used for?

A

To avoid interference from nearby structures and to better protect sections of bare or poorly coated metal

40
Q

What is Conventional/Remote Groundbed used for?

A

To distribute protective current over a wide area of the protected structure.

41
Q

Deep Anode Groundbed

A

Group of anodes installed vertically in a drilled hole at depths of 15m+.

42
Q

When is Deep Anode Groundbed used?

A

When space isn’t available for a conventional groundbed or when the upper soil has a high resistivity and the lower soil has a low resistivity.

43
Q

Shallow Vertical Groundbed

A

Group of anodes installed vertically in a drilled hole at a depth of <15m.

44
Q

When is Shallow Vertical Groundbed used?

A

When horizontal space is limited

45
Q

What kind of monitoring is required on Sacrificial Anode Systems?

A

Voltage readings indicate if the system voltage is declining, which indicates anode wastage. At a specified minimum voltage, the anodes must be replaced.

46
Q

What is required of the voltmeter used to monitor a Sacrificial Anode System?

A

It must be very sensitive, since normal voltage levels are very low.

47
Q

What must Impressed Current Systems be monitored for?

A

Current Flow to ensure it is maintained at the most optimal for changing conditions

48
Q

What can be compensated for by adjusting the DC current from the rectifier in an Impressed Current System?

A
  1. Anode wastage
  2. Variations in soil resistance (due to groundwater changes)
  3. Changes in the protected structure
49
Q

If there is a clash between a material’s suitability for its application vs. its corrosion resistance, which takes precedence?

A

Suitability for application

50
Q

When is the application of a protective coating or lining necessary to control corrosion?

A

When traditional methods of corrosion control are not practical or possible

51
Q

What is the primary purpose of Protective Coatings?

A

To physically isolate a metal surface from the environment, so that corrosion chemistry and corrosion cells cannot occur at the surface

52
Q

Where are Protective Coatings most commonly applied?

A
  1. External surfaces

2. Vessel Internals

53
Q

How do Metallic Coatings interact with the surfaces upon which they are applied?

A

They change the surface properties of the parent metal to those of the metal coating. The parent metal becomes a composite material, which exhibits properties not achievable by either material on its own. The coating provides a durable, corrosion resistant layer, while the parent metal provides the strength.

54
Q

What are the most popular Metallic Coatings?

A
  1. Cadmium
  2. Chromium
  3. Nickel
  4. Aluminum
  5. Zinc
55
Q

What are 7 methods for applying Metallic Coatings?

A
  1. Electroplating
  2. Electroless Plating
  3. Thermal Spraying
  4. Hot Dipping
  5. Chemical Vapour Deposition
  6. Ion Vapour Deposition
  7. Galvanizing
56
Q

What is the most widely used method for applying Metallic Coatings?

A

Galvanizing

57
Q

What does Galvanizing Metallic Coatings entail?

A

Coating carbon steel with a thin layer of zinc

58
Q

How is Nickel Metallic Coating applied?

A

A chemical reducing agent reduced nickel ions in an aqueous solution, causing it them to deposit without the use of electrical energy.

59
Q

Why is Nickel Coating widely used to plate items like valves and pump parts?

A

Its exceptional corrosion resistance and hardness

60
Q

How does Metal Cladding improve the corrosion resistance of a material?

A
  1. Metallurgically bonds a surface layer of a metal or alloy with good corrosion resistance to a susceptible core material
  2. It is more anodic than the core material by about 80-100mV, providing cathodic protection thereby
61
Q

Non-Metallic Coating

A

Metallic Oxide produced on a metal surface by chemical treatment

62
Q

What are 3 chemical treatments that produce non-metallic coating?

A
  1. Anodizing
  2. Nitriding
  3. Phosphating
63
Q

What is used as a preparatory step prior to painting or coating a metal?

A

Chemical Treatment of the metal surface

64
Q

What are 3 main purposes that paints and coatings can be developed for?

A
  1. Impermeability
  2. Corrosion Inhibition
  3. Cathodic Protection
65
Q

How do coatings designed for impermeability function?

A

They separate the metal from the corrosive environment and have excellent adhesion to the metal

66
Q

How do coatings designed for corrosion inhibition function?

A

They provide a protective barrier that will react with the environment

67
Q

How do coatings designed for cathodic protection function?

A

They usually contain a base paint (with zinc pigment added) which acts as an anode

68
Q

What are 6 typical Non-Metallic Coatings that are used in industry?

A
  1. Epoxy resin coating
  2. Bituminous coating
  3. Thermoplastic coating
  4. Specialty coating
  5. Elastomeric coating
  6. Inorganic coatings and linings

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

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