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api 653 chapter 8 > Cathodic Protection (chap 15 - API RP 651) > Flashcards

Cathodic Protection (chap 15 - API RP 651) Flashcards

1
Q

What is an anode

A

electrode of an electrochemical cell at which OXIDATION occurs

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2
Q

Cathode

A

electrode of an electrochemical cell as which REDUCTION reaction occurs

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3
Q

Electrolyte

A

Chemical substance containing ions that migrate in an electric field. (refer to the soil or water adjacent to and in contact with the bottom)

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4
Q

Oxidation

A

loss of electrons

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5
Q

what happens at the anode?

A

metal corrodes by releasing electrons and forming positive metal ions

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6
Q

What happens at cathode?

A

Chemical reactions take place using electrons released at the anode

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7
Q

What is the metallic path?

A

for electrons released at the anode to flow to the cathode

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8
Q

What does electrolyte contains?

A

contains both negatively charges ion called anions and positively charge ions called cations.

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9
Q

What are the 4 elements of an electrochemical cell?

A
  1. anode
  2. cathode
  3. metallic path
  4. Electrolyte
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10
Q

What are most common form of corrosion?

A
  • localized corrosion

- general corrosion

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11
Q

Factors affecting the rate of corrosion? (2)

A
  1. Composition of the metal

2. Physical and chemicals properties of the electrolyte.

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12
Q

Composition of the metal (2)

A
  1. difference in electrochemical potential between adjacent areas can result from uneven distribution of annoying elements or contaminants within the metal structure.
  2. Potential and physical difference between the weld metal, HAZ, and based metal are the riving force behind preferential weld corrosion, with with mechanisms such as galvanic corrosions, stress corrosions.
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13
Q

Physical and chemical properties of the electrolyte (2)

A
  • Ion concentration gradients in the electrolyte can provide a potential.
  • Differential aeriation can also generate corrosion. Part of metal exposed to higher o2 acts a cathodic regions and regions with less o2 acts as a anodic regions.
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14
Q

Explain why soil characteristics is important? (3)

A

Soil corrosion is a damage mechanism.

  1. Soil resistivity (soil resists or conducts electric current.)
  2. Salt present in the soil electrolyte affects the current carry capacity of the soil
  3. Moisture content, pH, o2 concentration
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15
Q

What is Stray currents (interference currents)

A

travel through the soil electrolyte and on to the structure for which they are not intended. electricity flow via buildings, ground or equipment due to electrical supply system imbalances or wiring flaw.

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16
Q

What does the severity of corrosion from stray currents depends on? (4)

A
  1. magnitude and density of the current
  2. Quality or absence of a coating on the affected structures
  3. Separation and routing of the interfering and affected structures and location of the stray current source
  4. presences and location of mechanical joint having high electrical resistance.
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17
Q

What is galvanic corrosion?

A

When 2 metal soil different compositions are connected in an electrolyte. (usually soil). Current will flow from the more active metal (anode) to the less active metal (cathode).

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18
Q

What does it mean when current takes the path of lease resistance?

A

The most severe corrosion attack will occur in the area closest to the cathode.

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19
Q

What does the severity of corrosion from galvanic corrosion depends on? (3)

A
  1. temperature
  2. surface area of the cathode and anode
  3. relative potential difference between the materials
    Corrosion tends to be more sever anodic area is small with respect to the cathode surface and the 2 metals are far apart in the galvanic series.
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20
Q

How to operate CP for new storage tanks

A

must be “on” all the time, little interruption wont do harm

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21
Q

Can you add CP for existing tanks?

A

Studies need to be made, surveys to be performed. If a tank is already corrode , adding a CP can be harmful.

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22
Q

when to use sacrificial anodes

A

for internal corrosion.

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23
Q

any issues with coating and CP?

A

make sure that coating select doesn’t act as electrical shielding.

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24
Q

When is CP an effective means of corrosion control?

A

ONLY when it is possible to pass electrical current between the anode and cathode.

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25
Q

What are potential problems that could affect CP (impressed current/GC? (4)

A
  1. tank pads that are concrete, asphalt, oiled sanded
  2. impervious external linter between tank bottom and anodes
  3. high resistance soil or rock aggregate pads
  4. old storage tanks left in place when a new bottom is installed
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26
Q

What are the 2 type of CP?

A

Galvanic and impressed current.

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27
Q

Limitation of Galvanic?

A

limited in current output driving voltage and resistance.

more economical on D<60feet.

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28
Q

Advantages of GC?(6)

A
  1. no external power supply
  2. installation is easy
  3. capital cost is low
  4. maintenance is low
  5. interference problems are rare
  6. less frequency monitoring
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29
Q

Disadvantages of GC? (5)

A
  1. potential limited
  2. current output is limited
  3. not practical for large structures
  4. short anode life
  5. limited to low resistivity soils
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30
Q

What is impressed current?

A

Anode connect to a DC power source , and a transformer rectifier connect to AC power.

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31
Q

Advantage of impressed current? (3)

A
  1. large potential (50x> GC)
  2. applied to all soil resistivity
  3. capable of variable output current.
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32
Q

Disadvantage of impressed current? (5)

A
  1. stray current might be an issues
  2. loss of AC power cause of loss of protection
  3. high maint and ops costs
  4. frequent monitoring
  5. high initial capital cost.
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33
Q

types of anode installation for impressed current

A
  1. shallow bed

2. deep bed

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34
Q

What is shallow bed installation?

A

shallow mean “of little depth”, used when soil resistance is high

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35
Q

What is deep bed installation

A

burred deep in the ground, could protect a range of equipment. for when soil resistance is low.

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36
Q

What kind of measurement are to be made during CP surveys? (8)

A

a. structure to soil potential
b. anode current
c. native structure to soil potentials
d. structure to structure potentials
e. piping to tank isolation if protected separately
f. structure to soil potential on adjacent structures
g. continuity of structure if produced as a single structure
h, rectifier DC volts, DC amps, efficiency and tap settings

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37
Q
  1. Records demonstrating the need for CP

8. Record of effectiveness of CP system.

A
  1. as long as the facility is in service

8. keep5 years

38
Q
  1. Survey of Impressed current sources

6. surveryof impressed current protectivity facilities

A
  1. every 2 months

6. every year

39
Q
  1. Survey of the CP to verify that it satisfied applicable criteria
  2. Survey of CP system without regard of the system
A
  1. after adequate polarization

4. every year

40
Q
  1. Check all electrical parameters of the soil

2. Survey the CP system to determine it operated properly

A
  1. before energizing the system

2. immediately after the system is energized or repaired

41
Q

most common and potentially the most damaging stray currents are?

A

Direct Currents.

42
Q

Is static energy a source of stray current?

A

No

43
Q

3 major type of internal corrosion?

A
  1. general
  2. pitting
  3. environmental cracking
44
Q

Decisions governing the need for cathodic protection should be based on all but which of the
following?

A

National Board and ASME recommendations

45
Q

What is an advantage of using fine particles for the cushion material?

A

Fine particles provide a more dense cushion to help reduce the influx and outflow of oxygen

46
Q

What is the main disadvantage in using large size particles in the soil cushion material?

A

If large particles are used, differential aeration corrosion may result where the particles contact the tank bottom

47
Q

A properly designed concrete tank cushion constructed on stable, properly prepared subsoil may be effective in all but which of the following?

A

The need for cathodic protection

48
Q

Cracks through the concrete cushion may permit water and contaminants to permeate to the steel tank bottom and provide a path for proper electrical current flow. (T/F)

A

falsse

49
Q

Corrosion of the steel tank bottom installed on a concrete cushion may result from moisture accumulation caused by all but which of the following?

A

Humidity in the air surrounding the tank

50
Q 
Soil resistivity ?
<500 
500-1000
1000-2000
2000-10000
>10000
A
  1. very corrosion
  2. corrosive
  3. moderately corrosive
  4. mildly corrosive
  5. less corrosive
51
Q

The rate of corrosion increases appreciably when pH is < 5.0 and decreases when pH is > 8.0. pH (T/F)?

A

True

52
Q

what does chlorides affect?

A

soil resistivity and acts a depolarizing agent.

53
Q

pitting can occur at what chloride level?

A

10 ppm

54
Q

Sulfate level >200 ppm

A

indicate high concentration of organic matter

55
Q

sulfide level > 0.1 ppm

A

indicate that sulfates have been reduced by bacterial

56
Q

What is the most common material used as a cushion beneath storage tank bottoms?

A

clean sand

57
Q

The higher the resistivity of the sand, the lower the corrosivity of the sand; however, when designing cathodic protection,

A

the higher the sand resistivity the greater the impact on the overall system resistance and the electrical power required for the cathodic protection system

58
Q

What is an advantage of using crushed-limestone or clam-shell under tank bottoms?

A

Water from rain or groundwater makes the environment under the tank alkaline, which may reduce corrosion

59
Q

Heated tanks or tanks storing hot product can cause

A

Accelerated corrosion on the external surface due to elevated temperature is the area is wet

60
Q

Where on the galvanic series is clean and shiny mild steel?

A

Clean and shiny mild steel has a value of –0.5 to -0.8 volts

61
Q

The most common metals used as galvanic anodes in soil are

A

High potential magnesium alloys, standard magnesium, and zinc

62
Q

Impressed current cathodic protection systems use:

A

Direct current usually provided by a rectifier

63
Q

A cathodic protection rectifier has two major components; what are they?

A

A step-down transformer to reduce the AC supply voltage and rectifying elements to convert AC to DC output

64
Q

Impressed current anodes used in soil are not made of:

A

Zinc

65
Q

There is a membrane installed under the tank.

What would be an option for installing anodes under this tank?

A

Bore under the tank at a very shallow angle and install the anodes

66
Q

The number of anodes required to provide cathodic protection depends upon what two factors?

A

Total current requirements and the expected individual anode discharge rate in the soil

67
Q

How are impressed current anodes installed?

A

These anodes are installed either bare or in special backfill material

68
Q

which is not the best material for impressed current

anodes used in water installations

A

copper

69
Q

The current required for cathodic protection should be calculated using the results of current requirement tests. In lieu of this test, what is the
generally accepted protective current density?

A

Between 1 and 2 milliamps per square foot at ambient conditions

70
Q

The current requirement test can only be performed on existing tanks using a temporary groundbed and an appropriate source of direct current. Depending on the current required, the power source can vary by how much?

A

From a 12 volt storage battery to a 300-amp welding unit

71
Q

rectifiers with a moderated excess
capacity should be selected to allow for adjustments during the life of the cathodic protection system and to prevent damage due to overloads. What is
the typical excess capacity of such rectifiers?

A

10% to 50%

72
Q

API-651 provides criteria for determining the adequacy of cathodic protection of aboveground storage tanks. What document is referenced for a
more detailed description?

A

NACE RP-01-69

73
Q

in determining if adequate cathodic protection is being achieved, which of the following would be correct?

A

A negative (cathodic) potential of at least 850 mV with cathodic current applied

74
Q

How must the required negative potential be measured?

A

This potential shall be measured with respect to a saturated copper/copper sulfate reference electrode (CSE) contacting the electrolyte

75
Q

Name a common method of measuring polarized potential

A

Using the “instant off” method

76
Q

In determining if adequate cathodic protection is being achieved, which of the following would be correct?

A

A minimum of 100 mV of cathodic polarization measured between the tank bottom metallic surface and a standard reference electrode
contacting the electrolyte

77
Q

The standard method of determining the effectiveness of cathodic protection on a tank bottom is the __________ measurement

A

Tank-to-soil

78
Q

How is the tank-to-soil potential measurement performed?

A

This measurement is performed using a high-impedance voltmeter and a stable, reproducible reference electrode contacting the electrolyte

79
Q

Packaged galvanic anodes should be back-filled with ______.

A

Compacted native soil

80
Q

true statement with regards to installing

galvanic anodes?

A

Anodes should not be carried or lowered into the excavation by the lead wires

81
Q

For a typical vertical anode installation, the hole is excavated ____
inches in diameter by approximately ______ feet deep.

A

8-12” 10-20’

82
Q

For impressed current cathodic protection systems, when would installation of the anode in the horizontal position be acceptable or practical?

A

Where rock is encountered at a shallow depth or where soil resistivity
increases markedly with depth

83
Q

To improve current distribution to the center of the tank in an impressed current cathodic protection system, it may be desirable to do which of the
following?

A

Install anodes in holes drilled at an angle under the perimeter of the tank bottom

84
Q

An impressed current cathodic protection system is installed; the installing contractor connected the positive connection of the rectifier to the
tank. Is this a satisfactory installation?

A

No the connection is incorrect, the tank bottom will now act as the
anode resulting in rapid corrosion of the tank bottom

85
Q

What should be done if

mechanical connections are used?

A

Mechanical connections should not be buried

86
Q

What is the most common source of “constant current” interference currents?

A

Rectifiers energizing nearby cathodic protection system

87
Q

After a system has been energized, it may take _____ for polarization to a steady state to take place.

A

Several months

88
Q

AST is defined as a stationary container with a barrel capacity greater than?

A

500

89
Q

controls of interference currents can be done (4)

A
  1. design to minimize exposure
  2. drainage of collected current by use of galvanic anodes
  3. bonding to provide a metallic return of current collected from the interfered with structure
  4. managing current output of all sources to minimize interference currents
90
Q

pure hydrogen fluids are

A

usually not corrosive

api 653 chapter 8 (36 decks)

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