Cathodic Protection (chap 15 - API RP 651)

Question 1

What is an anode

Answer 1

electrode of an electrochemical cell at which OXIDATION occurs

Question 2

Cathode

Answer 2

electrode of an electrochemical cell as which REDUCTION reaction occurs

Question 3

Electrolyte

Answer 3

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

Question 4

Oxidation

Answer 4

loss of electrons

Question 5

what happens at the anode?

Answer 5

metal corrodes by releasing electrons and forming positive metal ions

Question 6

What happens at cathode?

Answer 6

Chemical reactions take place using electrons released at the anode

Question 7

What is the metallic path?

Answer 7

for electrons released at the anode to flow to the cathode

Question 8

What does electrolyte contains?

Answer 8

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

Question 9

What are the 4 elements of an electrochemical cell?

Answer 9

1. anode
2. cathode
3. metallic path
4. Electrolyte

Question 10

What are most common form of corrosion?

Answer 10

• localized corrosion

- general corrosion

Question 11

Factors affecting the rate of corrosion? (2)

Answer 11

1. Composition of the metal

2. Physical and chemicals properties of the electrolyte.

Question 12

Composition of the metal (2)

Answer 12

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.

Question 13

Physical and chemical properties of the electrolyte (2)

Answer 13

• 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.

Question 14

Explain why soil characteristics is important? (3)

Answer 14

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

Question 15

What is Stray currents (interference currents)

Answer 15

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.

Question 16

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

Answer 16

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.

Question 17

What is galvanic corrosion?

Answer 17

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).

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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.

Question 20

How to operate CP for new storage tanks

Answer 20

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

Question 21

Can you add CP for existing tanks?

Answer 21

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

Question 22

when to use sacrificial anodes

Answer 22

for internal corrosion.

Question 23

any issues with coating and CP?

Answer 23

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

Question 24

When is CP an effective means of corrosion control?

Answer 24

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

Question 25

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

Answer 25

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

Question 26

What are the 2 type of CP?

Answer 26

Galvanic and impressed current.

Question 27

Limitation of Galvanic?

Answer 27

limited in current output driving voltage and resistance. | more economical on D<60feet.

Question 28

Advantages of GC?(6)

Answer 28

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

Question 29

Disadvantages of GC? (5)

Answer 29

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

Question 30

What is impressed current?

Answer 30

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

Question 31

Advantage of impressed current? (3)

Answer 31

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

Question 32

Disadvantage of impressed current? (5)

Answer 32

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.

Question 33

types of anode installation for impressed current

Answer 33

1. shallow bed | 2. deep bed

Question 34

What is shallow bed installation?

Answer 34

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

Question 35

What is deep bed installation

Answer 35

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

Question 36

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

Answer 36

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

Question 37

7. Records demonstrating the need for CP | 8. Record of effectiveness of CP system.

Answer 37

7. as long as the facility is in service | 8. keep5 years

Question 38

5. Survey of Impressed current sources | 6. surveryof impressed current protectivity facilities

Answer 38

5. every 2 months | 6. every year

Question 39

3. Survey of the CP to verify that it satisfied applicable criteria 4. Survey of CP system without regard of the system

Answer 39

3. after adequate polarization | 4. every year

Question 40

1. Check all electrical parameters of the soil | 2. Survey the CP system to determine it operated properly

Answer 40

1. before energizing the system | 2. immediately after the system is energized or repaired

Question 41

most common and potentially the most damaging stray currents are?

Answer 41

Direct Currents.

Question 42

Is static energy a source of stray current?

Answer 42

No

Question 43

3 major type of internal corrosion?

Answer 43

1. general 2. pitting 3. environmental cracking

Question 44

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

Answer 44

National Board and ASME recommendations

Question 45

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

Answer 45

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

Question 46

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

Answer 46

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

Question 47

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

Answer 47

The need for cathodic protection

Question 48

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)

Answer 48

falsse

Question 49

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

Answer 49

Humidity in the air surrounding the tank

Question 50

``` Soil resistivity ? <500 500-1000 1000-2000 2000-10000 >10000 ```

Answer 50

1. very corrosion 2. corrosive 3. moderately corrosive 4. mildly corrosive 5. less corrosive

Question 51

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

Answer 51

True

Question 52

what does chlorides affect?

Answer 52

soil resistivity and acts a depolarizing agent.

Question 53

pitting can occur at what chloride level?

Answer 53

10 ppm

Question 54

Sulfate level >200 ppm

Answer 54

indicate high concentration of organic matter

Question 55

sulfide level > 0.1 ppm

Answer 55

indicate that sulfates have been reduced by bacterial

Question 56

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

Answer 56

clean sand

Question 57

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

Answer 57

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

Question 58

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

Answer 58

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

Question 59

Heated tanks or tanks storing hot product can cause

Answer 59

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

Question 60

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

Answer 60

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

Question 61

The most common metals used as galvanic anodes in soil are

Answer 61

High potential magnesium alloys, standard magnesium, and zinc

Question 62

Impressed current cathodic protection systems use:

Answer 62

Direct current usually provided by a rectifier

Question 63

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

Answer 63

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

Question 64

Impressed current anodes used in soil are not made of:

Answer 64

Zinc

Question 65

There is a membrane installed under the tank. | What would be an option for installing anodes under this tank?

Answer 65

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

Question 66

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

Answer 66

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

Question 67

How are impressed current anodes installed?

Answer 67

These anodes are installed either bare or in special backfill material

Question 68

which is not the best material for impressed current | anodes used in water installations

Answer 68

copper

Question 69

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?

Answer 69

Between 1 and 2 milliamps per square foot at ambient conditions

Question 70

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?

Answer 70

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

Question 71

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?

Answer 71

10% to 50%

Question 72

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

Answer 72

NACE RP-01-69

Question 73

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

Answer 73

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

Question 74

How must the required negative potential be measured?

Answer 74

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

Question 75

Name a common method of measuring polarized potential

Answer 75

Using the “instant off” method

Question 76

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

Answer 76

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

Question 77

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

Answer 77

Tank-to-soil

Question 78

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

Answer 78

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

Question 79

Packaged galvanic anodes should be back-filled with ______.

Answer 79

Compacted native soil

Question 80

true statement with regards to installing | galvanic anodes?

Answer 80

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

Question 81

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

Answer 81

8-12” 10-20’

Question 82

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

Answer 82

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

Question 83

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?

Answer 83

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

Question 84

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?

Answer 84

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

Question 85

What should be done if | mechanical connections are used?

Answer 85

Mechanical connections should not be buried

Question 86

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

Answer 86

Rectifiers energizing nearby cathodic protection system

Question 87

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

Answer 87

Several months

Question 88

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

Answer 88

500

Question 89

controls of interference currents can be done (4)

Answer 89

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

Question 90

pure hydrogen fluids are

Answer 90

usually not corrosive