Cathodic Protection (chap 15 - API RP 651) Flashcards
What is an anode
electrode of an electrochemical cell at which OXIDATION occurs
Cathode
electrode of an electrochemical cell as which REDUCTION reaction occurs
Electrolyte
Chemical substance containing ions that migrate in an electric field. (refer to the soil or water adjacent to and in contact with the bottom)
Oxidation
loss of electrons
what happens at the anode?
metal corrodes by releasing electrons and forming positive metal ions
What happens at cathode?
Chemical reactions take place using electrons released at the anode
What is the metallic path?
for electrons released at the anode to flow to the cathode
What does electrolyte contains?
contains both negatively charges ion called anions and positively charge ions called cations.
What are the 4 elements of an electrochemical cell?
- anode
- cathode
- metallic path
- Electrolyte
What are most common form of corrosion?
- localized corrosion
- general corrosion
Factors affecting the rate of corrosion? (2)
- Composition of the metal
2. Physical and chemicals properties of the electrolyte.
Composition of the metal (2)
- difference in electrochemical potential between adjacent areas can result from uneven distribution of annoying elements or contaminants within the metal structure.
- 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.
Physical and chemical properties of the electrolyte (2)
- 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.
Explain why soil characteristics is important? (3)
Soil corrosion is a damage mechanism.
- Soil resistivity (soil resists or conducts electric current.)
- Salt present in the soil electrolyte affects the current carry capacity of the soil
- Moisture content, pH, o2 concentration
What is Stray currents (interference currents)
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.
What does the severity of corrosion from stray currents depends on? (4)
- magnitude and density of the current
- Quality or absence of a coating on the affected structures
- Separation and routing of the interfering and affected structures and location of the stray current source
- presences and location of mechanical joint having high electrical resistance.
What is galvanic corrosion?
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).
What does it mean when current takes the path of lease resistance?
The most severe corrosion attack will occur in the area closest to the cathode.
What does the severity of corrosion from galvanic corrosion depends on? (3)
- temperature
- surface area of the cathode and anode
- 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.
How to operate CP for new storage tanks
must be “on” all the time, little interruption wont do harm
Can you add CP for existing tanks?
Studies need to be made, surveys to be performed. If a tank is already corrode , adding a CP can be harmful.
when to use sacrificial anodes
for internal corrosion.
any issues with coating and CP?
make sure that coating select doesn’t act as electrical shielding.
When is CP an effective means of corrosion control?
ONLY when it is possible to pass electrical current between the anode and cathode.
What are potential problems that could affect CP (impressed current/GC? (4)
- tank pads that are concrete, asphalt, oiled sanded
- impervious external linter between tank bottom and anodes
- high resistance soil or rock aggregate pads
- old storage tanks left in place when a new bottom is installed
What are the 2 type of CP?
Galvanic and impressed current.
Limitation of Galvanic?
limited in current output driving voltage and resistance.
more economical on D<60feet.
Advantages of GC?(6)
- no external power supply
- installation is easy
- capital cost is low
- maintenance is low
- interference problems are rare
- less frequency monitoring
Disadvantages of GC? (5)
- potential limited
- current output is limited
- not practical for large structures
- short anode life
- limited to low resistivity soils
What is impressed current?
Anode connect to a DC power source , and a transformer rectifier connect to AC power.
Advantage of impressed current? (3)
- large potential (50x> GC)
- applied to all soil resistivity
- capable of variable output current.
Disadvantage of impressed current? (5)
- stray current might be an issues
- loss of AC power cause of loss of protection
- high maint and ops costs
- frequent monitoring
- high initial capital cost.
types of anode installation for impressed current
- shallow bed
2. deep bed
What is shallow bed installation?
shallow mean “of little depth”, used when soil resistance is high
What is deep bed installation
burred deep in the ground, could protect a range of equipment. for when soil resistance is low.
What kind of measurement are to be made during CP surveys? (8)
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
- Records demonstrating the need for CP
8. Record of effectiveness of CP system.
- as long as the facility is in service
8. keep5 years
- Survey of Impressed current sources
6. surveryof impressed current protectivity facilities
- every 2 months
6. every year
- Survey of the CP to verify that it satisfied applicable criteria
- Survey of CP system without regard of the system
- after adequate polarization
4. every year
- Check all electrical parameters of the soil
2. Survey the CP system to determine it operated properly
- before energizing the system
2. immediately after the system is energized or repaired
most common and potentially the most damaging stray currents are?
Direct Currents.
Is static energy a source of stray current?
No
3 major type of internal corrosion?
- general
- pitting
- environmental cracking
Decisions governing the need for cathodic protection should be based on all but which of the
following?
National Board and ASME recommendations
What is an advantage of using fine particles for the cushion material?
Fine particles provide a more dense cushion to help reduce the influx and outflow of oxygen
What is the main disadvantage in using large size particles in the soil cushion material?
If large particles are used, differential aeration corrosion may result where the particles contact the tank bottom
A properly designed concrete tank cushion constructed on stable, properly prepared subsoil may be effective in all but which of the following?
The need for cathodic protection
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)
falsse
Corrosion of the steel tank bottom installed on a concrete cushion may result from moisture accumulation caused by all but which of the following?
Humidity in the air surrounding the tank
Soil resistivity ? <500 500-1000 1000-2000 2000-10000 >10000
- very corrosion
- corrosive
- moderately corrosive
- mildly corrosive
- less corrosive
The rate of corrosion increases appreciably when pH is < 5.0 and decreases when pH is > 8.0. pH (T/F)?
True
what does chlorides affect?
soil resistivity and acts a depolarizing agent.
pitting can occur at what chloride level?
10 ppm
Sulfate level >200 ppm
indicate high concentration of organic matter
sulfide level > 0.1 ppm
indicate that sulfates have been reduced by bacterial
What is the most common material used as a cushion beneath storage tank bottoms?
clean sand
The higher the resistivity of the sand, the lower the corrosivity of the sand; however, when designing cathodic protection,
the higher the sand resistivity the greater the impact on the overall system resistance and the electrical power required for the cathodic protection system
What is an advantage of using crushed-limestone or clam-shell under tank bottoms?
Water from rain or groundwater makes the environment under the tank alkaline, which may reduce corrosion
Heated tanks or tanks storing hot product can cause
Accelerated corrosion on the external surface due to elevated temperature is the area is wet
Where on the galvanic series is clean and shiny mild steel?
Clean and shiny mild steel has a value of –0.5 to -0.8 volts
The most common metals used as galvanic anodes in soil are
High potential magnesium alloys, standard magnesium, and zinc
Impressed current cathodic protection systems use:
Direct current usually provided by a rectifier
A cathodic protection rectifier has two major components; what are they?
A step-down transformer to reduce the AC supply voltage and rectifying elements to convert AC to DC output
Impressed current anodes used in soil are not made of:
Zinc
There is a membrane installed under the tank.
What would be an option for installing anodes under this tank?
Bore under the tank at a very shallow angle and install the anodes
The number of anodes required to provide cathodic protection depends upon what two factors?
Total current requirements and the expected individual anode discharge rate in the soil
How are impressed current anodes installed?
These anodes are installed either bare or in special backfill material
which is not the best material for impressed current
anodes used in water installations
copper
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?
Between 1 and 2 milliamps per square foot at ambient conditions
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?
From a 12 volt storage battery to a 300-amp welding unit
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?
10% to 50%
API-651 provides criteria for determining the adequacy of cathodic protection of aboveground storage tanks. What document is referenced for a
more detailed description?
NACE RP-01-69
in determining if adequate cathodic protection is being achieved, which of the following would be correct?
A negative (cathodic) potential of at least 850 mV with cathodic current applied
How must the required negative potential be measured?
This potential shall be measured with respect to a saturated copper/copper sulfate reference electrode (CSE) contacting the electrolyte
Name a common method of measuring polarized potential
Using the “instant off” method
In determining if adequate cathodic protection is being achieved, which of the following would be correct?
A minimum of 100 mV of cathodic polarization measured between the tank bottom metallic surface and a standard reference electrode
contacting the electrolyte
The standard method of determining the effectiveness of cathodic protection on a tank bottom is the __________ measurement
Tank-to-soil
How is the tank-to-soil potential measurement performed?
This measurement is performed using a high-impedance voltmeter and a stable, reproducible reference electrode contacting the electrolyte
Packaged galvanic anodes should be back-filled with ______.
Compacted native soil
true statement with regards to installing
galvanic anodes?
Anodes should not be carried or lowered into the excavation by the lead wires
For a typical vertical anode installation, the hole is excavated ____
inches in diameter by approximately ______ feet deep.
8-12” 10-20’
For impressed current cathodic protection systems, when would installation of the anode in the horizontal position be acceptable or practical?
Where rock is encountered at a shallow depth or where soil resistivity
increases markedly with depth
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?
Install anodes in holes drilled at an angle under the perimeter of the tank bottom
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?
No the connection is incorrect, the tank bottom will now act as the
anode resulting in rapid corrosion of the tank bottom
What should be done if
mechanical connections are used?
Mechanical connections should not be buried
What is the most common source of “constant current” interference currents?
Rectifiers energizing nearby cathodic protection system
After a system has been energized, it may take _____ for polarization to a steady state to take place.
Several months
AST is defined as a stationary container with a barrel capacity greater than?
500
controls of interference currents can be done (4)
- design to minimize exposure
- drainage of collected current by use of galvanic anodes
- bonding to provide a metallic return of current collected from the interfered with structure
- managing current output of all sources to minimize interference currents
pure hydrogen fluids are
usually not corrosive
