NACE CP1 Book Chapters 5-10
Do not use larger than a ____ charge for carbon steel pressure pipe when thermite welding wires onto steel
15 gram
Anode efficiency is generally about ____ for Mg, ____ for Zn, and ____ for Al (but can vary)
50%, 90%, 90% (range)
The chemical backfill for Mg anodes consists of ____% gypsum (CaSO4), ____% benonite clay, ____% sodium salfate, and for Zn ____% gypsum and ____% benonite clay. What is its purpose? (3)
75, 20, 5, 50, 50; low resistivity, absorbs and holds moisture, uniform environment
Always measure what voltage first?
AC
Typical sources of stray current (6)
CP systems, mass transit systems, welding, transmission lines, HVDC transmission systems, telluric currents
What type of current is the concern when it comes to stray current?
DC
Pipeline coatings designed for underground use do not have good resistance to ____ light, so ____ is to be avoided
UV, sunlight
Wherever possible, isolating joints should be installed....
above grade (or needs test station)
What is the purpose of backfill in prepackaged anodes? (2)
absorbs water, absorbs corrosion products
If anodes have not been backfilled properly, the backfill will not fully serve its purpose and deterioration of the anode will ____
accelerate
The efficiency of a galvanic anodes depends on the ____ of the anode and the ____ to which it is installed
alloy, environment
Coatings formulated from coal tar pitch or natural asphalt, reinforced with glass and a felt wrapper, used less today because of toxicity
bituminous enamels
If contact to only one side of an isolating joint is available, stray current control by means of a ____ could not be achieved
bond
CP levels: portion of structure intended for CP is isolated
broken continuity bonds
It's better to install more test stations rather than fewer, but where should test stations definitely be installed?
buried isolating fittings, crossings, electrical bonding of joints
If rectifier current output drops suddenly, maybe not to zero, this is caused by...
cable break (current returning to negative cable through electrolyte, accelerated corrosion at point of discharge)
What is the #1 cause of anode failure?
cable insulation defects
ICCP anodes are backfilled in ____ ____. What is its purpose? (3)
carbonaceous material (coke breeze); reduces anode-to-ground resistance, increase current output capacity (by extending anode surface area), reduces consumption of the anode (backfill conductive, becomes part of anode)
Before touching the rectifier case, measure....
case-to-ground voltage (should be virtually zero)
What can help avoid the use of bonds and their installation/maintenance problems to help overcome stray current problems?
cathodic protection
What are telluric currents?
caused by disturbances in the earth's magnetic field by sun spot activity
How does sunlight exposure affect a coating?
changes porosity, brittle (changes properties)
Underground mechanical connections are made with a ____ and a ____ epoxy for test leads
clamp, conductive (not preferred, easily broken by corrosion)
What is the first line of defense in corrosion control programs?
coating
What is stray current?
current flowing on a structure that is not part of the intended circuit
____ shapes are better for good coupon-soil contact
cylinders
What should be checked for visually during coating application?
disbonding, damage, checking, cracking, chalking, etc. (check if reconditioning is required)
Telluric currents ____ cause corrosion, just ____ error
don't, measurement
What are potential measurement test stations used to monitor? (3)
effectiveness of CP, stray current effects, locate areas of active corrosion (no. 12 AWG insulated wires, thermite-welded to pipe)
If a combustible atmosphere is present, an ____ ____ must be installed around location
electrical bond
If you have to use dissimilar metals, what must you do?
electrically isolate them
What occurs when a low resistance electrolyte such as water/mud gets into the annular space between the casing and pipe?
electrolytic couple
Coatings that consist of polyolefin or polyethylene extruded over butyl or asphalt mastic adhesive, high impact resistance and resist holiday development
extruded polyolefin and polyethylene
Dynamic stray currents are manifested by...
fluctuations in structure-to-electrolyte potentials and variations in line current flow
Coatings that consist of powdered resin electrostatically sprayed onto heated pipe, moisture absorption can be a problem
fusion boned epoxy (FBE)
What are the following materials used for: heat shrink sleeves, hot and cold applied tapes, two-component liquid resins, coal tar mastics, fusion bonded epoxy?
girth weld coatings
CP levels: groundbed resistance increases as anodes fail until rated voltage output of rectifier can't provide needed current
groundbed malfunctions
When there is a possibility of an explosive mixture of a hydrocarbon, the affected area will be designated as ____
hazardous
Solvents, acids, caustics, chemicals in reference electrodes are all ____ and should have ____
hazardous, SDS sheets
If the circuit breaker has tripped, a ____ current or ____ has occured
high, overload (reduce V output and reset breaker, if it trips again it's permanent short circuit)
If the structure is coated, it is necessary to protect only the metal exposed at ____
holidays
CP levels: high resistance contact between reference electrode and soil, contaminated reference electrodes, high structure contact resistance, cracked wires, faulty meters
inaccurate measurements
Groundbed resistance ____ as it dies out, needs to be replaced
increases
In times of drought or low soil moisture content, soil resistivity ____
increases
As anode size diminishes, resistance of anode to electrolyte ____, and as resistance ____, current output for given voltage ____. This is called...
increases, increases, drops; anode deterioration
Gas venting problems in deep groundbeds ____ anode-to-earth resistance, ____ rectifier current output
increases, reducing
Coupons allow for ____ ____ ____ measurements without interrupting the rectifier, and can measure protective current density of a ____ of known size
instant off potential, known size
Highest galvanic anode potential, but low efficiency, work best when discharging current
magnesium
What should be monitored through mitigation bonds?
magnitude and direction of current
What is the main reason for monitoring CP effectiveness?
make sure corrosion is under control
What causes a metallic short, and how can it effect a CP system? How is the severity determined? (if shorted structure is large, bare, unprotected structure, or if they are both protected)
metallic contact, potentials of the protected structure may not meet criteria; become dependent on each other to maintain CP; relative structure-to-electrolyte potentials and surface area of shorted structure (especially if less electronegative)
In complex structures, always select materials that are ____ each other in the galvanic series that will exhibit ____ voltage differences
near, little to no
With a functioning electrical isolating device, the side with the current source interrupted will have a more ____ "on" potential than the "off" potentials, and the two values will be cycling on the same timing as the current interrupter
negative
Cabling from structure to rectifier
negative (return) circuit
Rectifier zero current and voltage outputs and what to do
no input power to unit, or open circuit within rectifier; determine if input AC V is present, and if yes, open circuit exists (could be due to tripped circuit breaker)
If short circuit is external to rectifier, the circuit breaker will ____
not trip
Rectifier zero current output with unchanged voltage output
open fuse in output circuit (short), open positive or negative lead wire, failed groundbed
Rectifier significant current change with unchanged voltage (if increase, if decrease)
output circuit resistance changed; increased current=lower resistance (system additions, shorts to other structures, major coating damage); decreased current=higher resistance (inline isolators, groundbed deterioration, drying soil)
Rectifier significant changes in both voltage and current outputs (if one-half)
partial failure of rectifier stacks (half-waving=hill, space, hill on diode graph) (if operating properly, investigate transformer for winding-to-winding shorts)
A metallic short in a casing will cause an electro____ shift in the pipe-to-electrolyte potential along the structure in the area of the casing
positive
What is essential for the coating to bond properly to the metal?
proper surface preparation and application (hot or cold)
Mitigation bonds
provide metallic path between affected structure and source of stray current, permits current to be drained through cable back to source (prevents from leaving structure through earth), possibly with resistor to control the amount of current drained (way to control stray current) (diode needed to prevent reversal of current flow if dynamic)
Avoid situations where gases generated by a cathodic protection system might collect, referred to as ____ ____ and should be vented away from CP equipment
reaction products
CP levels: structure-to-electrolyte potentials vary in proportion to current applied
reduced CP current
The shape and size of galvanic anodes depend on the ____ of the soil
resistivity
Listen for unusual noises, look for signs of heat (discoloration), look for vent obstructions, look for significant output changes, smell for unusual odors, feel for unusual heat
routine rectifier maintenance
Travel, electrical, environment, hazardous materials, trenches, animals, working at heights, working underwater, etc.
safety hazards to consider
CP levels: high current drain on system, resulting in more electronegative potential in addition to shielding the pipe inside
shorted casing
Audio tone pipe locators are very useful for finding...
shorted isolating fittings, underground contacts (transmitter creates low frequency signal that is carried to pipe through tests wires)
CP levels: structure with less electronegative potential connected, potential of protected structure will become less electronegative
shorted isolation, underground contact to foreign structure
Wires should be installed with ____ to prevent strain
slack
CP interference and ground current from HVDC transmission electrodes is....
steady state (static stray) current
CP levels: potential becomes more electronegative at point of current pick up, less at point of discharge
stray current interference
How can influence current be recognized? (4)
structure-to-electrolyte potential changes, changes in current flow on pipeline, localized pitting near foreign structure, localized coating breakdown
What is the purpose of a casing?
support dynamic loads caused by road traffic or trains to prevent harm to the pipe
Coatings that consist of a primer, corrosion sealant inner layer, mechanical protective outer layer backing (usually polyethylene or polybutylene), prone to disbonding if not properly tensioned
tape coatings
What environmental conditions must be checked during initial inspection before coating application?
temperature, humidity, dew point
How are mitigation bonds connected?
through test station
Why are inhibitors added to liquids?
to change the characteristics of the liquid or form protective films on the metal surface
What are IR drop (current span) test station used for?
to measure magnitude and direction of current flow on pipeline (pipeline acts as resistor, current determine by measuring IR drop across span of test wires
If the short circuit is internal to rectifier, the circuit breaker will ____
trip (again)
Coatings that consist of liquid resin (epoxy) and a curing agent, resistant to impact and abrasion damage
two-component liquid resin coatings (two part epoxy)
Graphite (carbon), high silicon iron, platinum, aluminum, magnetite, mixed metal oxide, conductive polymer, scrap metal
types of ICCP anodes
What does a sudden increase in current on either side of a pipeline crossing indicate?
underground contact
What is the purpose of a chemical backfill? (Mg, Zn)
uniform environment to reduce self-consumption rate, reduces polarization (anode functions more efficiently)
