Chapter 3 - CP - how it works Flashcards
The driving voltage of CP system must be greater than ………………. to provide effective cathodic protection.
driving voltage of corrosion cell
The sole purpose of the cathodic protection grounded is to ……………
discharge current
When cathodic protection current is applied to a structure, which direction does the potential of the structure shift?
negative direction
List two methods utilized to produce cathodic protection current.
A. Impressed current
B. Galvanic/sacrificial
CSE
Copper - Copper Sulfate reference electrode
Pipe-to-soil potential
potential of a pipeline in a given location compared to reference electrode.
On potential
potential of pipeline when CP system is energized.
Off or Instant-Off Potential
estimates the polarized potential when the measurement is made within 1 second after simultaneously interrupting current output from all CP current sources.
How does the use of underground pipeline coatings affect the current requirement to achieve cathodic protection?
more coating - less current.
Why is it possible to protect more miles of large diameter pipelines from a single cathodic protection installation than small diameter pipelines?
resistivity inversely proportional to area.
Larger diameter pipe has lower resistance than small diameter pipe.
larger area and the resistance, conductor inversely proportional to cross sectional area.
Cathodic disbondment can be avoided if the off-potentials are kept below what level?
-1.1 V (CSE)
Which grounded construction method will produce the smallest area of full protection and provide the least effect on the potential compared to remote earth? (remote ground bed or closed ground bed)
close ground bed
close - smaller area
remote - larger area
A plastic material loosely wrapped around a pipeline that blocks the effectiveness of the cathodic protection current is a example of which type of shielding? (Insulating barrier or Metallic shield)
insulating barrier
A shorted pipeline casing on a pipeline road crossing which blocks the effectiveness of the cathodic protection current is an example of which shielding? (Insulating barrier or Metallic shield)
Metallic shields
Is it good idea to insure that reinforcing wire in concrete be allowed to contact the underlying steel which is under cathodic protection to avoid shielding problems? (True or False)
False
What is cathodic corrosion and give an example.
corrosion resulting from a cathodic condition of a structure, usually caused by the reaction of an atmospheric metal with the alkaline products of electrolysis.
if the metal is over-voltaged, its going to break down the passivating layer.
Which type of cathodic protection system is most likely to cause stray current damage? (Galvanic anode system or impressed)
impressed
because they are putting large volume of current
Describe 2 procedures that a corrosion engineer could use to avoid causing or experiencing damage from interference.
A. Select grounded sites accordingly
B. Inform neighbors, owners/facilities of the CP system
Describe how you can determine if there is damaging interference at a point of crossing a foreign pipeline.
when surveying potential drop in the area of damage, the potential of foreign crossing measures at close intervals.
crossing - more electro+
distance - more electro_
What is corrosion coordinating committee?
committee of representatives from many principle owners of underground metallic structures in an area that work together to solve corrosion problems.
Steel potential range
0.2 - 0.8
current shunt
resistor
calibrated - known restore value that does not change its value
CP types
- Galvanic System
- Impressed Current
Galvanic (Mg, Zn, Al)
Mg, Zn - use in fresh and zinc water
Al - most electropositive
Impressed Current
- rectifiers
- photovoltaic solar panels
- thermo-electric
linear scale
2,4,6,8,10,…….
logarithmic scale
2, 4, 8, 16, 32, 64, ….
current takes place of ….
least resistor
most of them but not at all
