RMGA Corrosion Control and Cathodic Protection 2 — Questions and Answers
Question 1: In a sacrificial anode cathodic protection system, what happens to the anode over time?
- It gains mass as it absorbs corrosion products from the protected pipe
- It gradually dissolves and must be periodically replaced (Correct answer)
- It converts AC power to DC power to maintain the protection circuit
- Its electrical resistance increases until it must be electronically reset
Correct answer: It gradually dissolves and must be periodically replaced
Sacrificial anodes are made of more active metals (zinc, magnesium, or aluminum) that corrode preferentially to protect the steel pipe, consuming themselves in the process and requiring periodic replacement.
Question 2: Which instrument setup is used to measure pipe-to-soil potential in cathodic protection surveys?
- A manometer connected between two buried test wires
- A voltmeter connected between the pipe lead and a reference electrode placed on the soil surface (Correct answer)
- An ammeter placed in series with the pipeline test wire
- A megohmmeter connected across an isolation fitting
Correct answer: A voltmeter connected between the pipe lead and a reference electrode placed on the soil surface
Pipe-to-soil potential is measured with a high-impedance voltmeter connected between the pipe lead at the test station and a portable reference electrode (typically a CSE) placed on the soil directly above the pipe.
Question 3: What is stray current corrosion in the context of buried gas pipelines?
- Corrosion caused by high-velocity gas flow eroding the interior pipe wall
- Corrosion resulting from electrical currents from external sources flowing through the buried pipeline (Correct answer)
- Corrosion caused by oxygen dissolved in groundwater contacting the pipe exterior
- Corrosion from microbes living in low-oxygen soil around the pipe
Correct answer: Corrosion resulting from electrical currents from external sources flowing through the buried pipeline
Stray current corrosion occurs when external DC current sources such as light rail systems, welding equipment, or other impressed current systems introduce electrical currents into the pipeline, which then discharge from the pipe causing accelerated corrosion at discharge points.
Question 4: Per 49 CFR Part 192, how frequently must cathodic protection levels on gas distribution systems be monitored?
- Monthly
- Every 6 months
- At least once each calendar year, not exceeding 15-month intervals (Correct answer)
- Every 5 years as part of a comprehensive integrity review
Correct answer: At least once each calendar year, not exceeding 15-month intervals
49 CFR §192.465 requires that cathodic protection on distribution systems be checked at least once per calendar year with no interval exceeding 15 months between readings.
Question 5: What is the primary function of a rectifier in an impressed current cathodic protection system?
- To continuously measure pipe-to-soil potential and trigger protection alarms
- To convert alternating current (AC) from a utility power source to direct current (DC) for the cathodic protection circuit (Correct answer)
- To detect coating holidays using electromagnetic induction
- To isolate pipeline sections into individual cathodic protection zones
Correct answer: To convert alternating current (AC) from a utility power source to direct current (DC) for the cathodic protection circuit
A rectifier converts AC utility power to DC current, which is essential because cathodic protection requires unidirectional (DC) current flow to drive the electrochemical protection reaction.
Question 6: What is the primary purpose of applying external coating to buried gas pipelines?
- To improve gas flow efficiency by reducing interior friction
- To increase the pipe's rated maximum allowable operating pressure
- To electrically isolate the pipe exterior from the soil, reducing the area that cathodic protection must defend (Correct answer)
- To prevent internal condensation and water slug accumulation in the pipeline
Correct answer: To electrically isolate the pipe exterior from the soil, reducing the area that cathodic protection must defend
External coatings serve as the first line of defense against corrosion by electrically isolating the pipe surface from the soil electrolyte; cathodic protection then handles any coating defects (holidays) that exist.
Question 7: What does the term 'IR drop' (or IR error) refer to in cathodic protection potential measurements?
- The internal resistance value of the copper-copper sulfate reference electrode
- A voltage error caused by current flowing through soil resistance that makes the measured potential appear more negative than the true pipe potential (Correct answer)
- The dissolution rate of the sacrificial anode expressed in milliamps per year
- The insulation resistance of a holiday detector probe tip
Correct answer: A voltage error caused by current flowing through soil resistance that makes the measured potential appear more negative than the true pipe potential
IR drop is an unwanted voltage component (current × soil resistance) that appears in pipe-to-soil measurements, overstating the protection level; it must be eliminated using instant-off or close-interval survey techniques to obtain a true reading.
RMGA Corrosion Control and Cathodic Protection 2
In a sacrificial anode cathodic protection system, what happens to the anode over time?