GPT Corrosion Control & Cathodic Protection 1 — Questions and Answers
Question 1: What is the minimum pipe-to-soil potential reading (measured with a copper/copper sulfate reference electrode) that indicates adequate cathodic protection on a buried steel gas pipe?
- -0.65 volts
- -0.85 volts (Correct answer)
- -1.20 volts
- -0.50 volts
Correct answer: -0.85 volts
NACE SP0169 specifies that a minimum negative 0.85 V (CSE) pipe-to-soil potential indicates adequate cathodic protection for buried steel pipelines.
Question 2: Which type of cathodic protection system uses an external DC power source to drive protective current onto a buried pipeline?
- Sacrificial anode system
- Impressed current system (Correct answer)
- Galvanic bonding system
- Passive isolation system
Correct answer: Impressed current system
An impressed current cathodic protection (ICCP) system uses a rectifier as an external DC power source to supply current through an anode to protect the pipeline.
Question 3: Magnesium and zinc anodes used in sacrificial anode cathodic protection systems work because they are:
- More noble than steel in the galvanic series
- Less noble (more active) than steel in the galvanic series (Correct answer)
- Chemically inert in soil environments
- Equal in electrochemical potential to steel
Correct answer: Less noble (more active) than steel in the galvanic series
Sacrificial anodes must be anodic (less noble, more active) relative to steel so they corrode preferentially, supplying protective current to the pipeline.
Question 4: What does a 'holiday' refer to in the context of pipeline coating inspection?
- A scheduled downtime for maintenance
- A discontinuity or defect in the protective coating (Correct answer)
- A special exemption in corrosion regulations
- A type of corrosion cell formed at pipe joints
Correct answer: A discontinuity or defect in the protective coating
A holiday is any gap, pinhole, or defect in a pipeline's protective coating that exposes bare metal to the corrosive environment.
Question 5: Stray current corrosion on a buried gas pipeline most commonly originates from:
- Natural soil moisture variation
- DC transit systems or cathodic protection systems on adjacent structures (Correct answer)
- High-pressure gas flow inside the pipe
- Thermal expansion and contraction cycles
Correct answer: DC transit systems or cathodic protection systems on adjacent structures
DC stray current from transit systems (electric rail) or neighboring CP systems can discharge from a buried pipe and cause accelerated anodic corrosion.
Question 6: Which instrument is used to measure pipe-to-soil potential for cathodic protection surveys?
- Megohmmeter
- High-voltage holiday detector
- High-impedance digital voltmeter with a reference electrode (Correct answer)
- Clamp-on ammeter
Correct answer: High-impedance digital voltmeter with a reference electrode
A high-impedance voltmeter connected between the pipe and a copper/copper sulfate reference electrode (CSE) placed on the soil surface measures pipe-to-soil potential.
Question 7: Which federal regulation governs corrosion control requirements for gas transmission and distribution pipelines in the United States?
- 49 CFR Part 191
- 49 CFR Part 192 Subpart I (Correct answer)
- 29 CFR Part 1926
- 40 CFR Part 60
Correct answer: 49 CFR Part 192 Subpart I
49 CFR Part 192 Subpart I specifically covers corrosion control requirements for gas pipeline facilities, including cathodic protection installation and monitoring.
GPT Corrosion Control & Cathodic Protection 1
What is the minimum pipe-to-soil potential reading (measured with a copper/copper sulfate reference electrode) that indicates adequate cathodic protection on a buried steel gas pipe?