RMGA Corrosion Control and Cathodic Protection 1 — Questions and Answers
Question 1: What is cathodic protection primarily used for in gas distribution systems?
- To prevent internal corrosion from gas contaminants
- To prevent external corrosion of buried metallic pipe (Correct answer)
- To reduce pressure fluctuations in distribution mains
- To control gas odorization levels
Correct answer: To prevent external corrosion of buried metallic pipe
Cathodic protection is an electrochemical technique applied to the exterior of buried metallic pipelines to prevent soil-side corrosion.
Question 2: What type of anode material is commonly used in impressed current cathodic protection systems?
- Zinc
- Magnesium
- Aluminum
- Graphite or mixed metal oxide (MMO) (Correct answer)
Correct answer: Graphite or mixed metal oxide (MMO)
Graphite and mixed metal oxide (MMO) anodes are preferred for impressed current systems because of their extremely low consumption rate and long service life under high current output.
Question 3: What is the minimum pipe-to-soil potential (vs. copper-copper sulfate electrode) indicating adequate cathodic protection per NACE SP0169?
- -0.50 volts
- -0.85 volts (Correct answer)
- -1.20 volts
- -0.65 volts
Correct answer: -0.85 volts
A minimum potential of -0.85 volts (vs. CSE) is the widely accepted industry criterion for adequate cathodic protection of steel pipe per NACE SP0169.
Question 4: In pipeline coating inspection, what does the term 'holiday' refer to?
- A scheduled maintenance shutdown period
- A pressure test interval required by regulation
- A defect, void, or break in the pipeline's external coating (Correct answer)
- A pipe fitting used at customer service connections
Correct answer: A defect, void, or break in the pipeline's external coating
A holiday is a discontinuity or flaw in the protective coating that leaves bare metal exposed to the surrounding soil environment, requiring cathodic protection to compensate.
Question 5: Which federal regulation governs cathodic protection requirements for natural gas distribution pipelines in the United States?
- 49 CFR Part 192 (Correct answer)
- 49 CFR Part 195
- 40 CFR Part 60
- 29 CFR Part 1910
Correct answer: 49 CFR Part 192
49 CFR Part 192 (Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards) establishes cathodic protection requirements for natural gas distribution and transmission systems.
Question 6: What is the primary purpose of a test station installed on a buried gas pipeline?
- To inject corrosion inhibitors into the pipeline
- To serve as a pressure relief point during over-pressure events
- To provide accessible connection points for measuring pipe-to-soil potential without excavation (Correct answer)
- To mark the location of mainline isolation valves
Correct answer: To provide accessible connection points for measuring pipe-to-soil potential without excavation
Test stations bring lead wires from the pipe to the surface, allowing corrosion technicians to measure pipe-to-soil potential and other cathodic protection parameters at regular intervals without excavating.
Question 7: What type of corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte?
- Uniform corrosion
- Pitting corrosion
- Galvanic corrosion (Correct answer)
- Crevice corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs due to the electrochemical potential difference between dissimilar metals, causing the more active (anodic) metal to corrode preferentially while the less active (cathodic) metal is protected.
RMGA Corrosion Control and Cathodic Protection 1
What is cathodic protection primarily used for in gas distribution systems?