GPT Corrosion Control & Cathodic Protection 2 — Questions and Answers
Question 1: What is the purpose of an electrical isolation fitting (insulating union or flange) installed on a buried gas service line?
- To reduce gas flow velocity at the meter
- To electrically isolate the service line from the customer's piping and prevent CP current loss (Correct answer)
- To act as a pressure regulator at the service entrance
- To provide a test point for leak detection
Correct answer: To electrically isolate the service line from the customer's piping and prevent CP current loss
Isolation fittings prevent cathodic protection current from draining off the protected main into unprotected customer piping, ensuring CP current stays on the intended structure.
Question 2: Microbiologically influenced corrosion (MIC) on buried gas pipelines is primarily caused by:
- Ultraviolet radiation degrading the pipe coating
- Sulfate-reducing bacteria (SRB) producing hydrogen sulfide in anaerobic soils (Correct answer)
- High operating pressures causing stress corrosion
- Electrostatic discharge from passing vehicles
Correct answer: Sulfate-reducing bacteria (SRB) producing hydrogen sulfide in anaerobic soils
Sulfate-reducing bacteria thrive in anaerobic, wet soils and produce H2S, which aggressively attacks steel pipe, causing MIC.
Question 3: A rectifier output current on an impressed current CP system has dropped significantly with no change in the circuit resistance. This most likely indicates:
- The pipeline is now fully protected and requires less current
- AC power supply to the rectifier has failed or the rectifier fuse has blown (Correct answer)
- Pipe-to-soil resistivity has decreased due to rainfall
- The sacrificial anodes need replacement
Correct answer: AC power supply to the rectifier has failed or the rectifier fuse has blown
A drop in output current at constant resistance most commonly means the rectifier has lost its AC power supply or an internal fuse/component has failed.
Question 4: Which pipe coating system is commonly applied at the mill and provides excellent protection against soil-side corrosion on steel gas mains?
- Tar paper wrap
- Fusion-bonded epoxy (FBE) (Correct answer)
- Asphalt enamel over kraft paper
- Black iron oxide primer only
Correct answer: Fusion-bonded epoxy (FBE)
Fusion-bonded epoxy (FBE) is a widely used mill-applied coating that provides a high-quality, pinhole-resistant barrier against soil-side corrosion on steel pipelines.
Question 5: When performing a close-interval potential survey (CIPS), readings are taken:
- At 1,000-foot intervals using a GPS-tagged test station
- At intervals of approximately 1 to 5 feet along the pipeline route using a trailing reference electrode (Correct answer)
- Only at valve and tee locations where current drain is highest
- Annually at rectifier test points only
Correct answer: At intervals of approximately 1 to 5 feet along the pipeline route using a trailing reference electrode
CIPS involves walking the pipeline route and taking pipe-to-soil potential readings at close intervals (typically 1–5 ft) with a reference electrode dragged along the surface to pinpoint CP deficiencies.
Question 6: Galvanic corrosion between two dissimilar metals connected in a buried environment is driven by:
- Differences in pipe wall thickness
- A difference in electrochemical potential (voltage) between the two metals (Correct answer)
- Differences in operating gas pressure
- The relative depth of burial of each metal
Correct answer: A difference in electrochemical potential (voltage) between the two metals
Galvanic corrosion occurs because dissimilar metals have different electrode potentials; the more active (anodic) metal corrodes while the more noble (cathodic) metal is protected.
Question 7: What is the primary reason cathodic protection test stations are installed along a buried gas pipeline?
- To provide emergency shut-off access below ground
- To allow periodic measurement of pipe-to-soil potential without excavation (Correct answer)
- To equalize pressure between pipeline segments
- To anchor the pipe against thermal movement
Correct answer: To allow periodic measurement of pipe-to-soil potential without excavation
Test stations bring wire leads from the pipe to the surface so technicians can measure pipe-to-soil potential and monitor CP effectiveness without digging.
GPT Corrosion Control & Cathodic Protection 2
What is the purpose of an electrical isolation fitting (insulating union or flange) installed on a buried gas service line?