NACE - National Association of Corrosion Engineers Cathodic Protection Principles Questions and Answers — Questions and Answers
Question 1: According to NACE SP0169, which of the following is the most commonly accepted minimum protective potential for carbon steel in soil or water, when measured with respect to a copper-copper sulfate electrode (CSE)?
- -550 mV
- +850 mV
- -850 mV (Correct answer)
- 0 mV
Correct answer: -850 mV
NACE Standard Practice SP0169 establishes -850 mV relative to a CSE as the primary criterion for adequate cathodic protection of steel. Achieving this potential indicates that the net electrochemical potential across the structure's surface has been shifted to a point where corrosion is effectively mitigated.
Question 2: An engineer is designing a cathodic protection system for a large, poorly coated, cross-country pipeline with high current requirements. Which type of CP system is generally more suitable and cost-effective for this application, and why?
- Sacrificial Anode Cathodic Protection (SACP), because it requires no external power source.
- Impressed Current Cathodic Protection (ICCP), because it can provide a higher, adjustable current output. (Correct answer)
- Sacrificial Anode Cathodic Protection (SACP), because it eliminates the risk of stray current interference.
- Impressed Current Cathodic Protection (ICCP), because its anodes are always consumed slower than sacrificial anodes.
Correct answer: Impressed Current Cathodic Protection (ICCP), because it can provide a higher, adjustable current output.
ICCP systems are ideal for large or poorly coated structures with high current demands. They use an external power source (rectifier) to drive a large and adjustable amount of current from long-lasting anodes, making them more economical and effective for such applications over the long term.
Question 3: Which of the following anode materials is most commonly used in a sacrificial anode cathodic protection (SACP) system for a steel structure in a high-resistivity soil environment?
- Zinc
- Magnesium (Correct answer)
- Aluminum
- Mixed Metal Oxide (MMO)
Correct answer: Magnesium
Magnesium has the most negative potential (highest driving voltage) of the common sacrificial anode materials. This high driving potential is necessary to supply sufficient protective current in high-resistivity electrolytes like many soils.
Question 4: A technician measures a structure-to-electrolyte potential of -950 mV CSE on a pipeline protected by an ICCP system while the rectifier is operating. Immediately after interrupting the current source, the potential reading shifts instantly to -750 mV CSE. What does this instantaneous voltage shift represent?
- Cathodic polarization
- Anodic depolarization
- Telluric current effect
- IR drop error (Correct answer)
Correct answer: IR drop error
The instantaneous potential shift that occurs when the protective current is interrupted is the voltage drop (IR drop) through the resistive electrolyte. This value is an error in the 'on' potential measurement and is not part of the structure's protective polarization. 'Instant-off' potentials are taken to eliminate this error and determine the true polarized potential.
Question 5: In an impressed current cathodic protection (ICCP) system, what is the primary function of the rectifier?
- To serve as the consumable anode that protects the structure.
- To measure the structure-to-electrolyte potential.
- To convert alternating current (AC) to direct current (DC). (Correct answer)
- To create a dielectric shield between the anode and the structure.
Correct answer: To convert alternating current (AC) to direct current (DC).
The rectifier is the power supply for an ICCP system. Its main function is to convert commercially available high-voltage alternating current (AC) into the low-voltage, high-amperage direct current (DC) required to drive the protective current from the anodes to the structure (cathode).
Question 6: Which of the following conditions is a primary cause of "cathodic shielding," a phenomenon that can prevent protective current from reaching the underlying metal surface?
- A disbonded coating that remains intact and shields the steel from the electrolyte. (Correct answer)
- High soil resistivity that limits the overall flow of current.
- Using a sacrificial anode material with a very high driving voltage.
- Stray DC currents from a nearby transit system interfering with the CP system.
Correct answer: A disbonded coating that remains intact and shields the steel from the electrolyte.
Cathodic shielding occurs when a defect, such as a disbonded but otherwise non-conductive coating, physically blocks the protective current from reaching the metal surface underneath. Although shielded from the protective current, the area may still be exposed to a corrosive environment, leading to corrosion in a location believed to be protected.
NACE - National Association of Corrosion Engineers Cathodic Protection Principles Questions and Answers
According to NACE SP0169, which of the following is the most commonly accepted minimum protective potential for carbon steel in soil or water, when measured with respect to a copper-copper sulfate electrode (CSE)?