Free NACE Cathodic Protection Principles Questions and Answers — Questions and Answers
Question 1: What is the primary purpose of cathodic protection (CP)?
- To increase the electrical resistance of the metal
- To provide a decorative finish to the metal surface
- To control corrosion by making the metal a cathode (Correct answer)
- To insulate the metal from all environments
Correct answer: To control corrosion by making the metal a cathode
The primary purpose of cathodic protection (CP) is to control corrosion by making the metal a cathode. By supplying an external current or connecting the protected metal to a more electrochemically active metal (sacrificial anode), CP forces the entire surface of the structure to act as a cathode, preventing the anodic reactions (corrosion) from occurring.
Question 2: Which of the following is a type of cathodic protection system?
- Only galvanic anode systems
- Only impressed current systems
- Both galvanic anode and impressed current systems (Correct answer)
- Neither galvanic nor impressed current systems
Correct answer: Both galvanic anode and impressed current systems
Cathodic protection systems are broadly categorized into two main types: galvanic (or sacrificial) anode systems and impressed current systems. Both methods effectively control corrosion by making the protected metal a cathode, but they differ in how the protective current is generated and supplied to the structure.
Question 3: What is the typical material used for sacrificial anodes in seawater environments?
- Copper
- Stainless steel
- Zinc or aluminum alloys (Correct answer)
- Carbon steel
Correct answer: Zinc or aluminum alloys
Zinc and aluminum alloys are commonly used as sacrificial anode materials in seawater environments. These metals are more electrochemically active than steel, causing them to corrode preferentially and sacrifice themselves to protect the steel structure, thereby making the steel the cathode and preventing its corrosion.
Question 4: What is the criterion for effective cathodic protection of steel in soil?
- A positive potential of +850 mV vs. CSE
- A negative potential of -850 mV vs. CSE (Correct answer)
- Zero voltage potential
- A resistance of 1000 ohms
Correct answer: A negative potential of -850 mV vs. CSE
A negative potential of -850 mV vs. CSE (Copper-Copper Sulfate Electrode) is a widely accepted NACE criterion for effective cathodic protection of steel in soil or water. Achieving this potential indicates that the steel surface has been sufficiently polarized to prevent active corrosion, ensuring long-term asset integrity.
Question 5: What is the purpose of a reference electrode in CP systems?
- To supply current to the system
- To measure the electrical potential of the protected structure (Correct answer)
- To replace sacrificial anodes
- To increase soil resistivity
Correct answer: To measure the electrical potential of the protected structure
A reference electrode is a crucial component in CP systems used to accurately measure the electrical potential of the protected structure relative to a stable, known potential. This measurement allows engineers to assess the effectiveness of the CP system, ensuring adequate corrosion control and compliance with protection criteria.
Question 6: Which NACE standard covers the control of external corrosion on underground or submerged metallic piping systems?
- NACE SP0177
- NACE SP0169 (Correct answer)
- NACE TM0102
- NACE RP0188
Correct answer: NACE SP0169
NACE SP0169 is the foundational standard practice for the control of external corrosion on underground or submerged metallic piping systems. It provides comprehensive guidelines for cathodic protection, coatings, and other mitigation strategies to ensure the integrity and longevity of pipelines and other buried or submerged metallic structures.
Question 7: What is a common anode material for impressed current CP systems?
- Magnesium
- Cast iron
- Mixed Metal Oxide (MMO) anodes (Correct answer)
- Copper
Correct answer: Mixed Metal Oxide (MMO) anodes
Mixed Metal Oxide (MMO) anodes are a common anode material for impressed current CP systems due to their excellent current output, low consumption rate, and long lifespan. They are highly efficient and durable, making them suitable for various environments, including soil, freshwater, and seawater applications.
Question 8: What is the 100 mV polarization criterion in cathodic protection?
- A 100 mV increase in soil resistivity
- A 100 mV negative shift from the native potential (Correct answer)
- A 100 mV positive shift from the native potential
- A 100 mV reduction in current output
Correct answer: A 100 mV negative shift from the native potential
The 100 mV polarization criterion in cathodic protection assesses effectiveness by requiring a minimum 100 mV negative (cathodic) shift in the structure's potential from its native (unprotected) state. This shift indicates that sufficient current has been applied to polarize the metal surface, effectively mitigating corrosion.
Question 9: What is stray current corrosion and how can CP mitigate it?
- Corrosion caused by bacteria, mitigated with biocides
- Corrosion due to alternating current, mitigated with insulators
- Corrosion caused by external DC sources, mitigated by proper CP current distribution (Correct answer)
- Corrosion from high temperatures, mitigated with cooling systems
Correct answer: Corrosion caused by external DC sources, mitigated by proper CP current distribution
Stray current corrosion is caused by external DC sources, where current flows through unintended metallic paths, causing accelerated corrosion where it leaves the structure. Cathodic protection can mitigate this by ensuring proper current distribution, bonding, and drainage, diverting the stray current safely away from the protected structure.
Free NACE Cathodic Protection Principles Questions and Answers
What is the primary purpose of cathodic protection (CP)?