Cathodic Protection Principles Flashcards

What is the primary purpose of cathodic protection (CP)?

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.

Which of the following is a type of cathodic protection system?

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.

What is the typical material used for sacrificial anodes in seawater environments?

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.

What is the criterion for effective cathodic protection of steel in soil?

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.

What is the purpose of a reference electrode in CP systems?

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.

Which NACE standard covers the control of external corrosion on underground or submerged metallic piping systems?

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.

What is a common anode material for impressed current CP systems?

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.

What is the 100 mV polarization criterion in cathodic protection?

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.

What is stray current corrosion and how can CP mitigate it?

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.