API 653 Cathodic Protection Systems 2 — Questions and Answers
Question 1: What is stray current corrosion in the context of cathodic protection systems for storage tanks?
- Corrosion caused by excessive CP current on the protected structure
- Corrosion caused by electrical currents from external sources flowing through the tank structure (Correct answer)
- Corrosion due to improper anode placement
- Corrosion resulting from alternating current fluctuations in the rectifier
Correct answer: Corrosion caused by electrical currents from external sources flowing through the tank structure
Stray current corrosion occurs when electrical currents originating from external sources (such as DC transit systems or neighboring CP systems) flow through the tank structure and discharge at unintended locations, causing accelerated corrosion.
Question 2: What test is performed to evaluate whether a cathodic protection system is providing adequate protection at a tank bottom?
- Hydrostatic pressure test
- Pipe-to-soil potential survey (Correct answer)
- Ultrasonic thickness measurement
- Radiographic inspection
Correct answer: Pipe-to-soil potential survey
A pipe-to-soil (or structure-to-soil) potential survey measures the electrochemical potential of the tank bottom relative to a reference electrode in the soil to determine whether the -850 mV (CSE) protection criterion is met.
Question 3: What is the purpose of a 'close-interval potential survey' (CIPS) for an aboveground storage tank bottom?
- To measure the thickness of the tank shell at close intervals
- To obtain detailed potential readings at small spacing intervals around the tank perimeter to identify CP shielding or deficiencies (Correct answer)
- To inspect the close proximity welds on the tank bottom
- To test the dielectric isolation of the CP system at close range
Correct answer: To obtain detailed potential readings at small spacing intervals around the tank perimeter to identify CP shielding or deficiencies
A close-interval potential survey provides detailed potential measurements at small spacing intervals, allowing identification of areas where CP protection may be deficient or where CP current is being shielded.
Question 4: Which of the following would indicate OVERPROTECTION by a cathodic protection system on a coated steel tank bottom?
- Potentials more positive than -850 mV vs. CSE
- Potentials more negative than -1200 mV vs. CSE (Correct answer)
- Uniform potential distribution around the tank
- Anode current output within design specifications
Correct answer: Potentials more negative than -1200 mV vs. CSE
Potentials more negative than approximately -1200 mV vs. CSE indicate overprotection, which can cause hydrogen embrittlement of high-strength steel or disbondment of coatings due to excessive hydrogen generation.
Question 5: Why is electrical isolation (isolation joints/flanges) important in a cathodic protection system for a storage tank?
- To prevent thermal expansion stresses from damaging the CP anode
- To prevent CP current from being drained by connected pipelines or structures not intended to be protected (Correct answer)
- To ensure the tank foundation is grounded properly for safety
- To measure the CP system current output accurately
Correct answer: To prevent CP current from being drained by connected pipelines or structures not intended to be protected
Electrical isolation prevents CP current from being diverted to connected metallic structures (such as piping systems) that are not part of the intended protection zone, ensuring efficient use of CP current on the tank bottom.
Question 6: According to NACE/API recommendations, how often should cathodic protection system performance be evaluated for an aboveground storage tank?
- Every 10 years during scheduled tank inspection
- At least annually, with rectifier inspections more frequently (Correct answer)
- Only when visible corrosion is detected on the tank bottom
- Every 5 years during major turnaround inspections
Correct answer: At least annually, with rectifier inspections more frequently
NACE SP0193 and API 653 recommend that CP system performance be evaluated at least annually (with rectifier checks typically monthly or quarterly) to ensure the protection criterion is continuously met.
Question 7: What type of interference test is performed to assess the impact of one CP system on an adjacent structure?
- Hydrostatic interference test
- Electrical interference (stray current) survey (Correct answer)
- Acoustic emission interference test
- Magnetic flux interference test
Correct answer: Electrical interference (stray current) survey
An electrical interference (stray current) survey measures the potential and current changes on adjacent structures caused by a nearby CP system to determine if harmful interference is occurring.
API 653 Cathodic Protection Systems 2
What is stray current corrosion in the context of cathodic protection systems for storage tanks?