API 653 Cathodic Protection Systems 1 — Questions and Answers
Question 1: What is the minimum negative potential criterion for adequate cathodic protection of a steel tank bottom, as measured against a copper/copper sulfate reference electrode (CSE)?
- -750 mV
- -850 mV (Correct answer)
- -950 mV
- -650 mV
Correct answer: -850 mV
API 653 and NACE SP0193 specify a minimum potential of -850 mV (vs. CSE) as the criterion for adequate cathodic protection of steel tank bottoms.
Question 2: Which of the following is a type of cathodic protection system used on aboveground storage tank bottoms?
- Anodic impressed current system
- Sacrificial anode (galvanic) system (Correct answer)
- Passive inhibitor system
- Electrolytic resistance system
Correct answer: Sacrificial anode (galvanic) system
Sacrificial anode (galvanic) systems and impressed current systems are the two primary types of cathodic protection; sacrificial anode systems use more active metals such as magnesium or zinc that corrode in place of the tank steel.
Question 3: In an impressed current cathodic protection (ICCP) system for a tank bottom, what is the function of the rectifier?
- To measure soil resistivity
- To convert AC power to DC current for the CP circuit (Correct answer)
- To provide a sacrificial anode reaction
- To isolate the tank from stray current
Correct answer: To convert AC power to DC current for the CP circuit
The rectifier in an ICCP system converts alternating current (AC) from a power source into direct current (DC), which is then applied to the tank bottom to provide cathodic protection.
Question 4: According to API 653, when should cathodic protection be considered for the external bottom of an aboveground storage tank?
- Only when the tank is over 100 ft in diameter
- When corrosion on the tank bottom is detected or anticipated (Correct answer)
- Only during the initial construction phase
- Only when the tank stores corrosive products
Correct answer: When corrosion on the tank bottom is detected or anticipated
API 653 states that cathodic protection should be considered when corrosion on the external bottom is detected or anticipated based on soil corrosivity, inspection findings, or operating history.
Question 5: What reference electrode is most commonly used for measuring cathodic protection potentials in soil environments?
- Silver/silver chloride electrode
- Zinc reference electrode
- Copper/copper sulfate electrode (CSE) (Correct answer)
- Calomel electrode
Correct answer: Copper/copper sulfate electrode (CSE)
The copper/copper sulfate electrode (CSE) is the standard reference electrode used for measuring CP potentials in soil environments due to its stability and reliability.
Question 6: What does 'IR drop' refer to in the context of cathodic protection potential measurements?
- The resistance of the rectifier transformer
- The voltage error caused by current flowing through the soil resistance (Correct answer)
- The current loss due to anode coating deterioration
- The potential shift caused by stray AC current
Correct answer: The voltage error caused by current flowing through the soil resistance
IR drop (also called ohmic drop) is the voltage error introduced in a potential measurement when current flows through the resistance of the electrolyte (soil) between the structure and the reference electrode.
Question 7: Which material is commonly used as a sacrificial anode for cathodic protection of steel tank bottoms in soil environments?
- Copper
- Magnesium (Correct answer)
- Titanium
- Stainless steel
Correct answer: Magnesium
Magnesium is commonly used as a sacrificial anode in soil environments because it is more electrochemically active (anodic) than steel, causing it to corrode preferentially and protect the steel.
API 653 Cathodic Protection Systems 1
What is the minimum negative potential criterion for adequate cathodic protection of a steel tank bottom, as measured against a copper/copper sulfate reference electrode (CSE)?