NACE (National Association of Corrosion Engineers) CP2 EXAM
ACTUAL EXAM QUESTIONS AND CORRECT DETAILED ANSWERS
WITH RATIONALES ALREADY GRADED A+ 2025
Question 1
According to NACE SP0169, which of the following is a valid criterion for the cathodic
protection of steel and cast iron piping?
A) A potential of -850 mV (CSE) with the cathodic protection current applied (ON potential).
B) A polarized potential of -850 mV (CSE) or more negative.
C) A potential shift of 300 mV in the negative direction.
D) A maintained current density of 10 mA/sq ft.
E) A potential of -1000 mV relative to a Zinc reference electrode.
Correct Answer: B) A polarized potential of -850 mV (CSE) or more negative.
Rationale: One of the primary criteria in NACE SP0169 is a polarized (IR-free) potential of
-850 mV or more negative relative to a saturated copper/copper sulfate electrode (CSE).
Option A is incorrect because an "ON" potential includes IR drop error and is not a valid
criterion by itself without considering IR drop. Option C describes an old criterion (300
mV shift) that is no longer standard for all situations. Option D specifies a current density,
which is a design parameter, not a protection criterion.
Question 2
You are measuring a pipe-to-soil potential using a Copper-Copper Sulfate (CSE) reference
electrode. The temperature is 30°F (-1°C). How does this low temperature affect the
potential reading of the reference electrode compared to 77°F (25°C)?
A) The potential of the reference electrode increases (becomes more positive), causing the pipe
reading to appear more negative than it actually is.
B) The potential of the reference electrode decreases, causing the pipe reading to appear less
negative.
C) Temperature has absolutely no effect on CSE half-cells.
D) The potential reading will fluctuate wildly due to freezing of the gel.
E) The potential becomes unstable and cannot be used below 40°F.
Correct Answer: A) The potential of the reference electrode increases (becomes more
positive), causing the pipe reading to appear more negative than it actually is.
Rationale: The potential of a CSE increases by approximately 0.5 mV for every degree
Fahrenheit decrease in temperature below 77°F. If the reference electrode potential is
higher (more positive), the measured potential difference between the pipe and the
electrode will appear larger (more negative). Therefore, in cold weather, uncorrected
readings may erroneously suggest a pipeline meets criteria when it actually does not.
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Question 3
When performing a Wenner 4-pin soil resistivity test, you calculate the resistance (R) to be
2.5 ohms. The pin spacing (a) is 10 feet. Using the formula
𝜌 = 191.5 × 𝑎 × 𝑅
(where
𝜌
is in ohm-cm), what is the soil resistivity?
A) 478.75 ohm-cm
B) 1,915 ohm-cm
C) 4,787.5 ohm-cm
D) 9,575 ohm-cm
E) 47,875 ohm-cm
Correct Answer: C) 4,787.5 ohm-cm
Rationale: The formula for pin spacing in feet to obtain resistivity in ohm-cm is
𝜌 = 191.5 × 𝑎 × 𝑅
.
Calculation:
191.5 × 10 (feet) × 2.5 (ohms) = 4,787.5 ohm-cm
.
Question 4
In an Impressed Current Cathodic Protection (ICCP) system, which backfill material is
commonly used around the anodes to lower the resistance-to-earth and extend anode life?
A) Bentonite clay and Gypsum
B) Metallurgical Coke Breeze
C) Clean sand
D) Native soil
E) Concrete
Correct Answer: B) Metallurgical Coke Breeze
Rationale: Coke breeze (carbon) is used in ICCP groundbeds. It is highly conductive, which
increases the effective surface area of the anode (lowering resistance-to-earth).
Furthermore, the oxidation reaction (consumption) transfers from the anode surface to the
coke breeze, significantly extending the life of the primary anode material.
,[Type here]
Question 5
What is the primary chemical reaction occurring at the cathode surface on a protected
pipeline?
A) Oxidation of iron (
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
)
B) Evolution of Chlorine gas
C) Reduction reaction (e.g.,
2𝐻2 𝑂 + 𝑂2 + 4𝑒 − → 4𝑂𝐻 −
)
D) Formation of Carbon Monoxide
E) Consumption of Carbonic Acid
Correct Answer: C) Reduction reaction (e.g.,
2𝐻2 𝑂 + 𝑂2 + 4𝑒 − → 4𝑂𝐻 −
)
Rationale: The cathode is where reduction occurs. In aerated neutral or alkaline soils, the
primary reaction is the reduction of oxygen, which produces hydroxyl ions (
𝑂𝐻 −
). This increases the pH at the pipe surface (making it more alkaline). Oxidation (loss of
electrons/corrosion) occurs at the anode.
Question 6
You are troubleshooting a rectifier and notice the efficiency has dropped significantly. The
ripple voltage is measured to be 60% of the output DC voltage. What is the most likely
cause?
A) The transformer oil is low.
B) The groundbed resistance has increased.
C) The AC supply frequency has changed.
D) One or more diodes in the bridge stack have failed open.
E) The AC circuit breaker is undersized.
Correct Answer: D) One or more diodes in the bridge stack have failed open.
Rationale: High ripple voltage indicates that the rectification from AC to DC is incomplete.
In a full-wave bridge rectifier, if a diode fails (specifically fails open) or a stack degrades,
, [Type here]
the unit may effectively operate as a half-wave rectifier, significantly increasing the AC
ripple and reducing the efficiency of the unit.
Question 7
To apply the 100 mV polarization shift criterion properly, how must the measurement be
taken?
A) Measure the "ON" potential, then measure the "OFF" potential immediately after interruption;
the difference must be 100 mV.
B) Measure the "Instant OFF" potential, then allow the pipe to depolarize; the difference between
the "Instant OFF" and the depolarized (static) potential must be at least 100 mV.
C) Measure the "ON" potential and compare it to the native potential; the difference must be 100
mV.
D) Measure the "Instant OFF" potential and ensure it is more negative than -900 mV.
E) Measure the potential between the pipe and a zinc anode.
Correct Answer: B) Measure the "Instant OFF" potential, then allow the pipe to
depolarize; the difference between the "Instant OFF" and the depolarized (static) potential
must be at least 100 mV.
Rationale: The 100 mV shift criterion refers to polarization decay (or formation). You must
eliminate the IR drop to find the polarized potential ("Instant Off"). Then, you compare
this "Instant Off" value to the native/depolarized potential. If the difference is
≥ 100
mV, the criterion is met.
Question 8
Which of the following reference electrodes is most suitable for use in high-chloride
environments such as seawater?
A) Copper-Copper Sulfate (CSE)
B) Silver-Silver Chloride (Ag/AgCl)
C) Standard Hydrogen Electrode (SHE)
D) Zinc Reference
E) Calomel Electrode
Correct Answer: B) Silver-Silver Chloride (Ag/AgCl)
Rationale: Copper-Copper Sulfate electrodes become contaminated in high-chloride
environments, leading to drifting readings. Silver-Silver Chloride electrodes are chemically
stable in seawater and are the industry standard for offshore/marine CP applications.
ACTUAL EXAM QUESTIONS AND CORRECT DETAILED ANSWERS
WITH RATIONALES ALREADY GRADED A+ 2025
Question 1
According to NACE SP0169, which of the following is a valid criterion for the cathodic
protection of steel and cast iron piping?
A) A potential of -850 mV (CSE) with the cathodic protection current applied (ON potential).
B) A polarized potential of -850 mV (CSE) or more negative.
C) A potential shift of 300 mV in the negative direction.
D) A maintained current density of 10 mA/sq ft.
E) A potential of -1000 mV relative to a Zinc reference electrode.
Correct Answer: B) A polarized potential of -850 mV (CSE) or more negative.
Rationale: One of the primary criteria in NACE SP0169 is a polarized (IR-free) potential of
-850 mV or more negative relative to a saturated copper/copper sulfate electrode (CSE).
Option A is incorrect because an "ON" potential includes IR drop error and is not a valid
criterion by itself without considering IR drop. Option C describes an old criterion (300
mV shift) that is no longer standard for all situations. Option D specifies a current density,
which is a design parameter, not a protection criterion.
Question 2
You are measuring a pipe-to-soil potential using a Copper-Copper Sulfate (CSE) reference
electrode. The temperature is 30°F (-1°C). How does this low temperature affect the
potential reading of the reference electrode compared to 77°F (25°C)?
A) The potential of the reference electrode increases (becomes more positive), causing the pipe
reading to appear more negative than it actually is.
B) The potential of the reference electrode decreases, causing the pipe reading to appear less
negative.
C) Temperature has absolutely no effect on CSE half-cells.
D) The potential reading will fluctuate wildly due to freezing of the gel.
E) The potential becomes unstable and cannot be used below 40°F.
Correct Answer: A) The potential of the reference electrode increases (becomes more
positive), causing the pipe reading to appear more negative than it actually is.
Rationale: The potential of a CSE increases by approximately 0.5 mV for every degree
Fahrenheit decrease in temperature below 77°F. If the reference electrode potential is
higher (more positive), the measured potential difference between the pipe and the
electrode will appear larger (more negative). Therefore, in cold weather, uncorrected
readings may erroneously suggest a pipeline meets criteria when it actually does not.
,[Type here]
Question 3
When performing a Wenner 4-pin soil resistivity test, you calculate the resistance (R) to be
2.5 ohms. The pin spacing (a) is 10 feet. Using the formula
𝜌 = 191.5 × 𝑎 × 𝑅
(where
𝜌
is in ohm-cm), what is the soil resistivity?
A) 478.75 ohm-cm
B) 1,915 ohm-cm
C) 4,787.5 ohm-cm
D) 9,575 ohm-cm
E) 47,875 ohm-cm
Correct Answer: C) 4,787.5 ohm-cm
Rationale: The formula for pin spacing in feet to obtain resistivity in ohm-cm is
𝜌 = 191.5 × 𝑎 × 𝑅
.
Calculation:
191.5 × 10 (feet) × 2.5 (ohms) = 4,787.5 ohm-cm
.
Question 4
In an Impressed Current Cathodic Protection (ICCP) system, which backfill material is
commonly used around the anodes to lower the resistance-to-earth and extend anode life?
A) Bentonite clay and Gypsum
B) Metallurgical Coke Breeze
C) Clean sand
D) Native soil
E) Concrete
Correct Answer: B) Metallurgical Coke Breeze
Rationale: Coke breeze (carbon) is used in ICCP groundbeds. It is highly conductive, which
increases the effective surface area of the anode (lowering resistance-to-earth).
Furthermore, the oxidation reaction (consumption) transfers from the anode surface to the
coke breeze, significantly extending the life of the primary anode material.
,[Type here]
Question 5
What is the primary chemical reaction occurring at the cathode surface on a protected
pipeline?
A) Oxidation of iron (
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
)
B) Evolution of Chlorine gas
C) Reduction reaction (e.g.,
2𝐻2 𝑂 + 𝑂2 + 4𝑒 − → 4𝑂𝐻 −
)
D) Formation of Carbon Monoxide
E) Consumption of Carbonic Acid
Correct Answer: C) Reduction reaction (e.g.,
2𝐻2 𝑂 + 𝑂2 + 4𝑒 − → 4𝑂𝐻 −
)
Rationale: The cathode is where reduction occurs. In aerated neutral or alkaline soils, the
primary reaction is the reduction of oxygen, which produces hydroxyl ions (
𝑂𝐻 −
). This increases the pH at the pipe surface (making it more alkaline). Oxidation (loss of
electrons/corrosion) occurs at the anode.
Question 6
You are troubleshooting a rectifier and notice the efficiency has dropped significantly. The
ripple voltage is measured to be 60% of the output DC voltage. What is the most likely
cause?
A) The transformer oil is low.
B) The groundbed resistance has increased.
C) The AC supply frequency has changed.
D) One or more diodes in the bridge stack have failed open.
E) The AC circuit breaker is undersized.
Correct Answer: D) One or more diodes in the bridge stack have failed open.
Rationale: High ripple voltage indicates that the rectification from AC to DC is incomplete.
In a full-wave bridge rectifier, if a diode fails (specifically fails open) or a stack degrades,
, [Type here]
the unit may effectively operate as a half-wave rectifier, significantly increasing the AC
ripple and reducing the efficiency of the unit.
Question 7
To apply the 100 mV polarization shift criterion properly, how must the measurement be
taken?
A) Measure the "ON" potential, then measure the "OFF" potential immediately after interruption;
the difference must be 100 mV.
B) Measure the "Instant OFF" potential, then allow the pipe to depolarize; the difference between
the "Instant OFF" and the depolarized (static) potential must be at least 100 mV.
C) Measure the "ON" potential and compare it to the native potential; the difference must be 100
mV.
D) Measure the "Instant OFF" potential and ensure it is more negative than -900 mV.
E) Measure the potential between the pipe and a zinc anode.
Correct Answer: B) Measure the "Instant OFF" potential, then allow the pipe to
depolarize; the difference between the "Instant OFF" and the depolarized (static) potential
must be at least 100 mV.
Rationale: The 100 mV shift criterion refers to polarization decay (or formation). You must
eliminate the IR drop to find the polarized potential ("Instant Off"). Then, you compare
this "Instant Off" value to the native/depolarized potential. If the difference is
≥ 100
mV, the criterion is met.
Question 8
Which of the following reference electrodes is most suitable for use in high-chloride
environments such as seawater?
A) Copper-Copper Sulfate (CSE)
B) Silver-Silver Chloride (Ag/AgCl)
C) Standard Hydrogen Electrode (SHE)
D) Zinc Reference
E) Calomel Electrode
Correct Answer: B) Silver-Silver Chloride (Ag/AgCl)
Rationale: Copper-Copper Sulfate electrodes become contaminated in high-chloride
environments, leading to drifting readings. Silver-Silver Chloride electrodes are chemically
stable in seawater and are the industry standard for offshore/marine CP applications.
