5.3 Converting Potentials Between Reference Electrodes

AMPP CP1 Cathodic Protection Tester Study Guide Exam

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1 Chapter 1: Introduction & CP Exam Overview

About the AMPP CP1 Tester Exam & Certification Path How to Study for CP1 & Navigate This Guide Corrosion Science & Industry Terminology Glossary Field Mathematics & Ohm's Law Fundamentals AMPP Ethics for the Corrosion Professional

2 Chapter 2: Fundamentals of Electrochemistry & Corrosion Cells

2.1 The Corrosion Cell: Anode, Cathode, Electrolyte & Metallic Path 2.2 Anodic & Cathodic Reactions on Carbon Steel 2.3 Galvanic Series & Electromotive Force (EMF) Series 2.4 Forms of Corrosion: Uniform, Galvanic, Pitting & Crevice 2.5 Factors Affecting Corrosion Rates: Oxygen, pH, Temperature & Flow

3 Chapter 3: Principles of Cathodic Protection

3.1 Cathodic Protection Theory & Mixed-Potential Diagrams 3.2 Galvanic (Sacrificial Anode) CP Systems 3.3 Impressed Current Cathodic Protection (ICCP) Systems 3.4 Sacrificial Anode Materials: Mg, Zn, & Al Alloys 3.5 ICCP Anode Materials: MMO, Graphite, HSCI, & Platinum

4 Chapter 4: Electrical Circuit Analysis for CP

4.1 Ohm's Law Applications in CP Circuits 4.2 Series & Parallel Resistance in CP Groundbeds & Piping 4.3 Kirchhoff's Voltage & Current Laws in Network Piping 4.4 Shunts: Types, Ratings & Current Calculations 4.5 Rectifier Operation & Transformer Tap Adjustments

5 Chapter 5: Reference Electrodes & Half-Cell Potentials

5.1 Copper/Copper Sulfate (CSE) Reference Electrode Construction & Maintenance 5.2 Silver/Silver Chloride (Ag/AgCl) & Zinc Reference Electrodes 5.3 Converting Potentials Between Reference Electrodes 5.4 Reference Electrode Care, Temperature Correction & Contamination Checks 5.5 High-Impedance Voltmeters & Contact Resistance Errors

6 Chapter 6: Field Measurement & Potential Criteria

6.1 Structure-to-Electrolyte Potential Measurement Procedures 6.2 The -850 mV CSE Polarized Criterion (NACE SP0169) 6.3 The 100 mV Polarization Decay Criterion 6.4 Understanding IR Drop & The Instant-Off Interruption Technique 6.5 Native, On, Polarized, & Off Potentials Comparison

7 Chapter 7: Advanced Field Survey Techniques

Soil Resistivity Testing: Wenner 4-Pin Method (ASTM G57) Soil Pin Spacing, Layering & Corrosivity Classification Close Interval Potential Surveys (CIPS) Direct Current Voltage Gradient (DCVG) Coating Fault Surveys Cathodic Protection Coupons for IR-Free Potential Measurements

8 Chapter 8: Test Stations, Isolation & Stray Current

8.1 Test Station Types, Lead Wires & Commissioning Inspection 8.2 Electrical Isolation Joints & Dielectric Flange Testing 8.3 Casing Isolation & Shorted Casing Testing 8.4 DC Stray Current Interference: Detection & Mitigation 8.5 AC Stray Current Interference & Induction Hazards (NACE SP0177)

9 Chapter 9: Industry Standards & Field Documentation

9.1 NACE SP0169: External Corrosion Control Overview 9.2 NACE TM0497: Measurement Techniques Standard 9.3 Regulatory Compliance: PHMSA, DOT, & EPA Requirements 9.4 CP Data Collection, Logging & Quality Assurance 9.5 Rectifier Maintenance & Routine Inspection Logs 9.6 Anode Groundbed Testing & Resistance Measurements

10 Chapter 10: Electrical & Operational Safety for CP Personnel

10.1 Rectifier Electrical Safety, Lockout/Tagout (LOTO) & NFPA 70E 10.2 AC Induction Hazards Near High-Voltage Power Lines 10.3 Hazardous Atmospheres & Confined Space Safety at Test Vaults 10.4 Personal Protective Equipment (PPE) & Arc Flash Protection 10.5 Excavation, Trenching & Underground Utility Location (811) 10.6 Stop-Work Authority & Emergency Response Procedures

5.3 Converting Potentials Between Reference Electrodes

Key Takeaways Explain This Topic AI

AMPP CP1 EPG lists Ag/AgCl (3.5% SSC) at −0.060 V vs CSE and pure zinc at −1.100 V vs CSE—use that table on the exam.

Convert with E_vs_target = E_vs_CSE − (E_target_vs_CSE); −0.850 V CSE ≈ −0.790 V Ag/AgCl ≈ +0.250 V vs zinc.

SHE is −0.316 V vs CSE on the EPG sheet (CSE ≈ +0.316 V vs SHE); do not treat zinc as −1.10 V vs SHE.

Always record which reference electrode produced each reading before applying criteria.

Last updated: July 2026

• Converting Potentials Between Reference Electrodes
• Why Conversions Matter

A structure-to-electrolyte potential is meaningless unless the reference electrode is named. The same polarized steel can read about -0.850 V vs CSE , about -0.790 V vs Ag/AgCl (3.5% SSC) , or about +0.250 V vs zinc . AMPP's CP1 Exam Preparation Guide (November 2025) supplies the conversion chart used on the CBT reference PDF. Memorize that table—do not invent alternate SHE offsets that conflict with it.

Official EPG Relative Values (to CSE)

The EPG lists typical half-cell potentials relative to saturated copper–copper sulfate (CSE) :

• Electrode (Half-Cell) Potential vs CSE (V)
• Copper–Copper Sulfate, saturated (CSE) 0.000
• Silver–Silver Chloride, 3.5% (SSC / Ag/AgCl) −0.060
• Saturated Calomel (SCE) −0.072
• Standard Hydrogen Electrode (SHE) −0.316
• Pure Zinc (ZN) −1.100

These values assume the seawater-resistivity context noted on the EPG sheet (~20 Ω·cm) for the marine electrodes. Field Ag/AgCl cells can vary slightly with chloride concentration; for exam math, use −0.060 V vs CSE unless a problem states otherwise.

Consistency check with SHE

Because SHE is listed as −0.316 V vs CSE , CSE sits at approximately +0.316 V vs SHE . That matches the common textbook figure (~+0.316 to +0.318 V). Do not place pure zinc at −1.10 V vs SHE—the EPG places pure zinc at −1.100 V vs CSE , which is roughly −0.78 V vs SHE (+0.316 − 1.100).

Conversion Rule Used on the Exam

When the EPG gives each reference's potential versus CSE , convert a structure reading as follows:

E_vs_target = E_vs_CSE − (E_target_vs_CSE)

Equivalently, when you measured against some other reference and need CSE:

• E_vs_CSE = E_vs_measured + (E_measuredRef_vs_CSE)
• Example 1 — CSE to Ag/AgCl (3.5% SSC)

Measured: −0.850 V vs CSE . Target: Ag/AgCl at −0.060 V vs CSE .

E_vs_AgAgCl = −0.850 − (−0.060) = −0.790 V

So −0.850 V CSE ≈ −0.790 V vs Ag/AgCl . The structure did not change; the ruler shifted by 60 mV.

Example 2 — CSE to Zinc

Measured: −0.850 V vs CSE . Pure zinc reference: −1.100 V vs CSE .

E_vs_Zn = −0.850 − (−1.100) = +0.250 V

Protected steel often reads positive versus zinc because zinc is far more negative than steel. +0.250 V vs zinc corresponds to the familiar −0.850 V CSE polarized criterion.

Example 3 — Ag/AgCl reading back to CSE

Offshore drop-cell reading: −0.790 V vs Ag/AgCl (SSC) .

• E_vs_CSE = −0.790 + (−0.060) = −0.850 V
• Practical Field Tips

Always write the reference on the data sheet (CSE, Ag/AgCl/SSC, Zn, SCE).

Prefer the EPG CSE-relative table for CP1 exam calculations over memorized SHE chains that conflict with the official sheet.

Permanent zinc electrodes are convenient underground, but interpret their positive protected readings correctly—do not treat +0.25 V vs Zn as underprotected.

When comparing historical CSE surveys to new Ag/AgCl marine data, convert before judging criteria.

Quick Criterion Equivalents (using EPG −0.060 / −1.100 offsets)

• Criterion intent Approx. CSE Approx. Ag/AgCl (3.5%) Approx. Zinc
• −850 mV polarized −0.850 V −0.790 V +0.250 V

Exact field acceptance still follows the governing standard (e.g., NACE SP0169) and company procedures; the table above is for consistent unit conversion on the CP1 theory exam.

Sign Errors to Avoid

Students commonly reverse the zinc conversion and report −0.250 V instead of +0.250 V , or they subtract 60 mV the wrong way and turn −0.850 CSE into −0.910 Ag/AgCl. Write the EPG offsets with signs (−0.060, −1.100) before arithmetic. If your Ag/AgCl equivalent is more negative than the CSE reading, you flipped the sign—Ag/AgCl readings for the same structure are about 60 mV less negative than CSE readings.

When a problem gives SHE potentials that conflict with the EPG CSE-relative sheet, prefer the EPG sheet: it is the document mirrored in the CBT reference PDF for CP1.

SCE Calomel Bridge Values

The EPG also lists saturated calomel (SCE) at −0.072 V vs CSE . The same arithmetic applies: −0.850 V CSE ≈ −0.778 V vs SCE (−0.850 − (−0.072)). SCE appears more often in laboratory polarization studies than in ditch surveys, but recognizing the EPG offset prevents mixing calomel charts with CSE criteria.

Worked Mixed-Fleet Scenario

A marine terminal stores both onshore CSE survey data and offshore Ag/AgCl drop-cell data. An onshore riser reads −0.920 V CSE (instant-off). The matching subsea lateral reads −0.860 V vs Ag/AgCl (3.5% SSC). Convert the subsea reading to CSE before comparing:

E_vs_CSE = −0.860 + (−0.060) = −0.920 V

The two measurements describe the same polarized level once both are on the CSE scale. Without conversion, a reviewer might wrongly conclude the lateral is 60 mV underprotected relative to the riser.