Introduction
The Evans diagram is a graphical tool that illustrates the relationship between electrode potential and current density for anodic and cathodic reactions occurring on a metal surface. It provides insight into the kinetics of corrosion processes, polarization behavior, and the effects of cathodic protection.
For corrosion technicians, knowing the Evans diagram is a practical skill that bridges electrochemical theory with field applications. It helps technicians interpret polarization data, assess corrosion rates, and optimize cathodic protection systems. This article provides a detailed and comprehensive explanation of the Evans diagram, tailored to the needs of corrosion technicians, emphasizing actionable insights and field relevance.
Understanding the Basics of the Evans Diagram
At its core, corrosion is an electrochemical process involving anodic metal dissolution (oxidation) and cathodic reduction reactions occurring in an electrolyte environment. The Evans diagram plots electrode potential (E) on the vertical axis against the logarithm of current density (log i) on the horizontal axis. This plot represents the kinetics of the anodic and cathodic half-reactions on the metal surface.
Anodic Curve: Represents the oxidation reaction where metal atoms lose electrons and dissolve into the electrolyte. The current density increases exponentially with increasing potential in the anodic direction.
Cathodic Curve: Represents the reduction reaction where electrons are consumed, typically by oxygen reduction or hydrogen evolution. The current density increases exponentially with decreasing potential in the cathodic direction.
The intersection of the anodic and cathodic curves defines the corrosion potential (Ecorr) and the corrosion current density (icorr). At this point, the rate of anodic metal dissolution equals the rate of cathodic reaction, establishing the steady-state corrosion rate.
Detailed Explanation of the Evans Diagram Components
1. Electrode Potential (E)
Electrode potential is the driving force for electrochemical reactions and is measured relative to a reference electrode such as the copper-copper sulfate electrode (CSE). It reflects the energy state of the metal-electrolyte interface.
Positive shifts in potential indicate anodic polarization (more oxidizing).
Negative shifts indicate cathodic polarization (more reducing).
2. Current Density (i)
Current density is the rate of electron flow per unit area of the metal surface, directly related to the corrosion rate via Faraday’s Law. It is plotted logarithmically to capture the wide range of reaction rates.
3. Exchange Current Density (i₀)
At the equilibrium potential of each half-reaction, there is an exchange current density where the forward and reverse reactions occur at equal rates, resulting in zero net current.
The magnitude of i₀ depends on the metal surface condition and reaction kinetics.
Higher i₀ indicates faster reaction rates and less polarization.
4. Tafel Slopes (βa and βc)
The anodic (βa) and cathodic (βc) Tafel slopes describe the rate at which current density changes with potential in the logarithmic region. They are expressed in millivolts per decade of current change.
Typical anodic Tafel slope for steel is about 100 mV/decade.
Tafel slopes are used to extrapolate corrosion rates from polarization data.
Effect of pH on Cathodic Polarization
How to Interpret the Evans Diagram
Corrosion Potential (Ecorr): The potential where anodic and cathodic currents are equal; indicates the natural corrosion tendency of the metal in the environment.
Corrosion Current Density (icorr): The current density at Ecorr; proportional to the corrosion rate.
Polarization: Shifts in potential away from Ecorr due to applied current or cathodic protection.
Cathodic Protection Effect: Application of CP shifts the potential negatively, reducing icorr by suppressing anodic reaction kinetics.
Effect of Corroding Metal Ion Concentration (M)
Application of Evans Diagram in Corrosion Control
Cathodic Protection (CP) Mechanism
The Evans diagram visually demonstrates how CP reduces corrosion:
CP shifts the metal potential from Ecorr to a more negative potential (Ecp).
This shift decreases the anodic current density exponentially, lowering the corrosion rate.
The difference between the anodic and cathodic currents at Ecp equals the applied CP current.
Polarization and Environmental Effects
Activation Polarization: Due to slow charge transfer at the metal surface; dominant in concentrated electrolytes.
Concentration Polarization: Due to depletion of reactants (e.g., oxygen) near the surface; dominant in stagnant or dilute electrolytes.
Environmental Polarization: Changes in pH and ion concentration at the metal surface caused by CP current, further reducing corrosion.
Effect of Oxygen Concentration on Cathodic Polarization
Summary and Key Takeaways
The Evans diagram is a fundamental tool for visualizing and understanding the electrochemical kinetics of corrosion.
It graphically represents the balance between anodic metal dissolution and cathodic reduction reactions.
The intersection of anodic and cathodic curves defines corrosion potential and rate.
Cathodic protection shifts the potential negatively, reducing corrosion current exponentially.
Tafel slopes and exchange current densities are key parameters for quantifying corrosion kinetics.
Proper measurement techniques and environmental considerations are essential for accurate interpretation.
Understanding and applying the Evans diagram enables corrosion technicians to optimize CP systems and effectively control corrosion in the field.
Effect of Temperature on Polarization
Effect of Electrolyte/Electrode Movement (V) on Polarization
Referenced Standards
NACE SP0169-2024: Control of External Corrosion on Underground or Submerged Metallic Piping Systems — Provides criteria and methods for corrosion control and CP assessment.
ASTM G59: Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements — Supports polarization data collection.
NACE TM0177: Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H₂S Environments — Relevant for understanding polarization effects in sour environments.
Polarization Curves in Aerated and Deaerated Solutions of pH 7