Corrosion Rate Calculation From Tafel Plot

Formula Used:

The corrosion rate is calculated from the corrosion current density (i_corr) using the following fundamental relationship derived from Faraday's laws of electrolysis:

Corrosion Rate = (EW * i_corr * 3.27 * 10^4) / ρ

Where:

• EW is the Equivalent Weight of the corroding metal (g/equivalent).
• i_corr is the Corrosion Current Density (A/cm²).
• ρ is the Density of the metal (g/cm³).
• 3.27 x 10^4 is a conversion factor that incorporates Faraday's constant and unit conversions to yield mils per year (mpy).

The rate in mm/year is then derived from mpy.

What is Corrosion Rate Calculation from Tafel Plot?

Corrosion rate calculation from a Tafel plot is a critical electrochemical technique used to quantify the speed at which a metal or alloy deteriorates due to chemical or electrochemical reactions with its environment. A Tafel plot, generated from potentiodynamic polarization measurements, allows for the extrapolation of the corrosion current density (i_corr) from the anodic and cathodic Tafel regions. This i_corr is the linchpin for determining the corrosion rate, a vital parameter in materials science, engineering, and asset management across various industries.

Understanding and accurately calculating corrosion rates are essential for:

• Predicting the service life of metallic components.
• Designing effective corrosion protection strategies (e.g., coatings, inhibitors, cathodic protection).
• Monitoring the performance of protective systems.
• Ensuring the safety and reliability of infrastructure and equipment.
• Selecting appropriate materials for specific operating environments.

Professionals in fields such as materials engineering, chemical engineering, oil and gas, aerospace, civil engineering, and manufacturing rely on these calculations. Common misunderstandings often arise from unit conversions and the correct interpretation of the Tafel extrapolation method itself, which assumes linear behavior in the Tafel regions.

Key Inputs for Corrosion Rate Calculation

The accuracy of the corrosion rate hinges on precise input values:

• Corrosion Current Density (i_corr): This is the most crucial parameter, directly derived from the Tafel plot. It represents the rate of electrochemical reactions at the corrosion potential. Units are typically Amperes per square centimeter (A/cm²).
• Equivalent Weight (EW): This property relates to the electrochemical equivalent of the metal. It's calculated by dividing the atomic weight of the metal by the number of electrons transferred during the electrochemical reaction (valence). Units are grams per equivalent (g/equivalent).
• Density of the Material (ρ): The physical density of the metal or alloy being studied. Units are grams per cubic centimeter (g/cm³).
• Faraday's Constant (F): A fundamental physical constant representing the magnitude of electric charge per mole of electrons. Its value is approximately 96485 Coulombs per equivalent (C/equivalent). While a constant, it's included in the calculation for completeness and clarity.

Tafel Plot and Corrosion Rate Formula Explained

The core of this calculator lies in the conversion of the electrochemical parameter, corrosion current density (i_corr), into a more tangible measure of material loss over time. This is achieved through established electrochemical principles, primarily derived from Faraday's laws.

The Fundamental Formula

The general formula to calculate the corrosion rate from i_corr is:

Corrosion Rate (mass/time) = (EW * i_corr * Conversion Factor) / ρ

The "Conversion Factor" depends on the desired units for the corrosion rate. For common units like mils per year (mpy), a specific factor is used that incorporates Faraday's constant and necessary unit conversions:

Corrosion Rate (mpy) = (EW * i_corr * 3.27 * 10^4) / ρ

To convert this to millimeters per year (mm/year), we use the conversion: 1 mpy = 0.0254 mm/year.

Variable Explanations

Variables Used in Corrosion Rate Calculation

Variable	Meaning	Unit (Standard)	Typical Range
i_corr	Corrosion Current Density	A/cm²	10⁻¹² to 10⁻³
EW	Equivalent Weight	g/equivalent	10 to 100+ (depends on metal)
ρ	Density of Material	g/cm³	1 to 20+ (depends on metal)
F	Faraday's Constant	C/equivalent	96485 (constant)
3.27 x 10⁴	Unit Conversion Factor (for mpy)	(mpy * g * cm * s) / (A * year * equivalent)	Constant

The value for i_corr is obtained by extrapolating the linear Tafel regions of the polarization curve back to the corrosion potential (E_corr). The accuracy of this extrapolation significantly impacts the calculated corrosion rate. The conversion factor (3.27 x 10⁴) is derived from:

(F [C/equiv] * 3600 [s/hr] * 24 [hr/day] * 365.25 [day/year]) / (1000 [mA/A] * 10 [mm/cm] * 10000 [cm²/m²]) which simplifies to approximately 3.27 x 10⁴ when considering the appropriate unit conversions to arrive at mpy.

Practical Examples

Example 1: Mild Steel in Saline Solution

A common scenario involves evaluating the corrosion of mild steel in a simulated seawater environment.

• Inputs:
  • Corrosion Current Density (i_corr): 5.0 x 10⁻⁶ A/cm²
  • Equivalent Weight of Iron (Fe): 27.92 g/equivalent
  • Density of Steel (ρ): 7.85 g/cm³
  • Time: 1 Year
• Calculation:
  • Corrosion Rate (mpy) = (27.92 * 5.0e-6 * 3.27e4) / 7.85 ≈ 58.2 mpy
  • Corrosion Rate (mm/year) = 58.2 mpy * 0.0254 mm/mpy ≈ 1.48 mm/year
• Result Interpretation: The mild steel is corroding at a rate of approximately 58.2 mils per year or 1.48 millimeters per year. This rate suggests moderate corrosion, which might require protective measures in a long-term application.

Example 2: Stainless Steel in Acidic Environment

Assessing the corrosion resistance of a specific stainless steel grade in an industrial acid.

• Inputs:
  • Corrosion Current Density (i_corr): 8.0 x 10⁻⁸ A/cm²
  • Equivalent Weight of Stainless Steel (approximated): 27.5 g/equivalent
  • Density of Stainless Steel (ρ): 7.9 g/cm³
  • Time: 1 Year
• Calculation:
  • Corrosion Rate (mpy) = (27.5 * 8.0e-8 * 3.27e4) / 7.9 ≈ 0.91 mpy
  • Corrosion Rate (mm/year) = 0.91 mpy * 0.0254 mm/mpy ≈ 0.023 mm/year
• Result Interpretation: The stainless steel exhibits excellent corrosion resistance in this acidic environment, with a calculated rate of about 0.91 mils per year or 0.023 millimeters per year. This is considered very low corrosion.

How to Use This Corrosion Rate Calculator

This calculator simplifies the process of converting electrochemical data (i_corr) into practical corrosion rate metrics. Follow these steps:

1. Obtain i_corr: Perform a potentiodynamic polarization test to generate a Tafel plot. Analyze the plot to determine the corrosion current density (i_corr) by extrapolating the Tafel slopes to the corrosion potential. Ensure the units are A/cm².
2. Identify Material Properties: Find the Equivalent Weight (EW) and Density (ρ) for the specific metal or alloy you are analyzing. These values are crucial for accurate calculations. Typical values can be found in material handbooks or through material specifications.
3. Input Values:
   • Enter the obtained Corrosion Current Density (i_corr) in Amperes per square centimeter (A/cm²) into the 'Corrosion Current Density' field.
   • Enter the Equivalent Weight (EW) in grams per equivalent (g/equivalent) into the 'Equivalent Weight' field.
   • Enter the Density (ρ) in grams per cubic centimeter (g/cm³) into the 'Density of Material' field.
   • The Faraday's Constant field is pre-filled with the standard value.
   • Enter the numerical duration for which you want to calculate the corrosion rate (e.g., '1' for one year).
   • Select the appropriate unit for the time duration (Hours, Days, or Years) from the dropdown. The calculator assumes a 1-year period for the primary mpy and mm/year calculations, but this input can conceptually represent a rate over that selected period. For instance, if you input '10' days and select 'Days', the calculation still outputs a mpy/mm/year rate, which is a standard practice for expressing long-term corrosion potential.
4. Calculate: Click the "Calculate Corrosion Rate" button.
5. Interpret Results: The calculator will display the corrosion rate in two common units: mils per year (mpy) and millimeters per year (mm/year). It will also show the intermediate values used in the calculation for verification.
6. Reset: Use the "Reset" button to clear all fields and return them to their default or initial state.

Unit Selection: Always ensure your input values for i_corr, EW, and ρ are in the specified units (A/cm², g/equivalent, and g/cm³ respectively) for the calculator to function correctly. The time unit primarily affects the interpretation of the rate, as mpy and mm/year are annualized rates.

Key Factors Affecting Corrosion Rate

While the Tafel plot provides a direct measure of i_corr, the actual corrosion rate experienced in a real-world application is influenced by numerous environmental and material factors. These factors can either accelerate or mitigate corrosion:

1. Electrolyte Composition: The type and concentration of ions in the corrosive medium (e.g., chlorides, sulfates, acids, bases) significantly affect conductivity and electrochemical reactions. Higher concentrations of aggressive ions often lead to higher corrosion rates.
2. Temperature: Generally, increasing temperature accelerates electrochemical reaction rates, thus increasing the corrosion rate. However, in some cases, increased temperature might decrease oxygen solubility, which could have a mitigating effect.
3. pH: The acidity or alkalinity of the environment plays a crucial role. Many metals are more susceptible to corrosion in acidic conditions, while some (like stainless steels) can passivate in specific pH ranges.
4. Oxygen Availability: Dissolved oxygen is a common cathodic reactant. In many environments, the corrosion rate is controlled by the diffusion rate of oxygen to the metal surface. Areas with higher oxygen concentration can lead to differential aeration cells and localized corrosion.
5. Flow Rate and Velocity: Fluid flow can increase the supply of corrosive species to the surface or remove protective films, potentially increasing the corrosion rate. High velocities can also cause erosion-corrosion.
6. Presence of Microbial Contaminants (MIC): Microbially Influenced Corrosion (MIC) occurs when microorganisms alter the local chemical environment or directly participate in electrochemical reactions, often leading to unexpectedly high and localized corrosion rates.
7. Surface Condition and Microstructure: Surface roughness, presence of impurities, grain boundaries, and phase distribution within the alloy can create micro-galvanic cells, influencing localized corrosion initiation and propagation.
8. Protective Films: The formation and stability of passive films (like the chromium oxide layer on stainless steel) or applied coatings are critical. Breakdown of these films dramatically increases the susceptibility to corrosion.

Frequently Asked Questions (FAQ)

What is the primary output unit for this calculator?

The primary outputs are corrosion rates in Mils Per Year (mpy) and millimeters per year (mm/year). These are standard industry units for expressing long-term corrosion severity.

How is i_corr determined from a Tafel plot?

i_corr is found by extrapolating the linear segments (Tafel regions) of the anodic and cathodic curves on a polarization plot back to the corrosion potential (E_corr). The current density at this intersection point is the i_corr.

Can I use i_corr values in mA/cm²?

No, this calculator strictly requires i_corr in Amperes per square centimeter (A/cm²). If your measurement is in mA/cm², divide it by 1000 to convert to A/cm² before entering it.

What does Equivalent Weight (EW) mean?

Equivalent Weight is the atomic weight of an element divided by its valence (the number of electrons transferred in the electrochemical reaction). For alloys, an effective or average EW is often used.

Is Faraday's Constant adjustable?

No, Faraday's Constant (F) is a fundamental physical constant (approximately 96485 C/equivalent) and is fixed in the calculation.

Does the 'Time' input affect the mpy/mm/year result directly?

The mpy and mm/year units are inherently annual rates. The 'Time' input and its unit (Hours, Days, Years) are conceptually used to define the period over which the observed i_corr is representative. The calculator outputs a rate that assumes this i_corr would persist over a full year. It helps users contextualize the rate based on their experiment duration.

What if my material's density is very different from the examples?

Material density (ρ) varies significantly between metals and alloys. Always use the accurate density for the specific material you are testing. Using an incorrect density will directly impact the calculated corrosion rate.

What are the limitations of Tafel extrapolation?

Tafel extrapolation assumes that the anodic and cathodic reactions follow true Tafel kinetics in the measured potential range. This assumption may not hold true if the potential range is too narrow, if the passive film is unstable, or if diffusion-limited currents interfere. It also doesn't inherently account for localized corrosion mechanisms.

Related Tools and Resources

Explore these related resources for a comprehensive understanding of corrosion and materials science: