Unit
- Corrosion is an electrochemical process: It involves oxidation of metal at an anode and reduction of an oxidizing agent at a cathode, with ionic current flowing through an electrolyte. The metal dissolves as ions, releasing electrons that travel through the metal to the cathode.
- Anode and cathode reactions: At the anode, metal atoms lose electrons to become cations (e.g., Fe -> Fe2+ + 2e-). At the cathode, electrons are consumed by reduction reactions such as oxygen reduction (O2 + 2H2O + 4e- -> 4OH-) or hydrogen evolution (2H+ + 2e- -> H2) in acidic environments.
- Electrolyte role: The electrolyte conducts ionic current between anodic and cathodic sites. Common electrolytes include water with dissolved salts, acids, or bases. Higher conductivity and aggressive ions like chloride accelerate corrosion.
- Galvanic cell analogy: A corrosion cell is similar to a battery, with separate anodic and cathodic areas. The potential difference drives the reaction. In uniform corrosion, anodic and cathodic sites shift randomly over the surface.
- Mixed potential theory: The corrosion potential (Ecorr) and corrosion current (Icorr) are determined by the intersection of anodic and cathodic polarization curves. This theory explains how oxidizers and inhibitors affect corrosion rate.
- Faraday's law application: The mass of metal lost due to corrosion is proportional to the total charge passed. Using Faraday's law, corrosion rate can be calculated from current density and equivalent weight.
- Factors affecting electrochemical corrosion: Temperature increases reaction rates; pH influences cathodic reactions; dissolved oxygen accelerates oxygen reduction; and fluid velocity can enhance mass transfer of reactants.
- Practical implication: Understanding the electrochemical nature enables prediction of corrosion behavior and design of protection systems such as cathodic protection or anodic passivation.
- Polarization curves and Tafel analysis: Plotting potential versus log current density reveals anodic and cathodic Tafel slopes, which indicate reaction kinetics. The intersection of extrapolated linear Tafel regions at the corrosion potential gives the corrosion current density. Differences in Tafel slope help identify rate controlling steps.
- Passivity and localized corrosion: Many metals form a thin protective oxide film that drastically reduces corrosion in certain environments. This passive state appears as a region of low current on the polarization curve. Breakdown by aggressive ions like chloride can cause pitting or crevice corrosion.
- Electrochemical measurement techniques: Common methods include potentiodynamic polarization, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). LPR applies a small potential sweep near Ecorr to obtain polarization resistance, which is inversely proportional to corrosion rate. EIS provides mechanistic information.
Code Example
Python
def corrosion_rate(current_density, equivalent_weight, density):
# Faraday constant in C/mol
F = 96485.0
# Rate in mm/year (simplified)
rate = (current_density * equivalent_weight * 87600) / (F * density)
return rate
# Example: steel (Fe, EW=27.92 g/eq, density=7.87 g/cm3)
i_corr = 0.1 # mA/cm2
rate = corrosion_rate(i_corr, 27.92, 7.87)
print('Corrosion rate: {:.2f} mm/year'.format(rate))This function calculates uniform corrosion rate from measured current density using Faraday's law. In practice, corrosion current is obtained from polarization resistance measurements.
Quick Revision
- Corrosion is an electrochemical process with distinct anodic and cathodic reactions.
- An electrolyte conducts ions between anode and cathode, enabling current flow.
- Mixed potential theory helps interpret polarization behavior and predict corrosion rates.
- Faraday's law links mass loss to electric charge, forming the basis for rate calculations.