Understanding Corrosion Cells

• Corrosion is an electrochemical process involving electron and ion flow.

The Corrosion Cell

• Metal loss happens at the anode during corrosion. The cathode is protected.
• Electrochemical corrosion involves ion transfer across metal/electrolyte interfaces.
• The four components of a corrosion cell are the anode, cathode, electrolyte, and an electronic/metallic path.
• Electrons generated at the anode pass through the electronic path to cathodic immersed areas.
• Electrons sustain electrical balance by reacting with positive ions in the electrolyte.
• In a microscopic corrosion cell in water, the metal provides the anode, cathode, and electronic path.
• Water acts as the electrolyte in microscopic corrosion cells.
• Metal ions leave the anode, entering the electrolyte and leaving electrons behind, which then flow to the cathode.
• At the cathode, hydrogen ions meet electrons, forming hydrogen atoms, which may combine to form hydrogen gas.
• Hydrogen embrittlement happens if hydrogen atoms enter the metal lattice.
• Metal ions combine with hydroxyl ions to form metal hydroxide (rust), which precipitates.
• Oxidation of the metal occurs continuously at the anodic areas and reduction of hydrogen occurs at the cathodes as corrosion continues.
• A graphite-zinc dry cell battery exemplifies a corrosion cell. Zinc (anode) is electrically connected to carbon (cathode) through a load in a corrosive electrolyte, causing a completed circuit until the zinc is consumed.

Driving Force for Corrosion

• Corrosion involves electric charge movement from the anode into the electrolyte and onto the cathode.
• Voltage signifies the electrochemical difference between two electrodes in an electrolyte.
• A metal immersed in an electrolyte corrodes and exhibits an "electrode potential".
• Electrode potential is measured against a standard reference cell; it is the work to move a unit charge from electrode surface to the reference electrode.

Metal Driving Voltage and Activity

• Voltage differences on a metal surface arise from variations in grain structure, composition during alloying, or temperature/deformation, even in a single metal.
• Metals are found as chemical compounds known as ores.
• Obtaining pure metal from ore involves mining and refining to produce a nearly pure metal state, using mechanical, chemical, or electrical processes.
• Metals absorb energy during transformation from ore.
• The amount of energy absorbed during refining determines the metal's voltage/activity.
• Higher voltage means higher activity and corrosion tendency.
• Iron's cycle involves: ore (hematite), energy input, conversion to metallic iron, energy return upon corrosion (rust).
• Iron, thermodynamically unstable, readily corrodes to return to its lower energy state, rust.

Electromotive/Galvanic Series

• The standard EMF series represents the potential difference between metals in solutions with unit activity ions.
• Metals are arranged from most noble (gold) to most active (magnesium).
• Placing two electrodes in an electrolyte results in electrochemical reactions.
• Electromotive force (EMF) is the potential between two electrodes and the driving force for electrochemical corrosion.
• The EMF of a corrosion cell cannot always be predicted from the standard EMF series.
• The EMF series is produced under standardized conditions. Metals can polarize when connected which affects the voltage.

Electrically Interconnected Electrodes

• A metallic path between two electrodes with a voltage difference results in current flow.
• In a common electrolyte, more active metals electrically interconnected tend to become the anode of the corrosion cell.
• The standard EMF series helps organize the tendency of metals to corrode.
• The galvanic series is based on metal behavior in a particular electrolyte and is more useful than the standard EMF series.

Nernst Equation

• The Nernst Equation expresses the exact electromotive force of a cell (half-cell potential) in terms of the activities of the electroactive species:
  • E = E° + (RT/nF)ln(aM+ne/aM)
  • Where
    • E° = Standard state half-cell electrode potential (SHE)
    • E = Electrode potential in existing solution (SHE)
    • aM+ne = Activity of metal ions in solution
    • aM = Activity of the metal (aM=1 for pure metal)
    • R = Universal gas constant = 8.31431 joules K-1 mole-1
    • T = Absolute temperature (K) = 298.2°k
    • n = Number of electrons transferred
    • F = Faraday's constant (96,500 C/equivalent)
• The metal ion concentration in the electrolyte affects the electrode's potential.
• Standard conditions for EMF are based on unit activity of metal ions in electrolyte at 25°C with no impurities.

Simplified Nernst Equation

• The simplified Nernst Equation converts to logarithms to base 10 and condenses the coefficient RT/F to 0.0592 V:
  • E = E0 + (0.059/n) × log(aM+ne/aM)
  • Differences in ion concentration cause variations in measured metal potentials vs. published data.
• Higher concentration of metal ions makes the metal's activity less.

Temperature Differences

• In a temperature cell, the anode and cathode are the same metal type, but one is at a higher temperature.
• The electrode at a higher temperature usually becomes the anode in a temperature cell.
• A gas transmission line from a compressor station is an example. The compressed gas at the station outlet (i.e., downstream of) side is hot and loses heat to transfer to the surrounding soil. The hot pipe near the compressor is the anode; the cooler pipe down the line is the cathode; the soil is the electrolyte; and the pipe is the connecting circuit.

Oil and Gas Well Casings

• Oil and gas well casings undergo similar attacks with the pipe casing deep below the surface that is at a higher temperature than the pipe near the surface turning into the anode.

Effects of Alloying

• During alloying, areas along the grain boundaries can become enriched or depleted of specific alloying elements.
• These differences in alloy composition may cause intergranular corrosion.
• Selective leaching, such as dezincification (zinc leaching from brass) or iron leaching from cast iron could also occur.

Metal Fabrication

• Metals may experience variations in stress during fabrication, making highly stressed areas more active.
• Improper welding procedures can sensitize adjacent metal and be responsible for corrosion.

Concentration Cells

• Concentration cells are often responsible for much of the corrosion in soils.
• Concentration cell corrosion arises when two or more areas of a metal surface meet different concentrations of the same solution.
• The four general types of concentration cell corrosion are oxygen, salt, metal ion, and active-passive.

Oxygen Concentration Cells

• Oxygen is the most common cathodic depolarizer and influences the corrosion amount of iron and other metals.
• Areas with higher oxygen concentrations become cathodic.
• Buried pipes can experience oxygen concentration cells. Loose backfill facilitates oxygen diffusion, leading to corrosion. The bottom surface of the pipe becomes the anode as the rest is the cathode.
• Pavement reduces oxygen access. Thus, the metal under pavement becomes the anode, corroding more near the pavement's edge.
• Well casings form oxygen cells. The casing at depth is the anode, and the surface piping is the cathode and the soil there serves as the electrolyte.
• Isolating surface piping controls the oxygen concentration cells.
• Oxygen concentration cells commonly occur near waterline pilings where wave action provides high oxygen and deeper levels have low oxygen.
• Different soils can create concentration cells. Pipelines through various soils can have the pipe in heavier soil as the anode and more porous as the cathode.

Salt Concentration Cells

• If there is a difference in salt concentration. Areas with higher concentration corrode more.
• De-icing salts on roads or bridge decks may contribute to this process.

Metal-Ion Concentration Cells

• Metal-ion concentration cells occur when a metal is in contact with a solution with ions of that metal, but the ion concentration differs on each end.
• This is most commonly seen in buried metal structures.
• The area with lower concentration of metal ions is the anode, and the area with higher concentration of metal ions is the cathode.

Active-Passive Cells

• Active-passive cells involve metals with the ability to form a protective film.
• The passive film protects the metal reducing the corrosion rate.
• Damage to the passive film creates an active-passive cell since the broken film area is anodic.
• Stainless steel is a metal that displays active-passive behavior.

Anodic and Cathodic Reactions

• Anodic and cathodic reactions are the two fundamental types of electrochemical corrosion.
• The anodic reaction takes place at the anode. The actual metal will oxidize losing electrons to become metal ions.
• The cathodic reaction takes place at the cathode. Electrons are consumed and a species (e.g. hydrogen ions or oxygen) is reduced in the electrolyte.

Anodic Reactions

• At the anode, the metal dissolves into the electrolyte releasing electrons: M -> Mn+ + ne-
• Oxidation means to lose electrons.

Cathodic Reactions

• At the cathode, electrons are consumed, and the electrolyte is reduced. Reduction here means a gain of electrons.
• Common Cathodic Reactions: Hydrogen evolution (acidic solutions), oxygen reduction (neutral/alkaline solutions).
• Example Hydrogen Reduction: 2H+ + 2e- -> H2.
• Example Oxygen Reduction: O2 + 2H2O + 4e- -> 4OH-.

Polarization

• Polarization is the change in the electrode potential caused by the passage of current.
• Polarization affects the corrosion process by opposing and slowing corrosion current.

Activation Polarization

• Activation polarization is related to the energy barrier for reactions at the electrode surface.
• It is significant in reactions involving electron transfer, like hydrogen evolution.
• Activation polarization decreases as temperature increases.

Concentration Polarization

• Concentration polarization arises when the concentration of reacting species at the electrode differs from the electrolyte's bulk.
• This difference occurs when replenishment isn't enough to meet the needs for production.
• If metal ions are consumed quickly, the cathode is depleted.
• Agitation of electrolyte can reduce polarization due to facilitating ion surface transport.

Resistance Polarization

• Resistance (Ohmic) polarization is the potential drop in the electrolyte due to current flow resistance.
• Electrolyte resistance, any surface films, and resistance at the metal/electrolyte interface all contribute.
• Electrolyte conductivity decreases resistance polarization.

Corrosion Rate

• Corrosion rate measures metal loss per area per time (mpy, mm/yr).
• Key influence factors are materials, environment, and temperature.

Electrolyte Conductivity

• Electrolyte conductivity impacts corrosion; highly conductive electrolytes help ion flow between the anode and the cathode.

Presence of a moisture film

• Electrochemical corrosion requires a moisture film as an electrolyte.
• Anodic and cathodic reactions cannot be sustained without conductive film.

Relative Humidity

• Relative humidity regulates the formation of moisture film.
• Increasing humidity creates more metal surface moisture.
• Significant corrosion begins at 60-70% relative humidity.

Temperature

• Higher temperature increases the rate of chemical reactions.
• Kinetic rates and diffusion of ions in the electrolyte are increased with temperature.
• Increased temperature can decrease oxygen solubility and slow oxygen reduction and thus may slow down corrosion.
• Temperature impacts moisture film properties (viscosity, conductivity).

Concentration Changes

• Imbalance between electrochemical reaction rate and mass transport causes concentration changes.
• If electrochemical reactions consume or generate ions quicker than the rate of supply or remove, then this causes a concentration gradient.

Diffusion

• Diffusion is ion transport from high to low concentrations driven by the concentration gradient.
• Diffusion reduces concentration polarization.

Migration

• Migration is the movement of charged ions in an electrolyte under the influence of an electric field.
• In a corrosion cell, an electric field forms between the anode and cathode for ions to move.
• Ions that react can increase or decrease concentration polarization.

Convection

• Convection involves electrolyte physical movement.
• Stirring, flow, or temperature gradients that cause density differences are common convection causes.
• Convection reduces concentration polarization by mixing the electrolyte and minimizing the gradients.

Limiting Current Density

• Limiting current density is the maximum current by a given set of conditions.
• This is caused by drop to near zero for reacting species at the electrode surface.
• Past this value, the addition in increasing potential will not increase the current because the reaction is limited by the reactants.

Factors Affecting Concentration Polarization

• Primary factors: electrode kinetics, transport, temperature, and electrolyte.

Electrode Kinetics

• Faster electrode reactions cause larger polarization.

Mass Transport

• Slower transport leads to great changes causing greater polarization.