Understanding Corrosion Cells

Corrosion Cell

• It is an electrochemical process involving electron and ion flow.
• Metal loss (corrosion) occurs at the anode.
• No metal loss occurs at the cathode (protected).
• Electrochemical corrosion involves ion transfer across metal/electrolyte interfaces.
• Consists of an anode, cathode, electrolyte, and electronic/metallic path.
• Electrons from metallic ions at the anode travel through the electronic path to cathodic areas in the electrolyte.
• These electrons restore electrical balance by reacting with positive ions in the electrolyte.
• In a microscopic corrosion cell, metal provides the anode, cathode, and electronic path.
• Water is the electrolyte, containing hydrogen (H+) and hydroxyl (OH-) ions.
• Metal ions leave the anode, releasing electrons that flow to the cathode.
• At the cathode, electrons meet hydrogen ions from the solution.
• Hydrogen ions accept electrons and become hydrogen atoms which may form hydrogen gas or enter the metal lattice, leading to hydrogen embrittlement.

Driving Force for Corrosion

• Voltage: Electric charge flows from anode to electrolyte to cathode.
• Voltage is the electrochemical difference between two electrodes in an electrolyte.
• Metal corrodes and exhibits an electrode potential when immersed.
• Electrode potential is the reversible work to move a unit charge from the electrode surface to the reference electrode.
• It's the potential difference where 1 coulomb of electricity does 1 joule of work.
• Measured against a standard reference cell.

Metal Driving Voltage and Activity

• Metal itself can drive the corrosion cell.
• Variations in voltage arise from differences in grain structure, composition from alloying, temperature, or deformation during fabrication.

Refining and Corrosion Processes

• Metals occur as ores and are refined to a nearly pure state.
• Processes include mechanical, chemical, and electrical methods.
• Metals absorb energy during transformation.
• The amount of energy needed to refine the pure metal determines its voltage and active state.
• High voltage/active state metals (magnesium, aluminum, iron) corrode more easily.
• Low voltage metals such as copper and silver corrode slowly.
• Iron cycle: Iron ore (hematite, Fe2O3) converted to metallic iron requires energy.
• The most common product of the corrosion of iron is rust.
• Iron corrodes back to iron oxide, "returning" the energy.

Electromotive/Galvanic Series

• Potential difference between metals in solutions containing ions at unit activity.
• Arranges metals from most noble (gold) to most active (magnesium).
• When two electrodes are placed in the electrolyte, electrochemical reactions occur on each causing corrosion.
• The potential between the two electrodes - electromotive force (emf) - equals the anode's electrode potential less the cathode's.
• This emf is the driving force for electrochemical corrosion.
• The emf of a corrosion cell can't always be predicted from a standard emf series, as this series is produced under set environmental conditions and metals polarize when connected, affecting voltage.
• A metallic path connecting two electrodes causes current flow.
• Current is the net transfer of electric charge per unit time.

Practical Galvanic Series and Nernst Equation

• Galvanic series is based on a metal's behavior in a particular electrolyte (seawater).
• The Nernst Equation expresses the electromotive force based on the activities of products and reactants in the cell.

Temperature Differences

• In a temperature cell, the anode and cathode are the same metal, but have different temperatures.
• The electrode is maintained at the higher temperature, which becomes the anode.
• Compressed gas line example: Hot pipe near the compressor is the anode; the cooler pipe down the line is the cathode.
• The soil acts as the electrolyte, and the pipe provides the connection.
• High temperatures may damage coatings near the compressor, accelerating corrosion.

Effects of Alloying

• During alloying, grain boundaries can become enriched or depleted of alloying elements.
• These differences can cause intergranular corrosion.
• Selective leaching, such as dezincification of brass (zinc leaching) or graphitic corrosion in cast iron (iron leaching), is another common problem.

Metal Fabrication

• Variations in stress can occur during fabrication which also causes variations in corrosion.
• Highly stressed areas are more active.
• Improper welding can also cause corrosion due to incompatible welding rods or sensitizing the adjacent metal.

Electrolyte Differences

• Corrosion can result from variations in the electrolyte.
• Differences include soil types, chemical substances, concentrations, or temperature.
• Reinforced concrete example: salt contamination causes a voltage difference.

Concentration Cells

• Concentration cells have a dilute and concentrated salt solution, where electrode potential is the electrolyte concentration.
• A similar cell forms when electrolytes contain completely different substances.

Concentration Cells in Soils

• Pipelines pass through different soils which creates corrosion.
• Different soils in contact with a bare or poorly coated metal allows for corrosion.
• In naturally occurring soils, the pipe section in conductive soil is usually the anode, and that in less is the cathode.
• Moist soils act as the electrolyte, while the pipe is the connecting circuit.

Types of Concentration Cells

• Higher salt concentration may be the anode if simple salts are involved, not of the metal that makes up the electrodes.

Metal Ion Concentration Cell

• Exists where a lower and higher concentration of ions at one point.
• The surface in contact with lower metal ions acts as the anode.

Oxygen Concentration Cell

• Oxygen content influences corrosion of iron.
• Necessary for cathodic reactions.
• Buried pipes example: the pipe resting on undisturbed soil is the anode, and the sides/top with loose backfill is the cathode.
• Pavement example: The section under the pavement is the anode, and the section outside the pavement is the cathode.

Effects of pH

• Theoretically, the potential of a structure changes by 60 mV for each pH unit.
• Acidic pH environments can harm concrete.

Corrosion Rate

• It's directly proportional to current flow.
• Current density is important in gauging the potential destructiveness caused by current flow.

Faraday's Law

• Relates weight loss of metal to corrosion with time and current flow.

Electrolyte/Electrode Movement Effect

• Increased movement between the electrodes and electrolyte removes reaction products at the anode and cathode, also replenishing reactants.
• Increased movement results in decreased polarization and more current.
• In stagnant electrolytes, decreased movement results in increased polarization and decreased current.

Electrolyte Concentration Effect (Interface Area)

• Polarization of the anode is affected by the concentration of corroding metal ions produced by metal oxidation, and is named anodic polarization.
• Increase in metal ion concentration results in increase in polarization of the anode and decrease in current.

Cathodic Polarization

• Cathode polarisation is affected by the concentration of cathodic reactants such as oxygen or hydrogen ions.
• Increase reactant concentration will decrease polarisation of the cathode and increase current.

Passivity

• Passivity is the loss of chemical reactivity.
• Specific metals and alloys under specific environmental conditions.
• Caused by a surface film acting as a barrier to further corrosion.
• Stainless steels, chromium, titanium, and nickel alloys become inert.

Passivating vs. Nonpassivating

• Passivating metals demonstrate passivity.

Microbiologically Influenced Corrosion (MIC)

• Bacteria under anaerobic conditions can reduce sulfates and consume hydrogen.
• Hydrogen consumption at cathodic areas depolarizes steel, enabling more metal consumption by galvanic corrosion cells.
• Bacteria don't directly attack the structure.

Forms of Corrosion

• Uniform Attack: Even metal loss over the entire area.
• Crevice Corrosion: Occurs where two tightly spaced surfaces are exposed to a corroding environment.
• Galvanic Attack: Dissimilar metals are connected, creating a voltage difference.
• Pitting: Localized corrosion over a small area.
• Intergranular Attack: Local corrosion at grain boundaries due to element composition differences.
• Selective Leaching: Selective removal of one element from an alloy.
• Velocity Phenomena: Rapid movement of fluid over metal.
• Environmental Cracking: Reaction of metal with a corrosive environment and stress.

Environmental Cracking Types

• Corrosion Fatigue: Fatigue under alternating stress in a corroding media.
• Hydrogen Embrittlement: Reduction in ductility from diffused hydrogen.
• Hydrogen Damage: Atomic hydrogen causes blistering, hydride formation, embrittlement.
• Prevention involves keeping potential below the hydrogen evolution potential.
• Stress Corrosion Cracking (SCC): Cracking under tensile stress in a corrosive environment.

Reference Electrodes

• Important devices that measure the potential of a metal surface exposed to an electrolyte.
• Standard Hydrogen Electrode (SHE) is a primary reference electrode that is used to determine the potential of other reference electrodes.
• Structure-to-electrolyte potential is a potential difference between the structure and a reference electrode.
• The electrolyte itself has no value against which the potential of a structure can be measured independently of the potential of the reference electrode used.

Standard Hydrogen Electrode (SHE)

• Potentials are based relative to it
• The SHE half-cell is not practical for field use so other metal electrodes are used in their place

Copper-Copper Sulfate Electrode (CSE)

• The electrode consists of copper rod, immersed in a saturated solution of copper sulfate, held in a non-conducting cylinder with a porous plug at the bottom
• Most commonly used reference electrode for underground and fresh water structures.
• Not suitable for chloride electrolytes.

Other Reference Electrodes

• Silver-Silver Chloride (SSC) reference electrodes are used for measurements in seawater and concrete.
• Calomel (SCE) reference electrode consists of mercury-mercurous chloride in a saturated potassium chloride solution and used for laboratory electrochemistry experiments.
• Zinc (ZRE) reference electrode is used because zinc’s potential is relatively stable, however Zinc is not stable in carbonates or at high temperatures.

Polarity

• Polarity of measurement must be noted to ensure accurate results.
• Most instruments now have automatic polarity display to compensate for polarity.