Corrosion Measurement Techniques Quiz

Advanced Research Methods in the Engineering of Materials

• Lecture 7 covered basic polarization techniques for testing corrosion resistance of materials.
• Lecture 8 introduced electrochemical impedance spectroscopy (EIS) in material corrosion study.
• Lecture 6 focused on coating and thin film thickness measurement methods.
• Lecture 10a discussed the basics of scanning electron microscopy (SEM).
• Lecture 11 explores energy-dispersive (EDS) and wavelength-dispersive (WDS) X-ray spectroscopy in material engineering.

Electrochemical Nature of Corrosion

• Corrosion in aqueous or any other ionic conducting medium is electrochemical.
• Corrosion reaction requires four elements: anode, cathode, metallic conductor, electrolytic conductor.
• Oxidation reaction occurs at the anode, where the metal loses electrons.
• The anodic reaction can be written as: M → Mn^+ + ne⁻.
• Metallic ions are carried from the anode to the cathode by the electrolyte.
• Electrolytes are mainly liquids but can also be solids.
• Reduction reaction occurs at the cathode.
• Two ion types exist: anions and cations. Anions are negatively charged and move towards the anode. Cations are positively charged and move towards the cathode.
• Common reduction reactions at the cathode are hydrogen ion and oxygen reduction.
• Electrons from the anode are carried to the cathode via a metallic conductor.
• Electrochemical corrosion requires all four components to occur simultaneously for a reaction to take place.
• The rate of anodic reaction (oxidation) is equal to the rate of cathodic reaction (reduction).
• Electrochemical polarization measurements are used to determine the rate of anodic or cathodic reactions, either individually or collectively.
• Gibb's free energy change (ΔG) is negative for a spontaneous process, making the product more stable. A positive value of ΔG indicates the reaction is not spontaneous.

Electrochemical Polarization Measurement

• E (potential) is related to ΔG (Gibbs free energy change) using the equation E = −ΔG / nF, where n is the number of electrons transferred, and F is the Faraday constant.
• A positive potential corresponds to a negative ΔG and vice-versa.
• Arrangement of metals based on standard potentials is described by Standard Oxidation-Reduction (redox) potential, Standard Equilibrium Reduction Potential, Standard Potential, Electro-Motive Force (EMF), and Standard Reversible Potential.
• Standard potential is the potential of a metal in contact with its own ions at a concentration equal to unit activity.
• Several types of polarization techniques are employed, including potentiostatic, potentiodynamic, galvanostatic and galvanodynamic. Most electrochemical measurements control potential (potentiostatic or potentiodynamic) rather then controlling current (galvanostatic or galvanodynamic).
• Linear Polarization Resistance (LPR) and Tafel extrapolation used for corrosion rate determination via electrochemical polarization techniques.

Nernst Equation

• Nernst equation is used to calculate the potential of a metal when reactants are not at unit activity.
• The equation is E = E⁰ + (RT/nF) ln(a oxid/a red), where E⁰ is the standard redox potential, R is the gas constant, T is absolute temperature, n is the number of electrons transferred, and a and a are activities (concentrations) of the oxidized and reduced species.
• The more oxidized species present, the more positive the potential.

Mixed Potential

• Corrosion needs all four components in contact with each other, simultaneously.
• Corrosion potential (Ecorr) is the potential where the reaction rates match.
• Methods to determine mixed potential and corrosion current are important tools in studying general corrosion rates.

Potential vs Reference Electrode

• Corrosion potential is measured in relation to a reference electrode (like SCE, Ag|AgCl, or CCS electrodes).
• All reference results need to be reported against a reference electrode.
• Conversion can be done using relevant equations.
• Modern potentiostats report potentials as referenced to a reference electrode, and sometimes to the open-circuit potential.

Corrosion current and potential.

• A relation exists between the potential for activation-controlled processes and current, given by E − Ecorr = η = ±βlog(I/Icorr).
• η stands for overpotential
• Icorr depicts corrosion rate
• β is a constant.

Tafel Equation

• The rates of anodic and cathodic processes in corrosion are considered controlled by the kinetics of the electron-transfer reaction at the metal surface.
• Electrochemical reactions under kinetic control are described by the Tafel equation, 1 = Io exp(±2.303(E − E⁰/β)), where i is the current, Io is the exchange current, and β is the Tafel constant.
• The Tafel Equation reflects the behavior of one isolated reaction in a corrosion system, considering anodic and cathodic reactions that oppose each other.

Types of Polarization Control

• Methods for controlling potential or current in polarization techniques are characterized by potentiostatic and potentiodynamic measurement.
• Galvanostatic and galvanodynamic methods control current and the potential respectively.
• Linear polarization resistance (LPR) and Tafel are common methods for determining general corrosion rates via polarization resistance techniques.
• Cyclic potentiodynamic polarization can be used to examine localized corrosion.

Linear Polarization Resistance

• Rp is defined mathematically as ΔV/ΔI at Ecorr.
• It's associated with the corrosion current (Icorr) through Icorr = B/Rp.
• The constant B is defined as (βaβc)/(2.303(βa + βc)).
• Given the values of βa and βc, the corrosion rate can be calculated from Rp.
• Only a small potential perturbation is applied for this technique, so it does not substantially affect the corrosion reaction.

Tafel extrapolation method

• A linear relationship exists between the potential (E) and the logarithm of the current (log I) for electrode polarization, both anodic and cathodic. This relationship is known as the Tafel relation.
• The Tafel equation is a mathematical description of the relationship between potential and current.
• The equation often assumes kinetic control of the reaction at the metal surface.

Electrochemical impedance spectroscopy (EIS)

• EIS is used for fine-tuning mechanisms, determining kinetics of processes, resistances, and capacitances, and for in-situ determination of real surface areas.
• EIS is a very sensitive electrochemical technique, but needs careful use to avoid errors.

Electrochemical impedance spectroscopy (EIS) - Applications

• EIS is applied in interfacial processes (redox reactions, adsorption, electrosorption, kinetics of homogeneous reactions).
• It's used in geometric effects (like linear, spherical, cylindrical, and limited-volume electrodes) to determine solution resistance and porous electrodes,
• Also in power sources (batteries, fuel cells, supercapacitors); corrosion for, coatings, paints, electrocatalytic reactions, and various other applications such as in membranes, water electrolysis, halogen evolution or for studying conductive polymers, self-assembled monolayers, biological or semiconductor materials and sensors.

Ohm's Law

• Ohm's law describes the relationship between voltage (V), current (I), and resistance (R) in a simple electrical circuit.
• The law states that the current (I) through a conductor between two points is directly proportional to the voltage (V) across the two points.
• The constant of proportionality is the resistance (R). This relationship is expressed mathematically as V = IR.

Kirchhoff's laws

• The algebraic sum of currents entering any node (junction) in a circuit is equal to zero.
• The algebraic sum of voltage drops around any closed loop in a circuit is equal to zero.
• These laws are fundamental principles in circuit analysis and understanding the flow of current and voltage within complex circuits.

Impedance Definition

• Impedance is a measure of how much a circuit or material resists the flow of alternating current.
• Unlike resistance, impedance considers the phase shift between current and voltage in AC circuits, and can be a complex number, containing both magnitude (resistance) and phase angle components.
• Impedance may be used to measure more complex electro-chemical behavior.
• Measurements are usually conducted by applying an AC potential.

Corrosion Current and Potential

• Describes the relationship between corrosion potential and current during activation-controlled corrosion using an equation;
• This method is related to the calculation of the corrosion rate.

Basics of polarization techniques

• Potentiostatic method involves keeping the potential constant, while the corresponding current is monitored.
• Potentiodynamic method varies the potential actively at a constant rate and continually monitors the current.
• Galvanostatis and galvanodynamic methods actively control current.
• Linear polarization resistance (LPR) and Tafel extrapolation can examine corrosion rate changes over time through polarization resistance techniques.

Coating and Film Thickness Measurement Methods

• Various techniques for determining coating and film thickness can use optics, X-rays, or physical methods.
• Reflectometry analyzes intensity of reflected light corresponding to the wavelength.
• Ellipsometry uses polarized light to analyze reflectivity changes.
• Step height measurement is a direct technique that measures the difference in height with a stylus.
• Energy dispersive X-ray flourescence (XRF) analysis uses high energy X-rays that produce fluorescence in materials identifying chemical elements.
• Magnetic induction (MI) based on measuring induced magnetic fields as a result of material property changes.

Electrochemical Impedance Spectroscopy Analysis

• The electrochemical impedance spectroscopy (EIS) method is used for fine-tuning mechanisms, determining kinetics of processes, resistances and capacitances.
• It is used for measuring the in-situ areas of real surfaces. Its high sensitivity requires care during use.

Control Questions

• Includes questions to characterize various material testing methods: electrochemical corrosion, polarization measurements, Nernst equation, mixed potential, potential reference electrodes.