🔌Electrochemistry Unit 10 – Electrochemistry: Materials and Corrosion

Key Concepts and Fundamentals

• Electrochemistry studies chemical reactions involving the transfer of electrons between substances
• Redox reactions involve the oxidation (loss of electrons) and reduction (gain of electrons) of chemical species
• Electrochemical cells convert chemical energy into electrical energy (galvanic cells) or vice versa (electrolytic cells)
• Electrodes are the sites where electrochemical reactions occur, with the anode being the site of oxidation and the cathode being the site of reduction
• Electrolytes are ionic conductors that allow the flow of ions between electrodes in an electrochemical cell
• Standard electrode potentials ($E^0$) measure the tendency of a half-reaction to occur relative to the standard hydrogen electrode (SHE)
• The Nernst equation relates the electrode potential to the standard electrode potential and the concentrations of the oxidized and reduced species: $E = E^0 - \frac{RT}{nF} \ln \frac{[Red]}{[Ox]}$
• Faraday's laws of electrolysis relate the amount of substance produced or consumed in an electrolytic cell to the quantity of electricity passed

Electrochemical Materials and Their Properties

• Electrochemical materials are used in various applications, such as batteries, fuel cells, sensors, and corrosion protection
• Metals are commonly used as electrodes due to their high electrical conductivity and ability to participate in redox reactions
  • Examples of metal electrodes include copper, zinc, iron, and aluminum
• Semiconductors, such as silicon and germanium, can also be used as electrodes in specialized applications
• Electrolytes can be liquid (aqueous or non-aqueous), solid, or gel-based, depending on the specific application
  • Aqueous electrolytes include solutions of salts, acids, or bases in water
  • Non-aqueous electrolytes are used in lithium-ion batteries and include organic solvents like ethylene carbonate and dimethyl carbonate
• Ionic liquids are molten salts with low melting points that can serve as both electrolytes and solvents in electrochemical systems
• Conducting polymers, such as polyaniline and polypyrrole, exhibit electrical conductivity and can be used in electrochemical sensors and actuators
• Nanostructured materials, like graphene and carbon nanotubes, offer high surface area and unique properties for electrochemical applications

Corrosion Mechanisms and Types

• Corrosion is the deterioration of a material, usually a metal, due to chemical or electrochemical reactions with its environment
• Uniform corrosion occurs when the entire surface of a metal corrodes at a similar rate, leading to an even loss of material thickness
• Galvanic corrosion happens when two dissimilar metals are in electrical contact in the presence of an electrolyte, with the less noble metal acting as the anode and corroding preferentially
• Pitting corrosion is a localized form of corrosion that results in the formation of small, deep pits on the metal surface
  • Pitting corrosion is often associated with the breakdown of protective passive films on metals like stainless steel and aluminum alloys
• Crevice corrosion occurs in confined spaces or crevices where the local environment becomes depleted of oxygen, leading to accelerated corrosion
• Stress corrosion cracking (SCC) is the formation and propagation of cracks in a metal due to the combined action of tensile stress and a corrosive environment
• Intergranular corrosion is the preferential corrosion along the grain boundaries of a metal, often due to the precipitation of impurities or the formation of galvanic couples between the grain boundary and the adjacent grains
• Erosion-corrosion is the accelerated corrosion caused by the relative motion between a corrosive fluid and the metal surface, leading to the removal of protective films and increased corrosion rates

Electrochemical Reactions in Corrosion

• Corrosion involves the oxidation of a metal (anodic reaction) coupled with the reduction of a species in the environment (cathodic reaction)
• The anodic reaction in corrosion is the dissolution of the metal, which releases electrons: $M \rightarrow M^{n+} + ne^-$
• Common cathodic reactions in corrosion include oxygen reduction ($O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$) and hydrogen evolution ($2H^+ + 2e^- \rightarrow H_2$)
• The rate of corrosion is determined by the kinetics of the anodic and cathodic reactions, which can be influenced by factors such as temperature, pH, and the presence of inhibitors or accelerators
• Passivation is the formation of a protective oxide layer on a metal surface that reduces the corrosion rate by acting as a barrier between the metal and the environment
  • Passivation is important for the corrosion resistance of metals like stainless steel, titanium, and aluminum
• The stability of the passive film depends on the composition of the metal and the environment, with factors like chloride ions and acidity promoting the breakdown of the film and the initiation of localized corrosion
• Polarization curves, which plot the current density versus the electrode potential, provide valuable information about the corrosion behavior of a metal, including the corrosion potential, passivation range, and pitting potential

Prevention and Protection Strategies

• Material selection involves choosing metals or alloys with inherent corrosion resistance for a given environment, such as stainless steels for marine applications or titanium for chemical processing
• Corrosion-resistant coatings, like paints, epoxies, and polymers, act as a barrier between the metal and the corrosive environment, preventing direct contact and reducing corrosion rates
• Cathodic protection is a technique that uses an external power source (impressed current) or a sacrificial anode to supply electrons to the metal structure, shifting its potential to a more negative value where corrosion is thermodynamically unfavorable
  • Sacrificial anodes, typically made of zinc or magnesium, preferentially corrode to protect the metal structure they are connected to
• Anodic protection involves the application of an external current to maintain the metal in a passive state, effectively reducing the corrosion rate
• Corrosion inhibitors are chemical substances that, when added to the environment, reduce the corrosion rate by adsorbing onto the metal surface and forming a protective film or by interfering with the anodic or cathodic reactions
  • Examples of corrosion inhibitors include chromates, phosphates, and organic compounds like amines and thiols
• Design considerations, such as avoiding crevices, minimizing stress concentrations, and ensuring proper drainage, can help prevent or mitigate specific forms of corrosion
• Regular inspection and maintenance, including cleaning, painting, and replacing sacrificial anodes, are essential for maintaining the effectiveness of corrosion protection systems over time

Measurement and Testing Methods

• Visual inspection is the most basic method for detecting corrosion, involving the examination of the metal surface for signs of rust, pitting, or other forms of degradation
• Weight loss measurements involve exposing a metal sample to a corrosive environment for a specified time and measuring the change in mass to determine the corrosion rate
• Electrochemical techniques, such as linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS), provide quantitative information about the corrosion rate and the properties of the metal-electrolyte interface
  • LPR measures the slope of the polarization curve near the corrosion potential to determine the polarization resistance, which is inversely proportional to the corrosion rate
  • EIS uses alternating current (AC) to probe the impedance of the metal-electrolyte interface over a range of frequencies, providing information about the corrosion mechanism and the properties of the passive film
• Potentiodynamic polarization (PDP) is a technique that involves sweeping the potential of a metal sample and measuring the resulting current to generate a polarization curve, which provides information about the corrosion potential, passivation behavior, and pitting susceptibility
• Accelerated corrosion testing methods, such as salt spray testing and cyclic corrosion testing, expose metal samples to aggressive environments to evaluate their corrosion resistance in a shorter time frame compared to real-world conditions
• Non-destructive testing (NDT) methods, like ultrasonic testing and radiography, can detect and quantify corrosion damage without destroying the metal structure
  • Ultrasonic testing uses high-frequency sound waves to measure the thickness of the metal and detect subsurface corrosion or defects
  • Radiography uses X-rays or gamma rays to create images of the internal structure of a metal, revealing the presence and extent of corrosion or other anomalies

Real-World Applications and Case Studies

• Corrosion in the oil and gas industry is a major challenge, with pipelines, storage tanks, and offshore structures exposed to harsh environments containing carbon dioxide, hydrogen sulfide, and chlorides
  • Case study: The Trans-Alaska Pipeline System (TAPS) relies on a combination of material selection (high-strength, low-alloy steels), coatings, and cathodic protection to mitigate corrosion in the extreme Arctic conditions
• Automotive corrosion is a concern for vehicle manufacturers and owners, with exposure to road salt, moisture, and temperature fluctuations leading to the degradation of body panels, frames, and underbody components
  • Case study: Modern automotive coatings, such as electrophoretic deposition (EPD) primers and clear coats, provide enhanced corrosion protection and improved aesthetics compared to traditional paint systems
• Corrosion in the aerospace industry can have severe consequences, with the structural integrity of aircraft components being critical for safety and performance
  • Case study: The development of advanced aluminum alloys, like Al-Li alloys, and the use of anodization and conversion coatings have significantly improved the corrosion resistance of aircraft structures while reducing weight
• Biomedical implants, such as orthopedic prostheses and dental implants, must withstand the corrosive environment of the human body while maintaining their functionality and biocompatibility
  • Case study: Titanium and its alloys, particularly Ti-6Al-4V, have become the materials of choice for many biomedical applications due to their excellent corrosion resistance, high strength-to-weight ratio, and osseointegration properties
• Marine corrosion affects structures exposed to seawater, including ships, offshore platforms, and coastal infrastructure
  • Case study: The use of impressed current cathodic protection (ICCP) systems and sacrificial anodes, combined with the application of antifouling coatings, helps protect marine structures from the aggressive saltwater environment and biofouling

Advanced Topics and Current Research

• Microbiologically influenced corrosion (MIC) is caused by the presence and activities of microorganisms, such as sulfate-reducing bacteria (SRB), which can accelerate corrosion through the formation of biofilms and the production of corrosive metabolites
  • Current research focuses on understanding the complex interactions between microbes, metals, and the environment, as well as developing new strategies for preventing and monitoring MIC
• Corrosion in extreme environments, such as high-temperature, high-pressure, or highly acidic conditions, poses unique challenges for material selection and protection
  • Research efforts aim to develop new alloys, coatings, and inhibitors that can withstand these aggressive conditions, such as high-entropy alloys (HEAs) and nanocomposite coatings
• Atmospheric corrosion is the degradation of materials exposed to the atmosphere, influenced by factors like humidity, temperature, and pollutants
  • Recent studies investigate the use of self-healing coatings, which can autonomously repair damage and restore protection, as well as the application of machine learning techniques to predict and monitor atmospheric corrosion
• Corrosion in renewable energy systems, such as wind turbines, solar panels, and fuel cells, is an emerging area of concern as these technologies become more widely adopted
  • Researchers are working on developing corrosion-resistant materials and coatings specifically tailored for these applications, considering factors like high-temperature operation, exposure to hydrogen, and the need for long-term durability
• Computational modeling and simulation techniques, such as density functional theory (DFT) and molecular dynamics (MD), are increasingly being used to study corrosion mechanisms at the atomic and molecular level
  • These approaches enable the prediction of corrosion behavior, the design of new corrosion-resistant materials, and the optimization of protection strategies, complementing experimental studies and accelerating the development of advanced corrosion solutions