Corrosion and Control Basics

Corrosion and Control

• Corrosion is the destructive and unintentional attack on a metal, electrochemical in nature
• It's a significant economic problem, costing industrialized nations approximately 5% of their income for prevention and replacement of products
• Corrosion is commonly seen in automotive components like body panels and radiators
• Electrochemical reactions are central to the corrosion process involving electron transfer between species
• Oxidation involves the loss of electrons by metal atoms, forming positively charged ions
• Reduction involves the gain of electrons by other species
• Anodes are locations of oxidation; cathodes are locations of reduction
• Hydrogen ions (H+) are reduced to hydrogen gas (H2) in acidic solutions.
• Electrochemical half-reactions determine the direction and magnitude of spontaneous reactions
• Electrochemical potential difference, or voltage, between metal electrodes generates electrical current used for corrosion calculations

Electrode Potentials

• Not all metal materials oxidize at the same rate to form ions; different metals have different degrees of ease
• The standard hydrogen electrode (SHE) is a reference point for measuring electrode potentials
• Potential difference, or voltage, between two half-cells (electrodes) results in a galvanic couple.
• Voltage values for different electrode pairs differ.
• Standard half-cell measurements are done at 25°C (77°F), with 1 M molarity of each respective ion

Influence of Concentration and Temperature on Cell Potential

• Standard emf series and half-cell potential values are for idealized cells (1 M solutions, 25°C).
• Changes in temperature or ion concentrations alter cell potentials and sometimes even change the spontaneous reaction direction.
• Nernst Equation is used to predict cell potentials with different concentrations and temperatures.
• Calculating corrosion rates requires using appropriate formulations.

Corrosion Rates

• Real corroding systems are not at equilibrium, current flow through external circuit exists.
• Corrosion potential parameters (Table 7.1) thus cannot be readily applied.
• Corrosion penetration rate (CPR) is a critical corrosion parameter, useful in determining material removal rates.
• Related to current density (current / area).
• Activation and Concentration Polarization influence corrosion rates. These mechanisms determine the rate of electrochemical reactions.

Polarization

• Activation polarization refers to rate-limited reactions due to steps needing activation energy.
• Concentration polarization occurs when reaction rates are affected by the rate of diffusion of reactants or products, often causing depletion zones around electrodes.

Passivity

• Some normally active metals, like Chromium, Nickel, and Titanium, lose reactivity under specific environmental conditions (e.g. high oxidizing conditions).
• This is often due to a protective oxide film forming on the surface.
• Loss of passivity can cause significant increases in corrosion; hence protecting the passive film is important.

Environmental Effects

• Various environmental variables (velocity, temperature, composition) impact corrosion rates.
• Higher fluid velocity often increases corrosion rates.
• Corrosion rates generally increase with temperature, but in some metals, raising corrosive species can initiate passive action.
• Cold working on ductile materials may increase their susceptibility to corrosion.

Forms of Corrosion

• Uniform Attack: Equal corrosion intensity across entire metal surfaces (rust).
• Galvanic Corrosion: Unequal electrochemical behavior leads to corrosion of dissimilar metals (contact).
• Crevice Corrosion: Stagnant conditions in crevices lead to concentration differences (localized depletion).
• Pitting Corrosion: Localized corrosion, often penetrating deeply.
• Intergranular Corrosion: Grain boundaries are preferential corrosion sites; frequently in stainless steel.
• Selective Leaching: Specific material constituents preferentially dissolve over others (e.g., brass).
• Erosion Corrosion: Mechanical wear combined with chemical attack impacts corrosion (piping, turbine blades)
• Stress Corrosion: Corrosion is exacerbated by tensile stresses.
• Hydrogen Embrittlement : Molecular hydrogen weakens material properties and leads to cracking

Cathodic Protection

• Applied electrons reduce the oxidized metal to protect it (makes it cathode).
• Often uses more reactive material as an anode.
• This process prevents corrosion.

Scale Types

• Pilling-Bedworth ratio: Helps predict the protective or non-protective nature of metal oxide scale.
• Important for understanding the impact of metal oxide scales on corrosion and overall material behavior.

Ceramic Materials

• Ceramic materials are generally resistant to corrosion, especially at room temperature.
• Corrosion of ceramics may involve chemical dissolution, rather than electrochemical reactions dominant in metals.
• Useful material due to high temperature resistance and resistance to several environments.

Weathering

• Weathering is a specific form of degradation related to environmental conditions like sunlight, water, and atmospheric pollutants.
• Corrosion resistance varies depending on the material and specific environments.

This quiz explores the fundamental concepts of corrosion and its electrochemical processes. Learn about the economic impact of corrosion and the key reactions involved, including oxidation and reduction. Test your knowledge of corrosion in automotive components and the principles behind electrochemical potential.