Corrosion: Types, Factors, and Control

Unit IV: Corrosion and its Control

• This section covers the electrochemical theory of corrosion, types of corrosion (dissimilar metal cells, differential aeration cells, uniform, pitting, stress corrosion cracking), factors affecting corrosion rate (pH, temperature, dissolved oxygen), and prevention methods (cathodic protection, protective coatings, paints).

Definition of Corrosion

• Corrosion is the destruction of metals or alloys via chemical or electrochemical reactions with their surrounding environment.
• Rust on iron is an example of corrosion.
• Corrosion is the reverse process of extractive metallurgy.
• Irreversible corrosion is a natural process where unstable pure metals (excluding noble metals) convert to stable compounds like oxides, sulfides, hydroxides, and carbonates.
• Slow deterioration of metal surfaces due to atmospheric gases, leading to stable compounds.

Types of Corrosion

• Dry corrosion results from a direct chemical reaction between the metal and corrosive gases, such as oxygen, sulfur dioxide, carbon dioxide, and hydrogen sulfide.
• Wet corrosion is common in aqueous corrosive environments, happening when metal connects with conductive liquids, or when dissimilar metals are immersed in a solution.

Dry Corrosion (Chemical/Direct Corrosion)

• Dry corrosion involves a direct chemical action of the environment.
• It can also result from atmospheric gases like O₂ or halogens reacting with metal surfaces.
• Direct attack of oxygen is possible at both low and high temperatures.

Oxidation corrosion mechanism

• Oxidation of the metal happens until a metal oxide layer develops, and it can stop further corrosion.
• Metal ions and oxygen move outwards and inwards respectively, leading to continuous corrosion.

Metal Oxide Layer Characteristics

• Stable metal oxide layers are fine-grained and tightly adhere to the parent metal, making them impervious to further corrosion (e.g., Al, Pb, Cu).
• Unstable layers decompose back into metal and oxygen, ceasing additional corrosion (e.g., Ag, Au, Pt).
• Volatile layers volatilize, exposing the metal and enabling ongoing corrosion (e.g., MoO₃).
• Porous layers allow atmospheric oxygen to reach and corrode the metal surface (e.g., ZnO, PbO₂).

Corrosion by Other Gases

• Gases like Sulfur Dioxide, Carbon Dioxide, and Chlorine can corrode metal surfaces, depending on the nature of the metal oxide layer.
• Non-porous layers reduce corrosion rate by protecting from the metal's corrosion, for example, an AgCl layer formed on Ag metal from Chlorine.

Liquid Metal Corrosion

• In liquid metal dissolution or penetration of a solid metal happens by a flowing liquid metal at a high temperature.
• This corrosion is observed in nuclear power stations.

Electrochemical Series

• The Electrochemical Series is an arrangement of elements based on their standard reduction potentials.
• It measures the tendency of elements to gain or lose electrons in aqueous solutions.

Wet (Electrochemical) Corrosion

• Wet corrosion only occurs when a conducting liquid is in contact with a metal.
• It happens via electron transfer, oxidation, and reduction.
• During oxidation, metal atoms lose electrons; the surrounding environment gains these electrons via reduction.
• Anode is where metal atoms lose electrons: anode.
• Cathode is where the other metal, liquid, or gas receives electrons.
• Corrosion is found in the anodic areas and is not found in the cathodic areas.

Hydrogen Evolution Corrosion (Acidic)

• A type of corrosion occurs in acidic environments within large anodic and small cathodic areas.
• At the anode, oxidation occurs: Fe → Fe²⁺ + 2e⁻.
• At the cathode, reduction occurs, depending on the environment. Hydrogen gas commonly evolves.
• Metals above H₂ in the electrochemical series corrode this way.
• An example is Iron metal in contact with Hydrochloric Acid: Fe + 2H⁺ → Fe²⁺ + H₂.

Oxygen Absorption Corrosion (Neutral/Basic)

• Occurs where small anodic areas and large cathodic surfaces are in a neutral (NaCl solution) or basic.
• At the anode: Fe → Fe²⁺ + 2e⁻.
• At the cathode: ½ O₂ + 2e⁻ + H₂O → 2OH⁻.
• The net reaction: Fe²⁺ + 2OH⁻ → Fe(OH)₂.
• If enough oxygen is available, it leads to the formation of yellow rust (Fe₂O₃.H₂O), but if oxygen is limited, it causes black anhydrous magnetite.

Differences Between Dry and Wet Corrosion

• Dry corrosion occurs without moisture while wet corrosion requires moisture or an electrolyte.
• Dry corrosion results from direct chemical attacks while wet corrosion follows from a large anodic and cathodic number formation.
• Homogeneous metal surfaces are corroded in dry corrosion, while heterogenous surfaces can corrode while wet. Corrosion product accumulates on corrosion location in dry corrosion, but moves to the anode in wet corrosion.
• Dry corrosion is a self-controlled process while wet corrosion is continuous.

Galvanic Corrosion

• Galvanic Corrosion occurs when two dissimilar metals (like Zinc and Copper) are electrically connected and exposed to an electrolyte.
• The metal higher in the electrochemical series corrodes more.
• The more active metal (anode) has a more negative electrode potential, while the less active (cathode) has a less negative potential.
• In an acidic solution, corrosion results from hydrogen leaving; oxygen is absorbed in neutral or slightly alkaline solutions.
• Oxidation of Zinc happens when passing electrons to the cathode, Copper, in Zinc to Copper cells.

Differential Aeration Corrosion

• Differential Aeration Corrosion happens when parts of a metal are in contact with varying oxygen concentrations.
• Potential difference exists between areas resulting in areas with small oxygen concentration become anodically.
• The differential aeration of metal causes current flow, also called the differential current.
• A partially immersed metal in a neutral solution shows greater oxygen concentration and becomes more cathodic.
• The metal in the anodic regions dissolves. Oxygen then takes up electrons to form hydroxyl ions in cathodic areas.

Uniform Corrosion

• Uniform corrosion is a general attack across the entire metal surface, converting it to its oxide form.
• Results in to the uniform thinning of metal.
• This common corrosion is responsible for most material loss, though is less dangerous than other corrosion.

Pitting Corrosion

• Pitting corrosion results in localized accelerated, leading to the creation of cavities surrounding the relatively unattached metal.
• Extraneous impurities or the uneven amount of oxygen comes in contact with the metal, starting small anodic areas and surrounds, causing pitting.
• Increase of corrosion is likely form if a small pit grows.

Stress Corrosion

• Stress corrosion happens when the combined effect of static tensile stresses and corrosive environment exist on a metal.
• A highly localized attack is characterized.
• The corrosion type is common to the fabricated articles that result from heavy work like rolling and drawing.
• The resulting crack grows until failure.

Crevice Corrosion (Concentration Corrosion)

• Happens in joints, crevices, or narrow gaps between two surfaces.
• Corrosion-causing agents such as oxygen are then trapped in crevices.
• Seawater diffusion and chemical aggression due to undiffused corrosive ions are likely to increase corrosion.

Inter-granular Corrosion

• Inter-granular Corrosion attacks along grain boundaries, rather than affecting the grains themselves.
• Noticeable difference in grain boundary reactivity with impurities is the cause, common in defective welding, or heat treatment of stainless steels and copper.

Corrosion Fatigue

• The failure of materials under repeated stress in a corrosive environment, where the development of metal fatigue is worsened due to the corrosion.

Galvanic or Electro-potential Series

• Determines the nobility of metals. When submerged in an electrolyte and connected by a conductor, the less noble metal corrodes galvanically. The electrolyte, nobility difference, and exposed areas determine the rate.

Factors Affecting Corrosion

• Metallic
• Nature of the specific metal being considered.
• Position of the metals in galvanic series/ higher position leads to undergoing corrosion.
• Pure metals resist corrosion better; impurities encourage the formulation of local galvanic cells.
• Less corrosion is more likely in alloys.
• Smaller anode and increase cathode leads to rapid corrosion.
• Structural factors influence the corrosion rate. Stressed metals corrode more quickly.
• Environmental
• Nature of surrounding.
• Oxide film porosity can accelerate corrosion if it allows Oxygen to permeate.
• Volatile corrosion of high oxygen can accelerate in solubility.
• Higher the humidity more electrolytes, which cause galvanic corrosion and vice versa.
• Acidic pH or pH > 7 can form some metals.

Corrosion Controlling Methods

• Methods include addressing the great loss of material and money of lost materials.

Cathodic Protection

• A protective metal or alloy converts to cathodicity to prevent contact with the anode.
• Eliminating anodic sites prevents corrosion.

Sacrificial Anodic Protection

• Metal connects to more active metal to protect, the block corrodes, protecting pipe.
• Used for the protection of ships, boats, and underground cables.
• Easy simple method that doesn't require energy.
• However, recurring replacement costs required.

Impressed Current Cathodic Protection

• Changes the corroding metal from anodic to cathodic by changing corroding current.
• Negative terminal in the battery connects metal, the positive connects the inert anode, to prevent corrosion.
• One large cost and area installation.
• However, if corrosion isn't consistent, corrosion can happen at the localized area.

Metallic Coatings

• A coated surface separates the underlying metal from corrosion.
• This coating must be an inert and penetrative metal.

Coatings Types

• Anodic coatings on base metal must be the anodically most active.
• Cathodic coatings are made of noble, corrosion resistive metal.

Galvanizing

• Iron sheet process coating sheet with zinc.
• This protects from corrosion as molten Zink dries across sheet.

Tinning

• Involves coating steel articles with corrosion resistant tin and is known as is tinning.
• Involves an iron sheet with the removal of oxide film using acid, before coating with Tin at a temperature of 430°C..

Organic Coating

• Insulative compounds, varnishes, lacquers, enamels applied to metallic surfaces
• Serves as decoration/corrosion protection

Paints

• A mechanical dispersion mixture of pigments and a film forming drying oil and other materials to assist.
• Should spread well across the protected surface.
• It protects surface from corrosion and color and should also be durable.

Paint Constituents

• Pigment
  • Essential for paint in protection of light from UV and decoration.
• Functions must be desirable color, opacity, strength, and resistance from water and elements.
• Vehicle/drying oil
  • Must coat film and has to be pigment-dispersed.
• Has to have water resilience, toughness and durability to film.

Contituents Continued

• Turpentine is essential paint to prevent corrosion. Some have oil, hydrocarbons, and alothere.
• Film formation of a paint has to be viscous and elastic to perform those benefits.
• The elements have to suspend in pigments for their overall benefit.
• Drying oil accelerates catalyst performance
• Linoleates help coat and prevent corrosion.

Fillers

• Insert paint materials improve or reduce cost. For opacity, and elastic make-up and nature.
• Fill the voids, and make things random, to improve the durability and performance of paint.

Anti Skinning

• Skin agents prevent the film gelling in the paint.
• Plasticizers allow the film to stretch safely when coating paint by providing durability.

Varnishes

• A film like wood is coated with a varnish like resin or thinner that helps by drying quickly.
• Produce and maintain a resilient dry coating film.

Oil Varnish

• A varnishing mix of solvents that dries slowly to be durable in drying and maintenance.
• Copal is used in varnishing exterior and interiors for this reason.

Spirit Varnish

• The solvent dries varnish like a resin to strengthen wood as it dries.

Varnish Elements

• Resins high anti-weather chemicals for better protection.
• Drying oil must be dry by oxidation to preserve product.
• Solvents help by keeping elasticity and maintain a film, but most add resin to solvents.
• Alcohols enhance dryness to protect from environmental attacks.

Viscosity

• How a liquid surface acts and the force with to what degree it's needed.