Corrosion

Corrosion involves the deterioration of a material as it reacts with its environment. Corrosion is the primary means by which metals deteriorate. Corrosion literally consumes the material reducing load carrying capability and causing stress concentrations. Corrosion is often a major part of maintenance cost and corrosion prevention is vital in many designs. Corrosion is not expressed in terms of a design property value like other properties but rather in more qualitative terms such as a material is immune, resistant, susceptible or very susceptible to corrosion.

The corrosion process is usually electrochemical in nature, having the essential features of a battery. Corrosion is a natural process that commonly occurs because unstable materials, such as refined metals want to return to a more stable compound. For example, some metals, such as gold and silver, can be found in the earth in their natural, metallic state and they have little tendency to corrode. Iron is a moderately active metal and corrodes readily in the presence of water. The natural state of iron is iron oxide and the most common iron ore is Hematite with a chemical composition of Fe₂O₃.  Rust, the most common corrosion product of iron, also has a chemical composition of Fe₂O₃.

The difficulty in terms of energy required to extract metals from their ores is directly related to the ensuing tendency to corrode and release this energy.  The electromotive force series (See table) is a ranking of metals with respect to their inherent reactivity. The most noble metal is at the top and has the highest positive electrochemical potential. The most active metal is at the bottom and has the most negative electrochemical potential.

Corrosion involve two chemical processes: oxidation and reduction. Oxidation is the process of stripping electrons from an atom and reduction occurs when an electron is added to an atom. The oxidation process takes place at an area known as the anode. At the anode, positively charged atoms leave the solid surface and enter into an electrolyte as ions. The ions leave their corresponding negative charge in the form of electrons in the metal which travel to the location of the cathode through a conductive path. At the cathode, the corresponding reduction reaction takes place and consumes the free electrons. The electrical balance of the circuit is restored at the cathode when the electrons react with neutralizing positive ions, such as hydrogen ions, in the electrolyte. From this description, it can be seen that there are four essential components that are needed for a corrosion reaction to proceed. These components are an anode, a cathode, an electrolyte with oxidizing species, and some direct electrical connection between the anode and cathode.   Although atmospheric air is the most common environmental electrolyte, natural waters, such as seawater rain, as well as man-made solutions, are the environments most frequently associated with corrosion problems.

A typical situation might involve a piece of metal that has anodic and cathodic regions on the same surface.  If the surface becomes wet, corrosion may take place through ionic exchange in the surface water layer between the anode and cathode.  Electron exchange will take place through the bulk metal.  Corrosion will proceed at the anodic site according to a reaction such as

M → M⁺⁺ + 2e^-

where M is a metal atom.  The resulting metal cations (M⁺⁺) are available at the metal surface to become corrosion products such as oxides, hydroxides, etc.  The liberated electrons travel through the bulk metal (or another low resistance electrical connection) to the cathode, where they are consumed by cathodic reactions such as

2H⁺ + 2e^- → H₂

The basic principles of corrosion that were just covered, generally apply to all corrosion situation except certain types of high temperature corrosion. However, the process of corrosion can be very straightforward but is often very complex due to variety of variable that can contribute to the process. A few of these variable are the composition of the material acting in the corrosion cell, the heat treatment and stress state of the materials, the composition of the electrolyte, the distance between the anode and the cathode, temperature, protective oxides and coating, etc.

Types of Corrosion

Corrosion is commonly classified based on the appearance of the corroded material. The classifications used vary slightly from reference to reference but there is generally considered to be eight different forms of corrosion. There forms are:

Uniform or general – corrosion that is distributed more or less uniformly over a surface.

Localized – corrosion that is confined to small area. Localized corrosion often occurs due to a concentrated cell. A concentrated cell is an electrolytic cell in which the electromotive force is caused by a concentration of some components in the electrolyte. This difference leads to the formation of distinct anode and cathode regions.

• Pitting – corrosion that is confined to small areas and take the form of cavities on a surface.
• Crevice – corrosion occurring at locations where easy access to the bulk environment is prevented, such as the mating surfaces of two components.
• Filiform – Corrosion that occurs under some coatings in the form of randomly distributed threadlike filaments.

Intergranular – preferential corrosion at or along the grain boundaries of a metal.

• Exfoliation – a specific form of corrosion that travels along grain boundaries parallel to the surface of the part causing lifting and flaking at the surface. The corrosion products expand between the uncorroded layers of metal to produce a look that resembles pages of a book. Exfoliation corrosion is associated with sheet, plate and extruded products and usually initiates at unpainted or unsealed edges or holes of susceptible metals.

Galvanic – corrosion associated primarily with the electrical coupling of materials with significantly different electrochemical potentials.

Environmental Cracking – brittle fracture of a normally ductile material that occurs partially due to the corrosive effect of an environment.

• Corrosion fatigue – fatigue cracking that is characterized by uncharacteristically short initiation time and/or growth rate due to the damage of corrosion or build-up of corrosion products.
• High temperature hydrogen attack – the loss of strength and ductility of steel due to a high temperature reaction of absorbed hydrogen with carbides. The result of the reaction is decarburization and internal fissuring.
• Hydrogen Embrittlement – the loss of ductility of a metal resulting from absorption of hydrogen.
• Liquid metal cracking – cracking caused by contact with a liquid metal.
• Stress corrosion – cracking of a metal due to the combined action of corrosion and a residual or applied tensile stress.

Erosion corrosion – a corrosion reaction accelerated by the relative movement of a corrosive fluid and a metal surface.

Fretting corrosion – damage at the interface of two contacting surfaces under load but capable of some relative motion. The damage is accelerated by movement at the interface that mechanically abraded the surface and exposes fresh material to corrosive attack.

Dealloying – the selective corrosion of one or more components of a solid solution alloy.

• Dezincification – corrosion resulting in the selective removal of zinc from copper-zinc alloys.