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corrosion,atmospheric oxidation of metals (see oxidation and reductionoxidation and reduction,
complementary chemical reactions characterized by the loss or gain, respectively, of one or more electrons by an atom or molecule. Originally the term oxidation
..... Click the link for more information. ). By far the most important form of corrosion is the rusting of ironiron,
metallic chemical element; symbol Fe [Lat. ferrum]; at. no. 26; at. wt. 55.845; m.p. about 1,535°C;; b.p. about 2,750°C;; sp. gr. 7.87 at 20°C;; valence +2, +3, +4, or +6. Iron is biologically significant.
..... Click the link for more information. . Rusting is essentially a process of oxidation in which iron combines with water and oxygen to form rust, the reddish-brown crust that forms on the surface of the iron. Rust, a chemical compound, is a hydrated ferric oxide Fe2O3·nH2O, where n is usually 1 1-2. The chemical mechanism of rusting is not fully known, but is thought to involve oxidation of metallic iron to ferrous ion (Fe++) and reaction of the ferrous ion with oxygen and water to form rust. The reaction is catalyzed by water, acids, and metals (e.g., copper and tin) below iron in the electromotive serieselectromotive series,
list of metals whose order indicates the relative tendency to be oxidized, or to give up electrons (see oxidation and reduction); the list also includes the gas hydrogen. The electromotive series begins with the metal most easily oxidized, i.e.
..... Click the link for more information. . Because iron is so widely used, e.g., in building construction and in tools, its protection against rusting is important. Although metals (e.g., aluminum, chromium, and zinc) above iron in the electromotive series corrode more readily than iron, their oxides form a tenuous coating that protects the metal from further attack. Rust is brittle and flakes off the surface of the iron, continually exposing a fresh surface. Rusting can be prevented by excluding air and water from the iron surface, e.g., by painting, oiling, or greasing, or by plating the iron with a protective coating of another metal. Metals used for plating include chromium, nickel, tin, and zinc. Zinc plating is called galvanizing. Many alloys of iron are resistant to corrosion. Stainless steels are alloys of iron with such metals as chromium and nickel; they do not corrode because the added metals help form a hard, adherent oxide coating that resists further attack. The iron hulls of ships can be protected against rusting by attaching magnesium strips to the underside of the vessel. An electric current is generated, with the magnesium and iron acting as electrodes and seawater acting as the electrolyte. Because magnesium is above iron in the electromotive series, it serves as a "sacrificial anode" and is oxidized in preference to the iron. This is called cathodic protection, since the iron serves as the cathode and thus escapes oxidation. This method is also used to protect the pipes of electric generating plants where saltwater is used as a coolant.
See J. Waldman, Rust (2015).
(1) The destruction (dissolution) of rock by the chemical action of water, with the formation of crevices, channels, sinkholes, basins, caverns, caves, and other hollows and depressions. It is particularly clearly expressed in areas of readily soluble rocks, such as rock salt, gypsum, and limestones.
(2) The eating away and partial dissolution by magmatic melts or lava of phenocrysts that separated out in the first stage of their crystallization and of rock detritus (xenoliths) engulfed by magma during intrusion.
the destruction of metals as a result of their chemical or electrochemical interaction with a corrosive medium. Annual losses from corrosion are 1.0–1.5 percent of all the metal accumulated and used by man. In monetary terms, the direct losses from corrosion (for replacement of inoperative machinery) were about $5.5 billion in the USA in 1955 and about 250 billion francs in France in 1959. Annual losses in the USSR in the late 1960’s were not less than 5–6 billion rubles. It is difficult to calculate the greater indirect losses from idle time and reduction in the efficiency of equipment affected by corrosion, from the disruption of technological processes, and from acci-dents resulting from reduction in the strength of metal structures. In industry and agriculture, all possible measures are being taken to combat corrosion.
The cause of corrosion is the thermodynamic instability of a system consisting of a metal and the components of the sur-rounding corrosive medium. A measure of the thermodynamic instability is the free energy liberated by the reaction of a metal with the components. However, the free energy alone does not determine the rate of the corrosion process, which is the most important value for measuring the corrosion resistance of a metal. In a number of cases adsorbed or phase layers (films) that form on the surface of a metal as the result of an initial corrosion process become a barrier of sufficient density and impenetrability to stop or strongly impede corrosion. Therefore, under conditions of use, metals with greater affinity for oxygen may prove to be more, not less, resistant (thus, the free energy for oxide formation for chromium or aluminum is greater than for iron, but these metals are often superior to iron in corrosion resistance).
Corrosion processes are classified according to the type (geometric nature) of the corrosive damage on the surface or in the volume of the metal, the mechanism of the reactions of the metal with the medium (chemical and electrochemical corrosion), the type of corrosive medium, and the nature of additional effects acting on the metal simultaneously with the action of the corrosive medium.
Types of corrosion damage. Corrosion that has spread over the entire surface of a metal is called continuous corrosion. It may be even or uneven, depending on the uniformity of the depth of the corrosion damage in various areas. In local corrosion, the damage is localized and a significant part of the surface (often most of it) remains virtually unaffected. A distinction is made among corrosion spots, lesions, and pits, depending on the degree of localization. Pits may give rise to subsurface corrosion, which propagates in all directions under a very thin (for example, cold-worked) layer of the metal, which then blows up into bubbles or peels. The most dangerous forms of local corrosion are intergranular corrosion, which penetrates deeply along the less resistant grain boundaries without destroying the metal grains, and transcrystalline corrosion, which forms fissures directly through the grains. These types of damage may lead to complete loss of strength and the destruction of parts or structures without visible marks on the surface. Knife corrosion, which cuts through the metal like a knife along a weld seam during the use of certain alloys in particularly corrosive solutions, is similar to intercrystalline and transcrystalline corrosion. Sometimes a distinction is made between surface filament corrosion, which develops, for example, under nonmetallic coatings, and laminar corrosion, which proceeds primarily in the direction of plastic deformation. Selective corrosion, in which even individual components of solid solutions may selectively dissolve in an alloy (for example, the dezincing of brass), is specific.
Chemical and electrochemical corrosion. Corrosion is chemical if, after destruction of the metal bonds, the metal atoms form a direct chemical bond with the atoms or groups of atoms making up the oxidizing agents that remove the valence electrons of the metal. Chemical corrosion is possible in any corrosive medium, but it is most often observed in cases in which the corrosive medium is not an electrolyte (gas corrosion and corrosion in nonconducting organic liquids). The rate of chemical corrosion is most often determined by the diffusion of particles of the metal and the oxidizing agent through the surface film of corrosion products (high-temperature oxidation of most metals by gases) and sometimes by the dissolution or evaporation of the film (high-temperature oxidation of tungsten or molybdenum), by disintegration of the film (oxidation of niobium at high temperatures), and by convective transfer of the oxidizing agent from the external medium (at very low concentrations of the oxidizing agent).
Corrosion is electrochemical if the cation, upon leaving the metal lattice, bonds not to the oxidizing agent but rather to other components of the corrosive medium; the electrons liberated in forming the cation are transferred to the oxidizing agent. Such a process is possible in cases in which there are two types of reagents in the surrounding medium, one of which (a solvating or complexing reagent) is able to form stable bonds with the metal cation without participation of the metal valence electrons, and the others (oxidizing agents) are able to add valence electrons of the metal without binding the cations. These properties are found in electrolyte solutions or melts in which the solvated cations maintain considerable mobility. Thus, in electrochemical corrosion the removal of an atom from the metal lattice, which is the essence of any corrosive process, takes place as a result of two independent but coupled electrochemical processes that are connected by an electron balance: the anodic process, which is a transfer of the solvated metal cations to the solution, and the cathodic process, which is the binding of freed electrons by the oxidizing agent. Hence, the process of electrochemical corrosion may be slowed not only by direct hindrance of the anodic process but also by influences on the rate of the cathodic process. The two most prevalent cathodic processes are the discharge of hydrogen ions (2e + 2H+ = H2) and the reduction of dissolved oxygen (4e + O2 + 4H+ = 2H2O or 4e + O2 + 2H2O = 4OH−), which are often called hydrogen and oxygen depolarization, respectively.
The anodic and cathodic processes take place with a certain probability and in a certain sequence at any points on the metal surface where the cations and electrons may react with components of the corrosive medium. If the surface is uniform, the cathodic and anodic processes have equal probability over the entire surface; in such an ideal case, the corrosion is called homogeneous electrochemical corrosion (thus taking into ac-count the absence of any nonuniformities in the probability distribution of the electrochemical processes at any point on the surface but not, of course, excluding the possible thermodynamic heterogeneity of the reacting phases). In reality, there are areas on the surface of the metal with differing conditions of supply of the reacting components, with differing atomic energy states, or with differing impurities. More intense anodic and cathodic processes are both possible in such areas, and the corrosion becomes heterogeneous electrochemical corrosion.
The conductivity of metal is very high, and in case of the appearance of an excess charge, the electrons redistribute almost instantaneously, so that the charge density and electric potential of the metal vary simultaneously over the entire surface, in-dependent of the points at which the electrons were released after the departure of cations and at which the electrons are captured by the oxidizing agent. In particular, this means that the electrons move from areas in which the anodic reaction predominates to areas in which the cathodic reaction predominates. Thus, the solution close to the anodic areas acquires the excess positive charge of the dissolved cations, and the solution close to the cathodic areas is negatively charged as a result of electron capture by the dissolved oxidizing agent. These charges are not as easily redistributed in the solution as in the metal. Therefore, with an increase in the rate of the corrosion process, the solution potential becomes more positive in the immediate vicinity of the anodic areas, which impedes the further exit of positively charged cations from the metal, and more negative near the cathodic area, which impedes the cathodic process. In other words, this behavior may be represented as a current-induced ohmic potential drop between the anolyte and catholyte layers of the solution, making the metal potential relative to the anolyte layer somewhat more negative and relative to the catholyte layer more positive than to the entire solution. In cases in which such ohmic potential drops are great (with very great current density, low solution conductivity, or a great separation of the cathodic and anodic areas), the corrosion system is more conveniently seen as a system of short-circuited microgalvanic and macrogalvanic cells. In other cases, when determining the average rate of dissolution of metal over the surface, modern theory also permits representation of an electrochemically heterogeneous surface as a quasi-homogeneous surface. In this case the surface is assigned specific anodic and cathodic characteristics equal to the values of the analogous characteristics of the model heterogeneous surface
averaged integrally over the surface and graphically represented on a corrosion diagram as anode and cathode polarization curves. The curves indicate the way in which the electrode potential affects the rates of cation and electron departure from a given surface into a given electrolyte, averaged over the surface and expressed in units or logarithms of current density. The diagram may be very complicated, since in real systems many factors—including the diffusion of the oxidizing agent or the cations passing into the solution, passivation of the metal, and various disruptions of the passive state—may affect the shape of the curves. A schematic corrosion diagram for the simplest hypothetical case, in which none of the factors mentioned above has any effect, is shown in Figure 1.
The anodic and cathodic processes, as mentioned above, are related by an electron balance. The electrons left by the departing cations impart a negative charge to the metal, which impedes the outflow of cations into the solution but simultaneously accelerates the cathodic process. The latter, in turn, leading to a reduction in the negative charge of the metal, retards itself but facilitates the anodic reaction. Thus, self-regulation of the charge on the surface of the metal takes place; this is one of the most important elements of the mechanism for establishing a stationary corrosion potential (ϕst), at which the polarization curves of the cathode (C ) and anode (A) intersect (point S ).
Although the rate of electrochemical corrosion also depends on the potential, this relationship is far from unequivocal, as may be seen from the following example: if additional active cathodes appear on the metal surface, with unchanged anode characteristics (curve A), then the facilitation of the cathodic process (now described by curve C’) caused by the additional cathodes may lead to acceleration of the dissolution of the metal until the current density i*st is reached, with a potential shift toward the positive (up to ϕ*st). On the other hand, if the cathodic characteristics (curve C) remain unchanged and additional anodic segments appear (corresponding to the process described by curve A′), corrosion accelerates (up to i**st), with a shift in potential toward the negative (to ϕ**st) However, given proportional facilitation of both processes (curves A′ and C′), significant acceleration of corrosion (up to i***st) is possible without a change in potential. More complicated cases are observed when both passivation and disruptions of the passive state play a role.
Corrosion in various mediums; effect of additional factors. Some corrosive mediums and the damage caused by them are so characteristic that the corrosive processes that take place in them are classified according to the name of the medium—for example, gas corrosion, which is chemical corrosion under the influence of hot gases at temperatures much higher than the dew point. Some cases of electrochemical corrosion (mainly with cathodic reduction of oxygen) in natural mediums are characteristic—for example, atmospheric corrosion, which takes place in pure or polluted air in the presence of moisture sufficient for the formation of an electrolyte layer on the surface of the metal (particularly in the presence of corrosive gases such as sulfur dioxide, chlorine, or acid and salt aerosols); sea corrosion, which takes place upon the action of seawater; and underground corrosion, which occurs in soils.
Corrosion under stress develops in regions of tensile or bending mechanical stresses, as well as in regions of other deformations or thermal stress, and usually leads to transcrystalline corrosive splitting (for example, in steel cables and springs under atmospheric conditions, carbon and stainless steel in steam-power installations, and high-strength titanium alloys in sea-water). Corrosion fatigue, which is manifested in a fairly sharp drop in the endurance limit of a metal in a corrosive environment, may occur under an alternating stress. Corrosive erosion (or friction corrosion) is the accelerated wear of metal by simultaneous and mutually reinforcing corrosive and abrasive factors, such as sliding friction or a stream of abrasive particles. A similar type is cavitation corrosion, which arises during cavitational flow of a corrosive medium around the metal, when the continuous formation and bursting of tiny vacuum bubbles creates a flow of destructive microhydraulic shocks acting on the metal sur-face. Fretting corrosion may be considered a closely related type; it is observed at the points of contact of tightly compressed or rolling parts when microscopic shear displacements arise be-cause of vibrations between their surfaces.
Leakage of electric current across the boundary of a metal and a corrosive medium leads to additional anodic and cathodic reactions, depending on the nature and direction of the leakage; the reactions may lead directly or indirectly to accelerated local or general damage to the metal (stray current corrosion). Similar damage localized around a contact may result from the contact of two different metals in an electrolyte that form a short-circuited galvanic cell (contact corrosion). Crevice corrosion may arise in narrow gaps between parts or under coatings or growths that have pulled apart, where an electrolyte can penetrate but access for the oxygen necessary for passivation of the metal is more difficult. In this type of corrosion, dissolution of the metal occurs mainly in the crevice, whereas the cathodic reactions take place partially or completely on the open surface near the crevice.
A distinction is also made between biological corrosion, which takes place as a result of the effect of the metabolic processes of bacteria and other organisms, and radiation corrosion, which takes place upon the action of radioactive emission.
Quantitative evaluation. The total rate of corrosion is measured by the loss of metal K per unit area (for example, in g/m2 . hr) or according to the rate of corrosion penetration—that is, according to the unidirectional decrease II in the thickness of unaffected metal (for example, in mm/yr). For uniform corrosion, Π = 8.75K/ρ, where p is the density of the metal in g/cm3. For nonuniform and local corrosion, the maximum penetration is measured. According to GOST (Ail-Union State Standard) 13819–68, a ten-point scale of total corrosion resistance has been established (see Table 1). In special cases, corrosion may be evaluated by other indexes, such as the loss of mechanical strength and plasticity, increase in electric resistance, or decrease in reflectivity, which are selected in accordance with the type of corrosion and the function of the part or structure.
|Table 1. Ten-point scale for measuring corrosion resistance of metals|
|Rate of corrosion (mm/yr)||Points|
|Completely resistant...............||< 0.001||1|
|Highly resistant.............||> 0.001–0.005||2|
|Less resistant..............||> 0.1–0.5||6|
In the selection of materials resistant to the action of various corrosive mediums under specific conditions, reference tables for the corrosion and chemical resistance of materials are used or laboratory tests and field experiments under the conditions of future use are carried out, both for samples and for pilot plants and devices. Tests under conditions more severe than those en-countered in actual use are called accelerated tests.
REFERENCESAkimov, G. B. Osnovy ucheniia o korrozii i zashchite metallov. Moscow, 1946.
Tomashov, N. D. Teoriia korrozii i zashchita metallov. Moscow, 1959.
Evans, E. R. Korroziia i okislenie metallov. Moscow, 1962. (Translated from English.)
Rozenfel’d, I. L. Atmosfernaia korroziia metallov. Moscow, 1960.
Bialobzheskii, A. V. Radiatsionnaia korroziia. Moscow, 1967.
A. V. BIALOBZHESKII and V. M. NOVAKOVSKII
In broad terms, the interaction between a material and its environment that results in a degradation of the physical, mechanical, or even esthetic properties of that material. More specifically, corrosion is usually associated with a change in the oxidation state of a metal, oxide, or semiconductor.
Electrolytic corrosion consists of two partial processes: an anodic (oxidation) and cathodic (reduction) reaction (see illustration). In the absence of any external voltages, the rates of the anodic and cathodic reactions are equal, and there is no external flow of current. The loss of metal that is the usual manifestation of the corrosion process is a result of the anodic reaction, and can be represented by reaction (1).
This reaction represents the oxidation of a metal (M) from the elemental (zero valence) state to an oxidation state of Mn+ with the generation of n moles of electrons (e-). The anodic reaction may occur uniformly over a metal surface or may be localized to a specific area. If the dissolved metal ion can react with the solution to form an insoluble compound, then a buildup of corrosion products may accumulate at the anodic site.
In the absence of any applied voltage, the electrons generated by the anodic reaction (1) are consumed by the cathodic reaction. For most practical situations, the cathodic reaction is either the hydrogen-evolution reaction or the oxygen-reduction reaction. The hydrogen-evolution reaction can be summarized as reaction (2).
In this case, protons (H+) combine with electrons to form molecules of hydrogen (H2). This reaction is often the dominant cathodic reaction in systems at low pH. The hydrogen-evolution reaction can itself cause corrosion-related problems, since atomic hydrogen (H) may enter the metal, causing embrittlement, a phenomenon that results in an attenuation of the mechanical properties and can cause catastrophic failure. See Embrittlement
The second important cathodic reaction is the oxygen-reduction, given by reactions (3) and (4).
Reactions (2)– (4) represent the overall reactions which, in practice, may occur by a sequence of reaction steps. In addition, the reaction sequence may be dependent upon the metal surface, resulting in significantly different rates of the overall reaction. The cathodic reactions are important to corrosion processes since many methods of corrosion control depend on altering the cathodic process. Although the cathodic reactions may be related to corrosion processes which are usually unwanted, they are essential for many applications such as energy storage and generation.
Corrosion rates are usually expressed in terms of loss of thickness per unit time. General corrosion rates may vary from on the order of centimeters per year to micrometers per year. Relatively large corrosion rates may be tolerated for some large structures, whereas for other structures small amounts of corrosion may result in catastrophic failure. For example, with the advent of technology for making extremely small devices, future generations of integrated circuits will contain components that are on the order of nanometers (10-9 m) in size, and even small amounts of corrosion could cause a device failure.
In some situations, corrosion may occur only at localized regions on a metal surface. This type of corrosion is characterized by regions of locally severe corrosion, although the general loss of thickness may be relatively small.
Pitting corrosion is usually associated with passive metals, although this is not always the case. Pit initiation is usually related to the local breakdown of a passive film and can often be related to the presence of halide ions in solution.
Crevice corrosion occurs in restricted or occluded regions, such as at a bolted joint, and is often associated with solutions that contain halide ions. Crevice corrosion is initiated by a depletion of the dissolved oxygen in the restricted region. As the supply of oxygen within the crevice is depleted, because of cathodic oxygen reduction, the metal surface within the crevice becomes activated, and the anodic current is balanced by cathodic oxygen reduction from the region adjacent to the crevice. The ensuing reactions within the crevice are the same as those described for pitting corrosion: halide ions migrate to the crevice, where they are then hydrolyzed to form metal hydroxides and hydrochloric acid.
Corrosion can also be accelerated in situations where two dissimilar metals are in contact in the same solution. This form of corrosion is known as galvanic corrosion. The metal with the more negative potential becomes the anode, while the metal with the more positive potential sustains the cathodic reaction. In many cases the table of equilibrium potential can be used to predict which metal of galvanic couple will corrode. For example, aluminum-graphite composites generally exhibit poor corrosion resistance since graphite has a positive potential and aluminum exhibits a highly negative potential. As a result, in corrosive environments the aluminum will tend to corrode while the graphite remains unaffected.
Stress corrosion cracking and hydrogen embrittlement are corrosion-related phenomena associated with the presence of a tensile stress. Stress corrosion cracking results from a combination of stress and specific environmental conditions so that localized corrosion initiates cracks that propagate in the presence of stress. Mild steels are susceptible to stress corrosion cracking in environments containing hydroxyl ions (OH-; often called caustic cracking) or nitrate ions (NO3-). Austenitic stainless steels are susceptible in the presence of chloride ions (Cl-) and hydroxyl ions (OH-). Other alloys that are susceptible under specific conditions include certain brasses, aluminum and titanium alloys. Hydrogen embrittlement is caused by the entry of hydrogen atoms into a metal or alloy, resulting in a loss of ductility or cracking if the stress level is sufficiently high. The source of the hydrogen is usually from corrosion (that is, cathodic hydrogen evolution) or from cathodic polarization. In these cases the presence of certain substances in the metal or electrolyte can enhance the amount of hydrogen entry into the alloy by poisoning the formation of molecular hydrogen. Metals and alloys that are susceptible to hydrogen embrittlement include certain carbon steels, high-strength steels, nickel-based alloys, titanium alloys, and some aluminum alloys. See Alloy
A reduction in the rate of corrosion is usually achieved through consideration of the materials or the environment. Materials selection is usually determined by economic constraints. The corrosion resistance of a specific metal or alloy may be limited to a certain range of pH, potential, or anion concentration. As a result, replacement metal or alloy systems are usually selected on the basis of cost for an estimated service lifetime.