Embrittlement

Type of physical science: Embrittlement, Metals, Chemistry

Field of study: Chemical processes

Embrittlement denotes a series of physical and chemical phenomena that make metals less able to deform or to absorb energy during fracture. Embrittlement can be caused by numerous factors, including altered temperature, corrosive environments, and presence of surface notches.

Overview

Most of the elements, the simplest forms of matter in the world, are metals. Metals are exceptionally useful to human society because of their great luster, their ability to conduct electricity, their malleability, their ductility, and their great structural strength. Because of these properties, metals have a great many industrial uses. These applications include the use of rare, corrosion-proof metals such as gold to manufacture fine jewelry and the use of more common metals such as iron, copper, and aluminum to create wires that transmit electrical energy, strong metal beams for building construction, fuel and water storage tanks, ships, motor vehicles, aircraft, cutlery, and surgical instruments. In each application, it is crucial that the metal object that is manufactured have an extensive life throughout which it retains the physical properties it exhibited when new. This objective is often severely compromised when the embrittlement of metals occurs.

Embrittlement of metals denotes a group of physical and chemical phenomena that make the metals less able to deform in shape or to absorb energy during fracture. These changes make the metals much less malleable, less ductile, and less tough. The reduction in these qualities caused by embrittlement diminishes the usefulness and longevity of the affected metal objects, which are used in applications that range from everyday societal functions to environmental protection, including containers designed for the safe containment of dangerous wastes over very long storage periods. Understanding the causes of metal embrittlement and finding solutions for the problems associated with it are important to society at virtually every level.

It is well known that the embrittlement of metals can be induced by both external factors present in the environment and internal factors associated with the properties of the metals as pure solids or as alloys. Several issues germane to the causation of embrittlement of metals are the alteration of the temperatures to which metal objects will be subjected, various modifications of their internal structures (usually grain size), variation in internal structures due to the presence of other elements in alloys, the appearance of corrosive environments of various types in service conditions, and the presence of irregularities such as notches in metal surfaces. These issues are often interrelated and act together in the actual embrittlement processes.

Two common types of packing of metal atoms in materials are termed "body-centered cubical arrays" and "hexagonal arrays." These arrays are arranged in metal grains (or crystallites). Two widely used metals, pure iron and pure zinc, are found as body-centered cubical arrays and as hexagonal arrays, respectively. In both types of array, metals exhibit distinctive temperatures that are called their "critical temperatures." When a metal is used below its critical temperature, its toughness is diminished and it becomes embrittled. This internal metal embrittlement is caused by the decreased ability of the grain arrays of such metals to deform without being fractured. Because metals become embrittled below their critical temperatures, all uses for metals should preclude any conditions that will even approach their critical temperatures. This form of embrittlement is purely a physical process.

Metals can also become embrittled when they are exposed to elevated temperatures. When metals are heated, internal structural changes take place, embrittling their grain boundaries and weakening them. For example, steels (various alloys of iron and other elements) are often treated by cooling them (quenching) from a very stable, high-temperature, face-centered form called austenite to a room-temperature form called martensite. Martensite is a distorted, body-centered cubic form that is very hard and strong. However, it is also brittle because of the distortion of its grain units. The production of martensite and its grain size is affected by the composition of the alloy used and the rate of quenching. The brittleness of the steel made is minimized by a process termed "tempering," in which the alloy sample is heated, worked on, and cooled several times. Many other metals are also embrittled by heat treatment. This form of embrittlement is a physical process.

Another external factor that embrittles metals is called "stress-corrosion cracking." A metal is stressed and, at the same time, exposed to a corrosive environment. The corrosive environment varies. It may be virtually any liquid, including pure water, salt water, ammonia or solutions of stronger bases, and appropriate organic solvents. Stress-corrosion cracking can occur in a wide variety of metals, including many steels and brass. The mechanism is thought to be the dissolution, after corrosion occurs, of metal atoms that are located between metal grains, leading to intergrain cracks. After these cracks occur, the stress that is also being applied causes the embrittled metal sample to fracture at the cracks. Therefore, both corrosion and stress are required for this type of embrittlement, which is the result of both chemical and physical processes.

A related process is the embrittlement of a metal object when a thin film of some other metal is applied to its uncorroded surface. As with stress-corrosion cracking, the main process involved is intergrain cracking. Embrittlement is believed to occur because the atoms of the liquid metal that is applied decrease the force needed to break apart atoms of the recipient metal, perhaps by dissolution processes. One example of such embrittlement is the interaction of liquid lead with steels. The metal embrittlement observed in all such cases is entirely a physical process.

Another form of this phenomenon that is caused by a physical process is embrittlement due to hydrogen. Hydrogen atoms enter a metal object and cause the embrittlement to occur. Such an embrittlement is very often observed in steels and in metals such as titanium and nickel. It is particularly common in alloys. The hydrogen atoms may be retained in the affected metals when the metals are melted and cast into desired objects. In addition, hydrogen atoms may be produced when a metal is electroplated. Furthermore, hydrogen atoms may arise in metal objects already in use from traces of hydrogen left over in industrial plants when hydrogen is used in an operation such as coal gasification. A related topic is helium embrittlement produced via irradiation processes in nuclear reactors. This specialized instance of metal embrittlement is very important to the manufacture of safe nuclear reactor bodies and cores and uranium fuel rods.

A final embrittlement-related issue is the presence of notches in the surface of metal objects. These notches arise from imperfections in the metal-casting processes used to fabricate a metal object. In general such surface flaws increase the extent of embrittlement of metal objects. They elevate the stress on metal objects and provide sites of entry for environmental embrittlers such as corrosive liquids, liquid metals, and hydrogen atoms.

The overall embrittlement of metals is most frequently a combination of factors. Solving any particular case of metal embrittlement first involves careful study of the processes that are involved and the planned use of the metal object. In many cases the nature of the causative effects can be determined and appropriate corrective measures can be taken. If corrective measures are not possible, the embrittled metal must be replaced with a metal or metal alloy less subject to the embrittling factors to which the object will be exposed.

Applications

Each metal or metal alloy possesses an intrinsic critical temperature below which it is embrittled. Because the critical temperatures of most metals and their alloys are known, embrittlement can be avoided in several ways. One method is to be sure that a given metal is not used under conditions in which the critical temperature will be reached. Another solution is to replace the metal with a metal or alloy that has a critical temperature that does not fall within the temperature range of the desired use. If no better metal or alloy is available, at least the consumer can be made aware of potential problems and the probable product life.

Many metals and their alloys are embrittled by heat treatment during the fabrication process. One solution to this problem is to alter the heat-treatment regimen used in a way that maximizes metal toughness and minimizes embrittlement. Metalsmiths (and modern metal manufacturers) avoid excessive embrittlement by producing tempered steels designed for different uses. Medieval weapons makers used a tempering process to produce fine swords and other weapons that were very tough and minimally brittle. The metal to be used for the blade was heated, hammered into shape, and quenched. The overall process was repeated again and again before the blade was considered finished. This process altered the intergrain organization of the metal object and the size of its grains to produce the toughest and least embrittled form of the metal possible. In modern applications, multiquench procedures are used in the manufacture of metal objects in various ways, depending on the toughness and other properties that are needed in the final product. An alternative to this procedure, which may be overly expensive, is to find another, more suitable metal or metal alloy that is not prohibitively expensive and that can be prepared in a minimally embrittled (and therefore very strong) form.

When it is important to protect against stress-corrosion cracking, the basis of the cracking must be ascertained. Most often, the corrosion is caused by the production of tiny electrochemical cells (more commonly thought of as batteries) between the metal surface and the corrosive agent. The metal dissolves at the corrosion sites after becoming metal ions in an anodic reaction. In the presence of a great enough stress, the embrittled metal object cracks at the points where corrosion has occurred. A common treatment of such problems is to cover the main body of the metal with a coating of some other metal that will protect it from such processes. For example, chromium or zinc plating is used to cover the surfaces of steel objects. Chromium is quite expensive, so it is used only in very high-cost, ornamental objects. Less attractive zinc plating (galvanization) is utilized on larger or less costly items that are primarily functional rather than ornamental (for example, mop pails, outdoor metal piping, and ship propellers). A factor that can complicate the electroplating process is the production of hydrogen and the resultant potential for hydrogen embrittlement. The metal application involved must be examined carefully so that the solution of one metal-embrittling problem does not cause another.

Embrittlement due to notches will occur when the metal-casting process creates irregularities in the surface of the object. One way to alleviate this problem is to make sure that the surface of the die or mold used to cast a metal object is as smooth as possible. This will diminish the number of notches that occur. Another way to preclude embrittlement due to notches is to polish the object carefully and make sure that notch formation is minimal. Alternatively, notches can be covered over by another metal, however, this may introduce additional factors that cause embrittlement. For example, coating an object with a second metal can lead to the addition of hydrogen. Therefore, after any modification procedure is attempted, it is essential to test prototypes of the object to make sure that it will work well for the extent of its desired service life.

Hydrogen embrittlement results from hydrogen being picked up by the metal object during manufacture and from the environment during use. The pickup of hydrogen during manufacture can often be minimized by altering the fabrication procedure so that it will produce hydrogen in smaller quantities or not at all. Alternate procedures should be analyzed to make sure that they do not introduce other metal-embrittling factors that might cause more severe problems. If a suitable alternate procedure cannot be found, another metal or alloy must be used. In many instances, hydrogen pickup occurs in the service environment. This cause of metal embrittlement is more difficult to counteract because often the service environment cannot be altered so as to lessen hydrogen emissions. Often the best solution is to use a metal that resists hydrogen embrittlement and still enables manufacture at a cost that can be supported by the metal object's users.

Context

Problems associated with the embrittlement of metals have occurred since ancient humans began to work them in the Copper, Bronze, and Iron Ages. The embrittlement problem reached a high point in the Iron Age, after iron had replaced softer copper and bronze. Metals were used most in creating weaponry, and embrittlement endangered soldiers and knights when their weapons failed in battle. Little was known about the science of metals; however, metalsmiths combated the main embrittlement problem associated with iron weapons, heat embrittlement, by tempering the steel that they used. They employed a process that involved repeated cycles of heating, hammering, and cooling. Tempering made the steel maximally hard and tough. This early solution of an embrittlement problem later was understood to work because it altered grain size in steel objects.

By the time the Industrial Revolution flowered, metal (mostly carbon steel) was used to produce large industrial machines such as steam engines and for some building purposes. Advancements in the fields of chemistry, engineering, and metallurgy led to treatments that prolonged the useful lives of metal objects by systematically protecting them from aspects of high- and low-temperature embrittlement and from corrosion-stress cracking. These treatments included better tempering processes, some use of alloys, and efforts to replace certain metals with others more suitable for particular uses.

In the nineteenth and twentieth centuries, expansion of the number of metals that were available in their pure forms and as alloys much broadened their possible applications. The discovery, study, and identification of the critical temperatures of metals and alloys and the electrochemical basis for corrosion-stress cracking, hydrogen embrittlement, and other metal embrittlement enabled manufacturers to more appropriately choose metals for specific uses. In addition, the use of coatings on metal objects became widespread.

The last half of the twentieth century brought further advances in the fight against problems caused by embrittlement of metals. Numerous synthetic polymers were created that could be used to coat metal objects and protect them. This advancement has led to a wide range of polymer-coated metal items (for example, Teflon-coated ball bearings). Use of these coatings, along with much wider alloy choice, has solved many previously insurmountable embrittlement problems. In addition, ceramics are being used to replace metals in certain embrittlement situations. It is presumed that use of metal-ceramic combinations, more understanding of the basis for metal embrittlement in various situations, and development of new alloys will bring about further advances in solving metal embrittlement problems and further extend the useful lives of metal objects. These advances are expected to involve productive interaction of chemists, metallurgists, materials scientists, and engineers.

Principal terms

ALLOY: A microscopically homogeneous solid solution of two or more metals in which the atoms of one metal occupy regular positions between atoms of the other

ANODIC REACTION: A reaction in an electrochemical cell that causes the loss of electrons (oxidation); in the case of metals, it converts them to positively charged ions that dissolve in water

AUSTENITE: A form of steel arising at high temperatures, this face-centered crystalline material contains iron carbide or carbon carbide and is used in making corrosion-resistant steels

COAL GASIFICATION: A process wherein finely powdered coal is treated with hydrogen to make natural gas

CORROSION: The process of wearing away a metal as a result of interaction with its environment

CRITICAL TEMPERATURE: The temperature characteristic of a given metal or metal alloy below which its toughness is diminished and embrittlement occurs

DUCTILITY: The ability of a metal to be hammered into thin sheets or drawn into fine wires

ELECTROCHEMICAL CELL: A device, often called a "battery," which utilizes the ability of chemical substances to lose and gain electrons in chemical reactions that produce electric current

MALLEABILITY: The ability to be shaped or formed (as by hammering) without breaking or cracking

MARTENSITE: A solid solution of iron and carbon that is the chief component of hardened steels; produced by quenching, this body-centered cubic form of carbon steels is very hard and strong

QUENCHING: The process of cooling a hot metal object by thrusting it into water

TEMPERING: A process whereby the brittleness of steel or another alloy is minimized by alternately heating and quenching it

Bibliography

Brady, James E., and John R. Holum. Chemistry: The Study of Matter and Its Changes. New York: John Wiley & Sons, 1993. This college chemistry text covers many interesting aspects of the chemistry of metals and crystal structures. It also provides many definitions of terms that will be useful to the reader.

Briant, C. L., and S. K. Banerji. Treatise on Materials Science and Technology. Vol. 25. New York: Academic Press, 1983. This edited, technical text covers in depth the various types of embrittlement of metals.

British Nuclear Energy Society. Irradiation Embrittlement and Creep in Fuel Cladding and Core Components. London: Author, 1973. This conference-proceedings document covers many relevant aspects of metal embrittlement, including the production of irradiation, fast neutrons, helium generation, and aspects of concurrent high-temperature embrittlement. Solutions are suggested for embrittlement problems, and much information is made available to interested readers.

Colombier, L., and J. Hochman. Stainless and Heat-Resisting Steel. New York: St. Martin's Press, 1967. This text for scientists covers interesting aspects of steel fabrication, steel chemistry, and embrittlement associated with heat-related processes.

Evans, Ulick R. The Corrosion and Oxidation of Metals: Scientific Principles and Practical Applications. London: Edward Arnold, 1960. This text for scientists covers many aspects of metal chemistry and definitions. Of value for its consideration of metal structures and the basis for embrittlement.

Fontatna, Mars G. Corrosion Engineering. 3d ed. New York: McGraw-Hill, 1987. This classic text for scientists and engineers covers many aspects of the chemistry, corrosion, and embrittlement of metals. Contains definitions, details on metals, and discussion of embrittlement-related corrosion processes.