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Corrosion
sometimes good, is really mostly bad.
On the bad side, for which scientists are seeking remedies, corrosion invades all aspects of life from the exotic to the mundane• space satellites, teeth, bones, food containers, cars, and razor blades On the good side corrosion leads to chemical machining and milling, the aesthetically pleasing discolorations of metal objects, and the conversion of metals to useful chemicals. DR. HENRY LEIDHEISER, Jr., Virginia Institute for Scientific Research, Richmond A field of research with the unglamorous name "corrosion" is achieving a degree of appreciation from the consumer, engineer, marketing specialist, and basic scientist which it did not have a decade ago. Failures in space exploration because of corrosion of a vital part, the revolution in the razor blade industry brought on by the use of stainless steel, competition between container materials for the storage of foodstuffs, the rusting of automobile chrome trim, tragic destruction of aircraft, all have caught the public attention and have focused attention on the deterioration of materials. The demands of modern industry for products with superior strength, low density, specific neutron-absorption behavior, resistance to phase changes under pressure, or other characteristics have placed little-known materials into new environments. The corrosion behavior of these materials has often been the limiting factor in determining acceptability. 78
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The importance of corrosion to the national welfare prompted Prof. H. H. Uhlig of MIT to propose the establishment of a National Institute of Corrosion Control with support provided by U.S. industry, by the Federal Government, or by both. This institute would provide a means to carry out both long-range and shortrange research on corrosion and corrosion-related problems. Unfortunately, this worthy proposal has not been received with the enthusiasm it so well deserves. Corrosion is defined as "the deterioration of a substance, usually a metal, because of a reaction with its environment." This definition is so broad that it is impossible to do justice to the entire subject in an article of this length. Thus, I have restricted the scope of the article to recent findings in areas of my major interests. No claim is made for completeness; and the fine work of many authors has been omitted. Research within the U.S. has been emphasized although it
should be understood that significant corrosion research is being carried on in Canada, England, Belgium, France, Germany, Russia, and Japan. The presentation is in three sections: The first is concerned with the prevention of corrosion; the second with destructive, catastrophic corrosion; and the third with several corrosion problems of industrial significance. The corrosion of materials during use has important economic consequences. Therefore, research has been concentrated on understanding the kinetics of corrosion and on understanding the processes, such as inhibition, which can be used to reduce the rate of the deterioration reaction. The approaches which are now being taken to an understanding, and hence hopefully to control, of corrosion will be described. Early Stages of Corrosion A knowledge of the exact positions which atoms assume when present on
the surface of a foreign solid has always interested surface chemists. Field ion emission studies, such as those of Mueller at Pennsylvania State University, have given much information in this regard. New approaches using the tool which earned C. J. Davisson the Nobel Prize in Physics in 1937, namely, low-voltage election diffraction, are furnishing other longsought data. An example of such efforts is the work of MacRae, of Bell Telephone Laboratories, on the position of oxygen atoms on various crystal faces of nickel. The Bell Laboratories' instrument accelerates electrons to energies ranging from 2 to 500 e.v. The focused electron beam strikes the surface at normal incidence. The diffracted electrons are accelerated by 2000 volts after they have passed through a pair of fine grids, which transmit approximately 7 5 % of the electrons, and are then observed visually or photographically on a fluorescent screen. Diffraction theory is utilized to correlate positions of the diffraction spots with lattice positions of the atoms. These studies reveal that oxygen atoms adsorbed on nickel at low coverage are arranged in specific twodimensional structures. The dimensions of these structures are dependent on the surface oxygen concentration and the crystalline orientation of the substrate. For instance, five different atomic arrangements caused by oxygen have been seen on the (110) face of nickel. With an increase in temperature, these arrangements degrade to structures characteristic of lower oxygen contents. Low-voltage electron diffraction studies, such as the one described, furnish information useful in postulations on the earliest stages of the growth of thin, continuous films on a metal surface. Study of Thin Films Under mildly corrosive conditions, many metals react with oxygen, forming a continuous film of oxide. The presence of such a thin-film reaction product limits the rate at which oxygen and metal come into contact. Solid-state diffusion through the film
Six Unsolved Corrosion Problems Provide a Glimpse into the M u l t i t u d e of P r o b l e m s F a c i n g Corrosion W o r k e r s The development of means for protection of intermetallic compounds from destructive oxidation (pesting) at 0 500° to lOOO C. The development of a tarnish- and corrosion-resistant copper alloy which retains the color of copper. Such an alloy would find wide usefulness in indoor and outdoor use. (The International Copper Research Association is carrying out an active program in this field.) The development of means for the self-cleaning of building and ornamental trim which itself is subject only to superficial corrosion. The development of coatings which will protect molybdenum and tungsten alloys from high-temperature oxidation for a long time. The development of a silver alloy, which contains at least 92*5% silver so as to retain the designation of sterling, which resists tarnish. The alloying element should not appreciably change the color of silver and should be acceptable in contact with foodstuffs. The development of means of reducing internal oxidation of metals such as tantalum.
Electron Diffraction Patterns Show Film Structure One type of electron diffraction photograph obtained from the (100) face of nickel partially covered with oxygen is shown above. This pattern was obtained using 110-e. v. electrons. The four spots forming an approximate square nearest the center of the photograph result from diffraction of the electrons by the surface structure
The picture just above shows surface structure as deduced from the electron diffraction pattern at the top. The structure contains equal numbers of oxygen (dark marbles) and nickel (white marbles) atoms in the topmost layer. The repeat distance of the surface structure in the diagonal direction is twice that of the underlying substrate
(Photographs from Dr. A. U. MacRae of the Bell Telephone Laboratories) 80
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is the limiting factor in many reactions; and the amount of corrosion product is logarithmically or parabolically related to time. This type of relationship shows that the rate of corrosion per unit time decreases continuously. Perhaps a fuller understanding of this decrease can be used to prepare metals and alloys which resist rapid oxidation at high temperatures. Much effort has been expended in developing mathematical relationships which explain the exact form of the oxidation rate curve. Such an approach has been abandoned, except for engineering investigations, because of the awareness that different crystal faces oxidize at different rates, and that the oxidation is not homogeneous on a microscale. Furthermore, the state of the surface is vitally important during the formation of the first few atom layers of products. Emphasis during the past two decades has shifted to crystallographic and electrical studies of the corrosion product, microstructural studies using electron microscopy, and studies of the stoichiometry of the product. Benard in France and Gwathmey and co-workers in this country have focused attention on the so-called "nuclei" which are observable in the earliest stages of oxidation. The exact nature of these nuclei is unknown. They appear to be microscopically small regions in which oxidation occurs rapidly and results in formation of a small volume of product which grows at a much faster rate normal to the surface than the thin films as a whole. The high reactivity in the neighborhood of dislocations has suggested that the nuclei are associated with dislocations in the substrate metal. However, no adequate proof of this has been obtained. Young, of Oak Ridge National Laboratory, has shown that dislocations purposely introduced into a copper crystal can be detected by oxidation. In this case, microscopically high oxidation rates appear to be associated with the dislocations. A versatile and nondestructive technique for studying the rate of growth of thin films (10 to 200 A.) utilizes the optical characteristics of the growing film. This method, generally referred to as ellipsometry, takes advantage of a property of plane-polarized light. Such light, when reflected from the surface of an absorbing medium which is covered with a
film of another medium (either transparent or absorbing) and surrounded by a transparent third medium (air or oxygen), becomes elliptically polarized. The mathematical formulations which express this behavior are complex. Kruger, of the National Bureau of Standards, has developed a computer program which renders the equations much more tractable. The ellipsometer is mounted on a spectrometer table and the polarizer and analyzer are set at fixed angles of incidence and reflection. Incident light is elliptically polarized in such a way that the reflected light is planepolarized. The instrument readings are used to determine the relative amplitude reduction and the phase retardation in the perpendicular and horizontal components of the electric vector of the polarized light, when it is reflected from a film-covered surface. These properties of the reflected beam may be expressed as functions of the indexes of refraction of the substrate, the film, and the atmosphere above the film, the angles of incidence in these three mediums, the wave length of the light, and the thickness of the film. The thickness and refractive index of the oxide may be evaluated by an iterative process. Computed values for the amplitude reduction and phase retardation for a series of film thicknesses, and assumed values for the refractive index of the film, are compared with a similar experimentally determined set. Cathcart, of Oak Ridge National Laboratory, observed in studies of the oxidation of single copper crystals that the oxide film that forms on two of its faces [the (110) face and the (311) face] is optically anisotropic; that is, the optical properties are different in different directions. This result was totally unexpected, since the optical properties of cubic cuprous oxide should be isotropic. Two co-workers, Borie and Sparks, made accurate measurements of the lattice constant of these thin films using x-ray diffraction methods and found that the oxide film is highly strained. The oxide lattice expanded in the direction perpendicular to the film, but contracted in the plane of the film. These results have been rationalized in terms of the epitaxial relationships between the oxide and the metal and of the strains from epitaxially induced stresses. These observations repre-
Electron Beam Probe Used To Identify Constituent At Grain Boundary Aluminum-zinc alloys, used as sacrificial galvanic anodes, suffer intergranular corrosion, which decreases their electrochemical efficiency as an anode from the theoretical value of 1350 ampere-hours per pound to about 800 amperehours per pound. The addition of about 0.1% tin improves this value to about 1150 ampere-hours
sent a major step toward understanding the anisotropic oxidation of metals, such as copper, in which the rates of oxidation of different crystal faces are greatly different. Passivity Many metals which form thin-film corrosion products can become passive, or inactive, under conditions where high activity is expected. This phenomenon was studied by Michael Faraday, who observed the inactivity of iron in concentrated nitric acid. The fascination that passivity has for research workers is indicated by the fact that it has captured the interest of outstanding scientists such as Frumkin in Russia; Evans and Hoar in England; Bonhoeffer, Vetter, and Gerischer in Germany; Cohen in Canada; and Cartledge, Uhlig, and Hackerman in the U.S. International conferences on passivity were held in Germany in 1957 and in Toronto in 1962. Metals probably became passive as
per pound. The constituent at the grain boundary as shown in the photomicrograph was identified as tin by means of the electron beam probe, whose trace is shown as a dark line across the center of the photo, originating from the black, diamond-shaped impression used as a marker. The chart presents the tin concentration on
a result of the formation of a surface film which affords protection to the metal below. Kruger at the Bureau of Standards has shown that the passive film on iron is 20 to 30 A. thick. Many workers have shown that the film contains iron in two oxidation states. Hackerman believes the formation of the passive film on iron involves adsorption of solution species which is followed by a transformation brought about by the migration of metal ions. The net effect is the formation of a protective film capable of resisting further external attack. Cohen is one of the few workers who has stated a viewpoint, based on many years of research, in a sufficiently detailed form that one can come to grips with it: "When an anodic potential is first applied to a film-free iron specimen, the iron is oxidized to form both a film of magnetite ( F e a 0 4 ) and ferrous ion in solution. When the applied anodic potential is above the range where y - F e 2 0 3 is stable, y-FeL»03 is formed on top of the magnetite, and any ex-
the vzrtical component {arbitrary scale reading) versus the trace position on the horizontal. The peaks in the chart reading as the trace crosses the grain boundaries show tin enrichment at those locations
(Photographs are from the Metallurgical Research Division of Reynolds Metals Co.)
cess ferrous ion in the solution over the equilibrium value is also oxidized at the oxide-solution interface to form y-FL,0;». In the initial stages the film is thin so that there is a high potential gradient across the film and thickening occurs rapidly. "However, as the potential gradient decreases with thickening, the rate of migration also decreases and the anodic current drops. The second factor leading to a decrease in the film growth is a change in the composition of the outer layer of oxide. As the equilibrium concentration of Fe+ + in the solution drops to a very low number it becomes possible to form an oxide with cation defects at the surface because of the strong oxidizing conditions. The higher equilibrium potential associated with this defect oxide leads to a further decrease in the potential gradient across the film which is finally almost independent of applied potential because of the combination of thicker films and increased cation vacancies (accompanied by a higher 'equilibrium poAPRIL
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tential') at the higher applied anodic potential. "Only those films formed close to the passivation potential will give equilibrium behavior with regard to Fe+ + concentration and pH. Films formed at higher potentials tend to change to the equilibrium type film on cessation of anodic polarization. This decay of the potential can occur naturally by diffusion or autoreduction through 'pores' and can be accelerated by the addition of F e + + to the solution or slight cathodic treatment. In the long run, all of the oxide may be re-
Control of Electrochemical Corrosion Derives from Study Of fundamentals
moved by autoreduction and the reaction of iron to give F e + + in solution will predominate. This latter condition, of course, is the active state." The passivity of iron is of little industrial usefulness because the passivity of the unalloyed metal is limited to a narrow range of environmental conditions. However, the passive state of aluminum is within the range of industrial control. In fact, many applications of aluminum, where appearance is important, are possible because of the development of an inactive surface formed by anodic treatment in
an electrolyte. Electrolytes containing oxalate, borate, tartrate, or strong acids such as sulfuric and phosphoric are extensively used. Aluminum is made anodic at potentials between several volts and several hundred volts. An oxide film forms under these conditions. The films formed in the neutral range are compact, whereas those formed in sulfuric acid, for example, are much thicker and more porous at lower applied voltages. The porous films are used where dyeing of the film is needed for decoration.
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