by Anton I. Petrovich Lukens Steel Co.
CORROSION A
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Metallurgy—Important Factor in Corrosion Science Phenomena occurring inside metals have a direct influence on hydrogen embrittlement and stress corrosion
WORROSION science has had closer relations with chemistry and electrochemistry than with metallurgy. Recent progress in physical and mechanical metallurgy, however, has been of such a character that metallurgy will have an increasing influence upon corrosion theory. The classical approaches to corrosion have tacitly bypassed the internal workings of the metal for a consideration of the corroding medium and the liquid-metal interface. This situation can be attributed to the rudimentary state of the development of metallurgy. Corrosion science now must extend its consideration to phenomena occurring inside metals, particularly to the portion of the metal immediately adjacent to the surface.
the factor of stress was usually almost completely ignored and a typical surface-chemical treatment was afforded. Recent experience (3), however, strongly points to the hypothesis that hydrogen embrittlement and certain types of stress corrosion cracking are manifestations of an interaction between hydrogen diffusing in the metal, and voids and imperfections of a specific type known as dislocations, also occurring within the metal. Tensile stresses, consequently, are only a means of generating the internal imperfections; corrosion or other means of charging hydrogen into the metal are simply means of supplying hydrogen to the metal.
Both hydrogen embrittlement and stress corrosion appear to be best interpreted from a unified viewpoint, considering both metallic and corrosion processes. In the case of stress corrosion, it was obvious that stresses in the metal played a major role. This experimental fact was admitted, but was inadequately related to what was occurring in the interior of the metal. However, in the case of hydrogen embrittlement,
Dislocation Theory
Dislocations were originally invented to explain plastic deformation (7, 4). Well-defined slip bands are formed when metals are strained plastically—i.e., when deformation is so great that stress is no longer proportional to strain. Moreover, there is evidence that plastic deformation by slip does not occur by the simultaneous movement of groups or I/EC
blocks of atoms. Slip, instead, goes forward by a progressive movement or dislocation of a few atoms at a time, very much in the manner of a progressive wave action. T h e boundary between the slipped and unslipped regions is defined as a "dislocation line," or "dislocation." This apparently simple concept has many ramifications; a good case can be made for considering the theory of dislocations as the atomic theory of plasticity. Dislocation theory is now at the stage where a precise mathematical form can be given to its concepts (2). Thus, it has been shown that when stresses are applied to a metal, mobile dislocations course through the metal and tend to pile up at obstructions within the metal. Indeed, these dislocations can become immobilized just under the surface of the metal. Thepiling of dislocations near the surface is accompanied by a summation and intensification of the force fields surrounding individual dislocations (7). Equilibrium is established between applied and residual stresses of the metal and the force fields from dislocations. T h e tensile stresses generated by a pile-up of dislocations are internal in a metal and
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are additive, in the usual way, with other stresses. As a corollary, the presence of these dislocation-gener ated tensile stresses, internally in the material, makes the metal incapable of carrying its normal load, as in a typical tensile test. The metal is said to be embrittled.
notches are found only on metallic surfaces where large tensile stresses are present and where plastic defor mation has taken place; and also where large concentrations of hydro gen exist—i.e., where large internal "osmotic pressures" can be exerted by hydrogen gas. In contrast, the figure shows that the outside of the specimen in contact with air or nitrogen has none of the notches of the inner surface in contact with the hydrogen, even though the outer sur face may have undergone greater plastic flow.
The Role of H y d r o g e n
In interacting with occluded dis locations, hydrogen diffuses through a perfect lattice, jumping from one lattice site to another, in atomic or protonic form. However, when an appreciable void is encountered, the atomic hydrogen tends to combine to form gaseous hydrogen, which is then desorbed in the metal. The new stress field resulting from the pressure in internal voids is in addi tion to stresses already existing in the metal—i.e., from external tensile or residual internal stresses. It is not difficult to see why embrittlement oc curs in specimens charged with hy drogen under relatively large charg ing pressures. Experiment demonstrates that a reduction in tensile properties oc curs as a consequence of corrosion, when hydrogen is generated at the metal surface and then diffuses into the metal. This can likewise occur in electrolysis systems and in high pres sure hydrogen systems (3). For ex ample, charging hydrogen into car bon steel by electrolysis under ap propriate conditions has reduced the fracture stress to approximately 7 5 % of the usual tensile value; under more intensive charging with hy drogen, rupture at 50 or even at 2 5 % of the ultimate strength can be achieved. High pressure hydrogen gas can act in a similar way; in ex periment, the ultimate tensile strength has been reduced to below 75% of its normal values. It is of interest to compare the sur faces of ruptured, embrittled speci mens with surfaces of specimens sub jected to corrosion. Frequently, the surface of the corroded specimen contains notches and pockets of de bris and products of corrosion. One of the theories of stress corrosion cracking postulates that the pressure from products of corrosion is re sponsible for lowering the ultimate strength of the material. This, how ever, appears to be an incomplete 74 A
Cross section o f T y p e 3 0 4 thimble, Vz inch in outside d i a m e t e r , '/β-inch w a l l thickness, near point o f fracture under influence of h y d r o g e n gas under pressure
generalization, because the products are usually not impounded in com pletely enclosed pockets, and have full freedom to escape by extrusion into the solution. Specimens ruptured during the simultaneous application of external tensile loads and charging with hy drogen by electrolysis in arseniccontaining IN sulfuric acid show the same notches, particularly at the points where appreciable plastic de formation has occurred. The same phenomenon has been found in speci mens in which corrosion in' arseniccontaining sulfuric acid has replaced electrolysis. The same pattern of nicks and notches has been observed in specimens tested to destruction by high pressure hydrogen. The figure shows a section through a specimen of annealed Type 304 stainless steel that has failed in gaseous hydrogen at a pressure near 42,000 p.s.i. within the tubular specimen. The appearance of the specimens has been taken as partial evidence that a dislocation mechanism is act ing. The specimens ruptured by high pressure hydrogen gas present no plausible mechanism by which "corrosion" can occur, where the holes fill up and the products and debris can exert sufficient pressure to cause local rupture and break through. On the contrary, hydro gen must have diffused into the metal and found voids which subse quently were filled with high pressure hydrogen gas, and local rupture with no subsequent healing must have occurred. T h e nicks and
A p p l i c a t i o n of M e t a l l u r g i c a l T h i n k i n g to T h e o r y of Corrosion
This type of effort is being intensi fied, at present, toward general con sideration of the interaction of solute atoms and dislocations, and partic ularly the chemical interaction (2, 5) between solute atoms and dislo cations and metal imperfections. Much more, however, is to be ex pected from the joining of interest of corrosion and metallurgical scien ces, particularly in the field of applied science. Thus, an explanation of embrittlement will also provide clar ification in other fields such as caus tic embrittlement, embrittlement by high pressure gaseous hydrogen, electrolysis embrittlement, sulfide stress corrosion cracking, sour crude cracking, hydrogen attack, and even metallic creep and fatigue in contact with electrolytes. T h e possible bene fits of a unified corrosion-metal lurgical consideration are manifold. Literature
Cited
(1) Cottrell, A. H . , "Dislocations- a n d Plastic Flow in Crystals," Oxford U n i versity Press, Oxford, 1953. (2) Fisher, J. C , others, eds., "Disloca tions a n d Mechanical Properties of Crystals," Wiley, New York, 1957. (3) Petrovich, A. I., unpublished work. (4) R e a d , W . T . , "Dislocations in Crys tals," M c G r a w - H i l l , New York, 1954. (5) Suzuki, Hideji, Set. Repts. Research Inst. Tohoku Univ. A4, 455-63 (1952).
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