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(11) | EP 0 026 863 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Corrosion resistant glassy metal alloys |
| (57) Corrosion resistant glassy metal alloys are provided which evidence a greater degree
of corrosion resistance than prior art glassy metal alloys and conventional crystalline
stainless steel alloys. The corrosion resistant glassy metal alloys consist essentially
of from about 0 to 18 atom percent nickel, from 7 to about 21 atom percent chromium,
from 0 to about 8 atom percent molybdenum, from about 13 to 18 atom percent of at
least one element selected from the group consisting of phosphorus, carbon and boron,
other than phosphorus plus carbon, and the balance essentially iron with the proviso
that the ratio of phosphorus to the sum of phosphorus, boron and carbon is greater
than or equal to 0.64. |
BACKGROUND OF THE INVENTION
Field of the Invention
Description of the Prior Art
[0002] The corrosion resistance of any given metal or alloy in a reducing medium is often sharply different from its corrosion resistance in an oxidizing medium, with some metals and alloys being more resistant to reducing media and others to oxidizing media. These differences in behavior are thought to be attributable to differences between the corrosion mechanism in a reducing medium and the corrosion mechanism in an oxidizing medium. Thus, corrosive attack by a reducing acid is generally considered to involve attack on the metal by hydrogen ions, resulting in the oxidation of metal to soluble ions and release of hydrogen gas. Metals of relatively high nobility, therefore, as indicated by their positions in the galvanic series are generally resistant to corrosion by reducing acid. Attack by oxidizing media, on the other hand, does not involve release of hydrogen but commonly results in the formation of metal oxides or other metallic compounds at the metal surface. Unlike the situation with reducing acids, a favorable position relative to hydrogen in the electromotive series provides no insurance that a metal will not be rapidly attacked by an oxidizing medium. However, certain elements, such as chromium, aluminum and silicon, form tough insoluble oxide films upon initial contact with an oxidizing medium, and such films serve as barriers against further reaction between the medium and the metal to prevent further corrosion from taking place.
[0003] Sulfuric acid solutions are not only very corrosive generally, but the nature of their corrosion properties varies markedly with both acid concentration and temperature. This variability relates at least in part to sulfuric acid's ambivalent assumption of both reducing and oxidizing properties as its concentration temperature and the nature and proportion of various contaminants are altered. As a consequence of this variability in its corrosive properties, few materials are available which are reasonably resistant to sulfuric acid solution over a wide range of concentrations and temperatures.
[0004] Of the known alloys which are demonstrably effective over wide ranges of sulfuric acid concentrations, many contain relatively high proportions of nickel and chromium and are, thus, rather expensive.
[0005] Corrosion resistant crystalline alloys are well known and are exemplified by stainless steels, for example. Corrosion resistant glassy metal alloys are also well known, see, for example, U.S. Patent . 3,856,513, which discloses a corrosion resistant glassy metal alloy, Fe40Ni38P14B6Al2 the subscripts are in atom percent), as being several orders of magnitude less reactive than stainless steels with concentrated hydrochloric acid. Other prior art corrosion resistant glassy metal alloys include iron-nickel-chromium- phosphorus-carbon alloys. However, these alloys evidence stress corrosion cracking and thus are not suitable in many applications, even though their corrosion resistance is superior to many other glassy metal alloys.
[0006] A continuing need exists for corrosion resistant alloys having a relatively low expensive metal content. In particular, a need has existed for such alloys in which the nickel and chromium content is relatively low, since these are both expensive materials. At the same time, there is a need for such alloys which are not only low in nickel and chromium, but also have low proportions of other expensive components such as molybdenum.
Summary of the Invention
[0007] In accordance with the invention, a metal alloy is provided that is substantially glassy and resistant to corrosion in acid media. The glassy metal alloy consists essentially of from about 0 to 18 atom .percent nickel, from 7 to about 21 atom percent chromium, from 0 to about 8 atom percent molybdenum, from about 13 to 18 atom percent of at least one element selected from the group consisting of phosphorus, carbon and boron, other than phosphorus plus carbon, and the balance essentially iron with the proviso that the ratio of phosphorus to the sum of phosphorus, boron and carbon present is greater than or equal to 0.64.
Brief Description of the Drawings
[0008]
FIG. 1, on coordinates of potential and current density, is a plot of a typical curve for an active-passive alloy; and
FIG. 2, on coordinates of potential and current density, is a plot of typical alloys of the invention compared with prior art stainless steel and prior . art glassy metal alloys.
Detailed Description of the Invention
[0009] Potentiostatic anodic polarization measurements are performed by immersing a metallic electrode in an electrolyte solution and varying its potential in a stepwise manner with a special feedback power supply (i.e.., a potentiostat). If the current corresponding to each potential is recorded, an anodic polarization curve can be constructed. A typical curve for an active-passive alloy is shown in FIG. 1. Potential is ploted on the ordinate and the logarithm of the current density on the abscissa. The current density, which is equivalent to the alloy dissolution rate, at first increases with increasing potential (active region 10). At more noble (positive) potentials, the dissolution current density decreases and then remains at a low value (passive region 11). At very positive potentials, the dissolution rate again increases with increasing potential (transpassive region 12). The current maximum which occurs at the primary passive potential Epp is termed the critical anodic current density Ic. The passive region begins at the passive potential Ep and is characterized by the passive current density Ip;
[0010] Potentiostatic anodic polarization curves may be viewed as plots of solution oxidizing power (E) versus corrosion rate (current density). Thus, a typical active-passive alloy possesses maximum corrosion resistance under moderately oxidizing conditions. The corrosion resistance is lower under reducing conditions and in the presence of very strong oxidizers which correspond to the, active and transpassive states, respectively. Passivation becomes easier as Epp becomes more negative and as Ie decreases. Therefore, it is possible to compare the ease of passivation of alloys on the basis of their anodic polarization curves. Also, corrosion rates in the passive state can be directly compared on the basis of I values. Finally, the range of useful corrosion resistance can be estimated by noting the width of the passive region and the potential at which the transpassive region begins. Although actual anodic polarization curves sometimes deviate from the schematic illustration of FIG. 1, they can be compared on the same basis.
[0011] The corrosion resistant alloys of the invention consist essentially of from about 0 to 18 atom percent nickel, from 7 to about 21 atom percent chromium, from 0 to about 8 atom percent molybdenum, from about 13 to 18 atom percent of at least one element selected from the group consisting of phosphorus, carbon and boron, other than phosphorus plus carbon, and the balance essentially iron with the proviso that the ratio of phosphorus to the sum of phosphorus, carbon and boron is greater than or equal to 0.64.
[0012] Glassy metal alloys of the invention are formed by cooling a melt of the desired composition at a rate of at least about l05oc/sec. A variety of rapid quenching techniques, well known to the glassy metal alloy art, are available for producing glassy metal powders, wires, ribbon and sheet. Typically, a particular composition is selected, powders or granules of the requisite elements in the desired portions are melted and homogenized, and the molten alloy is rapidly quenched on a chill surface, such as a rapidly rotating cylinder, or in a suitable fluid medium, such as water. Under these quenching conditions, a metastable, homogeneous, ductile material is obtained. The metastable material may be amorphous or glassy, in which case there is no long range order. X-ray diffraction patterns of glassy metal alloys show only a diffuse halo, similar to that observed for inorganic oxide glasses.
[0013] The amorphous metal alloys are at least 50% amorphous, and preferably at least 80% amorphous, as measured by X-ray diffraction. However, a substantial degree of amorphousness approaching 100% amorphous is obtained by forming these amorphous metal alloys in a partial vacuum. Ductility and corrosion resistance are thereby improved, and such alloys possessing a substantial degree of amorphousness are accordingly preferred.
Examples
[0014] A number of iron-based glassy metal alloys were studied and compared with prior art glassy metal alloys and crystalline stainless steel alloys. Exposed electrode areas for as-cast glassy metal ribbons ranged from 0.3 to 1.0 cm2. The ribbons were mounted (dull side out) on lucite rods. The plastic-metal interfaces were coated with lacquer to prevent crevice effects. Stainless steel standards were in rod form with an exposed area of about 5 cm2. The experiments were performed in nitrogen or hydrogen saturated electrolyte prepared from distilled water and reagent-grade chemicals. The electrodes were inserted into a Princeton Applied Research polarization cell and pre-exposed until the corrosion potential became nearly constant. Polarization measurements were conducted in the noble direction using a Potentiodyne. The scanning rate for glassy metal alloys was generally 1.5 V/hr.
[0015] Polarization curves for all alloys were obtained in IN H2SO4 at 22°C. Materials exhibiting interesting characteristics were studied in other environments such as IN H2SO4 plus 5 percent NaCl, ION H2SO4, and 38 percent HC1. In addition, weight losses were determined in 6% FeCl3 at 60°C for 20 hours.
[0016] Table I compares alloy performance in hydrogen-saturated 1N H2SO4. Included are prior art crystalline and glassy metal alloys, as well as glassy metal alloys having compositions outside the scope of the invention.
[0017] Table II compares weight loss of alloys in 6% FeCl3 at 60°C for 20 hours. Again, prior art crystalline and glassy metal alloys having compositions outside the scope of the invention are included for comparison.
[0018] On the basis of Ip values, the glassy metal alloys of the invention evidence values less than 1x10 A/cm , with many values less than 10 A/cm2. Further, these alloys are stable in 6% Fe/Cl3. Alloys outside the invention are seen not to possess this combination of corrosion resistance.
[0019] Having thus described the invention in rather full detail, it will be understood that this detail need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the present invention as defined by the subjoined claims.
1. A metal alloy that is substantially glassy and resistant to corrosion in media
over a wide range of oxidizing power consisting essentially of from about 0 to 18
atom percent nickel, from 7 to about 21 atom percent chromium, from 0 to about 8 atom
percent molybdenum, from about 13 to 18 atom percent of at least one element selected
from the group consisting of phosphorus, carbon and boron, other than phosphorus plus
carbon, and the balance essentially iron with the proviso that the ratio of phosphorus
to the sum of phosphorus, boron and carbon present is greater than or equal to 0.64.
2. A metal alloy as recited in claim 1, wherein: Cr + Mo + 0.4 Ni> 20 atom percent,
and P + C + B > 16 atom percent.
3. A metal alloy as recited in claim 1, having a formula consisting essentially of
Fe50.2Ni17.5Cr14P18B0.3.
4. A metal alloy as recited in claim 1 having a formula consisting essentially of
Fe60.2Cr14p18B0.3Mo7.5.
5. A metal alloy as recited in claim 1, having a formula consisting essentially of
Fe59.6Cr20P13C7B0.4.