Corrosion resistant amorphous ferrous alloy compositions

Abstract

 An amorphous metal alloy composition exhibiting corrosion resistance in acidic and alkaline environments of the formula:

Fe.Cr_bM_cM'd wherein

M is at least one metal selected from the group consisting of

Mo and Ta;

M' is at least one metal selected from the group consisting of

V, Ti, Zr, W and Nb; and wherein

a ranges from about 0.2 to about 0.8;

b ranges from 0.0 to about 0.3;

c ranges from about 0.2 to about 0.8; and

d ranges from 0.0 to about 0.8.

Description

Field of the Invention

[0001] The present invention relates to amorphous iron-containing metal alloys that exhibit excellent corrosion resistance in both acidic and alkaline environments.

Background of the Invention

[0002] The tendency of metals to corrode has long been a recognized concern. As used herein, the term.corrosion refers to the degradation of a metal by the environment by either chemical or electrochemical processes. A large number of crystalline alloys have been developed with various degrees of corrosion resistance in response to various environmental conditions on to which the alloys must perform. As examples, stainless steel contains nickel, chromium and/or molybdenum to enhance its corrosion resistance. Glass and metals such as platinum, palladium, and tantalum are also known to resist corrosion in specific environments. The shortcomings of such materials are that they are not entirely resistant to corrosion and that they have restricted uses. As an example, tantalum and glass resist corrosion In acidic environments but are rapidly corroded by hydrogen fluoride and strong base solutions.

[0003] The corrosion resistance of an alloy is found generally to depend on the protective nature of the surface film, generally an oxide film. In effect, a film of a corrosion product functions as a barrier against further corrosion.

[0004] In recent years, amorphous metal alloys have become of interest due to their unique characteristics. While most amorphous metal alloys have favorable mechanical properties, they tend to have poor corrosion resistance. An effort has been made to identify amorphous metal alloys that couple favorable mechanical properties with.corrosion resistance. Binary metal-metalloid amorphous alloys were found to have improved corrosion resistance with the addition of elements such as chromium or molybdenum, M. Naka et al, Journal of Non-Crystalline Solids, Vol. 31, page 355, 1979. Naka et al. noted that metalloids such as phosphorus, carbon, boron and silicon, added in large percentages to produce the amorphous state, also influenced its corrosion resistance.

[0005] T. Masumoto and K. Hashimoto, reporting in the Annual Review of Material Science, Vol. 8, page 215, 1978, found that iron, nickel and cobalt-based amorphous alloys containing a combination of chromium, molybdenum, phosphorus and carbon were found to be extremely corrosion-resistant in a variety of environments. This has been attributed to the rapid formation of a highly protective and uniform passive film over the homogenous, single-phase amorphous alloy which Is devoid of grain boundaries and most other crystalline defects.

[0006] Many amorphous metal alloys prepared by rapid solidification from the liquid phase have been shown to have significantly better corrosion resistance than their conventionally prepared crystalline counterparts, as reported by R. B. Diegle and J. Slater in Corrosion, Vol. 32, page 155, 1976. Researchers attribute this phenomena to three factors: structure, such as grain boundaries and dislocations; chemical composition; and homogeneity, which includes composition fluctuation and precipitates.

[0007] A thorough discussion of the corrosion properties of amorphous alloys can be found in Glassy Metals: Magnetic, Chemical, and Structural Properties, Chapter 8, CRC Press, Inc., 1983. In spite of advances made to understand the corrosion resistance of amorphous metal alloys, few alloys have been identified that exhibit little or no corrosion under extremely harsh acidic and/or alkaline environments. Those few alloys which do exhibit such properties utilize expensive materials such as ruthenium in the alloy composition and so are prohibitive for many applications where their properties are desired. What is lacking in the field of amorphous metal alloys are economical alloy compositions that exhibit a high degree of corrosion resistance in acidic and alkaline environments.

[0008] It is, therefore, one object of the present invention to provide amorphous metal alloy compositions having excellent corrosion resistance in acid and alkaline environments.

[0009] It is another object of the invention to provide such amorphous metal alloy compositions in a cost effective manner.

[0010] These and other objects of the present invention will become apparent to one skilled in the art from a reading of the following description of the invention and the appended claims.

Summary of the Invention

[0011] The present invention relates to an amorphous metal alloy of the formula:
Fe_aCr_bM_cM'_d wherein M is at least one metal selected from the group consisting of: Mo and Ta; M' is at least one metal selected from the group consisting of:

V, Ti, Zr, W and Nb; and wherein a ranges from about 0.2 to about 0.8;

b ranges from zero to about 0.3;

c ranges from about 0.2 to about 0.8; and

d ranges from zero to about 0.8.

Detailed Description of the Invention

[0012] The compositions described herein are substantially amorphous metal alloys. The term "substantially" as used herein with reference to amorphous metal alloys indicates that the metal alloys are at least 50 percent amorphous as indicated by x-ray diffraction analysis. Preferably, the metal alloy is at least 80 percent amorphous, and most preferably about 100 percent amorphous, as indicated by x-ray diffraction analysis. The use of the phrase "amorphous metal alloy" herein refers to amorphous metal-containing alloys that may also comprise traces of non-metallic elements.

[0013] In accordance with the present invention there are provided amorphous metal alloy compositions having the ability to withstand corrosion under acidic and alkaline conditions. These amorphous metal alloys are represented by the emperical formula:

wherein M is at least one metal selected from the group consisting of Mo
and Ta;

[0014] M' is at least one metal selected from the group consisting of V,

Ti, Zr, W and Nb; and wherein a ranges from about 0.2 to about 0.8;

b ranges from zero to about 0.3,

c ranges from about 0.2 to about 0.8; and

d ranges from zero to about 0.8.

[0015] Iron is a mandatory element of the foregoing substantially amorphous metal alloy compositions. Chromium may be absent from the compositions of this invention, although generally chromium is present. Preferably, the ranges of a, b, c and d are as follows:

a ranges from about 0.2 to about 0.7;

b ranges from 0.0 to about 0.25;

c ranges from about 0.2 to about 0.65; and

d ranges from 0.0 to about 0.5.

Most preferably, the ranges of a, b, c and d are as follows:

a ranges from about 0.3 to about 0.6;

b ranges from about 0.0 to about 0.2;

c ranges from about 0.2 to about 0.5; and

d ranges from 0.0 to about 0.3.

[0016] Amorphous metal alloy compositions of the present invention include Fe_aCr_bTa_c, Fe_aCr_bMo_c, Fe_aCr_b(MoTa)_c, Fe_aTa_c, Fe_a(MoTa)_cZr_d, Fe_a(MoTa)_cTi_d, and Fe_a(MoTa)_c. The foregoing list is not to be construed as limiting but merely exemplary. The amorphous metal alloy compositions taught herein are different from most amorphous compositions in the literature that claim corrosion resistance in that the compositions herein demonstrate resistance to corrosion under both acidic and alkaline environments. The compounds taught herein are also conspicuous in the absence of a metalloid element as is taught in the literature. However, it is to be recognized that the presence of other elements as impurities in these amorphous metal alloy compositions are not expected to significantly impair the ability of the alloy to resist corrosion. Thus, trace impurities such as 0, N, C, B, S, Se, Te, Si, Al, P, Ge, Sb, Sn, As, and Ar are not expected to be seriously detrimental to the preparation and performance of these materials.

[0017] To insure the desired corrosion resistant properties of these amorphous metal alloy compositions, it is important to maintain the integrity of the amorphous state, and so it is not intended that these materials be exposed to an environment wherein the temperature of the alloy may reach or exceed its crystallization temperature.

[0018] The substantially amorphous metal alloys taught herein may exist as powders, solids or thin films. The alloys may exist separately or in conjunction with a substrate or other material. A coating of the amorphous metal alloy may be provided onto a substrate to impart the necessary corrosion resistance to the substrate material. As a coating, these amorphous metal alloys may be used on the interior surface of chemical reaction vessels on structural metal exposed to sea water or other strongly corrosive environments, and on the surface of pipelines and pumps that transport acidic and/or alkaline chemicals. The amorphous metal alloy, because of its inherent hardness, may also be fabricated into any shape and used freestanding for applications in harsh environments. Additional uses for these corrosion-resistant amorphous metal alloys will be evident to those skilled in the art.

[0019] The compositions taught herein can be prepared by any of the standard techniques for the synthesis of amorphous metal alloy materials. Thus, physical and chemical methods such as electron beam deposition, chemical reduction, thermal decomposition, ion cluster deposition, ion plating, liquid quenching, RF and DC sputtering may be utilized to form the compositions herein.

[0020] The following examples demonstrate the corrosion resistance of the compositions taught herein. It is to be understood that these examples are utilized for illustrative purposes only, and are not intended, in any way, to be limitative of the present invention.

EXAMPLES

[0021] The following examples contrast known corrosion resistant materials with several representative corrosion resistant amorphous metal alloys in accordance with the present invention. In the examples, Examples 1-10 tested the corrosion resistance of amorphous compositions reported in the literature, Examples 11-15 evaluated the corrosion resistance of crystalline and elemental films, and Examples 16-23 tested the corrosion resistance of several amorphous metal alloy compositions taught herein.

[0022] Each of the amorphous metal alloys, Examples 1-10 and 16-23, were prepared by RF sputtering in argon gas. A 2" research S-gun manufactured by Sputtered Films, Inc. was employed. As is known, DC sputtering can also be employed to achieve similar results. For each of the examples, a glass substrate was positioned to receive the deposition of the sputtered amorphous metal alloy. The distance between the target and the substrate in each instance was about 10 cm. The thickness of each film was measured by a quartz crystal monitor located next to the deposition site. The average film thickness was about 2500 Angstroms. Confirmation of film thickness was done with a Dektak II, a trade name of the Sloan Company.

[0023] The compositions were then tested under five strenuous environmental settings:

--12N HCl at room temperature;

--6.49N HC1 azeotrope (108.5°C):

--in the vapor of the refluxing 6.49N HCl azeotrope;

--50/50 KOH/H₂0 (by weight) solution at room temperature; and

--refluxing 6N aqueous KOH solution.

[0024] Samples of each of the materials to be tested were subjected to the various sets of environmental conditions for a time sufficient to measure corrosion. In cases where no corrosion was detected, as In examples 0, 12, 14, and 16-21, the samples remained under test conditions for from about four hours to about 48 hours, with the exceptions of Examples 22 and 23 that were tested for about 189 hours. The results are displayed In Table I which reports corrosion rates in millimeters per year, as extrapolated from actual measurements. No data is presented where the test was not performed; those compositions not demonstrating corrosion resistance to acidic conditions were not further tested for corrosion resistance under strongly basic conditions.

[0025] As can be seen, of the 10 compositions listed in the literature and tested herein. only Mo₄₈Ru₃₂B₂₀, reported in Example 6, withstood corrosion attack under the five corrosive test conditions. This composition was reported by R.M. Williams, et al. in the Journal of the Electrochemical Society, Volume 131, pp. 791-2794, 1984. Unfortunately, its excellent corrosion resistance is offset by its prohibitive cost for most applications. Other control materials, such as Ta₇₅Si₂₅, Example 10, and tantalum foil. Example 12, withstood corrosion in the acidic environments but could not maintain their integrity in a strongly alkaline environment.

[0026] The compositions in accordance with the present invention, however, are excellent materials for use in highly acidic and alkaline environments, showing virtually no corrosion under any conditions. No corrosion was detected at all in four of the eight compositions in accordance with the present invention. The composition Fe₆₀Cr₁₀Mo₃₀. Example 19, was slightly corroded when tested in 6.49 normal HC1 azeotrope. having a corrosion rate of about 0.12 mm per year, however, it should be emphasized that this corrosion rate is lower than the corrosion rate of most of the compositions represented in the literature and tested under the same conditions. The composition depicted in Example 19 was not corroded in the alkaline environment of 50/50 KOH/H₂0 at room temperature.

[0027] Fe₆₀Ta₄₀, Example ₂₀, was slightly corroded in the alkaline environment of 50/50 KOH/H₂0 at room temperature, having a corrosion rate of about 0.125 mm/year, but this corrosion rate can be favorably compared to the corrosion rates of Ta₇₅Si₂₅, Example 10, and tantalum foil, Example 12, which are approximately 0.876 mm/year and 0.26 mm/year, respectively.

[0028] Examples 22 and 23, which were tested for 189 hours In 6.49N HC1 azeotrope (108.5°C), exhibited slight corrosion rates of about 0.0046 mm/year and 0.0073 mm/year, respectively. These corrosion rates, under these conditions, are Indicative of a material having excellent corrosion resistance.

[0029] Thus It is seen that the compositions in accordance with the teachings herein exhibit excellent corrosion resistance to both acid and alkaline environments. In general, it is preferred to have Cr and/or Ta present when the environment comprises strong oxidizing acids and Cr and/or Mo present when the environment is strongly alkaline. The fact that the compositions are amorphous metal alloys also indicates that their mechanical properties are relatively good, and so the compositions should be quite useful in environments in which both erosion and corrosion resistance is needed. In addition, these compositions do not require the use of precious or semi-precious metals, and so are economically feasible for a wide range of practical applications.

[0030] Although several amorphous metal compositions have been exemplified herein, it will readily be appreciated by those skilled in the art that the other amorphous metal alloys encompassed in the teachings herein could be substituted therefore.

[0031] It is to be understood that the foregoing examples have been provided to enable those skilled in the art to have representative examples by which to evaluate the invention and that these examples should not be construed as any limitation on the scope of this invention. Inasmuch as the composition of the amorphous metal alloys employed in the present invention can be varied within the scope of the total specification disclosure, neither the particular M or M' components nor the relative amounts of the components in the alloys exemplified herein shall be construed as limitations of the invention.

[0032] Furthermore. while these alloys were prepared by a sputtering technique which is a useful means for depositing films of the alloys onto a substrate, it is to be understood that neither the process of sputtering nor the coating of substrates are to be construed as limitations of the present invention, inasmuch as the alloys can be prepared by other processes and have other forms.

[0033] Thus, it is believed that any of the variables disclosed herein can readily be determined and controlled without departing from the spirit of the invention herein disclosed and described. Moreover, the scope of the invention shall include all modifications and variations that fall within that of the attached claims.

Claims

1. An amorphous metal alloy having a formula consisting of:

Fe_aCr_bM_cM'd

wherein M is at least one metal selected from the group consisting of Mo and Ta; M' is at least one metal selected from the group consisting of V, Ti, Zr and W; and

wherein a ranges from about 20 to about 80 atomic percent; b ranges from zero to about 30 atomic percent; c ranges from about 20 to about 80 atomic percent; and d ranges from zero to about 80 atomic percent.

2. The amorphous metal alloy in accordance with Claim 1 wherein a ranges from about 20 to about 70 atomic percent; b ranges from zero to about 25 atomic.percent c ranges from about 20 to about 65 atomic percent; and d ranges from zero to about 50 atomic percent.

3. The amorphous metal alloy in accordance with Claim 1 wherein a ranges from about 30 to about 60 atomic percent; b ranges from zero to about 20 atomic percent; c ranges from about 20 to about 50 atomic percent; and d ranges from zero to about 30 atomic percent.

4. The amorphous metal alloy in accordance with Claim 1 wherein said amorphous metal alloy is at least 80 percent amorphous.

5. The amorphous metal alloy in accordance with Claim 1 wherein said amorphous metal alloy is about 100 percent amorphous.

6. A substantially amorphous metal alloy thin film having a formula consisting of: Fe_aCr_bM_cM'_d

wherein M is at least one metal selected from the group consisting of Mo and Ta; _M' is at least one metal selected from the group consisting of V, Ti, Zr and W; and

wherein a ranges from about 20 to about 80 atomic percent; b ranges from zero to bout 30 atomic percent; c ranges from about 20 to about 80 atomic percent; and d ranges from zero to about 80 atomic percent.

7. The thin film of Claim 17 wherein the thickness of said film is about 2500 Angstroms.

8. A corrosion resistant material comprising a substrate and an amorphous metal alloy coating thereon, which coating is a substantially amorphous metal alloy having a formula consisting of: Fe_aCr_bM_cM'd

wherein M is at least one metal selected from the group consisting of Mo and Ta; M' is at least one metal selected from the group consisting of V, Ti, Zr and W; and

wherein a ranges from about 20 to about 80 atomic percent; b ranges from zero to about 30 atomic percent; c ranges from about 20 to about 80 atomic percent; and d ranges from zero to about 80 atomic percent.