AUSTENITIC STAINLESS ALLOY WITH SUPERIOR CORROSION RESISTANCE

Abstract

Austenitic stainless alloys have been discovered that exhibit unexpectedly superior corrosion resistance, particularly to sulfuric acid solutions, when compared to that exhibited by conventional alloys with closely related compositions. These alloys advantageously are corrosion resistant to a relatively wide range of sulfuric acid concentration and temperature and are thus particularly suitable for use in the industrial production of sulfuric acid.

Claims

1. An austenitic stainless alloy characterized by the following composition in weight %: 36-40% chromium 32.5-36% nickel 1.5-2.0% manganese 0.35-0.6% nitrogen <0.3% silicon <0.02% carbon <0.02% phosphorus <0.04% molybdenum <0.02% copper <0.005% sulfur remainder consisting essentially of iron.

2. The alloy of claim 1 characterized by the following composition in weight %: 36-37% chromium 32.5-34% nickel 1.65-1.75% manganese 0.37-0.47% nitrogen.

3. The alloy of claim 1 characterized by the following composition in weight %: 38-40% chromium 34.5-36% nickel 1.55-1.65% manganese 0.38-0.48% nitrogen.

4. The alloy of claim 1 characterized by the following composition in weight %: 36-40% chromium 32.5-36% nickel 1.55-1.75% manganese 0.37-0.48% nitrogen.

5. The alloy of claim 1 characterized in that the alloy is hot worked, solution annealed, and quenched.

6. A method for making the austenitic stainless alloy of claim 1 comprising the steps of: obtaining sources of chromium, nickel, manganese, nitrogen, and iron in a selected ratio; vacuum induction melting the sources thereby forming a molten mixture; casting the molten mixture thereby creating a solid precursor alloy; hot working the solid precursor alloy; solution annealing and quenching the solid precursor alloy thereby creating a quenched alloy; and removing heavy oxide scale from the quenched alloy thereby creating the austenitic stainless alloy.

7. The method of claim 6 wherein the casting step is performed in air.

8. The method of claim 6 wherein the solution annealing step is done at greater than or equal to 1150 C.

9. The method of claim 6 comprising cold working the solid precursor alloy.

10. The method of claim 6 wherein the removing heavy oxide scale step comprises pickling the quenched alloy.

11. Use of the austenitic stainless alloy of claim 1 in a component exposed to high temperature, concentrated solution of sulfuric acid wherein the temperature of the solution is greater than or equal to 175 C. and the average concentration of sulfuric acid in the solution is greater than or equal to 98%.

12. The use of claim 11 wherein the temperature of the solution is in the range from 175 C. to 265 C.

13. The use of claim 11 wherein the concentration of sulfuric acid in the solution is in the range from 98% to 99.5%.

14. The use of claim 11 wherein the component is part of an industrial sulfuric acid production plant.

15. The use of claim 14 wherein the component is part of a steam generating system in an industrial sulfuric acid production plant.

16. The use of claim 11 wherein the component is a wrought or a cast product of the austenitic stainless alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 compares the corrosion rates of Inventive Sample 1 to those of commercial 310S when exposed to 99.5% H.sub.2SO.sub.4 at temperatures ranging from 175 C. to 265 C.

[0036] FIG. 2 compares the corrosion rates of Inventive Sample 1 to those of 310S when exposed to 99% H.sub.2SO.sub.4 at temperatures ranging from 175 C. to 265 C.

[0037] FIG. 3 compares the corrosion rates of Inventive Sample 1 to those of 310S when exposed to 98.5% H.sub.2SO.sub.4 at temperatures ranging from 175 C. to 265 C.

[0038] FIG. 4 compares the corrosion rates of Inventive Sample 1 to those of 310S when exposed to 98% H.sub.2SO.sub.4 at temperatures ranging from 175 C. to 265 C.

DETAILED DESCRIPTION

[0039] Unless the context requires otherwise, throughout this specification and claims, the words comprise, comprising and the like are to be construed in an open, inclusive sense. The words a, an, and the like are to be considered as meaning at least one and not limited to just one.

[0040] Stainless alloy refers to an alloy comprising at least chromium, nickel, and iron with a minimum of 10.5% chromium content by mass.

[0041] Herein, an austenitic stainless alloy is a stainless alloy primarily characterized as having an austenite crystalline structure (i.e. face centered cubic). While commonly known austenitic stainless steels contain from about 16 to 25% chromium, herein austenitic stainless alloys can include greater amounts of chromium (e.g. up to 40%).

[0042] Also herein, the term sulfuric acid solution refers to those solutions or compositions commonly known in the industry as sulfuric acid solutions. Thus, along with H.sub.2SO.sub.4 solutions up to 100% in concentration, the term is additionally intended to include oleum and/or fuming sulfuric acid which are compositions considered to be greater than 100% in concentration. In the industry, sulfuric acid solutions may be considered to be solutions of sulfur trioxide or SO.sub.3 in water and can be described for instance as ySO.sub.3.H.sub.2O. Oleum or fuming sulfuric acid refers to compositions in which y is greater than 1 (e.g. excessive sulfur trioxide is present). Such compositions may also be expressed in terms of a percentage of sulfuric acid strength, namely as a sulfuric acid solution whose concentration is greater than 100%. Such compositions are routinely encountered in the industrial production of sulfuric acid.

[0043] Alloys of the invention are austenitic stainless alloys comprising chromium, nickel, manganese, nitrogen, and iron in specially selected ratios that result in surprisingly superior corrosion resistance characteristics. In particular, the composition of these stainless alloys is chosen to obtain superior corrosion resistance in strongly oxidizing, acidic, liquid chemical environments such highly concentrated, hot sulfuric acid. However, the alloys also have potential to work very well in high temperature oxidizing furnace gas.

[0044] The elemental composition of the austenitic stainless alloys in weight % is: [0045] 36-40% chromium [0046] 32.5-36% nickel [0047] 1.5-2.0% manganese [0048] 0.35-0.6% nitrogen [0049] <0.3% silicon [0050] <0.02% carbon [0051] <0.02% phosphorus [0052] <0.04% molybdenum [0053] <0.02% copper [0054] <0.005% sulfur [0055] with the remainder consisting essentially of iron.

[0056] As illustrated in the Examples below, alloy samples comprising chromium, nickel, manganese, and iron at the low end of these ranges clearly show markedly improved corrosion resistance under harsh sulfuric acid conditions compared to preferred conventional stainless materials. Further, it is generally expected that workable stainless alloys may be prepared with modestly greater amounts of these elements (e.g. up to about 10% more of the major elements Cr and Ni and even slightly more of the minor elements Mn and N) and that, generally, the increased amount of these elements would result in additional improvement. This is evidenced by an alloy sample in the Examples comprising chromium and nickel at the high end of these ranges and which also appears to show similar improved corrosion resistance.

[0057] Without being bound by theory, the presence of high levels of chromium in the alloy, while otherwise maintaining a high level of alloy purity so as to prevent degradation of the influence of the chromium, is believed to provide the alloy's resistance to oxidizing chemicals. Such chromium levels, in combination with the specified nickel content, are considered to be about the highest available so as to obtain a workable and weldable wrought stainless alloy. The high Cr content gives the inventive alloys improved resistance to hot acid corrosion over known and traditionally used materials, while also giving them economy over conventional, higher nickel alloys. Further, the higher the Cr level in the alloy of the invention, the higher the expected corrosion resistance will be.

[0058] The nickel content of the alloy extends the stability of the austenitic structure and, along with the amount of incorporated nitrogen, allows for the inclusion of the required large amount of chromium in the alloy, without losing other important alloy characteristics like strength, weldability and workability. Increasing Ni content stabilizes the alloy structure with increasing Cr content and helps to prevent segregations and internal precipitations inside the material and welds of the material, thereby assisting in production, fabrication and improving corrosion resistance. (Segregations deplete tiny localized areas of key reactive elements, e.g. Cr, thereby decreasing corrosion resistance). Reasonable increases in Ni content are expected to provide improved results. However, too much Ni can be detrimental because Ni has a high affinity for sulfur and in hot, aggressive sulphuric acid environments this can stimulate unwanted corrosion. Thus, only enough Ni is desirable so as to achieve the inclusion of the desired high amount of Cr. An advantage of reduced Ni content is better economy.

[0059] Manganese is required in the alloy making process and the presence of Mn helps to reduce the amount of Ni required, stabilizes the alloy structure, promotes material uniformity, retards segregations and precipitations from forming, and controls impurities. Mn also assists in hot processing and scavenges impurities. The presence of nitrogen in the alloy also assists in reducing the amount of Ni required. Further, nitrogen retards unwanted chemical reactions from taking place in the material during production and fabrication. Mn and nitrogen work independently and synergistically with Ni to keep the alloy structure stable and homogeneous, and prevent segregations and precipitations inside the material and its welds, aiding in manufacture, fabrication and improving corrosion resistance. Mn, Ni, and N also can work synergistically in the right amounts to promote uniformity and retard segregations, thus improving corrosion resistance in the same way

[0060] Mo and Cu are synergistic, important alloy additions known to improve an alloy's corrosion resistance in reducing chemicals/environments. And even alone or in combination, Mo and Cu are known to improve an alloy's corrosion resistance to sulfuric acid. The corrosion resistance of the inventive alloys is surprising then, since it has been found that the inclusion of Mo and Cu is detrimental to corrosion performance. Alloys of the invention contain very little or essentially no Mo and Cu, and yet their corrosion resistance is outstanding in the highly oxidizing conditions experienced in high temperature, highly concentrated sulfuric acid. Unexpectedly then, Mo and Cu levels are thus desirably kept very low.

[0061] As for the other cited elements, i.e. Si, C, P, and S, these are considered as impurities and are present in very small quantities. In order to attain a desired alloy purity, these may be kept as low as practically possible, within controllable limits/variations, and except as required for the alloy making process.

[0062] A general method for making austenitic stainless alloys of the invention initially involves obtaining appropriate sources of chromium, nickel, manganese, nitrogen, and iron in a ratio selected to match that desired in the final alloy composition. These sources can be pure metals, combinations of metals or oxides; a higher percentage or pure metal raw material is preferred. Nitrogen may be incorporated as a gas, as an injected liquefied gas, and/or as complexes with other alloying element additions. The sources are then combined and melted via vacuum induction melting to form a molten mixture. The molten mixture is then cast thereby creating a solid precursor alloy. The casting can be done either in air or vacuum and either as ingot or continuous casting although ingot casting is preferred. Optionally the precursor alloy can be remelt refined via ESR (electro-slag remelting) or VAR (vacuum arc remelting) as an additional possible production step.

[0063] After casting, the precursor alloy is hot worked, such as by rolling, forging, or extruding. The alloy can also optionally be cold worked to dimension product pieces more precisely and/or to produce additional product forms by rolling and drawing complete with interstage annealing. The hot and optional cold working of the precursor alloy is then followed by solution annealing and quenching steps, thereby creating a quenched alloy. In the annealing step, a higher than typical annealing temperature (e.g. 1150 C.) helps to homogenize the material and put all the effective alloying elements in solid solution where they need to be in order to prevent corrosion. For quenching purposes, a water quenching step is preferred. Finally, the heavy oxide scale created on the quenched alloy is removed thereby creating the austenitic stainless alloy. For large pieces, the heavy oxide scale is preferably removed via a pickling procedure (an acid surface cleaning procedure e.g. using nitric-hydrofluoric acid mixtures at elevated temperatures). For small pieces, scale may alternatively be removed via sand blasting or bright/hydrogen atmosphere annealing.

[0064] Alloys of the invention have close to highest concentration of chromium and highest level of overall purity available in wrought alloy product forms. They are advantageous for use in pressure vessels and chemical plant equipment. Such alloys are highly corrosion resistant to concentrated nitric acid, high temperature oxidizing gases, and especially to high temperature, concentrated sulfuric acid. Thus, they are particularly suitable for use in sulfuric acid production but could also find application in nitric acid and in high temperature furnace gas service.

[0065] The present alloys provide longer service life, greater reliability, the potential for improved energy recovery/efficiency and greater flexibility in chemical plant operation. For instance, as discussed earlier, at the temperatures and concentrations involved in sulfuric acid production, even relatively small changes in concentration and temperature can markedly increase the rate of corrosion. Specifically, small decreases in concentration and/or small increases in temperature outside the normal required operating conditions can result in substantially greater corrosion rates. Thus, use of the present alloys may allow for acid at temperatures of 250 C. or greater to be used to generate higher pressure/quality steam (the present limit is about 210 to 225 C.). Also, reduced acid concentrations may be considered to provide for more efficient SO.sub.3 absorption that can lead to process equipment size reductions. Greater operating flexibility and turndown possibilities for a given system design are created by allowing operation over wider temperature and acid concentration ranges. In addition to reduced corrosion benefits, some process and economic benefits can also be expected by increasing the viable operating window for acid strength and temperature beyond that conventionally used.

[0066] Of particular advantage in sulfuric acid production is a potential improvement against plant temperature and/or concentration upsets and loss of control. As mentioned earlier, many plants employing 3 lOSS have been destroyed because of plant upsets and the massive corrosion that ensued. To avoid this, such plants must operate in very tight operating ranges (e.g. sulfuric acid concentrations between about 99.2 and 99.6% and temperatures between about 210 and 225 C.). However, controlling within these ranges can be difficult to do consistently with available instrumentation and operation error. As illustrated in the Examples below though, the corrosion resistance of the present stainless alloys is several times better than 310SS, and in some extreme cases, more than 30 times better. The present alloys may thus be acceptable for use under conditions representing major plant upsets or under conditions hitherto not believed possible (e.g. concentrations and temperatures of 98.5% and 250 C. respectively or of 98.5 to 99.5% and up to 265 C. respectively).

[0067] The following Examples have been included to illustrate certain aspects of the invention but should not be construed as limiting in any way.

EXAMPLES

[0068] In the following, inventive austenitic stainless alloys were prepared and their corrosion resistance characteristics were compared to those of certain commercial and prior art alloys specifically intended for use in applications involving exposure to sulfuric acid at high concentrations and temperatures (e.g. industrial sulfuric acid production plants).

[0069] A sample batch (denoted Inventive Sample 1) of an austenitic stainless alloy of the invention was made by obtaining sources of chromium, nickel, manganese, nitrogen, and iron in a ratio selected such that the weight % of Cr, Ni, Mn, and N in the product alloy would be at the lower ends of the ranges of inventive compositions. The sources were then melted using a vacuum induction technique and the resulting molten mixture was casted in air to form a solid precursor alloy. The precursor alloy was then hot worked, cold worked, solution annealed (at a temperature exceeding 1150 C.), and thereafter was quenched in water. Finally, oxide scale on the quenched alloy was removed by pickling.

[0070] The elemental composition of the inventive sample was determined using Inductively Coupled PlasmaOptical Emission Spectrometry (ASTM D1976 and E1086) and/or X-ray Fluorescence Analysis (JIS G 1256), Spark Spectrographic Analysis (JIS G 1253) and Combustion and Insert Gas Fusion Techniques (ASTM E1019). The results obtained appear in Table 1 below.

[0071] Samples of two commercially available stainless alloys commonly used in present-day sulfuric acid production plants were also obtained. These two alloys were premium 310S stainless steel (UNS S31008) and specialty stainless steel VDM Alloy 33 (Nicrofer 3033). The compositions of these commercial alloys also appear in Table 1.

[0072] For further comparison purposes, the corrosion resistance of certain stainless alloys reported on in U.S. Pat. No. 5,695,716 were considered below. These alloys are closely related in composition to those of the present invention and were also specifically intended for use in applications involving exposure to sulfuric acid at high concentrations and temperatures. The compositions of these alloys as reported on in U.S. Pat. No. 5,695,716 also appear in Table 1.

TABLE-US-00001 TABLE 1 Compositions of various samples Inventive Commercial Commercial Comparative Comparative Element Sample 1(%) premium 310S*(%) Alloy 33**(%) Sample 4 ***(%) Sample 5 ***(%) Cr 36.01 25.49 31.0-35.0 35.46 36.4 Ni 32.95 19.32 30.0-33.0 31.65 31.7 Mn 1.70 1.36 2.0 0.74 0.73 N 0.4210 0.04 0.35-0.5 0.51 0.58 Si 0.20 0.43 0.5 0.03 0.04 C 0.017 0.04 0.015 0.012 0.012 P 0.009 0.022 0.020 0.004 0.002 Mo <0.02 0.15 0.5-2.0 0.11 0.1 Cu <0.02 0.19 0.3-1.2 0.01 0.01 S 0.0007 0.001 0.010 0.002 0.002 Al Not obtained 0.019 Not reported 0.099 0.072 Fe Remainder Remainder* Remainder Remainder Remainder *Source: Mill certificate; also present were Co 0.38, Cb 0.013, Ti 0.004, Sn 0.008, Ta 0.007, Pb 0.001 **Source: VDM Metals GmbH data sheet March 2000 *** Source: U55695716

[0073] The corrosion characteristics of these samples was then determined when exposed to hot, concentrated sulfuric acid solutions at a variety of concentrations and temperatures representative of those experienced in commercial sulfuric acid production. In certain cases in the following, data was obtained from the literature. Otherwise, data was obtained by taking sample coupons approximately 8-10 cm.sup.2 in area and about 6 mm thick and then exposing them to the indicated solutions for a period of 14 days or as otherwise indicated. Corrosion rates were then determined based on the loss of sample thickness as a result of this exposure. The corrosion rates are expressed as mils (thousandths of an inch) per year or mpy.

Comparisons to 310S

[0074] FIG. 1 compares the corrosion rates of Inventive Sample 1 to those of 310S (the current industry standard for such process environments) when exposed to 99.5% H.sub.2SO.sub.4 at temperatures ranging from 175 C. to 265 C. FIG. 2 compares the corrosion rates of Inventive Sample 1 to those of 310S when exposed to 99% H.sub.2SO.sub.4 at temperatures over the same range. FIG. 3 compares the corrosion rates of

[0075] Inventive Sample 1 to those of 310S when exposed to 98.5% H.sub.2SO.sub.4 at temperatures over the same range. FIG. 4 compares the corrosion rates of Inventive Sample 1 to those of 310S when exposed to 98% H.sub.2SO.sub.4 at temperatures over the same range.

[0076] As is evident from the figures above, Inventive Sample 1 shows improved resistance to corrosion over commercial 310S over the entire temperature range tested. However, this is particularly so at the weaker acid strengths and higher temperatures tested where the corrosion characteristics of Inventive Sample 1 are markedly superior, sometimes being more than an order of magnitude better. It should be noted that these are ranges where the corrosion conditions are the most aggressive (i.e. at weaker acid strengths and/or higher temperatures).

Comparisons to Alloy 33

[0077] Table 2 below compares corrosion rates of Inventive Sample 1 to those of Alloy 33 when exposed to 99% H.sub.2SO.sub.4 at certain temperatures ranging from 155 C. to 265 C.

TABLE-US-00002 TABLE 2 Corrosion rates in 99% H.sub.2SO.sub.4 (in mpy) Temperature Sample 150 C. 175 C. 200 C. 215 C. 225 C. 240 C. 255 C. 265 C. Inventive Sample 1 0.18 0.29 0.35 1.56 2.53 2.73 1.21 Alloy 33* 4.00 4.00 4.00 *Data for Alloy 33 was obtained from Corrosion 53, 893-897 (2002)

[0078] Certain additional comparison data was obtained as summarized in Table 3 below. Here, the corrosion rates were determined for Inventive Sample 1 after exposure to the hot, concentrated acid for 14 days. In the case of Alloy 33, the rates were determined from an average of two results after exposure to the acid for 30 days.

TABLE-US-00003 TABLE 3 Additional corrosion rates in hot concentrated H.sub.2SO.sub.4 (in mpy) Concentration & temperature Sample 98.5% & 200 C. 98.5% & 215 C. 99% & 200 C. Inventive 1.42 0.29 Sample 1 Alloy 33 5.30

[0079] The available data in the tables above show that Inventive Sample 1 has improved resistance to corrosion over commercial Alloy 33 and again suggests it is particularly superior at weaker acid strengths and higher temperatures (e.g. 98.5% and 215 C.).

Comparisons to Alloys in U.S. Pat. No. 5,695,716

[0080] Table 4 below compares corrosion rates of Inventive Sample 1 to those provided for samples 4 and 5 in the aforementioned U.S. Pat. No. 5,695,716 (which are closely related in composition to those of the present invention). Data here is provided at closely related concentrations and temperatures as indicated.

TABLE-US-00004 TABLE 4 Corrosion rates in hot concentrated H.sub.2SO.sub.4 (in mpy) Concentration & temperature 215 C. for Inventive 99% for Inventive Sample 99% for Inventive Sample Sample 1 & 200 C. for 1 & 99.1% for others; all 1 & 99.1% for others; all Sample others; all @ 98.5% @ 175 C. @ 200 C. Inventive 1.4 0.18 0.29 Sample 1 Comparative 4.7 2.4 1.2 Sample 4 Comparative 2.4 4.3 6.3 Sample 5

[0081] As can be seen from the data in Table 4 above, Inventive Sample 1 surprisingly shows markedly superior resistance to corrosion when compared to closely related samples from U.S. Pat. No. 5,695,716. Again, it should be noted that weaker acid strengths and higher temperatures are more aggressive corrosive conditions here. Thus, it is expected that the large differences already seen in corrosion rates between Inventive Sample 1 and the comparative samples would just be greater if the data were obtained under exactly the same acid concentrations and temperatures.

[0082] A second sample batch (denoted Inventive Sample 2) of an austenitic stainless alloy of the invention was made in a like manner to Inventive Sample 1 above except that the sources of chromium, nickel, manganese, nitrogen, and iron were obtained in a somewhat different ratio and thus the composition of the second sample batch was also somewhat different. The elemental composition of Inventive Sample 2 was determined as before and the results obtained appear in Table 5 below.

TABLE-US-00005 TABLE 5 Composition of Inventive Sample 2 Inventive Element Sample 2 (%) Cr 38.93 Ni 35.4 Mn 1.62 N 0.4300 Si 0.23 C 0.016 P 0.011 Mo 0.03 Cu 0.01 S 0.0004 Al Not obtained Fe Remainder

[0083] As before, the corrosion characteristics of this sample were then determined when exposed to hot, concentrated sulfuric acid solutions at different concentrations and temperatures representative of those experienced in commercial sulfuric acid production. Data was obtained as described above by exposing coupons to the indicated solutions for a period of 14 days. Corrosion rates were again determined based on the loss of sample thickness as a result of this exposure (again in mils per year or mpy). Table 6 compares the corrosion rates of Inventive Sample 2 to those of 310S when exposed to two different sulfuric acid concentrations (i.e. 98.98% and 97.98% H.sub.2SO.sub.4) at a temperature ranging from 265 C. to 270 C. (a period of relatively minor temperature instability was experienced during testing such that the test temperature varied over this range) and to a 98.50% sulfuric acid concentration at 265 C.

TABLE-US-00006 TABLE 6 Corrosion rates in hot concentrated H.sub.2SO.sub.4 (in mpy) at 265 C. to 270 C. Concentration Sample 98.98% 97.98% 98.50% Inventive 4.9 8.9 6.4 Sample 2 310S 10.7 21.6 15.3

[0084] Inventive Sample 2 shows improved resistance results to corrosion that are similar to those obtained with Inventive Sample 1. Further, this demonstrates that alloys of the invention can be prepared with varied compositions over the claimed range and that they are characterized by unexpectedly superior corrosion resistance to hot concentrated sulfuric acid.

[0085] All of the above U.S. patents, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety.

[0086] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.