EP0378998A1 - Stainless wrought and cast alloys, and welding additives for parts contacted by concentrated hot sulfuric acid - Google Patents

Abstract

Iron-chromium-nickel-silicon alloys containing 13-32% by weight of Cr, 5-25% by weight of Ni and 4-9% by weight of Si and having a microstructure which contains more than 10% of delta-ferrite are highly resistant to hot concentrated sulphuric acid and can be used for structural components in contact therewith.

Description

The present invention relates to materials which are highly resistant to hot, concentrated sulfuric acid and 0-10% by weight oleum.

There is a lot of information in the literature on the corrosion resistance of materials against hot, concentrated sulfuric acid.

Because the solubility of lead sulfate increases with increasing sulfuric acid concentration, lead and its alloys can only be used at concentrations up to 78% H₂SO₄ and only up to 110 ° C (Ullmanns Encyclopedia of Technical Chemistry, 4th edition, Volume 21 (1982), p. 157).

Unalloyed steel can be used in 68-99% sulfuric acids up to 70 ° C, although removal rates of up to 1.3 mm / a can be expected (G. Nelson, Corrosion Data Survey, Shell Development Co., San Francisco, 1950, p. ZT-102A). In the concentration range of 99 to 100% H₂SO₄ resistance of mild steel decreases significantly higher flow rates are to be avoided on mild steel (Ullmann, loc. Cit .; Z. f. Werkst.-Techn. 4 (1973), S. 169 / 186; RJ Borges, Corrosion / 87, Paper No. 23, NACE, Houston, Texas, 1987).

Cast iron alloys alloyed with chrome or copper are stable at sulfuric acid concentrations of 90-99% up to about 120 ° C (Ullmann, loc. Cit.), However, the current dependence of the corrosion must also be taken into account here (Z. f. Werkst.-Techn ., loc. cit.). The iron-silicon casting material with 14-18% Si has a very good corrosion resistance in wide concentration and temperature ranges (Ullmann loc. Cit.); however, the hardness and brittleness of this special cast iron are of great disadvantage (RJ Borges, Corrosion / 87, loc. cit .; Ullmann, 4th edition, volume 3 (1973), p. 21). Standard austenitic stainless steels, as per material no. 1.4571, are used for concentrated sulfuric acids up to temperatures of 85 ° C. The removal rates increase steeply with increasing temperature. Even at 150 ° C, removal rates of around 1 mm / a can be expected (Z. f. Werkst.-Techn. 8 (1977), pp. 362/370 and 410/417), with the current dependence of corrosion being pronounced.

The use of nickel-based alloys has no advantages. For plate heat exchangers made of NiMo16Cr15W, Material no. 2.4819 (type Hastelloy alloy C-276), which are used for cooling concentrated sulfuric acid, the product temperature is limited to 95 ° C (N. Sridhar, Materials Performance March 1988, p. 40/46).

There has been no shortage of proposals to improve the resistance to sulfuric acid through alloying measures. The 3.7-4.3% Si-containing, austenitic stainless steel grade X 1 CrNiSi 18 15, material no. 1.4361, a much higher resistance than material no. 1.4571 in, for example, 98.5% sulfuric acid at 150 and 200 ° C (Ullmann, volume 3, p. 21); the flow dependence of corrosion is very low (Z. f. Werkst. Techn. 8 (1977), pp. 362/370 and 410/417; M. Renner and R. Kirchheiner, "Corrosion resistance of high-alloyed special stainless steels in strongly oxidizing media ", Lecture at the seminar" Nickel materials and high-alloy special steels ", Esslingen, April 7-8, 1986). By further increasing the Si content to 4.5 to 5.8%, the corrosion resistance of the austenitic, stainless steels in hot 85%, preferably 90%, sulfuric acids can be improved within certain limits (US Pat. No. 4,543,244; DE- OS 33 20 527). Such a special steel can hardly be considered for practical use at higher temperatures because of the pronounced temperature dependence of corrosion. The following removal rates were for a stainless, fully austenitic steel with the composition 17.5% Cr, 17.5% Ni, 5.3% Si, the rest essentially iron, in 98.2% sulfuric acid determined (named US '244 and DE' 527):
125 ° C 0.1 mm / a,
135 ° C 0.8 mm / a,
145 ° C 1.6 mm / a,
in 93.5% H₂SO₄ a corrosion rate of 0.25 mm / a was found at 85 ° C. Anodic protection of the systems can be provided to reduce corrosion; under these conditions, the rate of removal in 93.5% H₂SO₄ at 200 ° C is still 1.1 mm / a.

Furthermore, hardenable nickel-based alloys with 2-4% Si have been proposed for handling hot, at least 65% sulfuric acid (DE-PS 21 54 126). However, the removal rates in sulfuric acid heated to 120 ° C are quite high at around 0.6 mm / a. For a further hardenable and flow-insensitive nickel-based alloy, removal rates of 0.25 mm / a are given in 98% H₂SO₄ heated to 140 ° C. (R. J. Borges, Corrosion / 87, loc. Cit.).

An austenitic stainless steel with 17% Cr, 16%. Ni, 3.7% Si and 2.3% Mo, on the other hand, can only be used in cold sulfuric acids at concentrations below 10% and above 80% (CAFL document No. 235: Uranus, rust and acid-resistant steels for difficult corrosion problems, p 37). According to GB 1.534.926, austeniti alloyed with molybdenum may have to be used cal chrome-nickel-copper steels contain at least 4.1% or 4.7% silicon in order to ensure good corrosion resistance in heated to 110 ° C, 96.5% H₂SO₄; Similar findings apply to iron-chromium-nickel-cobalt-silicon alloys when subjected to 99% H₂SO₄ heated to 130 ° C (N. Sridhar, loc. cit.).

Finally, 4-6% silicon-containing iron-chromium-nickel alloys have been described, the delta ferrite content of which is limited to 5 to 10% so that no coherent delta ferrite network can be formed (DJ Chronister and TC Spence, Corrosion 85, Paper 305, NACE, Boston / Mas., March 1985). Such a network can be expected for delta ferrite shares from 10%. One of DJ Chronister et al. described, containing 4.8% Si alloy, the removal rates in heated to 110 ° C 95% H₂SO₄ are initially relatively low (0.4 mm / a), but quickly increase to 2.4 mm / a with longer exposure times. In alloys containing 5 to 5.2% Si, corrosion rates of 0.11 to 0.56 mm / a were found under these conditions. Only at 5.6% Si do ablation rates of about 0.1 mm / a occur. If the temperature of the 95% H₂SO₄ is increased to 130 ° C, with a Si content of 5.6% increasing removal rates are observed, which in the first test section (48 h) 0.66 mm / a, in the second section already 1 .24 mm / a; with a Si content of 5.9%, the removal rates are 0.45-0.54 mm / a.

It has now been found that the corrosion resistance of the silicon-containing iron-chromium-nickel alloys in hot, over 75% sulfuric acids or 0-10% by weight oleum can be markedly improved by setting an alloy structure which is more than 10 Contains% delta ferrite.

The invention accordingly relates to rust-proof kneading and casting materials as well as welding additives for components made of iron-chromium-nickel-silicon alloys with 13-32% by weight Cr content, with hot concentrated sulfuric acid or 0-10% by weight oleum, 5 -25 wt .-% Ni content and 4-9 wt .-% Si content and a structure that contains more than 10% delta ferrite.

The Si content is 4-9% by weight, preferably 4.3-7.5% by weight.

The Cr content is 13-32% by weight, preferably 15-24% by weight.

The Ni content is 5-25% by weight, preferably 10-23% by weight. Part of the nickel, e.g. 1-80%, can be replaced by cobalt.

The rest of 100% by weight consists of iron and the unavoidable accompanying elements such as carbon and / or sulfur and / or phosphorus. In addition to the alloy components mentioned and the unavoidable accompanying elements, the materials according to the invention can also contain one or more of the elements manganese, molybdenum, copper, silher, cobalt, tungsten, niobium, tantalum and nitrogen, preferably contain manganese, molybdenum, copper, silver, cobalt and nitrogen. The content of the latter elements is limited to the following% by weight: Mn 8%, Mo 3%, Cu 4%, Ag 2%, Co 20%, W 4%, Nb / Ta together 2% and N 0.2 %.

Contrary to the teaching of the prior art, the kneading, casting or welding additives according to the invention are characterized by a delta ferrite content of more than 10%. The delta ferrite content is preferably between 10 and 65% and is particularly preferably 11 to 55%. The delta ferrite content results from the ratio of chromium equivalent (alloy elements Cr, Si, Mo, W) to nickel equivalent (alloy elements Ni, Co, C, N, Mn, Cu), whereby the individual alloy elements have a different valence or Weighting comes up. This connection is basically known to the person skilled in the art.

The materials of the composition mentioned are preferably subjected to a heat treatment (solution annealing), for example at 1030-1250 ° C., before being used.

The materials of the invention are highly corrosion-resistant to over 75% H₂SO₄, preferably to 85-100%, particularly preferably to 90-100% H₂SO₄ and to 0-10% by weight oleum. This high corrosion resistance is given at high temperatures, for example at 90-350 ° C, preferably 150-340 ° C, particularly preferably at 200 ° C to the boiling point of the different concentrations trated sulfuric acids or oleum. The materials and the components made from them are often used at temperatures in the range of 180-335 ° C. The materials according to the invention can accordingly be used for components which are exposed to such hot, concentrated sulfuric acids or oleum. The materials or components can be pressurized with hot concentrated sulfuric acid or 0-10% by weight oleum at pressures from 0.1 bar to 10 bar. Such components are, for example, reaction vessels, pumps, fittings, lines, heat exchangers, etc. Such components can be manufactured by forging and rolling (kneading), by casting, by lining, by plating, by shaping welding or by cladding. Such components are used, for example, in the evaporation of sulfuric acid.

A high corrosion resistance is understood to mean a removal rate of at most 1 mm / a, in many cases at most 0.1 to 0.2 mm / a, under the difficult conditions mentioned.

The properties of the materials according to the invention are surprising, since DJ Chronister (loc. Cit.) Teaches that the delta ferrite content of iron-chromium-nickel-silicon alloys must be limited to 5 to a maximum of 10% for reasons of corrosion. In addition to the corrosion resistance found, an increased delta ferrite content has the advantage of facilitating welds, such as production welds on castings or joint welds, and also significantly improving the erosion resistance of the materials.

Examples

The materials I to XX according to the invention specified in Table 1 were produced. These materials are characterized by the properties given in Table 2. Here the yield strength R _{p0.2 means} the stress up to a non-proportional elongation of 0.2% (tensile test according to DIN 50 145), the tensile strength R _m the stress resulting from the maximum force related to the initial cross-section, the elongation at break A₅ the change in length after the sample broke based on the initial measurement length and the impact energy A _v the consumed impact energy on ISO-V samples, measured in J (impact test according to DIN 50 115).

1, 2 and 3 show the metallographic sections of the materials IV, V and VI, etched according to Murakami, with an enlargement of 50: 1, from which the structure can be seen. The etchant Murakami (10 g K₃ [Fe (CN) ₆], 10 g KOH and 100 g H₂O) makes the delta ferrite appear darker than the austenite.

Materials 1 to X were subjected to various corrosion tests over a period of 360 to 670 hours. The various corrosion tests were carried out in boiling 93.3% H₂SO₄ (297 ° C), boiling 95.3% H₂SO₄ (313 ° C), boiling 96.6% H₂SO₄ (316 ° C) and boiling 98.2% H₂SO₄ (334 ° C).

Table 3 shows the removal rates determined.

Further results from corrosion tests with materials IV to IX as well as XI and XII in sulfuric acid processing plants are shown in Table 4. Table 1 Chemical composition of the investigated materials mass fractions in% (rest 100% is Fe) material C. Si Mn Cr Ni Co Cu Mon P S Other I. 0.029 4.62 4.35 20.64 12.83 0.04 0.016 0.005 N: 0.093 II 0.031 5.57 4.27 17.40 13.16 0.02 0.04 N: 0.04 III 0.023 6.49 4.20 15.43 13.54 0.02 0.04 N: 0.031 IV 0.027 4.76 0.94 21.08 13.06 0.05 0.02 0.009 0.01 N: 0.084 V 0.024 5.09 1.0 18.50 9.30 4.0 0.04 0.011 0.008 N: 0.061 VI 0.025 5.23 1.01 22.35 8.30 9.20 0.04 0.012 0.009 VII 0.03 5.45 4.35 21.10 10.85 5.10 2.24 VIII 0.03 5.30 2.69 18.10 11.20 0.44 0.93 0.019 0.012 W: 0.42 Nb: 0.42 IX 0.03 6.45 1.03 21.10 12.30 7.10 X 0.011 5.1 0.01 19.3 11.2 11.8 2.2 1.5 XI 0.034 5.0 0.44 22.2 9.1 10.0 2.9 0.006 0.004 N: 0.08 XII 0.04 5.2 0.48 20.2 9.2 9.2 1.1 0.008 0.009 N: 0.07 XIII 0.033 5.5 0.16 21.4 9.29 9.25 1.28 XIV 0.029 5.65 0.12 15.25 4.37 10.65 XV 0.032 5.15 0.13 22.35 3.46 20.65 XVI 0.026 5.29 0.66 20.53 13.24 N: 0.034 XVII 0.017 5.0 0.08 22.9 10.8 8.4 4.26 0.004 0.006 N: 0.064 XVIII 0.027 5.18 0.08 22.4 10.9 7.1 4.28 0.004 0.005 N: 0.09 Ag: 0.13 XIX 0.022 5.16 0.05 17.7 10.9 8.0 4.19 1.62 0.004 0.006 N: 0.084 XX 0.07 5.20 0.06 20.0 13.8 11.2 2.2 2.5 0.005 <0.001 N: 0.005 Mechanical properties, ferrite content and heat treatment of the test materials material Heat treatment R _p0.2 N / mm² R _m N / mm² A₅% A _v J Delta ferrite% I. 1070 ° C / W 360 695 44.9 135 15 II 1120 ° C / W 403 778 41.0 118 19th III 1120 ° C / W 45 IV 1100 ° C / W 436 803 41 107 23 V 1100 ° C / W 466 840 40 129 26 VI 1100 ° C / W 527 848 23.7 51 36 VII 1100 ° C / W 515 770 10.0 18th 43 VIII 1100 ° C / W 53 IX 1100 ° C / W 28 X 1070 ° C / W 20th XI 1100 ° C / W 405 780 18.5 56 20th XII 1100 ° C / W 370 720 27.0 196 13 XIII 1100 ° C / W 450 815 40.2 53 15 XIV 1100 ° C / W 20th XV 1100 ° C / W 444 896 41.9 22 44 XVI 1100 ° C / W 427 782 36.0 143 28 XVII 1100 ° C / W 466 840 37.5 82 25th XVIII 1100 ° C / W 460 752 16.8 39 28 XIX 1100 ° C / W 27th XX 1100 ° C / W 16 Removal rates (mm / a) of the heat-treated materials in differently concentrated (% by weight) boiling sulfuric acids material 93.3% 95.3% 96.6% 98.2% I. 0.9 ¹⁾ 0.7 ¹⁾ 0.6 ³⁾ 0.2 II 0.6 ¹⁾ 0.4 ¹⁾ 0.25 ³⁾ 0.1 III 0.55 ¹⁾ 0.4 ¹⁾ 0.25 ³⁾ 0.1 IV 0.3 0.15 0.2 <0.1 V 0.3 0.15 0.2 0.1 VI 0.4 0.2 0.2 0.1 VII (0.9) ²⁾ (0.4) ²⁾ VII 0.45 0.3 0.25 0.1 VIII (0.85) ²⁾ (0.4) ²⁾ VIII 0.45 0.3 0.25 0.15 IX 0.45 0.3 0.15 ³⁾ X 0.4 0.35 0.27 0.15 The removal rates in brackets were determined on non-heat-treated materials. Test duration: ¹⁾ 360 h ²⁾ 410 h ³⁾ 530 h Rest: 670 h Results from operational corrosion tests in sulfuric acid concentrating plants Removal rates in mm / a Materials 96% H₂SO₄ 330 ° C / 59 d 93% H₂SO₄ 180 ° C / 231 d IV 0.12 0.07 V 0.12 0.03 VI 0.14 0.04 VII 0.1 0.08 VIII 0.13 0.11 IX 0.04 0.03 XI 0.14 XII 0.16

Claims (11)

1. Rustproof kneading and casting materials as well as welding filler materials for components made of iron-chromium-nickel-silicon alloys with 13-32% by weight Cr content, 5-25, exposed to hot concentrated sulfuric acid or 0-10% by weight oleum % Nickel content and 4-9% by weight Si content and a structure which contains more than 10% delta ferrite. 2. Materials according to claim 1 with a delta ferrite content between 10 and 65%. 3. Materials according to claim 2 with a delta ferrite content of 11-55%. 4. Materials according to claim 1, characterized in that in addition to the unavoidable accompanying elements such as carbon and / or sulfur and / or phosphorus one or more of the elements manganese, molybdenum, copper, silver, cobalt, tungsten, niobium, tantalum and nitrogen contain. 5. Materials according to claim 4, characterized in that they contain one or more of the elements manganese, molybdenum, copper, silver, cobalt and nitrogen. 6. Materials according to claim 4, characterized in that the content of the elements further contained is limited to the following wt .-%: Mn 8%, Mo 3%, Cu 4%, Ag 2%, Co 20%, W 4%, Nb / Ta together 2% and N 0.2%. 7. Materials according to claim 1, characterized in that they are subjected to a heat treatment at 1030-1250 ° C before use. 8. Use of materials according to claim 1 for components which are exposed to hot, concentrated sulfuric acid or 0-10 wt .-% oleum. 9. Use according to claim 8 for components which are exposed to over 75%, preferably with 85-100%, particularly preferably with 90-100% sulfuric acid or with 0-10% by weight oleum. 10. Use according to claim 8 for components with hot concentrated sulfuric acid or with 0-10 wt .-% oleum at temperatures of 90-350 ° C, preferably 150-340 ° C, particularly preferably 200 ° C to the boiling point differently concentrated sulfuric acids or oleum can be applied. 11. Use according to claim 8 for components which are subjected to hot concentrated sulfuric acid or 0-10% by weight oleum at pressures of 0.1 bar - 10 bar.

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