EP0334410A1 - Nickel chromium-molybdenum alloyd - Google Patents

Abstract

The use of an alloy containing 22.0 to 24.0% of chromium, 15.0 to 16.5% of molybdenum, up to 0.3% of tungsten, up to 1.5% of iron, up to 0.4% of vanadium, 0.1 to 0.4% of aluminium, 0.001 to 0.04% of magnesium and 0.001 to 0.01% of calcium, the remainder being nickel and including unavoidable accompanying elements and impurities, is proposed for the production of structural components which have very good resistance to material-removing corrosion and to pitting corrosion and crevice corrosion under very severe corrosive conditions, as prevail in present-day chemical process engineering and in current environment protection technology, such as, for example, in flue gas desulphurisation plants or plants for concentrating sulphuric acid, and which should be capable of being produced economically and without problems by hot-working and cold-working.

Description

The invention relates to the use of a NiCrMo alloy for the production of components which are subject to very corrosive conditions, such as those found in today's chemical process engineering and in current environmental protection technology, such as flue gas desulfurization plants or plants for the concentration of sulfuric acid must have good resistance to abrasive corrosion and pitting and crevice corrosion and which should be economical and easy to produce by hot and cold forming.

According to DE-AS 1 210 566 and the corresponding US-P 3 203 792 and FR-P 1 536 741, the following corrosion-resistant alloys with nickel, chromium and molybdenum have become known as main alloy components: DE-AS 1 210 566 US-P 3 203 792 FR-P 1 536 741 14-26 % Chrome 14.5-23 % Chrome 3 - 18 % Molybdenum 14-17 % Molybdenum Max. 5 % Tungsten Max. 5 % Tungsten Max. 20th % Cobalt Max. 2.5 % Cobalt Max. 0.1 % Carbon Max. 0.03 % Carbon Max. 0.2 % Silicon Max. 0.05 % Silicon Max. 3rd % Manganese Max. 1 % Manganese Max. 30th % Iron Max. 7 % Iron 40-65 % Nickel Max. 0.35 % Vanadium Rest of nickel

It is also known that such alloys can only be processed properly if they contain further reactive elements as deoxidizing agents. Such alloys could, according to information in Z. Metallkunde, Volume 53 (1962), 5.289, be flawlessly forged if they contained 0.16 to 0.71% aluminum or 0.09 to 0.11% magnesium. According to the teaching of DE-AS 1 210 566 or the corresponding US Pat. No. 3,203,792, which has the same origin, aluminum has proven to be very unfavorable as a deoxidation element, whereas additions of alkaline earth metal, i.e. Magnesium or calcium should be suitable.

Surprisingly, it has now been found that hot workability is best, ie completely crack-free, if the deoxidizing elements aluminum, magnesium and calcium are used in the following combination 0.1 to 0.4 % Aluminum 0.001 to 0.04 % Magnesium 0.001 to 0.01 % Calcium, the lower limit for magnesium and calcium can also be fallen below in the course of an electroslag remelting to lower contents. However, all three elements must be present at the same time and are not free or interchangeable optional components, for example according to the teaching of US Pat. No. 4,129,464.

An alloy of the following composition has also become known from DE-OS 31 25 301 for applications involving a wide variety of highly corrosive media: 20 to 24th % Chrome 12 to 17th % Molybdenum 2 to 4th % Tungsten less than 0.5 % Niobium less than 0.5 % Tantalum less than 0.1 % Carbon less than 0.2 % Silicon less than 0.5 % Manganese 2 to 8th % Iron less than 0.7 % Aluminum and titanium less than 0.5 % Vanadium

At the time it became known and when it was introduced to the market, this well-known alloy had the optimum combination of corrosion-resistant properties from the alloys available at the time. When testing for tasks in today's chemical process engineering and current environmental protection technology, however, it turns out that this alloy cannot meet all requirements. For example, due to the constantly increasing demands on environmental protection, it will no longer be possible to dump waste sulfuric acid, so-called thin acid, into the open sea. This waste sulfuric acid must therefore be worked up, which requires materials of particularly high resistance to hot contaminated sulfuric acid of medium concentration. On the other hand, with the increasing introduction of flue gas desulphurization in recent times, it has emerged that here too, aggressive conditions can occur which the alloys known from the prior art no longer meet with certainty. This is, among other things, a consequence of the circulation of the washing water with small amounts of discharge, so that it is too high Enrichments come especially from chloride ions. However, since the priority of environmental protection presupposes the functionality of the flue gas desulfurization systems for the operation of fossil-fired power plants, materials with a higher corrosion resistance than known from the prior art must be used here.

Another example is the current material requirements of biotechnology. Hydrochloric acid is the only mineral acid that is compatible with the human and animal body. The new material requirement is that of particularly high resistance to dilute hydrochloric acid.

It is therefore the task of specifying an alloy for the new working conditions of today's chemical process engineering and current environmental protection technology, which in terms of its corrosion properties meets the new requirements significantly better than that according to the state of the art in 1980 (Union priority to DE-OS 31 25 301) known alloy and which can be manufactured and processed economically.

Surprisingly, it has been found that this goal can be achieved if an alloy of the following composition is used: 22.0 to 24.0 % Chrome 15.0 to 16.5 % Molybdenum to 0.3 % Tungsten to 1.5 % Iron to 0.3 % Cobalt to 0.1 % Silicon to 0.5 % Manganese to 0.015 % Carbon to 0.4 % Vanadium 0.1 to 0.4 % Aluminum 0.001 to 0.04 % Magnesium 0.001 to 0.01 % Calcium Balance nickel including unavoidable impurities

As can be seen from the test results given in the attached tables 1 to 7, this alloy has a significantly better corrosion resistance under all test conditions than corresponds to the state of the art (stdT) according to DE-OS 31 25 301. The test results were obtained on four working examples Nos. 1 to 4 of the alloy according to the invention, the chemical analyzes of which are listed in Table 1. Table 1 also contains the analyzes of comparative samples No. 5 and 6 corresponding to the prior art according to DE-OS 31 25 301, which have already been produced in accordance with this invention with regard to the levels of deoxidation elements aluminum, magnesium and calcium which determine their processability were.

As a test solution for the tasks of thin acid concentration, a boiling aqueous solution with 23% H₂SO₄, 1.2% HCl, 1% FeCl₃ and 1% CuCl₂ according to ASTM G-28, method B can be used. As Table 2 makes clear, the alloy according to the invention has a 30% lower removal rate there than corresponds to the older prior art. Are not the own measurements for the prior art (comparative sample 6 in Table 2), but the specification of 0.17 mm / year in "Materials and Corrosion", Volume 37 (1986), pp. 137-145, the alloy according to the invention is even 59% above the prior art.

In a more dilute solution of sulfuric acid containing chloride ions, which is often used to determine the local corrosion resistance in the form of the critical pitting temperature under such conditions, the alloy according to the invention shown in Table 3 is essentially the same as the prior art. The critical pitting corrosion temperature determined for the prior art corresponds to the information in "Materials and Corrosion", Volume 37 (1986), pp. 137-145. The alloy according to the invention here has only a slight tendency to be superior, as the measurement results in working examples 3 and 4 show. The identical values for the local corrosion resistance in the known 10% FeCl₃ · 6H₂O solution given in the same table for the prior art and the invention are only identical because the test conditions do not allow measurements at a higher temperature. Therefore, the "greater than" sign must appear in both cases. In contrast, Table 4 shows the clear superiority of the alloy according to the invention over the prior art with regard to the susceptibility to crevice corrosion in the same solution at 85 ° C., measured with a conventional crevice block arrangement made of PTFE (cf. "Materials and Corrosion", Volume 37 ( 1986), p. 185).

With regard to the requirements that arise in flue gas desulfurization, the increased resistance of the alloy according to the invention to local corrosion, as shown in Table 4 comes, of great importance. The alloy according to the invention can thus be used where the alloy corresponding to the prior art can no longer be used due to local corrosion, for example in prewashers with particularly aggressive working conditions. Table 5 also gives the linear removal rates in typical flue gas desulfurization media. Here too, the far better suitability of the alloy according to the invention is evident, especially in the case of the dilute 2% sulfuric acid solution at high temperature (105 ° C.) and with a high chloride content, where the average removal rate is around 93% lower than that of the Alloy corresponding to the state of the art.

The higher resistance of the alloy according to the invention in dilute hydrochloric acid compared to the prior art is shown in Table 6. Accordingly, the alloy according to the invention behaves about 60% better in this reducing acid than the comparative samples corresponding to the prior art. Significant progress of around 25% is still given when the comparison shows the removal rate of 0 published elsewhere ("Materials and Corrosion", Volume 37 (1986), pp. 137-144) for the state of the art , 28 mm / year. Table 6 also gives information on the resistance in chloride-free 10% H₂SO₄ as another important reducing acid. The removal rate there is reduced by around 64% compared to the prior art and still by 50% if the comparison for the prior art is based on the information in DE-OS 31 25 301 of 0.36 mm / year. It is then astonishing that even in oxidizing media, such as the test solution according to ASTM G-28, method A, which is valid as the standard test solution for strongly oxidizing conditions, with the alloy according to the invention according to Table 7 the state of the art is clear, ie around 40 % superior corrosion resistance is observed. In the latter case, however, removal rates were measured for the prior art with an average of 0.91 mm / year higher removal rates than the 0.74 mm / year specified in DE-OS 31 25 301. But even if this lower value is taken as a basis for the prior art, there is still a considerable advance of 26% over the prior art for the alloy according to the invention.

The superior behavior of the alloy according to the invention over the prior art is particularly noteworthy because, according to the teaching of DE-OS 31 25 301, at least 2% tungsten and iron are alloyed and certain ratios of Mo / W and Fe / W have to be observed. However, tungsten is only added as an alloying element if certain goals cannot be achieved otherwise. In the case of the known alloy, it is expressly pointed out that a mutual exchange of molybdenum and tungsten is not possible, that both elements are required within the specified limits and that a Mo / W ratio of 3 to 5 must be observed. Against this background, it was not self-evident for the person skilled in the art to select an alloy for the application mentioned which only contains tungsten in amounts which are unavoidable in the case of economical production using recycle scrap, but do not impair the processability of the alloy.

Tabelle 2 Prüfbedingungen Abtragungsraten in mm/a StdT Erfindung Unterschied Lösung mit 23 % H₂SO₄, 1,2 % HCl, 1 % FeCl₃, 1 % CuCl₂, siedend (ASTM G-28, Methode B) Beispiel No. 6 1 2 3 4 Einzelwerte 0,10 0,06 0,07 0,08 0,06 Mittelwert 0,10 0,07 30 % Tabelle 3 Prüfbedingungen kritische Lochkorrosionstemperatur StdT Erfindung Lösung mit 7 % H₂SO₄,3 % HCl, 1 % FeCl₃ und 1 % CuCl₂ über 24 h Beispiel No. 5 6 1 2 3 4 Einzelwerte 120 120 120 120 120 >120 >120 10 %ige FeCl₃·6 H₂O Lösung über 72 h Beispiel No. 5 6 1 2 3 4 Einzelwerte 85°C >85 Tabelle 4 Prüfbedingungen Anfälligkeit gegen Spaltkorrosion* StdT Erfindung Unterschied 10 %ige FeCl₃·6 H₂O-Lösung über 72 h bei 85°C Beispiel No. 5 6 1 2 3 4 Einzelwerte 0,77 0,75 0,23 0,08 0,06 0 Mittelwert 0,76 0,09 88 % *Zahl der Spalten mit Korrosionsangriff dividiert durch die Gesamtzahl der Spalten (48) Tabelle 5 Prüfbedingungen Abtragungsraten in mm/a StdT Erfindung Unterschied 60 %ige Schwefelsäure mit 15 g/l, Cl⁻, 80°C, 14 Tage Beispiel No. 5 6 1 2 3 4 Einzelwerte 0,32 0,30 0,25 0,28 0,26 0,27 Mittelwert 0,31 0,27 13 % 2 %ige Schwefelsäure + 70.000 ppm Cl⁻, 105°C, 21 Tage Beispiel No. 5 6 1 2 3 4 Einzelwerte 0,08 0,04 0,004 0,012 0 0 Mittelwert 0,06 0,004 93 % Tabelle 6 Prüfbedingungen Abtragungsraten in mm/a StdT Erfindung Unterschied 1,5 % HCl, siedend, 14 Tage Beispiel No. 5 6 1 2 3 4 Einzelwerte 0,59 0,47 0,22 0,25 0,22 0,16 Mittelwert 0,52 0,21 60 % 10 % H₂SO₄, siedend, 14 Tage Beispiel No. 5 6 1 2 3 4 Einzelwerte 0,52 0,31 0,17 0,14 0,18 0,12 Mittelwert 0,42 0,15 64 % Tabelle 7 Prüfbedingungen Abtragungsraten in mm/a StdT Erfindung Unterschied Lösung mit 50 % H₂SO₄ und 42 g/l, Fe₂ (SO₄)₃, siedend, 120 h (ASTM G-28,Methode A) Beispiel No. 5 6 1 2 3 4 Einzelwerte 0,93 0,88 0,54 0,51 0,61 0,53 Mittelwert 0,91 0,55 40 % The alloy is preferably used under the conditions specified in the subclaims. The skilled person will be able to tap into other possible applications due to the corrosion resistance demonstrated in Tables 2 to 7.

Table 2 Test conditions Removal rates in mm / a StdT invention difference Solution with 23% H₂SO₄, 1.2% HCl, 1% FeCl₃, 1% CuCl₂, boiling (ASTM G-28, method B) Example No. 6 1 2nd 3rd 4th Individual values 0.10 0.06 0.07 0.08 0.06 Average 0.10 0.07 30% Test conditions critical pitting temperature StdT invention Solution with 7% H₂SO₄, 3% HCl, 1% FeCl₃ and 1% CuCl₂ over 24 h Example No. 5 6 1 2nd 3rd 4th Individual values 120 120 120 120 120 > 120 > 120 10% FeCl₃ · 6 H₂O solution over 72 h Example No. 5 6 1 2nd 3rd 4th Individual values 85 ° C > 85 Test conditions Susceptibility to crevice corrosion * StdT invention difference 10% FeCl₃ · 6 H₂O solution over 72 h at 85 ° C Example No. 5 6 1 2nd 3rd 4th Individual values 0.77 0.75 0.23 0.08 0.06 0 Average 0.76 0.09 88% * Number of gaps with corrosion attack divided by the total number of gaps (48) Test conditions Removal rates in mm / a StdT invention difference 60% sulfuric acid with 15 g / l, Cl⁻, 80 ° C, 14 days Example No. 5 6 1 2nd 3rd 4th Individual values 0.32 0.30 0.25 0.28 0.26 0.27 Average 0.31 0.27 13% 2% sulfuric acid + 70,000 ppm Cl⁻, 105 ° C, 21 days Example No. 5 6 1 2nd 3rd 4th Individual values 0.08 0.04 0.004 0.012 0 0 Average 0.06 0.004 93% Test conditions Removal rates in mm / a StdT invention difference 1.5% HCl, boiling, 14 days Example No. 5 6 1 2nd 3rd 4th Individual values 0.59 0.47 0.22 0.25 0.22 0.16 Average 0.52 0.21 60% 10% H₂SO₄, boiling, 14 days Example No. 5 6 1 2nd 3rd 4th Individual values 0.52 0.31 0.17 0.14 0.18 0.12 Average 0.42 0.15 64% Test conditions Removal rates in mm / a StdT invention difference Solution with 50% H₂SO₄ and 42 g / l, Fe₂ (SO₄) ₃, boiling, 120 h (ASTM G-28, method A) Example No. 5 6 1 2nd 3rd 4th Individual values 0.93 0.88 0.54 0.51 0.61 0.53 Average 0.91 0.55 40%

Claims (10)

1. Nickel-chrome-molybdenum alloy with 22.0 to 24.0 % Chrome 15.0 to 16.5 % Molybdenum to 0.3 % Tungsten to 1.5 % Iron to 0.3 % Cobalt to 0.1 % Silicon to 0.5 % Manganese to 0.015 % Carbon to 0.4 % Vanadium 0.1 to 0.4 % Aluminum 0.001 to 0.04 % Magnesium 0.001 to 0.01 % Calcium Balance nickel including unavoidable impurities for the production of components that have to have a very good resistance to abrasive corrosion as well as pitting and crevice corrosion under hot, highly corrosive conditions of today's chemical process engineering and environmental protection technology, such as those prevailing in flue gas desulfurization plants or plants for the concentration of sulfuric acid, and which must be warm - And cold forming should be easy to manufacture. 2. Use of an alloy according to claim 1 for the production of components in medium to medium-high concentration in hot sulfuric acid containing chloride ions, as u.a. occurs in flue gas desulfurization plants (for example 60% sulfuric acid with a chloride ion concentration of 15 g / l at an application temperature of 80 ° C), must have significantly lower removal rates than the alloy known from DE-OS 31 25 301. 3. Use of an alloy according to claim 1 for the production of components which in dilute sulfuric acid high chloride ion concentration, as u.a. occurs in flue gas desulfurization plants (for example 2% sulfuric acid with a chloride ion concentration of 70 g / l at 105 ° C) must have a very low removal rate. 4. Use of an alloy according to claim 1 for the production of components which, in hot chloride ion-containing sulfuric acid, have a low to medium concentration in the presence of strongly oxidizing additives, as occurs in plants for sulfuric acid concentration, and with the test medium 23% H₂SO₄, 1.2 % HCl, 1% FeCl₃, 1% CuCl₂, can be simulated boiling, must have a high corrosion resistance. 5. Use of an alloy according to claim 1 for the production of components which must have a high corrosion resistance in sulfuric acid solutions under conditions of concentration of so-called thin acid, for example in the test according to ASTM G-28, method B. 6. Use of an alloy according to claim 1 for the production of components which must have critical pitting temperatures of at least 120 ° C in a solution of 7% H₂SO₄, 3% HCl, 1% FeCl₃ and 1% CuCl₂ at a test duration of 24 h. 7. Use of an alloy according to claim 1 for the production of components which in 10% FeCl₃ · 6H₂O solution at 72 h test duration a critical pitting temperature of over 85 ° C and in contrast to the prior art at 85 ° C only a small to have a negligible tendency to crevice corrosion. 8. Use of an alloy according to claim 1 for the production of components which must be very corrosion-resistant in very corrosive reducing hot acid solutions, for example in boiling 1.5% HGl solution, a comparatively lower removal rate of on average 0.21 mm / Show year, ie considerably less than the alloy known from DE-OS 31 25 301 and described in this respect in "Materials and Corrosion", Volume 37 (1986), pp. 137-145. 9. Use of an alloy according to claim 1 for the production of components which show a good resistance in very corrosive reducing acidic solutions, such as in boiling 10% H₂SO₄ solution, with a removal rate of on average around 0.15 mm / Year, ie considerably less than the alloy known from DE-PS 31 25 301. 10. Use of an alloy according to claim 1 for the production of components which may have only a slight corrosion removal under oxidizing acidic conditions, for example in a boiling aqueous solution with 50% H₂SO₄ and 42 g / l Fe₂ (SO₄) ₃ (so-called string test according to ASTM G-28, method A) have a significantly lower removal rate than the alloy known from DE-OS 31 25 301.