Palladium-containing austenitic steel for use in contact with concentrated sulfuric acid at high temperatures

Abstract: An austenitic stainless steel for use for high temperature concentrated sulfuric acid which comprises, on weight basis 0.04% or less of C, 5 - 7% of Si, 2% or less of Mn, 15 - 25% of Cr, 4 - 24% of Ni, 0.01 - 1.07% of Pd and the rest consisting of Fe and unavoidable contaminant materials.

Description: The present invention relates to an austenitic stainless steel superior not only in its workability but also in the corrosion resistance for use in, for example, absorption towers, cooling towers, pumps, vessels and the like, which are to be employed in an environment of high temperature concentrated sulfuric acid in the sulfuric acid industry, and in particular, for dealing with sulfuric acid of a concentration of 90-100% at a temperature of up to 240°C. Sulfuric acid has in general a high corrosive effect on metals. Such attack of metals by sulfuric acid is quite considerable especially at medium concentrations of sulfuric acid from about 10 to about 80%. This is attributed mainly to the fact that such medium concentration sulfuric acid is a non-oxidative acid. Existing materials capable of withstanding such a sulfuric acid environment are quite limited and may be exemplified, for use at temperatures below 100°C, by lead and some nickel alloys, such as Hastelloy B and C276 (trade names). It is known, on the other hand, that sulfuric acid becomes oxidative when it is concentrated up to 90% or higher. Some metals which do not withstand a medium concentration of sulfuric acid may nevertheless tolerate such highly concentrated sulfuric acid. For example, at lower temperatures, mild steel has a better corrosion resistance to a highly concentrated sulfuric acid of 98% due to the formation of an anti-corrosive protective layer of FeSO₄ over the entire surface of the steel, so that it finds practical use for such highly concentrated sulfuric acid at room temperature (at around 20°C). At higher temperatures up to 240°C which are encountered in the sulfuric acid industry, the attacking action of sulfuric acid becomes violent. At such a high temperature, the protective FeSO₄ coating layer of mild steel will tend to dissolve in the highly concentrated sulfuric acid thereby destroying the anti-corrosive passive layer, resulting in loss of the corrosion resistance of mild steel. Usual austenitic steels, various ferritic steels and nickel alloys exhibit poor corrosion resistance in such highly concentrated high temperature sulfuric acid and even lead and Ni-alloys, such as Hastelloy B and C-276 (trade names), which exhibit relatively high corrosion resistance to a medium concentration of sulfuric acid become less resistant at high temperatures to highly concentrated sulfuric acid. To date, no material has been found which has sufficient corrosion resistance in such an environment and which may be utilized in various installations and instruments in the sulfuric acid industry. However it is known that high silicon (Si) cast iron (containing more than 14% of Si) exhibits a relatively superior corrosion resistance to high concentration sulfuric acid at lower temperatures (below about 120°C). It has been assumed that Si contributes to the development of the anti-corrosive property. It has recently been reported that ferritic stainless steels having a high content of chromium (Cr) also exhibit relatively better corrosion resistance in such an environment. This suggests that Cr may contribute to the development of effective corrosion resistance and that the content of nickel (Ni), which is assumed to have a negative effect on the development of anti-corrosive property, is low. However, these steels have poor mechanical workability and, in particular, high Si cast iron is scarcely able to be subjected to mechanical working and welding, so that it finds no practical use for large sized installations and instruments. Thus, in practice, large sized installations to be employed in an environment of highly concentrated sulfuric acid of above 90% at a temperature of up to 120°C, such as absorptions towers and so on, have hitherto been lined internally with acid-resistant bricks. Such internal linings suffer from the following problems: The binder material employed to fill up the interstices between the adjoining acid-resistant bricks become damaged in the course of long-term operation by the highly concentrated sulfuric acid, which may result in leakage of sulfuric acid, so that it is necessary to overhaul the entire installation every few years. Such damage to the binding material is markedly accelerated under the conditions with which the present invention deals, namely, sulfuric acid of a concentration of above 90% and a temperature of up to 240°C, and the durability of the brick itself will also be reduced. Also, high Cr ferritic stainless steels which have relatively better corrosion resistance as compared with other materials will suffer from corrosion attack under the condition mentioned above and will be subject to a corrosion rate exceeding the critical allowable value of 0.1 g/cm² hr for practical use. This is because in order to maintain a tolerable workability, the content of Cr is not allowed to reach the amount necessary, namely, over 35%, for attaining sufficient corrosion resistance under the condition mentioned above. When the content of Cr is increased, the resulting high Cr ferritic stainless steel becomes brittle and mechanical working, such as pressing and rolling, becomes difficult. In order to weld such a high Cr ferritic stainless steel, additional technical measures, such as preheating, after-heating, and so on, are necessary for avoiding the hardening of the material around the welded portion, resulting in a considerable increase in the costs of manufacturing and overhauling such installations, as compared with materials composed of austenitic stainless steels. As for high Si cast iron, the problem that effective mechanical working and welding is not really possible due to the brittleness of the high Si cast iron is left unsolved. In view of the aforementioned circumstances, it is an object of the present invention to provide a novel austenitic stainless steel which overcomes the disadvantage of poor corrosion resistance associated with the conventional material in the environment of highly concentrated high temperature sulfuric acid whilst permitting effective welding and mechanical working. Thus, the present invention provides an austenitic stainless steel containing a small amount of palladium and exhibiting a markedly increased corrosion resistance under the environment of highly concentrated high temperature sulfuric acid, which comprises, on weight basis, 0.04% or less of carbon (C), 5-7% of silicon (Si), 2% or less of manganese (Mn), 15-25% of chromium (Cr), 4-24% of nickel (Ni), 0.01-1.07% of palladium (Pd) and the rest consisting of iron (Fe) an unavoidable contaminant materials. The essential characteristic feature of the austenitic steel according to the present invention resides in that it comprises three basal elements of Cr, Ni and Si with the addition of a small but suitable amount of Pd for attaining a considerably increased corrosion resistance under the environment of highly concentrated high temperature sulfuric acid. In the following, the functions and effects of each component element of the alloy steel according to the present invention will be described with reference to the appended drawings by way of concrete embodiments. In the accompanying drawings: Fig. 1 is a graph showing the relationship between the Si content of steel and the corrosion rate of the steel in highly concentrated high temperature sulfuric acid. Fig. 2 shows the comparison of temperature dependence of the corrosion rate between the steel according to the present invention and conventional steels. Fig. 3 is a graph showing the relationship between the Pd content and the corrosion rate for the steel according to the present invention. Fig. 4 is a graph showing the comparison of corrosion resistance and mechanical workability between the steel according to the present invention and conventional steels. The experimental results and the composition of the steel for each of Examples 1 to 10 and Comparatives Examples 11 to 22 are summarized in Table 1. Each essential component element of the steel according to the present invention has been selected based upon the knowledge and consideration from the experiments as hereinafter described. It has been known that high Si cast iron has a relatively better corrosion resistance to highly concentrated (90-100%) sulfuric acid at higher temperatures (100-120°C). This suggests that Si has a certain effect on improving the corrosion resistance of a steel to such a sulfuric acid environment. It is also known that increase in the content of Cr in a stainless steel will impart to the steel and improved corrosion resistance to such sulfuric acid environment. However, in order to maintain the austenite phase which provides a better mechanical workability in an austenitic stainless steel, it is necessary to increase the Ni content in correspondence to an increase in the total content of the ferrite-forming elements, namely, Cr + Si. It is necessary and preferred to limit the content of Ni in the stainless steel according to the present invention to the minimum amount necessary for maintaining the austenite phase, since it is known that the content of Ni has a negative effect on a stainless steel in attaining corrosion resistance to an environment of highly concentrated high temperature sulfuric acid. Supported by such knowledge, the inventors made an investigation into the possibility of improving the corrosion resistance of an austenitic stainless steel in such an environment of highly concentrated high temperature sulfuric acid by increasing the content of Si in a basal austenitic stainless steel whilst preserving the austenite phase to which is attributed better weldability and higher workability of the steel. Consideration of Schaeffler's phase diagram (a diagram showing the relationship between the metal structure and equivalent proportion of each component alloy element), it was confirmed experimentally that increased content of Si in the austenitic basal alloy steel will bring about an improvement in the corrosion resistance of the basal austenitic steel against the environment of highly concentrated high temperature sulfuric acid, as shown in Fig. 1. It is seen from Fig. 1 that the anti-corrosive property of the basal austenitic steel is improved remarkably by the content of Si in an amount over 5%. However, an excessive content of Si in the steel brings about a considerable increase in the hardness of the steel and, when the Si content exceeds about 7%, the increase in the hardness goes beyond the permissible limit for allowing rolling work. Thus, the upper limit of Si content in an austenitic stainless steel for preserving permissible workability may be assumed to be at about 7%. While, as confirmed experimentally, a better anti-corrosive property is imparted to an austenitic stainless steel by adding Si, the Si content is preferably low enough to allow better mechanical working, such as rolling, pressing and so on. The inventors therefore looked for a measure that enabled a sufficient mechanical workability to be maintained whilst enabling enough corrosion resistance against said sulfuric acid environment to be achieved in a basal austenitic stainless steel having such a low Si content and have found that the addition of a small amount of palladium (Pd) to such a basal austenitic stainless steel provides the practical solution therefor. Thus, as shown in Fig. 2, it was discovered that the addition of a small amount of Pd to the basal austenitic stainless steel brings about a remarkable improvement in the anti-corrosive property of the basal austenitic steel under the environment of highly concentrated high temperature sulfuric acid. According to a further study carried out by the inventors, it was confirmed, as shown in Fig.3, that, with a fixed Si content of 5.5%, the maximum anti-corrosive effect was attained when the Pd content was in the range from 0.2 to 0.6%. Furthermore, it was shown, as seen in Table 1, that a better anti-corrosive property was attained at a Si content of 6.61%, even when the content of Pd amounted to only 0.01%. As to the content of carbon (C): While C has a negative effect on the anti-corrosive property of the basal austenitic steel, it has a positive effect on the development of the strength of the steel and some content thereof should preferably be present. Since the anti-corrosive property deteriorates marrkedly when the carbon content exceeds 0.04%, the pertinent content of C is preferably in the range from 0.004 to 0.04%. As to the content of silicon (Si): Si constitutes one of the essential elements of the basal austenitic stainless steel of the present invention and has a fundamental contribution to the development of not only the anti-corrosive property but also the anti-oxidative nature of the steel. The anti-corrosive property of the basal austenitic steel is improved remarkably by an Si content of above 5%. An increase in the Si content also results in an improvement in the anti-corrosive property. However, a Si content over 7% may cause a deterioration of mechanical workability. Therefore, the pertinent content of Si may be in the range from 5 to 7%. As to the content of manganese (Mn): Manganese serves as a deoxidizer and is employed in an amount below 2% of the alloy from the point of view of the anti-corrosive property of the steel. In the Examples, it was incorporated in the steel in an amount in the range from 0.49 to 0.60%. As to the content of chromium (Cr): Chromium constitutes one of the essential tertiary elements of the basal austenitic stainless steel according to the present invention. It is necessary, in general, to choose a content of chromium of at least 15%, in order to attain a sufficient anti-corrosive property according to the present invention under the environment of highly concentrated high temperature sulfuric acid. While the anti-corrosive property of the steel improves with increasing the content of chromium, a corresponding increase in the content of Ni becomes necessary for the maintaining the austenite phase of the steel and such an increase may counteract the development of anti-corrosive property due to the debasement of the corrosion resistance by the higher Ni content. Furthermore, when the content of Cr exceeds 25%, forging becomes difficult. Thus, the pertinent content of Cr should be in the range from 15 to 25%. As to the content of nickel (Ni): Ni is necessary for maintaining the austenite phase and should be present in an amount in the range from 4 to 24% As to the content of palladium (Pd): Palladium constitutes one of the essential elements of the austenitic stainless steel according to the present invention, though it is employed in a small amount. It provides a remarkable improvement of the corrosion resistance against the environment of highly concentrated high temperature sulfuric acid. The effect of improvement of the corrosion resistance at 220°C is attainable at a Pd content of at least 0.01% and such effect increases as the content of Pd becomes higher. However, at temperatures below 180°C, a Pd content over 1.07% becomes increasingly counter productive as the temperature decreases and is, in any event, uneconomical. Thus, the pertinent content of Pd is in the range of 0.01 to 1.07 per cent. As to the unavoidable contaminant materials: They include inter alia phosphorus (P), sulfur (S) and oxygen (O). The content of phosphorus (P) should preferably be as little as possible to maintain the anti-corrosive property and maintain hot workability. If it exceeds 0.03%, the hot workability deteriorates. Sulfur (S) has, like phosphorus, also a large effect on the mechanical workability of the steel and preferably should not be present in an amount higher than 0.014%. The content of oxygen should also be as little as possible in the steel for reasons similar to that for P and S and the content thereof should preferably be lower than 50 ppm. It is preferable that the sum of the contents of S and O does not exceed 150 ppm. Table 1 sets out the composition and experimental data for austenitic stainless steels according to the present invention exhibiting a higher anti-corrosive property together with a better mechanical workability (Examples 1-10) and those of conventional anti-corrosive steels (Comparison Examples 11-20) The experimental data given in Table 1 are plotted in the graph of Fig.4 for easy comparison between the steel according to the present invention (indicated by blacked out circle) and the conventional steel (indicated by whited out circle). The workability index used in Fig.4, -R is defined as follows: -R = - [ (equivalent of Cr) minus (equivalent of Ni) ] in which the equivalent of Cr is calculated by Cr + Mo + 1.5 Si and the equivalent of Ni is calculated by Ni + 0.5 Mn The value of R, namely (eq. of Cr)-(eq. of Ni) is an index for the degree of ease of mechanical working. In general, this value is greater for less workable materials having higher Cr content (for example, the materials SUS 447 J and EB26-1 as given in Fig.4) and it falls in the range from 7 to 20 for materials exhibiting a relatively better workability and supplied in the market in large amounts (for example, the materials SUS 316L, SUS 304L and so on as given in Fig.4). In the Comparative Examples, conventional steels widely produced and with good production records have been selected for the comparison. The values of R for Inconel 625 and C 276 are given only by numbers in the graph of Fig.4, since the values are too large and cannot be plotted on the proper position in the graph. The variation of the hot workability and the anti-corrosive property due to the variation of the alloy composition was investigated for alloy steels according to the present invention (Examples 1 to 10) and for alloy steels of the prior art (Comparison Examples 11 to 22). The alloy steels according to the present invention were prepared in such a manner that the metal component are melted in a vacuum arc smelting furnace and the resulting metal ingot is subjected to a surface treatment work before it is hot rolled under a condition normally used for a stainless steel, whereupon the resulting hot rolled strip is subjected to a solid solution treatment. Each specimen of the alloy steels was examined by a corrosion test in which the specimen was immersed in a 90% conc. sulfuric acid at a temperature in the range of, in most cases, 100-200°C for 24 hours and the weight loss due to the corrosion was determined by accurately weighing the specimen before and after the immersion. For the workability of the steels, the values of the workability index explained above were calculated only because such an index is convenient. As explained above, the calculation was based on the equation: -R = - [ (equivalent of Cr) minus (equivalent of Ni) ] in which the equivalent of Cr is calculated by Cr + Mo + 1.5 Si and the equivalent of Ni is calculated by Ni + 0.5 Mn From the data given in Table 1, it is clear that the austenitic stainless steels according to the present invention having a Pd content of 0.5% (Examples 2, 3 and 4) are superior in their corrosion resistance to a highly concentrated sulfuric acid as compared with a prior art steel having a similar composition without a Pd content (Comparative Example 17). It is seen further that the corrosion resistance of the steels according to the present invention having a Pd content of 0.5% (Examples 2, 3 and 4) is superior at a temperature of 180°C than that of the steels according to the present invention having a Pd content of 1.07% (Examples 5 and 6). It is seen moreover, that the workability of the steels according to the present invention may be comparable to that of the conventional steel for use in the environment of sulfuric acid employed practically and most frequently (Comparative Example 11). As described in detail above, an austenitic stainless steel for use in an environment of highly concentrated high temperature sulfuric acid which exhibits superior anti-corrosive property together with better workability and which is based upon a basal alloy steel containing the three elements of chromium, nickel and silicon with addition of a small amount of palladium can be provided by the present invention. The austenitic stainless steel according to the present invention offers a wider applicability in the sulfuric acid industry due to its superior corrosion resistance even at higher temperatures together with its better workability.

Claims: An austenitic stainless steel for use for high temperature concentrated sulfuric acid, comprising, on a weight basis, 0.04% or less of carbon, 5-7% of silicon, 2% or less of manganese, 15-25% of chromium, 4-24% of nickel, 0.01-1.07% of palladium and the rest consisting of iron and unavoidable contaminant materials. An austenitic stainless steel as claimed in claim 1, characterised in that the amount of phosphorus present as an unavoidable contaminant is not more than 0.03% by weight. An austenitic stainless as claimed in claim 1 or 2, characterised in that the amount of sulfur present as an unavoidable contaminant is not more than 0.014% by weight. An austenitic stainless steel as claimed in any of claims 1 to 3, characterised in that the amount of oxygen present as an unavoidable contaminant is less than 50 ppm. An austenitic stainless steel as claimed in claim 1 or 2, characterised in that the sum of the contents of sulfur and oxygen present as unavoidable contaminant materials is not greater than 150 ppm. An austenitic stainless steel as claimed in any of claims 1 to 5, in which the content of carbon is from 0.004 to 0.04% by weight.

Inventors: EBARA RYUICHIRO C O HIROSHIMA, NAKAMOTO HIDEO C O HIROSHIMA T, NISHIMURA YASUO C O MITSUBISHI, UKAWA NAOHIKO C O HIROSHIMA TE, YAMADA TAMOTSU C O HIROSHIMA T, EBARA, RYUICHIRO, C/O HIROSHIMA TECHNICAL INST., NAKAMOTO, HIDEO, C/O HIROSHIMA TECHNICAL INST., NISHIMURA, YASUO, C/O MITSUBISHI JUKOGYO K.K., UKAWA, NAOHIKO, C/O HIROSHIMA TECHNICAL INST., YAMADA, TAMOTSU, C/O HIROSHIMA TECHNICAL INST.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1991
Main classification: C22C38
IPC codes: C22C38