Chemical and Petroleum Engineering, Vol. 47, Nos. 9–10, January, 2012 (Russian Original Nos. 9–10, Sept.–Oct., 2011)
MATERIALS SCIENCE AND CORROSION PROTECTION USE OF CORROSION-RESISTANT STEELS AND ALLOYS IN SULFURIC ACID MEDIA
A. S. Zholud, A. S. Derbyshev, and Yu. N. Dulepov
Within the chemical industry, and also in some other branches, there is extensive use of sulfuric acid solutions of different concentration and corrosiveness, containing metal ions of varied valency (Cu2+, Fe3+, Ni2+, Cr3+, etc.), which appear in a medium, for example, as a result of equipment corrosion, etc. In sulfuric acid media, corrosion can be uniform or local, i.e., pitting, spot corrosion, intercrystalline corrosion (ICC). Whereas uniform corrosion develops as a gradual reduction in the thickness of an original vessel, equipment, and machine component elements, whose corrosion rate may be calculated previously from existing data for the corrosion resistance of structural materials in specific production media, it is almost impossible to predict ICC development, and therefore in many cases it leads to sudden structural breakdown. In order to predict corrosion, it is recommended to use for article manufacture corrosionresistant steels and alloys with alloying providing in sulfuric acid solutions resistance to uniform and local corrosion with the required heat treatment and monitoring for ICC resistance.
In the chemical, and also other branches of industry, there is extensive use of sulfuric acid solutions of different concentration and corrosiveness, containing additions of both a reducing and an oxidizing nature. The corrosiveness of sulfuric acid solutions is affected by metal ions of varied valency (Cu2+, Fe3+, Ni2+, Cr3+, etc.), which appear in a medium, for example, as a result of equipment corrosion, dissolution of rocks, etc. In addition in nonferrous metallurgy units there is production of sulfuric acid with the use of waste sulfurous gases as feedstock, formed during metallurgical melting of sulfide concentrates of different metals (copper, nickel, zinc, etc.). The corrosive activity of sulfuric acid with respect to materials depends mainly on its concentration, temperature, and presence of impurities. Sulfuric acid solutions of even low concentration cause corrosion of the majority of corrosionresistant materials. An increase in acid temperature and concentration in solution leads to intensification of the corrosion of equipment materials. A feature of sulfuric acid as a corrosive medium is the fact that it may develop, depending on concentration and temperature, both oxidizing and acid properties. Schematically the nature of the oxidation-reduction equilibrium in sulfuric acid may be presented as follows. Sulfuric acid, as a dibasic acid, may dissociate by equilibrium reactions: H2SO4 ↔ H+ + HSO4– ; H2SO4 ↔ H+ + SO42–.
Sverdlovskii Research and Design Institute of Chemical Machine Building (SverdNIIkhimmash), Ekaterinberg, Russia. Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 9, p. 37–39, September, 2011. 0009-2355/12/0910-0627 ©2012 Springer Science+Business Media, Inc.
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Fig. 1. Through pitting in welded joint metal of the lower ring of a scrubber body of an IN-50.4 unit after six months operation.
Fig. 2. Surface of welded specimens after corrosion testing in the section for cleaning industrial discharges from the sulfuric acid workshop lasting 660 h at 27–32°C: a) carbon steel St. 3 sp. (uniform corrosion); b) steel 08Kh18N10 (pitting 0.1–0.2 mm deep); c) steel 04Kh18N10 (pitting 0.2–0.5 mm deep); d) steel 03Kh17N14M3 (pitting <0.1 mm deep); e) steel 06KhN28MDT (EI943) (pitting 0.1 mm deep); ƒ) steel 03KhN28MDT (EP516) (pitting <0.1 mm deep).
In dilute solutions, dissociation proceeds more completely, which provides the maximum content of hydrogen ions in solutions, and sulfuric acid develops acid properties. With an increase in acid concentration and temperature of a solution, there is an increase in the content of SO42– ions, facilitating their reduction, and sulfuric acid starts to develop oxidizing properties. Presence within sulfuric acid solutions of chlorides, fluorides, or metal ions of varied valency (Fe3+, Cu2+, etc.) may facilitate an increase in the corrosion of steels and alloys. In sulfuric acid media, uniform or local corrosion may be observed. Whereas uniform corrosion develops as a gradual reduction in the thickness of the original vessel, equipment, and machine component elements, whose corrosion rate may be calculated previously from existing data for the corrosion resistance of structural materials in specific production media, it is almost impossible to predict the development of local corrosion, and therefore in many cases it leads to sudden structural breakdown. With local corrosion failure of metals and alloys, it occurs in individual areas, whereas the rest of the surface is almost undamaged. Corrosion damage differs in nature: crevice corrosion, contact corrosion, selective corrosion, spot corrosion, point corrosion (pitting); intercrystalline corrosion, and stress corrosion cracking. Point and intercrystalline corrosion are often encountered in sulfuric acid media and lead to breakdown of equipment and pipelines. Point corrosion is one of the critical forms of corrosion failure, which arises in solutions with oxidizing agents and activation ions (Cl–, Br–, etc.). Here a considerable part of the surface is in a passive condition, and in individual areas deep damage, pitting, or spots develop (Fig. 1), which may disrupt equipment sealing [2]. 628
Fig. 3. ICC (knife-line corrosion) of an 80 mm diameter shaft made of steel 10Kh17N13M2T in the area of a weld heat-affected zone of a shaft flange after operating for two weeks.
The external appearance of steel and alloy specimens after testing in real production solutions of a sulfuric acid workshop is shown in Fig. 2. Intercrystalline corrosion (ICC) is one of the most critical forms of local corrosion for steels and alloys, causing selective breakdown along grain boundaries, as a result which there is loss of alloy strength and ductility and premature failure of a structure. ICC is observed for many corrosion-resistant steels and alloys. The tendency of a steel or alloy towards ICC is due to electrochemical inhomogeneity of the structure, which as a rule is a consequence of separation of chromium carbide at grain boundaries during heat treatment or other forms of steel and alloy heating, for example during welding. As a result of separation of carbon from grains and formation of grain boundary chromium carbides there is impoverishment of boundary areas in chromium. With a chromium content of less than 12.5% in areas adjacent to boundaries, these areas are not passivated and their corrosion resistance is sharply reduced [3]. A variety of ICC is knife-line corrosion, which arises within a very narrow zone, normally at a welded joint – basic metal boundary. It may rise with welding of steels and alloys even those with stabilized with titanium and niobium. This is an exceptionally critical form of corrosion failure, since it may develop at a very considerable rate into the depth. This phenomenon is explained by impoverishment of grain boundaries in chromium, which occurs as a result of steel heating during welding. In a narrow welded area of metal, it is heated to 1300°C and above. At this temperature, titanium (or niobium) carbide is dissolved and as a consequence of rapid cooling it does not manage to precipitate again. With repeated heating in the temperature range 600–800°C due to a higher chromium concentration in solid solution in a precipitate there may be a drop in chromium carbide, and not titanium (or niobium), and this causes a reduction in chromium concentration at grain boundaries. Knife-line corrosion (Fig. 3) is mainly observed in multilayer joints as a result of heating to high temperature, close to the solidus, and it appears during welding of the first joint, subjected to further repeated heating. For reliable equipment working, operating within sulfuric acid media, it is necessary depending on component composition and concentration, and medium parameters, to select correctly a corrosion-resistant steel or alloy, providing not only low uniform corrosion, but exhibiting high resistance to local corrosion, i.e., pitting, ICC, and knife-line corrosion. Depending on the corrosiveness of a sulfuric acid medium and temperature various steels and alloys are used for equipment and pipeline manufacture. For equipment manufacture, only steels and alloys are required that have been monitored for resistance to ICC in sulfuric acid solutions according to GOST 6032–2003 and RD24.200.15–90. For equipment operating in solutions 629
with a sulfuric acid concentration up to 10–25%, chromium-nickel-molybdenum austenitic steels 10Kh17N13M2T, 03Kh17N14M3, and austenitic-ferritic steels 08Kh21N6M2T (EP54), 08Kh22N6T (EP53), 03Kh24N6AM3, etc., are used. With high concentrations (above 80%), sulfuric acid develops oxidizing properties and at elevated temperature it is a very corrosive medium with respect to the majority of structural materials. Therefore, for equipment operating in sulfuric acid of high concentration at elevated temperature special high-alloy materials based on iron-chromium-nickel, i.e., 06KhN28MDT (EI943) and 03KhN28MDT (EP516), are used. An increase in corrosion resistance for steels and alloys in sulfuric acid solutions is promoted by an increase of the content of nickel and chromium within them, and also additional alloying for alloys 06KhN28MDT (EI943) and 03KhN28MDT (EP516) with molybdenum and copper. These alloys have similar resistance to uniform corrosion in sulfuric acid solutions and high resistance to intercrystalline corrosion for alloy 03KhN28MDT (EP516) as a result of a reduced carbon content (≤0.03%). Alloy 06KhN28MDT (EI943) exhibits high corrosion resistance in 10–45% sulfuric acid solutions at up to 80°C. The rate of corrosion penetration for alloy under these conditions varies from 0.00003 to 0.88 mm/yr. At above 80°C with a sulfuric acid concentration more than 50% this alloy may be prone to intercrystalline corrosion. For branches of industry where working sulfuric acid media have high corrosiveness, including at elevated temperature, within which corrosion-resistant steels and alloys based on iron-chromium-nickel are inadequately stable, within Russia and abroad there has been development and introduction of a large group of alloys based on nickel. Contemporary high-alloy weldable, structurally-stable, corrosion-resistant alloys based on nickel include: 1) nickel-molybdenum alloys grades N65M-VI (EP982-VI), N70MFV-VI (EP814A-VI), Hastelloy B-2, Nimofen S6928, having exceptionally high resistance in media of a non-oxidizing nature, i.e., in sulfuric, hydrochloric, phosphoric acids, etc.; 2) nickel-chromium-molybdenum alloys of grades KhN63MB (EP758U), KhN65MVU (EP760), Hastelloy C-276, Hastelloy C-22, Nicrofen 5923hMo, exhibiting high corrosion resistance in a wide range of highly corrosive media of an oxidizing and reducing nature, in highly concentrated solutions of sulfuric, phosphoric, acetic, and formic acids, contaminated with ions of chlorine, fluorine, etc. The compositions of these alloys have a balanced content of the main (Mo, Cr, Cr + Mo) and supplementary (V, W, Fe) alloying elements, and also a controlled low content within them of impurity elements (C, Si, S, P). This gives alloys high resistance to uniform corrosion in the corresponding media and to forms of local corrosion, and technological efficiency in manufacturing various forms of equipment. Operating experience shows that use of alloys of this group for media of high corrosiveness makes it possible to increase considerably the service life and operating reliability of critical chemical equipment, although use of these alloys is limited by the tendency of welded equipment, even of these metallic materials, to local corrosion, i.e., intercrystalline and knife-line corrosion, failure of welded joints and very high cost. Thus, in order to avoid local corrosion in sulfuric acid media it is recommended: 1. To use the following corrosion-resistant steels and alloys: • austenitic chromium-nickel-molybdenum steels (10Kh17N13M2T, 10Kh17N13M3T, etc.) in solutions with a weight fraction of sulfuric acid of 10–25% at up to 75°C; • austenitic-ferritic steels (08Kh21N6M2T (EP54), 08Kh22N6T (EP53), 03Kh24N6AM3, etc.) in solutions with a weight fraction of sulfuric acid of 10–35% at up to 90°C, and also in concentrated sulfuric acid; • alloys based on Fe–Ni (06KhN28MDT (EI943) and 03KhN28MDT (EP516), developed for operation with especially corrosive solutions of sulfuric acid; here alloy 03KhN28MDT has higher resistance to ICC due to a reduced carbon content [4]; • highly alloyed materials based on nickel-molybdenum (N65M-VI (EP982-VI), N70MFV-BI (Ep814A-VI), Hstelloy B-2, Nimofen S6928) and nickel-chromium-molybdenum alloys (KhN63MB (EP758U), KhN65MVU (EP760), Hastelloy C-276, Hastelloy C-22, Nicrofen 5923hMo) in highly concentrated sulfuric acid solutions, contaminated with ions of chlorine and fluorine, etc. 2. Before manufacture or repair of objects there should be input monitoring of the steels and alloys used for ICC resistance. 3. To use steels and alloys after heat treatment providing ICC resistance. 630
REFERENCES 1. 2. 3. 4.
M. A. Shluger, F. F. Azhogin, and E. A. Efimov, Metal Corrosion and Protection [in Russian], Metallurgiya, Moscow (1981). N. D. Tomashov and G. P. Chernova, Theory of Corrosion and Structural Alloy Corrosion Resistance [in Russian], Metallurgiya, Moscow (1993). I. Ya. Klinov, P. G. Udyma, A. V. Molokanov, and A. V. Goryainova, Chemical Equipment in Corrosion-Resistant Construction: Handbook [in Russian], Mashinostroenie, Moscow (1970). A. P. Shlyamnev et al., Corrosion-Resistant, Heat-Resistant, and High-Strength Steels and Alloys: Reference Edition [in Russian], Prommet-Splav, Moscow (2008).
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