INVESTIGATION OF THE CORROSION RESISTANCE OF IRON--NICKELAND NICKEL-BASED ALLOYS IN SULFURIC, PHOSPHORIC, AND HYDROCHLORIC ACID MEDIA (REVIEW) Yu. S. Sidorkina, G. P. Bekoeva, T. V. Mankevich, and N. G. Zinchenko
UDC 669.15'24:620.193o4
The Scientific-Research Institute for Chemical-Equipment Manufacture has investigated equipment in eight chemical combines working with an apatite concentrate and a flotation concentrate from Karatau. It has been determined that in the manufacture of wet-process phosphoric acid the equipment manufactured from alloy 06KhN28MDT undergoes intergranular corrosion (IGC) both in the heat-affected zone and in the weld metal. An analysis of the literature has shown that there are different viewpoints concerning the reason for the formation of IGC of austenitic corrosion-resistant steels [i-3]o In most cases, in the authors' opinion, the steel tends to undergo IGC because of lowering of the chromium content of grain boundaries as a result of the formation of Cr23C6type chromium carbides. At the same time, there are publications in which it has been shown that IGC also appears when, according to this theory, it should not~ The authors explain the reason for IGC in this case by the formation of a o phase on the grain boundaries or by the formation of a o phase and a phase containing Mo, Cr, and Feo in addition, it is known that steels containing 18% Cr, 10% Ni, and 2% Mo undergo IGC only when chromium carbides segregate after exposure to heat along the grain boundaries. In the case of steels with higher contents of these elements, the authors consider the most probable reason for IGC to be lowering of the chromium content of boundary zones as a result of segregation, along the grain boundaries, of not only chromium carbides, but also of a ~ phase, ferrite, a < phase, a Laves phase, and other phases enriched in chromium and molybdenum and depleted in nickel. In these cases, an important role is played by the carbon content in the steel. Some authors consider that carbon hinders segregation of a o phase; therefore, in carbon-supersaturated steels it can segregate only after long-term annealing. The o phase forms faster if the carbon is bonded to titanium or niobium. Carbon solubility is higher in the o phase than in 9 and n phases. According to some authors' data, molybdenum and manganese facilitate segregation of a o phase and expand the region of its stability in the higher-temperature zone. In austenitic corrosion-resistant steels, as a rule, the o phase is highly enriched in molybdenum and other elements. Thus, there is stiil disagreement as to the reasons for the appearance of IGC of corrosion-resistant steels. In the preparation of new steels and alloys, it is necessary to take into account the available data of previous investigations, which was, in fact, done in the development of steel 03Kh21N21M4GB, resistant to IGC in the manufacture of wet-process phosphoric acid. Investigations by the Central Scientific-Research Institute for Ferrous Metallurgy have shown that the corrosion resistance of iron--nickel-based steels depends mainly on the molybdenum content, to a lesser degree on the chromium content, and insignificantly on the nickel and copper contents. Molybdenum in amounts greater than 2.5-3% significantly lowers the solubility of carbon in austenite; therefore, niobium was added as a stabilizing agent to steel containing 0.02-0.04% carbon to reduce the IGC tendency. Depending on the chromium and molybdenum contents, the residence time of the metal in the furnace,~ and the quenching time in the range I040-I070~ this steel may have a purely austenitic or two-phase structure containing a o phase. Welded joints of steel 03Kh21N21M4GB are corrosion resistant in wet-process phosphoric acid containing 32% P2Os, 0.2% CaO, I~67% SO3, 0.4% Fe2SO4, 0.35% A1203, and 2.28% F + at 60-80~ The corrosion rate of the steel is not more than 0.02 mm/yr, and there is no IGC. Extractors made of steel 03Kh21N21M4GB have Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. ii, pp. 26-27, November, 1992.
0009-2355/92/1112-0681512.50
9 1993 Plenum Publishing Corporation
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been used in the manufacture of wet-process phosphoric acid for more than 15 yr at the Almalyk Chemical Plant and the Fosforit Kingisepp Production Association. Steel 03Kh21N21M4GB is welded by argon-arc and electric-arc methods. When its structure is single-phase, the steel has rather good plastic properties. Equipment components can be manufactured from it by pressure shaping. Steel 03Kh21N21M4GB is corrosion-resistant in 10-20% H2SO 4 at temperatures up to 80~ In pure 85% phosphoric acid at temperatures up to 100~ and in the manufacture of 74-76% sulfuric acid with the presence of sodium bisulfite and bisulfate, the corrosion resistances of steel 03Kh21N21M4GB and alloy 06KhN28MDT virtually do not differ, but the use of steel 03Kh21N21M4GB ensures saving of nickel. The region of corrosion resistance of steel 03Kh21N21M4GB in pure 25-85% phosphoric acid at temperatures up to 100~ is significantly wider than in the case of steels 10KhI7NI3M2T and 12KhlSNIOT, but it is almost the same as in the case of alloy 06KhN28MDTo Reactors made of steel 03Kh21N21M4GB are widely used in the manufacture of hydroxylamine sulfate. The process for manufacture of hydroxylamine sulfate amounts to catalytic reduction of nitrogen oxide by hydrogen and dilute sulfuric acid at 40-45~ platinum deposited onto finely divided graphite is used as the catalyst, As a result of inspection of reactors made of this steel at the Cherkassy and Eustavi Nitrogen Production Associations, it was determined that they were in satisfactory condition after 2 years of operation. The search for new compositions has continued. Widely known alloy 06KhN28MDT tends to undergo IGC; therefore, in the beginning of the 1970's, metallurgists produced the new 03KhN28MDT, in which the carbon content was lowered to 0.04%. But it was found that the IGC tendency was retained after heating not only when the carbon content was 0.06%, after heating in the region of "dangerous" temperatures at 700~ phase and x-ray diffraction analyses revealed only the carbide Me23C ~. For alloy 03KhN28MDT, the region of "dangerous" temperatures was shifted toward higher temperature values (~8000C). Phase and x-ray diffraction analyses of this alloy after heating at 800~ revealed only a o phase or a a phase and the carbide Me23C 6. Taking into account the results of investigations, we attempted to prepare a new alloy in which segregation of excess phases causing the appearance of an IGC tendency would not occur during heating. As a result, we obtained a composition with stabilizing addition of Nb and produced an experimental lot of hot-rolled sheet alloy 03KhN26MDB with thickness 5-20 mm, which welded well and had rather good plastic properties, enabling the manufacture of equipment components by pressure shaping. Alloy 03KhN26MDB is resistant to IGC during testing by method VU of State All-Union Standard GOST 6032--89 with heating in the range from 500 to 950~ and holding for up to 8 h. It is more resistant to knife-line corrosion than is alloy 06KhN28MDT because of replacement of the carbide-forming element Ti by Nb. The new alloy and its welded joints are resistant to IGC in media for the manufacture of wet-process phosphoric acid. The resistance of alloy 03KhN26MDB to overall corrosion in solutions of sulfuric, nitric, and phosphoric acids and media for the manufacture of Nitrophoska differs little from the corrosion resistance of alloy 06KhN28MDT. Mixer blades for extractors have been manufactured from the new alloy and have been used at the Almalyk and Samarkand Chemical Plants. Besides the Soviet alloys, in the 1970's the Sandvik firm (Sweden) manufactured unstabilized Nb alloy Sanicro-28 with low carbon content (0.02%), from which equipment was manufactured for use in highly corrosive media of the chemical industry, mainly in media for the manufacture of phosphoric acid. The Sandvik firm mentions the very high resistance of this alloy to local kinds of corrosion, i.e., pitting, crevice, intergranular, and stress-corrosion cracking. Later, Soviet alloy KhN30MDB was also developed. Its properties are similar to those of alloy Sanicro-28, which is now manufactured as merchant shapes, hot- and cold-rolled sheets, welding wire, and tube preforms. During testing of alloy KhN30MDB in laboratory and industrial media, it was determined that both the alloy and its welded joints are resistant to IGC (unlike alloy 06KhN28MDT). There is no knife-line corrosion of welded joints of alloys KhN30MDB and 03KhN26MDB in a medium for the manufacture of Nitrophoska (H2SO ~ + HNO 3 + P205 + HF) at temperatures up to II0~ whereas welded joints of alloy 06KhN28MDT do undergo knife-llne corrosion~ In their resistance to overall corrosion in solutions of 10-94% sulfuric acid at 80 and 100~ alloys 03KhN26MDB and 06KhN28MDT, containing 2.5 and 3% copper, respectively, and
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their welded joints are superior to alloy KhN30MDB, containing 1.5% copper. In solutions of phosphoric acid (thermal and wet-process obtained by dihydrate and hemihydrate processes)~ the corrosion resistance of alloy KhN30MDB does not differ from the corrosion resistance of alloys 03KhN26MDB and 06KhN28MDT. However~ it has been determined that alloy 06KhN28MDT and its welded joints have corrosion resistance lower than that of alloy 03KhN26MDB. Alloy Kh30MDB is recommended for use in the manufacture of complex mineral fertilizers, for installations for concentration (when the temperature of the medium is ~120~ of wetprocess phosphoric acid, and in the manufacture of Nitrophoska. The mechanical and technological properties of alloys KhN30MDB and 03KhN26MDB and their welded joints at temperatures from --70 to 500~ satisfy the requirements of Special Standard OST 26-291--87. The optimum materials for manual argon-arc welding of alloy Kh30MDB are welding wire KhN30MDB and "spaghetti" rods with cross section 2 x 3 mm manufactured from sheet alloy KhN30MDB~ All kinds of welding and welding materials suitable for alloy 06K~N28MDT are suitable for alloy 03KhN26M~B. LITERATURE CITED i. 2.
3.
V . M . Knyazheva, V. Chigal, and Ya. M. Kolotyrkin, "Role of excess phases in the corrosion resistance of stainless steels," Zashch. Met., ii, No. 5, 531-552 (1975)0 N . D . Tomashov, V. M. Doronin, O. N. Markova, et al., "Accelerated potentiokinetic methods for determination of the intergranular-corrosion tendency of stainless steels," Zashch. Met., ii, No. 3, 290-295 (1975). E. Goudremont, Special Steels [Russian translation], Metallurgiya, Moscow (1959).
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