Chemical and Petroleum Engineering, Vol. 37, Nos. 3–4, 2001
MATERIALS SCIENCE AND CORROSION PROTECTION CORROSION STUDIES OF ALLOYS FROM KRUPP VDM IN CAUSTIC SODA PRODUCTION
Yu. B. Danilov, V. A. Kachanov, E. K. Gvozdikova, T. É. Shepil’, V. N. Khil’, T. A. Balak, and V. S. Gorlova
UDC 620.193:661.322.1
Caustic soda production is a large-tonnage enterprise with a trend toward steady growth. Caustic soda is produced in several ways applying electrolysis of sodium chloride. Maximum purity, in terms of such impurities as chlorides, sulfates, chlorates, etc., and concentration of caustic soda are attained by mercury electrolysis. The method is fairly popular, but is very seriously flawed in that the traces of the mercury left need to be removed. In spite of this, the volume of caustic soda production by the mercury method comprises 35–40%. Pure caustic soda is produced as well by the membrane method (without the demerits of mercury electrolysis), but, because of the high cost of membranes, the volume of its production by this method is only 10–15%. Roughly half of the caustic soda produced in the world is obtained by the diaphragm method which has another serious demerit, viz., heightened corrosion activity of the caustic soda produced due to formation of hypochlorite and chlorate in the analyte as well as of wastes of asbestos diaphragms that pollute the environment. Heightened corrosion activity of the alkali produced makes it extremely difficult to choose the structural materials for implementation of the process of concentrating caustic soda to as much as 99%. The main structural materials usable for concentrating caustic soda from 120 g/liter to 45–50% are the steels 12Kh18N10T (12Cr18Ni10Ti), 10Kh17N13M2T (10Cr17Ni13Mo2Ti), and 15Kh25T (15Cr25Ti), the alloy 06KhN28MDT (6CrNi28MoCuTi) [GOST (All-Union State Standard) 5632–72], nickel, and the steel 316L. For concentrating mercury caustic soda in melting boilers to the solid state, nickel is used with cathodic protection of the welded joints, and for evaporation concentration of diaphragm caustic soda, SChShch-2 cast iron alloyed with nickel to the extent of 3% is used. In view of the low corrosion resistance of the structural materials used, especially for producing solid caustic soda, we studied the corrosion behavior of the materials produced by Krupp VDM GmbH, viz., the nickel alloy 201 and the new chromium–nickel–molybdenum alloy 33 in media for obtaining caustic soda from the liquid (up to 45%) to the solid (94–99.5%) state, as well as the domestic alloy 6CrNi28MoCuTi. Volgograd Open Joint-Stock Company Kaustik (Caustic). Inspection showed that the equipment had been functioning stably since 1982 without noticeable damage. But in the course of operation, the evaporator pipes were partially replaced because of corrosive wear. The pipes in the concentrator are proposed to be replaced completely in 2001. Caustic soda obtained by mercury electrolysis was concentrated from 46 to 99% in two serially installed dropping-film evaporators (concentrators). The plant for concentrating diaphragm caustic soda from 120 g/liter to 50% has been functioning since 1986. The equipment of the first and second casings of the chamberless concentrator was nickel-plated and the heating pipes were made of nickel. The third casing was made of 316L type of steel and the heating pipes were made of nickel.
Open Joint-Stock Company UkrNIIKhimmash (Ukrainian Scientific Research Institute of Chemical Machine Building). Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 4, pp. 40–44, April, 2001. 0009-2355/01/0304-0253$25.00 ©2001 Plenum Publishing Corporation
253
TABLE 1 Chemical composition, % Apparatus NaOH
NaCl
Na2CO3
NaClO3
Pipeline past the preheater of the original solution
11
12
0.2
0.6
Casing 1 of the concentrator (above/below the heating chamber)
15/15
16/16
Casing 2 (above the heating chamber)
19
T, °C
p, MPa
Testing location
115–130
0.32–0.36
150/140–150
0.32–0.36
Pervomaisk state enterprise
0.26/0.26 0.83/0.85
Khimprom, Ukraine
18
0.34
Plate 1 (solution inlet/outlet) 46–50/60–65
2.7/2.9
Plate 2 (solution inlet/outlet) 65–70/75–80
2.98/3.18
0.7/0.8
1.15
120–130
0.06
Open joint-stock company 0.48/0.23 180–195/220–240 Atmospheric Khimprom (Volgo0.24 240–280 grad, Russia)
0.76/0.63 0.31/0.42 143–150/172–180
Plate 3 (solution inlet)
80–85
3.29
0.83
Preconcentrator
60
0.03
0.27
0.01
95
SCS1 tank
97.5
0.01
0.32
0.001
380
SCS2 tank
99.5
0.01
0.32
0.001
395
Caustic soda concentrating boiler
46–94
2.7–3.1
Plate 3 (solution outlet)
90–94
0.54–0.73 0.26–0.14
3.3
0.85
Atmospheric
Open joint-stock company Kaustik (Volgograd, Russia)
Atmospheric
Open joint-stock company Kaprolaktam (Dzerzhinsk, Russia)
140–360
0.24
340–360
TABLE 2 Element content, % Alloy C
Si
Mn
Cr
Ni
32.75 31.35
Mo
Ti
Cu
Fe
N
S
P
33
0.012
0.30
0.63
1.49
–
–
32.20
0.4
201
0.009
0.03
0.11
–
99.6
–
0.02
0.01
0.12
–
0.002
0.002 0.014
6CrNi28MoCuTi
0.06
0.44
0.44
23.1
27.15
2.7
0.52
2.92
Base
–
0.008 0.015
–
In 1998, because of failure of the lining of the cone of the first concentrator, it was replaced by the nickel alloy 201 supplied by Krupp VDM. Inspection of the lining after two years of operation showed it in a satisfactory state, but peeling-off was noticed in some places, which apparently is associated with the technology of lining of the casing. Volgograd Open Joint-Stock Company Khimprom (Chemical Industry). Solid diaphragm (SD) caustic soda is produced in a cascade-type installation (three plates placed one below the other). The plates are made of the steel 12Cr18Ni10Ti, the alloy 6CrNi28MoCuTi, or the carbon steel 20. The thickness of the plate bottom is 20–30 mm and the service life is not more than one month. Dzerzhinsk Open Joint-Stock Company Kaprolaktam (Caprolactam). Solid caustic soda is produced in melting boilers made of cast iron alloyed with up to 2% nickel. The service life of the boilers is also not long (not more than a few months). Industrial corrosion tests of specimens of welded joints of the test materials were performed by a testing program prepared jointly with Krupp VDM, taking account of the production conditions and the technological distinctions of each enterprise. 254
The test conditions are cited in Table 1. The nature of damage to the specimens after the tests was determined visually using a magnifying glass (magnification 20) and metallographically, and the pit depths were measured by a needle of the type of a clock arm and metallographically. The chemical composition of the tested materials is cited in Table 2. The welded joints of the alloys 201 and 33 were made by Krupp VDM. The specimens as supplied and the welded joints were submitted to tests. The specimens for industrial corrosion tests were made from welded plates supplied by Krupp VDM. For the tests, we cut out 20 mm × 80 mm specimens with a transverse weld and two drilled holes of diameter 8 mm at a distance of 5 mm from the shorter end. The specimens were ground, degreased, weighed on an analytical balance to a precision of 0.0002 g, and put in a cartridge with the help of a rod made of the nickel alloy 201. The specimens were interlaid with graphite, which is used as anodes to protect the welded joints of the nickel boilers for melting of the caustic soda. The specimens were hung in the apparatuses on an NP-2 nickel wire. The corrosion rate was determined by the gravimetric method. The electrochemical heterogeneity of the welded joints was assessed from the potentials measured on microsections. The site, at which the potential on the welded joint was not measured, was insulated by a coat consisting of a mixture of paraffin (wax) and colophony (rosin). The measurements were performed in 20% alkali and sulfuric acid solutions at ambient temperature, the reference electrode being saturated silver chloride. The corrosion potential was measured successively on the base metal, in the heat-affected area, and in the weld metal. After the industrial tests, microsections were cut out from the specimens in perpendicular to the weld for metallographic investigations. The macrostructure was studied on polished sections at a magnification of up to 20. On the macrosections the contours of weld penetration and the macroscopic structure of the weld (shape, dimensions, incomplete penetration, nonmetallic inclusions, etc.) were determined. The microstructure of the alloys was studied on electrolytically etched polished sections: alloy 33 in 10% aqueous oxalic acid solution and alloy 201 in a solution consisting of 5 ml of glacial acetic acid, 10 ml of nitric acid, and 85 ml of water. The investigation was carried out on a MIM-7 microscope at 100, 200, and 300 magnifications. The size of the base metal grains and the heat-affected area was determined in accordance with GOST 5639–88. Pervomaisk State Enterprise Khimprom. Industrial corrosion tests of the materials for a period of 2617 h demonstrated that, in concentrating electrolytic caustic soda produced by the diaphragm method under the conditions of the preheater of the starting product (11% NaOH solution), the alloys 201 and 33 suffered a negligible general corrosion at a rate lower than 0.01 mm/yr. Elevation of solution concentration to 15% at 150°C accelerates corrosion of the alloys 201 and 33 to some extent (to a greater extent above the heating chamber), but the nature of corrosion does not change: the alloys suffer corrosion from the stable inert state, having been submitted to a slight general corrosion. Further elevation of caustic soda concentration to 19% slows down corrosion of the alloy 33 but accelerates corrosion of the alloy 201, and also causes slight etching of the weld metal of the welded joints of the alloys 201 and 33 both. Volgograd Open Joint-Stock Company Khimprom. Corrosion testing of the test materials was carried out under the conditions of solid caustic soda production from diaphragm caustic soda concentrated to 45% by evaporation. The test duration was 2000 h. It was found that the base metal of the alloys 201 and 33 suffers a slight general corrosion (corrosion rate up to 0.072 mm/yr). The corrosion rate of welded joints is somewhat higher, being 0.082 mm/yr, whereupon the weld gets slightly etched. The alloy 6CrNi28MoCuTi suffers severe general damage (corrosion rate up to 2 mm/yr). As the caustic soda concentration is raised to 60–65% with elevation of the temperature of the solution being evaporated to 180°C, the rate of corrosion of the base metal of the alloy 201 rises, and the weld metal is also significantly etched, which may be attributed to increased concentration of chlorate ions and their activating effect. However, in this process, the alloy also undergoes general corrosion. Apparently, the passivating effect of chlorate ions may be the cause of even a slight slowing down of corrosion of the chromium–nickel–molybdenum alloy 33. At the same time, the corrosion rate of the alloy 6CrNi28MoCuTi attains 3 mm/yr with intense general dissolution of the alloy. Upon concentration of the caustic soda to 70% with elevation of temperature to 195°C, the alloy 201 becomes unstable, and suffers pointed-pitting (spot-pitting) and knife-line corrosion. In this case, there occurs selective etching of the metal on both the base and backing weld sides. The corrosion rate of the alloy 33 attains 0.285 mm/yr, but the alloy suffers gen255
TABLE 3 Apparatus
Alloy
Specimen type
Corrosion rate, mm/yr
Nature of damage
Preconcentrator
201 33 6CrNi28MoCuTi
Base/Welded Base/Welded Welded
0.0063/0.0066 0.0182/0.0203 0.0654
General corrosion/Etching of weld General corrosion/Etching of weld General corrosion
SCS1 tank
201 33 6CrNi28MoCuTi
Base/Welded Base/Welded Base
0.1198/0.1295 0.334/0.338 0.417
General corrosion/Etching of weld General corrosion/Etching of weld General corrosion
SCS2 tank
201 33 6CrNi28MoCuTi
Base/Welded Base/Welded Welded
0.0413/0.0557 0.0020/0.00237 0.0089
General corrosion/General corrosion General corrosion/General corrosion General corrosion
eral corrosion and the weld metal, etching. Note that the chlorate ion content on the second plate rises from 0.31% (at the inlet) to 0.48% (at the outlet). The corrosion rate of the alloy 6CrNi28MoCuTi is higher than 10 mm/yr; the specimens disintegrated completely. At the outflow from the second plate the caustic soda concentration rises to 80% and the temperature, to 240°C, whereupon the sodium chlorate content diminishes almost twofold, giving rise to a dramatic increase in the rate of general and pitting corrosion of the alloy 201 and severe etching of the weld. The corrosion rate of the welded joint of the alloy 201 is more than twice as much as that of the base metal. The alloy 33 experiences corrosion roughly at the same rate as at the plate inlet. The nature of corrosion changes from general to pointed-pitting. The depth of some pits attains 0.31 mm, which may comprise 1.32 mm/yr when pit depth is linearly dependent on time, i.e., when pitting prevails over general corrosion. The alloy 6CrNi28MoCuTi was surprisingly stable; in addition to general degradation, it suffered pointed-pitting corrosion (isolated pits may be as deep as 0.36 mm) at the rate of 1.44 mm/yr, which is slightly higher than the corrosion rate of the alloy 33. Raising concentration of diaphragm caustic soda to 85% (inlet to plate 3) renders the alloy 201 totally unstable (the corrosion rate of the base metal may be as high as 6.7 mm/yr in the case of selective etching of the weld metal and the heat-affected area). The corrosion resistance of the alloy 33 remained at the previous level in the case of pitting type of degradation. Subsequent concentration of the caustic soda to 94% (outlet from plate 3) toughens the operation conditions for nickel still further. Over the testing time, the specimens of the alloy 201 dissolved completely, whereas the alloys 33 and 6CrNi28MoCuTi, having been submitted to pitting corrosion, seemed relatively stable materials: the pitting corrosion rates were 0.96 and 1.44 mm/yr, respectively. The industrial corrosion tests of the materials at the Dzerzhinsk joint-stock company Kaprolaktam for a period of 740 h were performed under the conditions of operation of the melting boiler where 45% diaphragm caustic soda is fed and concentrated to a fusion cake (94% NaOH). Thus, all the corrosion conditions necessary for obtaining solid caustic soda are concentrated in this apparatus. In order to study the corrosion behavior of nickel in conditions of solid caustic soda production, a cartridge holding the specimens was suspended in the boiler on a nickel wire. After 740 h the tests were discontinued because of failure of the cartridge. The corrosion rate of the nickel wire determined from the change in its thickness was 10 mm/yr. The test results showed that the corrosion resistance of specimens of the alloys 201, 33, and 6CrNi28MoCuTi is unsatisfactory if diaphragm caustic soda is concentrated periodically from 45 to 94%. It is of interest to determine the corrosion behavior of the alloys 201 and 33 under the conditions of evaporation concentration of caustic soda produced by using mercury as the cathode. Volgograd Open Joint-Stock Company Kaustik (Caustic Soda). The test results in concentrating caustic soda from 45 to 99.5% (minimally contaminated with chlorides and chlorates) are cited in Table 3. Since presence of chlorates in 256
TABLE 4 Corrosion potential in 20% solution, V Alloy
Material
Base metal 201
33
NaOH
H2SO4
–0.42
+0.12
Heat-affected area
–0.31
+0.08
Weld metal
–0.26
+0.15
Base metal
–0.33
–0.08
Heat-affected area
–0.61
+0.17
Weld metal
–0.27
+0.10
the caustic soda fed into the evaporation concentrator (made of nickel) is impermissible, sugar was added to the caustic soda to reduce the chlorates. The test duration was 4100 h. The results of the corrosion tests in the preconcentrator indicate high corrosion resistance of the alloy 201. The alloys 33 and 6CrNi28MoCuTi also possess corrosion resistance, being submitted to only a slight general corrosion. Under the conditions of the solid caustic soda tank SCS1, the rate of corrosion of the alloys 201 and 33 increases almost 20 times. However, the alloy 201 is more preferable even though it suffers appreciable corrosion (as much as 0.13 mm/yr). Under the conditions of the solid caustic soda tank SCS2, where further dewatering takes place, in spite of the temperature rising to 395°C, the corrosion rate of the studied materials slowed down, and the alloy 33 exhibited the maximum corrosion resistance. Let us examine the electrochemical heterogeneity of the tested alloys. The potentials (referred to a saturated silver chloride electrode) are cited in Table 4. As will be seen, in 20% NaOH the base metal, which undergoes maximum dissolution in the given welded joint, acts as the anode in the alloy 201, whereas the heat-affected area acts as the anode in the alloy 33. In more corrosive conditions (20% H2SO4), in which nickel is corroded in the active region, it is again the heat-affected area that acts as the anode. In the alloy 33, the base metal acts as the anode. Thus, the tests performed showed electrochemical heterogeneity of the welded joints of the tested alloys and corrosion susceptibility (in drastic conditions) of the heat-affected area of the alloy 201. In the alloy 33, under more drastic conditions, the base metal that protects the heat-affected area and the weld acts as the anode. It has been proved by metallographic investigations of the welded joints of the alloys 201 and 33 that the former has a purely austenitic structure with typical twins and a grain index of 6–7, and that the grain grows appreciably (up to the index 2–3) in the zone of transition from the base metal to the weld metal (to the heat-affected area). Severely etched intercrystalline bands are noticed in the austenitic weld. The weld metal has the cellular structure of an inhomogeneous solid solution. Metallographic investigations of welded joints of the alloy 201, which suffered perceptible corrosion (corrosion rate up to 0.8 mm/yr), revealed that in the heat-affected area the metal experienced intense intercrystallite corrosion. Intense general and pitting corrosion was observed in both the base metal and the weld metal. Knife-line corrosion was noticed in the weld and base metal fusion zones. The weld metal suffered structurally selective corrosion at a much greater rate than the base metal. This is evident from both the depth of corrosion of the weld metal and the mass loss. Further increase in caustic soda concentration accelerates the corrosion rate of welded joints of the alloy 201 still more (to 3 mm/yr). The macro- and microstructure of welded joints of the alloy 201 after corrosion tests in 60% caustic soda suggest that the weld metal did not undergo corrosion. Increased corrosion of the grain boundaries were noted in the base metal and heat-affected area, i.e., there occurred very little intercrystallite corrosion. Unlike the nickel alloy 201, the alloy 33 suffered pitting corrosion in 80% NaOH, the overall corrosion rate of the alloy remaining almost unchanged. In this context, the nature of damage to the alloy (beginning with the given NaOH concentration) was studied. It was found that the base metal of the alloy 33 has an austenitic structure and a grain index of 5–6. 257
The weld metal structure is austenitic dendritic with a prevalence of cellular (in the base metal) and columnar dendritic (in the backing weld) structure. In the heat-affected area, the structure is austenitic and the grain index is 3–4. Nitrides of the alloying elements as well as intermetallide inclusions of the σ-phase type are observed in the microstructure of the alloy. Electrolytic etching of the alloy 33 in 10 N NaOH revealed presence of σ-phase: as liquation lines and isolated inclusions in the base metal and as very fine point-inclusions in the heat-affected area. The grain boundaries of the base metal (near the intermetallide segregations) are lean in the alloying elements, which, under certain operational conditions (corrosive media, elevated temperature, failure of the equipment-making technology, etc.), may become a cause for development of various types of damages of a spontaneous nature. Pitting corrosion to a depth of 0.12 mm was noticed in the alloy 33 specimens after tests in 80% caustic soda. Accumulation of nitrides and intermetallide σ-phase is observed in the microstructure of the alloy 33. In more drastic conditions (90–94% NaOH) the rate of general corrosion barely changed, and the alloy also suffered pitting corrosion. In the base metal, heat-affected area, and weld there were pits 0.09, 0.17, and 0.18 mm in depth, respectively. In this case, structurally selective etching of the weld metal takes place, apparently due to presence of intermetallides (σ-phase) which reduces in the adjoining areas the content of chromium and molybdenum which are the main passivating elements of the alloy and its welded joints. The alloy and its welded joints did not exhibit intercrystallite corrosion. The corrosion tests of the alloys 201 and 33 from Krupp VDM conducted in media for evaporation concentration of electrolytic caustic soda solutions (caustic lyes) make it possible to arrive at the following conclusions: – the alloy 201 and its welded joints possess high corrosion resistance in media for evaporation concentration of electrolytic caustic soda solutions (obtainable by both mercury and diaphragm methods) to 60% NaOH at temperatures up to 170°C; – increased grain growth (index 2–3) is noticed in the heat-affected area of the welded joints of the alloy 201, which may be one of the causes of failure of the welded joint of the alloy 201 in diaphragm caustic soda in more severe conditions (65–70% NaOH, 180–195°C). Intercrystallite and knife-line corrosion was noted in the welded joint, and the weld metal suffered structurally selective corrosion. The overall corrosion rate of the welded joint of the alloy attained 0.8 mm/yr; – the base metal and the welded joints of the alloy 201 are fairly corrosion resistant under the conditions of concentration (more than 60% NaOH) of caustic soda obtained by electrolysis with a mercury cathode; – the alloy 33 and its welded joints possess high corrosion resistance under the conditions of evaporation concentration of caustic soda produced by the diaphragm method (up to 65% NaOH), but, as the concentration is raised to 70%, the rate of corrosion of the welded joint rises to 0.3 mm/yr with general corrosion and marked etching of the weld metal; at higher concentrations of the diaphragm caustic soda the alloy suffers pointed-pitting corrosion; – under the conditions of concentration from 45 to 60% of caustic soda (free from chlorides) produced by electrolysis with a mercury cathode, the alloy 33 possesses high corrosion resistance, but somewhat less than the alloy 201. At caustic soda concentrations rising from 60 to 97.5%, the corrosion resistance of the alloy 33 drops almost three times below that of the alloy 201. Upon further dewatering (concentration) of the caustic soda to 99.5%, the corrosion resistance of the alloy 33 rises almost 20 times above that of the alloy 201. Recommendations. The alloy 201 is recommended for making equipment for evaporation concentration of caustic soda to 60% at 170°C. For making evaporators for concentration of caustic soda obtained by electrolysis with a mercury cathode, the alloy 201 is recommended for all stages of concentration up to 99.5% caustic soda. The alloy 33 is recommended for making equipment for concentration of diaphragm caustic soda and caustic soda obtained by electrolysis with a mercury cathode to 70% as well as for equipment for the stage of dewatering of ecaustic soda, produced by electrolysis using mercury as the cathode, from 97–99.5%. It must be emphasized that it is desirable to continue the work on evaluation of the effect of the σ-phase on the corrosion resistance of the alloy 33 as well as of the thermal welding cycle for the alloy 201 that ensures minimal grain growth in the heat-affected area.
258
