CORROSION RESISTANCE OF TITANIUM-BASED ALLOYS IN THE SULFURIC ACID MEDIA OF THE VISCOSE RAYON SECTOR T. M. Sigalovskaya, L. I. Tishkina, R. K. Burdonova, I. L. Voitenko, V. S. Mikheev, B. A. Goncharenko, V. I. Kazarin, and N. D. Tomashov

UDC 620.197.677.463.051.12

The choice of metallic materials for equipment components exposed to contact with the sulfuric acid media in viscose rayon production is rendered difficult by the high corrosion activity of the solutions relative to all available metals and alloys including the highalloy stainless steels [i, p. 49]. In an effort to find corrosion-resistant alloys suitable for service in the sulfuric acid media of the viscose rayon sector the All-Union Scientific-Research Institute for Light and Textile Machines and the Experimental Production Division of the Kalinin Man-Made Fibre Combine carried out corrosion resistance tests on experimental alloys b~sed on titanium and containing molybdenum, niobium, and zirconium. The alloys were developed by the Institutes of Metallurgy and Physical Chemistry of the Academy of Sciences of the USSR jointly with the All-Union Scientific-Research Institute for Light and Textile Machines [2]. The tests were carried out with over 50 alloys in which the ratio of the alloying components varied as follows: 4Mo/iNb/iZr, iMo/iNb/4Zr, and IMo/iNb/iZr. The total content of alloying components was varied from 0.3-0.5 to 40-60 wt.% (within the limits of malleability of the alloys). The alloys were produced, the bars fabricated, and the mechanical properties tested in the A. A. Baikov Institute of Metallurgy of the Academy of Sciences of the USSR while the corrosion and electrochemical properties were tested in the Institute of Physical Chemistry of the Academy. The tests of the corrosion resistance in industrial conditions were carried out by direct immersion in the precipitation bath for textile yarn (Table i) and in the stretching bath for technical yarn (Table 2). The test specimens were in the form of small cylinders 4.5-6 mm in diameter and 10-18 mm in height. Transverse holes 2 - 3 m m in diameter were b o r e d in the specimens and used to suspend them with acid-resistant Khlorin yarn in perforated polypropylene baskets. Before the tests the surface of the specimens was cleaned with fine emery cloth and degreased with acetone. The TABLE i. The Corrosion Resistance of Titanium Alloyed with Mo, N5, and Zr in the Precipitation Bath* of the Viscose Rayon Textile Yarn Sector

Corrosion rate ~!(m,.h)]durIng a ~$t period (h) of:

Com ,osition of the allo/, wt. % u

T,

91

82 76

70 64 58

I M________~oNb i ' ~ r 6 8 12 16

20 24 2S

1,~5

69

12'5 4 5

7

7

375 [ 701

2310

0,0370 0,210 0,130 [ 0,075 0,0230 0,010 ] O,0O6 0,0016 +0 064T 10,003 10,002 +0,0001 0,035 i 0,008 1O,O05 0,0009 0,270 i 0,007 1O,O04 -I-0,0004 0,013 0,006 ] 0,003 0,0003 0,950 0,610 0,055.

*Bath composition (g/liter): H=S04 140142, Na=SO~ 285-290, ZnSO~ 16-18, and Berol-Spin 0.0006; impurities H=S and CS=; temperature 50-52°C. %A plus sign denotes an increase in the weight. Translated from Khimicheskie Volokna, No. 3, pp. 39-41, May-June, 1978. cle submitted March 16, 1977.

0015-0541/78/1003-0255507.50

Original arti-

© 1979 Plenum Publishing Corporation

255

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Fig. i. The rate of corrosion of alloys TI--Mo--Nb--Zr in the precipitation bath of the viscose rayon textile yarn sector vs the amount of alloying components added in the proportion 4Mo/iNb/iZr. Bath composition (g/liter): HISO~ 140-142, NaISO~ 285-290, ZnS04 16-18, Berol Spin 0.006; impurities HiS and CSI. Temperature 50-52°C. Fig. 2. The rate of corrosion of alloys Ti--Mo--Nb--Zr in the stretching bath of the viscose rayon cord yarn sector vs the amount of alloying components added in the proportion 4Mo/iNb/iZr. Bath composition (g/llter): HIS04 35-45, ZnSO~, 11.5-15.5; impurities HiS and CS2. Temperature 85-90°C. specimens were weighed on a chemical balance precise to i.i0-~ g. Specimens which exhibited an unsatisfactory corrosion resistance in 66-73 h were eliminated from the tests. The results of the tests showed that the alloys with a dominant proportion of Zr are unsuitable for service in the sulfuric acid media of the viscose rayon sector. None of these alloys (20 compositions containing Zr 0.4-40 wt.%) were sufficiently corrosion-resistant, After testing for 66 h the corrosion rate of the alloys, in which the concentration of the alloying components varied within wide limlts, was 1.5-3 g/(ml.h) in the precipitation bath and 9-12 g/(ml.h) in the stretching bath. In addition to being corroded overall many specimens were pitted. In the case of equal amounts of alloying components (IMo/iNb/iZr) the corrosion resistance was quite high in both baths for the alloy with the maximum content of Mo, Nb, and Zr TABLE 2. The Corrosion Resistance of Alloys Ti--Mo--Nb--Zr in the Stretching Bath of the Viscose Rayon Cord Yarn Sector Composition of t i c alloy, wt. %

Ti

Corrosion r a ~ [g/(m2.h)] during a ~ s t period ~h) of:

Mo INb I Zr

76

58

28

7

7

78

397

707

2316

0,081 I" 1,560 0,057 0,097 0,(}65

0,043 0,360 0,041 0,061

0,048 0,220 0,044 0,050

0,056

0,059

0,018 0,074 0,015 0,020 0,040

*Bath composition (g/liter): HISO~ 35-45, ZnSO~ ii.5-15.5; impurities HiS and CSI; temperature 90°C. tThe results are given for two parallel tests because of the considerable discrepancy in the corrosion rate which is attributable to the quality of the experimental alloy.

256

TABLE 3. The Mechanical Properties of the Alloys Ti--Mo--Nb--Zr Alloycomposition° Ultimate Yield wt. U/o s~e~tI~ ~ i ~ ,

T,

76 70 64

58

Relative [Relative lelonga- Icomtac-

Mo INbIZ, MN/m' i MN/-m2 ')tion. % ]tlon, % 16 20 24

28

33

lO O

65 56 7 7

1020

44

900 841 949

1~ 830 939 1015

1~,,4 13,5 12,0 12,5

56,0 49,0 65,5 58,0 56,0

(13 wt.% of each). During 66 h the corrosion rate of the alloy was 0.04-0.08 g/(ma'h) in both baths. In the precipitation bath the alloys which contained less alloying components (8, 9, i0, or 12 wt.% of each) were also highly resistant. Some of the specimens were not passlvated, however, and were destroyed at a rapid rate, i.e., about 1.5 g/(ma'h). The best results were obtained with alloys containing a large proportion of Mo. An increase in the content of alloying components in the alloys of this group (Figs. i and 2) results in slower corrosion in both baths but the results obtained in a stretching bath varied considerably owing to the variation of the quality of the experimental alloys. Complete short-term corrosion resistance in the precipitation bath was attained with alloys containing at least 12 wt.% Mo and in the stretching bath withalloys containing atleast 20wt.%Mo. An investigation of the kinetics of the corrosion of these alloys (see Tables i and 2) showed that the corrosion rate decreases with an increase in the exposure in the solutions, notably in the precipitation bath, and after 2000 h the corrosion rate is low in the case of the alloys with smaller amounts of alloying components, viz., with 6 wt.% Mo in the precipitation bath and with 16 wt.% in the stretching bath. In the precipitation bath the specimens of all alloys ceased to lose weight after 700 h. According to the findings of the All-Union Scientific-Research Institute of Man-Made Fibres, during exposure for 720 h in a precipitation and stretching bath the corrosion rate of alloy 4201 (Ti--33% Mo) varies 0.05-0.2 g/(m2"h); the corrosion rate of the hlgh-alloy stainless steels 06KhN28MDT ~EI-943) and 03Kh21N21M4GB (EI-35) and the nlckel--molybdenum alloys N65M26 and N70MF (EP-496) in these conditions varies 0.i-i.0 and 0.9-2.0 g/(m2"h) in the precipitation and stretching baths respectively. In terms of the corrosion resistance in the sulfuric acid media of the viscose rayon sector the experimental titanium alloys are therefore not inferior to alloy 4201 and significantly better than high-alloy chrome -nickel --molybdenum steels and nickel --molybdenum alloys. The mechanical properties of the experimental alloys are also good (Table 3); the alloys are easily pressure-worked and machined and not prone to brittle failure (thermal cracking), and their technological properties and machinability are significantly better than those of alloy 4201 and nickel-based alloys. The corrosion resistance and the mechanical and technological properties of the alloy Ti--20Mo5Nb5Zr render it suitable forindustrial-scale testing. Alloy Ti--20MoSNbSZr welds well. The corrosion resistance of the welded joints of the alloy produced by automatic arc-weldlng with a tungsten electrode in an argon atmosphere is not inferior to that of the base metal. The welded joints are devold of flaws and cracks. The alloy can be rolled to sheet down to the thickness of foil. Specimens of welded sheet of alloy Ti--20Mo5Nb5Zr were subjected to testing in the processing liquors of the viscose rayon sector for a period of 7000 h. In the precipitation bath the corrosion rate of welded and unwelded specimens varied 0.0013-0.0025 g/(ma'h) and in the stretching bath 0.010-0.026 g/(ma'h). The welded joints were free of intercrystalllte and other types of corrosion. These findings demonstrate the advantages of using the new, tltanlum-based experimental alloys for the fabrication of the components and subassemblies of equipment operating in the sulfuric acid media of the man-made fibre industry. LITERATURE CITED i.

2.

B. Ya. Barochina et al., Corroson Protection of the Equipment and Structures of ManMade Fibre Plants [in Russian], Nauka, Moscow (1970). N. D. Tomashov et al., Zashch. Met., 13, No. I, 3-9 (1977). 257