Chemical and Petroleum Engineering, Vol. 31, Nos. 3-4, 1995
MATERIALS
SCIENCE AND CORROSION PROTECTION
CORROSION RESISTANCE OF ZIRCONIUM ALLOYS IN ACETIC ACID MEDIA I~. K. Malakhova, A. N. Kuzyukov, and A. V. Meshcheryakov
UDC 620.193:669.15'296-196
Zirconium and zirconium-based alloys have currently become important in various industries. Application of zirconium and its alloys for operation in media involved in a number of chemical production processes seems promising. Usually nickel-molybdenum alloys (Hastelloy B and Hastelloy B-2) are used in the very aggressive media for production of synthetic acetic acid. However, equipment made of Hastelloy (such as a synthesis reactor) is subject to enhanced corrosion (intercrystallite and knife-line corrosion along the weld line) [1]. At the same time, the good corrosion resistance of zirconium in acetic acid solutions has been established. At the Severodonetsk Scientific-Research and Design Institute of Chemical Machinery Manufacture, we have investigated the corrosion resistance of the zirconium alloys t~ll0 (with 1% niobium) and I~125 (with 2.5% niobium) and welded joints of these alloys under laboratory and industrial conditions in media used for acetic acid production. We have also tested the oxidized zirconium alloy t~ll0. Oxidation was accomplished according to a selected program: annealing at 500~ holding for 2 h. TABLE 1 Apparatus
Compositionof Temperamedium (major ture, ~ components), % Dehydration 97,5 CH3COOH, column up to 2,5 C2HsCOOH, 155 (still) up to 0,1 H20 155 Dehydration 45 CH3COOH, column 130 up to 2.5 CH3I (upper part) 100 rest It20 Flow separator 76 CH3COOH; of reactor 17 H20; 115 8,4 HI; 80 0,62 cn3I
Pressurel MPa
0,2 0,15
0,o5
0,1 CH3COOCH3; 0,02 CH3CHO
Still of light82 CH3COOH, fraction distilla- 6,4 H20, tion column 1,2 HI, 0,86 I2, 0,015CH3I,
125 90
0,05
0,002 CH3COOCH3 0,005 CH3CHO, 0,002 CH3OH
*In the numerator - in the apparatus; in the denominator under laboratory conditions.
Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 3, pp. 29-30, March, 1995. 0009-2355/95/0304-0183,$12.50 9
Plenum Publishing Corporation
183
TABLE 2 Corrosion rate of samples (mm/year) in media of different apparatus Type of test
Alloy
Characteristics of sample
still of dehydration column
~Base metal
El10 Laboratory
upper part of dehydration column
flow separator of reactor
still of lightfraction distillation column
0,003
0,0004
--
--
,welded, additive E l 1 0 wire
0,004
0,0007
--
--
,welded, additive E l 1 0 wire; oxidized after welding
0,0007
0,0006
0,003 0,005
0,004 0,004
1~125
Base metal ,welded, additive E125 wire
15"110
Base metal Welded, additive E l 10 wire
0,0019 0,0017
0,0007 0,0008
---
---
1~125
Base metal Welded, additive E125 wire
---
---
0,0005 0,0008
0,0008 0,0013
Industrial
Ii, m A / c m 2
+1
I
o
1
J
),'(
_
-J
- 4,o0
Ix{ ,I1 ,go
gO0
1zgo
16gO5 m V
Fig. 1. Potentiodynamic curves for zirconium alloy 1~110 in the medium of a dehydration column at a temperature of 80~ ) refers to the upper part of the column; - - - ) refers to the still of the column; 1, 2 refer to an unoxidized and an oxidized sample respectively. The test pieces were made from a rolled sheet of the alloys of thickness 4 mm. We tested 20 • 80 x 4 m m samples of the base metal and also welded samples (the weld was at the center of the sample). Welding was done by the argon-shielded arc technique. The tests were carried out a dehydration column, the flow separator of a reactor, and the still of a column for distillation of light fractions. The duration of thelaboratory tests was 500 h; the holding time in the apparatus was 2700-3500 h. The composition of the medium and the sample testing conditions are presented in Table 1; the corrosion rate of the samples after their testing under laboratory and industrial conditions are presented in Table 2. The results obtained show that in the medium found in the still and in the upper part of the dehydration column, the alloy 1~110 and welded joints of this alloy exhibit sufficient corrosion resistance. The alloy t~125 and welded joints of this alloy are stable in the media found in the flow separator and the still of the light-fraction distillation column. By means of metallographic investigations we established that in welded joints of zirconium we observe slight general corrosion at a depth down to 0.03 ram. The potentiodynamic curves obtained in the media found in the upper part and the still of the dehydration column are presented in Fig. 1. Judging from the very insignificant self-dissolution current, we can say that zirconium is found in the passive state, connected with the presence of an oxide film on the metal whose ionic conductivity is probably very small. An increase in the thickness of this film upon oxidation significantly increases its resistance.
184
The corrosion rate of titanium has been investigated in detail [2] in acetic acid media containing water; the corrosion rate of zirconium increased with a decrease inthe water content (the still of the dehydration column), but remains acceptable. Welded joints of zirconium alloys made by argon-shielded arc welding are practically as stable as the base metal in acetic acid media. Thus our investigations have shown that it is expedient to use zirconium alloys in acetic acid media, such as in manufacture of equipment for acetic acid media, such as in manufacture of equipment for acetic acid production.
REFERENCES .
2.
A. N. Kuzyukov, L. V. Zakseva, A. N. Kuzyukova, and V. I. Romaniv, "Corrosion of nickel-molybdenum alloys in media involved in acetic acid production," Khim. Prom., No. 7, 22-24 (1985). L. M. Pischik, A. M. Tsinman, and N. I. Bal'vas, "Application of titanium for manufacture of equipment in acetic acid production," Khim. Neft. Mashinostr., No. 5, 28-29 (1983).
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