AN I N V E S T I G A T I O N CONSTRUCTIONAL PRODUCING

OF

CORROSION

MATERIALS

CAUSTIC

V. Y u . B a b k i n a , I . K. V a s i l ' e v a , a n d E. I. K o g a n

RESISTANCE

IN A M E D I U M

OF

FOR

SODA E. O.

G. V.

Chub, Gapunina,

UDC 620.193.4:661.322.1

C o r r o s i o n r e s i s t a n t steels and copper are widely used for the production of equipment for the production of caustic soda. Experience in the use in this industry of evaporating equipment with copper tubing indicates that its service life depends to a significant degree upon the s e r v i c e conditions and quality of the metal. F o r example, there have been c a s e s of complete failure of the copper tubing of evaporation equipment after a y e a r of operation in the f i r s t stage of evaporation (NaOH content in the caustic soda solution 140-250 g/liter) at 120 and 149~ The tubes c o r r o d e d in the upper portion in the zone of the rolled region and failure of all of the tubes of the heating c h a m b e r was s i m i l a r . Metallographic and c h e m i c a l analyses established that the tubes have portions contaminated with copper oxide, phosphorus, and lead. On the portions of the tubing damaged by c o r r o s i o n grains of metal with t r a c e s of deformation in the f o r m of parallel slip lines are observed. In equipment the casing and tube sheets of which are made of c o r r o s i o n r e s i s t a n t steel and iron there is solution of the r i m s of tubing, which is not seen in equipment made of carbon steel. In some soda plants copper tubing lasts f r o m four to ten y e a r s . Cases are known in which copper tubing placed in carbon steel equipment has failed after eight or nine months as a r e s u l t of becoming thinner over its whole length. Such a difference in the c o r r o s i o n behavior of copper made it n e c e s s a r y to conduct investigations of the effect of technological f a c t o r s on the c o r r o s i o n r e s i s t a n c e of steels and copper in the medium for the p r o d u c tion of caustic soda. The c o r r o s i o n rate of 20, 12Kh18N10T, 08KhlSN10T, 20Kh23N18, 15Kh25T, 08Kh22N6T, and 12Kh21N5T steels and M1 copper was studied in relation to the t e m p e r a t u r e and concentration of caustic soda containing production impurities (NaC1, N H4C1, Na2CO~, F e 2+, Fe 3+, Na2S203) and the p r e s e n c e of oxygen. Samples w e r e made of tubing of the tested m a t e r i a l s . salts and production solutions w e r e used.

In the t e s t s solutions made of chemically pure

The r e s u l t s of investigation of the effect of t e m p e r a t u r e and concentration of NaOH solutions on the c o r rosion behavior of copper under stationary conditions and in boiling of the solution a r e shown in Fig. 1. It may be seen that with an i n c r e a s e in t e m p e r a t u r e the c o r r o s i o n rate of the copper samples i n c r e a s e s continuously. With an i n c r e a s e in solution concentration at a constant t e m p e r a t u r e the rate of solution of the copper samples d e c r e a s e s sharply. The data obtained is an indication of the significant effect of oxygen on the c o r r o s i o n p r o c e s s e s in copper. The investigation of the relationship of the c o r r o s i o n rate to the oxygen content was made in a solution containing 200 g/liter of NaOH at 95~ for 96h. The oxygen was r e m o v e d by boiling the solution for 20 min. In tests with aeration of the solution nitrogen containing 1% oxygen was p a s s e d through it continuously. As an analysis of the test r e s u l t s shows, the c o r r o s i o n rate of the copper samples i n c r e a s e d by 60 times with a e r a tion and agitation of the solution by the flow of gas. In the p r e s e n c e of oxygen the c o r r o s i o n rate of samples completely i m m e r s e d in a boiling solution i n c r e a s e s by 8.6 times and that of samples partially i m m e r s e d in the solution by 12 times. In the latter case pitting c o r r o s i o n is observed at the boundary between the phases. It was established that the addition of NaC1 does not influence the intensity of c o r r o s i o n of these metals. In the p r e s e n c e of NH4C1 the c o r r o s i o n rate of copper samples i n c r e a s e s by 1.2 times. With the addition to the solution of up to 0.8 g/liter of Na2S203 the intensity of c o r r o s i o n of 20 and 12Khl8N10T steels i n c r e a s e s as a r e s u l t of the fact that during evaporation of the NaOH solution decomposition of the Na2S203 o c c u r s . The r a t e s of c o r r o s i o n of 20 and 12KhlSN10T steels i n c r e a s e s by 3 and 2.5 t i m e s , respectively. I n t h e p r e s e n c e o f o x y g e n T r a n s l a t e d f r o m Khimicheskoe i Neftyanoe Mashinostroenie, No. 2, pp. 22-24, F e b r u a r y , 1978.

142

0009-2355/78/0102-0142 $07.50

9 1978 Plenum Publishing Corporation

TABLE

I

Corrosion T rate, g/m~ 9 ~o

"

.

.

Material of pair samples m

SO .

.

.

MI copper 20 steel

g~

.

;o ~ 12Kl'6.8N10T . . . . . . . . . .

+

17

0---5~l ~l 0,002

........... '

. .

MI copper

.........

20 smel

.........

SCh 18-36 iron

........

.......

1~1 c o p p e r

.......

12KhI8N10T

.......

MI c o p p e r

.......

+

o--
o,33

0,068

0,008

O,OI'

+

I

o,o~

0~,48

+

0-~

030 0,22

,, o-~ I~176I ~ 0,033 0,024

1,50 4 2-"~'6--!,,05 I

I o,o2 1 0,024

+

~I1 copper

SCh 18-36 iron

0,012 0,005 0-7 I

1,0

~0 steel

12Khl8N10T

0.36 I

0,79

0,~

12KhI8NIOT

I.~

0,04

.....

-

.....

+

.....

-

iolS

-

~

1,05

~1

o,0~

22 ~

0,m 0,O57

t

_

t

Note, The upper and lower Rgures are data for samples with ratios of area of 1 : 1 and 1 : 20, respectively.

I |

1 [

the c o r r o s i o n r a t e of c o p p e r a l s o i n c r e a s e s . A d e n s e b l a c k l u s t r o u s f i l m is f o r m e d on the c o p p e r . It is p o s s i b l e t h a t with the p a s s a g e of t i m e t h i s f i l m would p r o v i d e p r o t e c t i o n of the m e t a l f r o m the a c t i o n of the s o l u tion. On the r e m a i n i n g s a m p l e s d a r k g r a y f i l m s a r e f o r m e d which a r e e a s i l y r e m o v e d by w a s h i n g and m e c h a n ical cleaning. T h e e f f e c t of F e 2+ and F e 3+ i o n s on the c o r r o s i o n of 12Kh18N10T, 0 8 K h l 8 N 1 0 T , 20Kh23N18, 15Kh25T, 08Kh22N6T, and 12Kh21N5T s t e e l s a n d c o p p e r w a s s t u d i e d by a d d i n g f r o m 10 to 200 m g A i t e r of FeC12 o r FeC13 and 0.8 g / l i t e r of Na2S203 to the NaOH s o l u t i o n s . In s o l u t i o n s of m e d i u m c o n c e n t r a t i o n (350 g / l i t e r of NaOI-I) a t 95~ the p r e s e n c e of i r o n i o n s does n o t have a n e f f e c t on the c o r r o s i o n r e s i s t a n c e of t h e s e m a t e r i a l s . In s o l u t i o n s of h i g h e r c o n c e n t r a t i o n (650 g / l i t e r of NaOH) with a n i n c r e a s e i n the q u a n t i t y of i r o n i o n s the c o r r o s i o n r a t e of a l l of the t e s t e d s t e e l s i n c r e a s e s . F o r e x a m p l e , with the a d d i t i o n to the NaOH s o l u t i o n of 100 m ~ / l i t e r of i r o n ions the c o r r o s i o n r a t e i n c r e a s e s by 2 0 - 3 5 t i m e s . With the p r e s e n c e in the s o l u t i o n of Na2S2O 3 the n e g a t i v e e f f e c t of the i r o n i o n s a p p e a r s to a s i g n i f i c a n t l y g r e a t e r d e g r e e . With a c o n t e n t of 50 m g / l i t e r of i r o n i o n s in the s o l u t i o n the c o r r o s i o n r a t e i n c r e a s e s by 100-200 t i m e s . The s a m e p h e n o m e n o n is o b s e r v e d i n e v a p o r a t i o n of NaOH s o l u t i o n s f r o m 350 to 650 g / l i t e r .

143

K, m m / y r

~,

am 2

~,o

i

4,o

o,J?\\ \

a,o

o,~

2,0

o

-~ ;5

-

50 f

iO0 ~o0

600

75

-

-

~,o

-

100 t,~ r

r

8o0 roooe, gaiter

Fig. 1

o

24

~8

72

,,o8

I

r

Fig. 2

Fig. I. The relationship of the c o r r o s i o n rate K of copper s a m p l e s to t e m p e r a t u r e t and concentration c of an NaOH solution: - - - ) K = f(c) at t = 95~ 1) K = f(t) under stationary conditions with c = 200 g/liter; 2) under stationary conditions with c = 650 g/liter; 3) with boiling of a solution containing 650 g/liter of NaOH. Fig. 2. The change in density i of the anodic c u r r e n t with time T in testing p a i r s of contacting samples of metals in evaporating equipment: 1) 20 s t e e l - M 1 copper; 2) 12Kh18N10T s t e e l - M1 copper. To determine the effect of the rate of m o v e m e n t of the medium and the p r e s e n c e of a solid phase t e s t s were made on samples at 20, 12Kh18N10T, 08Kh22N6T, 12Kh21N5T, and 15Kh25T s t e e l s in a solution with a concentration of 650 g / l i t e r of NaOH made f r o m chemically p u r e salt with the addition of 0.8 g/liter of Na2S203 and 132 g/Jiter of Na2CO 3. A gate m i x e r was used for agitation of the solution. The r e s u l t s of these tests show an i n c r e a s e in the c o r r o s i o n rate of steels in c o m p a r i s o n with their c o r r o s i o n rate under static conditions, e s p e c i a l l y in the p r e s e n c e of Na2S203. The steels with a reduced nickel content have a significantly p o o r e r r e s i s t a n c e to the abrasive action of the medium. The c o r r o s i o n rate of 15Kh25T c h r o m i u m steel is p r a c t i c a l l y the same as that of 20 carbon steel. T h e r e f o r e the low c o r r o s i o n r e s i s t a n c e of heating tubes of 12Kh18N10T steel in evaporating apparatus operating in the last stage of evaporation may be explained by the eroding action of the soda going out of solution and also by the p r e s e n c e in the solution of reducing agents (lqa2S203 and Fe2+), which promote failure of the passive film on c o r r o s i o n r e s i s t a n t steel. F o r the purpose of solving the question of the acceptability of the contact of carbon steel, copper, and c o r r o s i o n r e s i s t a n t steel used as basic s t r u c t u r a l m a t e r i a l s in the manufacture of equipment for the p r o d u c tion of caustic soda a study was made of the basic c h a r a c t e r i s t i c s of galvanic p a i r s made of these m a t e r i a l s , the change in the strength of the c o r r o s i o n c u r r e n t with t i m e , the polarity of the e l e c t r o d e s , and the l o s s e s of weight of the e l e c t r o d e s in contact of them and in the isolated condition. L a b o r a t o r y tests were made in production solutions of NaOH with concentrations of 140 and 650 g/liter at 95~ for 96 h. The ratio of the a r e a s of the samples of the p a i r s was 1:20, which c o r r e s p o n d s to the ratio of the operating surface of the casing and the tube sheets of the evaporation apparatus to the surface of the heating tubes. The c h a r a c t e r i s t i c s of the p a i r s with a 1:1 ratio of a r e a s were also studied. The r e s u l t s of tests of the samples a r e shown in Table 1. The data in Table 1 indicates that as a rule the c o r r o s i o n rate of contacting samples is m o r e than the c o r rosion rate of isolated s a m p l e s , especially under conditions where the sample is the anode of the galvanic pair. Steel and iron in a pair with M1 copper and 12Kh18N10T steel in a concentrated solution of NaOH with a ratio of a r e a s of 1:20 are m e t a l s with low r e s i s t a n c e . The c o r r o s i o n rate of copper samples is p r a c t i c a l l y the same. The copper samples are covered with a black film which adheres strongly to the metal while on 20 and 12Kh18N10T s t e e l s a film is not formed. 144

A study was also made of the c o r r o s i o n behavior of 20 s t e e l - M 1 copper and 12Khl8N10T - M1 copper p a i r s (with a ratio of sample a r e a s of 1:20) directly in the equipment of the last stage of evaporation (with a concentration of 650 g/liter of NaOH) for seven and five days, r e s p e c t i v e l y . The e l e c t r o d e s were mounted in the c o v e r of the a c c e s s hatch. F o r 2-3 h after starting up the apparatus the copper samples of the tested p a i r s operated as the anodes, then their polarity changed, and until the end of the tests the copper samples operated as the cathodes. In these tests an interesting relationship was noted. In the period when the a p p a r a tus is empty or being s t a r t e d up, when the e l e c t r o d e s are in s p r a y s or the v a p o r s of boiling solution, there is a change in the poles of the e l e c t r o d e s with the copper sample becoming the anode of the galvanic element. In these periods the density of the anodic c u r r e n t i n c r e a s e d by s e v e r a l t i m e s (Fig. 2). The low c u r r e n t density during continuous operation of the apparatus is an indication of the acceptability of contact of copper with c o r r o s i o n r e s i s t a n t steel. Carbon steel in a pair with copper c o r r o d e s m o r e rapidly. On the basis of these investigations it is possible to conclude that the formation of local failures of copper tubing is caused by the low quality of the metal and mechanical s t r e s s e s o c c u r r i n g in the zone of the rolled band. The p r i m a r y f a c t o r s intensifying the c o r r o s i o n of the copper tubes are oxygen of the a i r , ammonia which periodically e n t e r s with the code solution, high t e m p e r a t u r e , and the p r e s e n c e of thiosulfate compounds in the solution. The installation of copper tubes into steel and iron tube sheets does not affect the c o r r o s i o n r e s i s tance of the tubing but it does a c c e l e r a t e failure of the tube sheets. Despite p r a c t i c a l l y the same rate of c o r rosion of steel and iron in contact with copper, it is p r e f e r a b l e that tube sheets be made of iron, which is not subject to c o r r o s i o n cracking in NaOH solutions. F o r the purpose of i n c r e a s i n g the s e r v i c e life of the tubes we may r e c o m m e n d the use for their p r o d u c tion of types of copper containing a small quantity of impurities (M1, MO), a reduction in the oxygen c o n c e n t r a tion in the solution entering the apparatus as a r e s u l t of p r e l i m i n a r y heating of the solution and supplying it to a section for improving the separation of a i r , elimination of the possibility of the e n t r y of ammonia into the apparatus with the solution, reducing the boiling point of the solution by providing a higher vacuum, using a method of heat t r e a t m e n t of the tubing which does not allow r e c r y s t a U i z a t i o n of the copper and the o c c u r r e n c e of internal s t r e s s e s , rolling of the tubing with m i n i m u m deformation of the metal, and providing stable o p e r a tion of the equipment which does not allow a reduction in the solution level below the upper tube plate. The fulfillment of these r e c o m m e n d a t i o n s would make it possible to i n c r e a s e the service life of copper tubes to 8-10 years. D e c r e a s i n g the c o r r o s i o n rate of heating tubes of c o r r o s i o n r e s i s t a n t steel in evaporation equipment operating in the last stage of evaporation is possible by reducing the iron concentration in the solutions, that is, by producing the equipment completely of c o r r o s i o n r e s i s t a n t steel, careful r e m o v a l of the solid phase f r o m the solution supplied to the e v a p o r a t o r , and a reduction in the concentration of thiosulfate compounds and the degree of their decomposition. Contact of copper with c o r r o s i o n r e s i s t a n t steel is acceptable in concent r a t e d caustic soda solutions but in solutions of weak o r medium concentration leads to failure of the n o n f e r rous metal.

145