duration is 192 and 504 h, respectively. Testing of specimens with a change of solution imitates the actual service conditions of equipment and the results of such tests provide a basis for a m ore objective evaluation of the possible service conditions of the coating. LITERATURE

1. 2. 3.

4.

CITED

G. Ya. Vorob'eva, Corrosion Resistance of Materials in Aggressive Media of t,~ Chemical Industry [~, Russian], Khimiya, Moscow (1967). V. V. Vargin, V. V. Grachev, I. Ya. Zorina, et al., "Resistance of an acid-resisting enamel coating at elevated temperatures and p r e s s u r e s , " Steklo Keram., 10, 25-26 (1971}. Yu. V. Rogozhin, V. M. Gladush, L. V. Markina, and A. K. Miglo, "Some technological p a r a m e t e r s of enameling and the properties of enamel coatings for high t e m p e r a t u r e s , " Steklo K e r a m , ~ 58-61, (1976). Enameled Equipment. Catalog of Ts]~TIKhinmefetmash (1974).

CORROSION RESISTANCE

IN N I T R I C

ACID MEDIA

L. P. Lozovatskaya, A. I. P i s k u n o v a , I . K. B u r t s e v a , M. N. S h m a t k o , Ya. E. Gol'dshtein, a n d I. A. L e v i n

UDC 669.018.841

At the Chelyabinsk Metallurgical Plant eight 40-ton ordinary heats of 03Kh18N11 steel with different contents of impurity silicon were produced. The method of steel melting with a reduced silicon content was developed by the Chelyabinsk Scientific-Research Institute of Metallurgy. The composition of heats is given in Table 1. After hot rolling the sheets intended for making specimens were hardened in water at temperatures of 1080, 1120, and 1150~ for 30 rain. After hardening some specimens were subjected to sensitization t e m p e r ing, and heated at 650~ for 1 h. The normally hardened specimen were tested for corrosion in 20, 40, 56, and 65% solutions of HNO3 at t e m p e r a t u r e s of 100, 120, and 130~ and at boiling point and also in a boiling solution of 27% HNO3 containing 4 g/liter of Cr e+ ions. The corrosion rate was determined from the mass loss of two simultaneously tested specimens, from one of which a metallograpl~ic specimen was ground for determining the type of etching of the metal under a microscope. After corrosion tests one of the two specimens was cut across and the resulting surface ground and polished for measuring the intercrystalline corrosion (ICC) penetration depth. Additional electrochemical tests were carri ed out to determine the effect of the steel potential in HNO3 solutions on the etching rate of grain boundaries as a function of silicon content in steel, using the method deScribed in [1]. The tempered specimens were tested for corrosion by the DU method (GOST 6032-75) in boiling 65% HNO3. The corrosion rate was calculated after each test cycle (five cycles of 48 h and each) from the mass loss of two simultaneously tested specimens. Corrosion tests on welded specimens were also c a r r i e d out by the DU method. We carried out the metallographic examination of specimens simultaneously with the corrosion tests in order to determine the type of corrosion and the time needed for ICC to become apparent in the weld zone and the thermal effect area. Welding was performed in argon atmosphere using a welding rod whose analysis was selected in accordance with the chemical composition of the heat concerned. The low silicon heats were welded with Sv-03Kh18Nll wire with a silicon content reduced to 0.14% and with 0.015% C, while the heats with a higher silicon content were welded with ordinary Sv-03KhlSNll wire. Table 2 gives the corrosion test results obtained in nitric acid media of varying oxidation ability of hardened 03Khl8Nll steel specimens with varying silicon contents. It may be seen that in boiling solutions of 56 and 65% HNO~ corrosion rates of steels with different silicon contents differ relatively little. After testing, the specimen surface remained smooth, and no falling-out of grains as a result of ICC Was observed. Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No.10, pp. 22-24, October, 1978.

914

0009-2355/78/0910-0914507.50

9 1979 Plenum Publishing Corporation

TABLE

i

No. IZ of heat 1

2 3 4 5 6

Analysis,% Mn

: 0,025 0,021 0,025 0,025 0,020

1,0l 0,74 0,90 0,95 1,00

7

0,023 0,030

0,42 3;32

8

0,020

I, I0

SI 0,14 O, 17 0,20 0,23 0,28 0,48 0,49":

0,76r 0,75

P 0,011 0,014 0,015 0,012 0,017 0,012 0,014 0,005

0,020 0,018 0,014 0,015 0,017 0,018 0,018 0,015

Cr 17,82 ] 16,25 [ I8,04 [ 18,32 [ 16,35 [ I7,80 t 17,66 ;18,16

" NI II,2I

1t,70 11,80 11,6"7 11,40 I1,15 10,8I 11,46

AI

03

CJIeabl 0,0230 0,004 0,006 0,005 0,028

0,~49

0,014

0,0054"~ 0,0056 ~

0,009 0,011

0,0194 O,0084

0,0043 0,0061";

N2

Cu I TI

i 0,050 [ 0,013 0,031 [

o,o3s oSiol -I o,loo I o,oi 0,03I t o , o 9 o I -

0,043 0,041.:I

0,140-[~ 0,012 ]

0,033

0,100 [ rr,3.cr

---

~In the first ingot. +In the second ingot.

Fig. 1. Metallographic specimens of hardened 0 3 K h l S N l l steel with different silicon contents after testing in boiling 56% HNO 3 for 35 h. • a) 0.14% Si; b) 0.78% Si. However, Fig. 1 shows that under these conditions steels with a higher silicon content suffer ICC, while steels containing --<0.2% Si show a different typeof corrosion: Generally an etching of twins and their boundaries is observed which, from the c o r r o s i o n point of view, is l e s s harmful; grain boundaries are etched to a l e s s e r d e g r e e , only in patches. The investigations into the effect of the potential of 0 3 K h l S N l l steel in 56% II N Q on the etching rate of grain boundaries confirmed beyond any doubt that silicon content affects this rate. If the potential is the same, the anode polarization causes an i n c r e a s e in silicon content from 0.14 to 0.78%, resulting in an i n c r e a s e in the etching rate of boundaries. The etching rate can i n c r e a s e up to 20 times depending on the potential. If for the acceleration of c o r r o s i o n tests Cr 6+ ions are introduced into the nitric acid solution, then the effect of silicon content on the c o r r o s i o n rate could be detected by the method of corrosion l o s s e s (see Table 2). The [CC was easily observed on the specimen c r o s s section prepared for the metallographic examination as early as after five test c y c l e s . The data of Table 2 shows that the depth of ICC of a steel containing 0.7~0 Si (second ingot of No.7 heat) e x c e e d s by m o r e than 10 times the depth of ICC in a steel containing 0.14% Si (heat No. 1). Figure 2 shows the results of 192-h corrosion tests carried out in nitric acid of various concentrations at temperatures of 100, 120, and 130~ on specimens of hardened 03Kh18Nll steel containing 0.2 and 0.78% Si. It shows that a reduction in silicon content results in a considerable d e c r e a s e of c o r r o s i o n l o s s e s . This can be seen with particular clarity in t e s t s which are, with respect to temperature and concentration conditions, m o s t rigid. The c o r r o s i o n test results of 0 3 K h l S N l l steel s p e c i m e n s in boiling 65% solution and HNO 3 are given in Table 3. A comparison of Tables 2 and 3 shows that the sensitizing tempering of 03Kh18N11 steel containing -< 0.025% Si has practically no effect on its c o r r o s i o n rate in a boiling 65% solution of HNO3, regardless of the silicon content in steel. Sensitizing tempering has a considerable effect on the corrosion rate of 0 3 K h l 8 N l l steel containing more than 0.025% Si (according to GOST 5632-72 the carbon content of 03Kh18Nll steel should not exceed 0.03%). With this carbon content a change in silicon content has a considerable effect on the c o r rosion rate of tempered steel (first and second ingots of heat No. 7).

915

TA BLE 2 Av. corrosionrate (mm/yr) of hardened 03KhlSNll steel in 5 test cycles in boiling solutions NO. of 65% containing 56% heat 4 g/liter of Hl~O3 HN03

0.188

Cr6 ions-

0,198 0,250 0.206 0,165 0.197 0,200

0.228

0.177 0.165 0.131 0.155

Depth of ICC (ram) of hardened 03t
ing 4 g/liter of cr ~ ions

1,370 1,090 2,540 3,325 3,815 5,864

0,025 0,043 0,078 0,I84 9 O, 167 O,292

*First ingot. r ingot.

k, m m / w r

i~, mm,'y~ar 0,8

2

,0 V 0

20

40

t 20

i

L I

~-0

5~ ,%

~0 C~%

Fig. 2, C o r r o s i o n rate K of hardened 03Kh18N13 steel w i t h d i f f e r e n t s i l i c o n eontents

as a function of concentration of HNO 3 at various t e m p e r a t u r e s : a) at 130~ b) at 120~ c) at 100~ l) 0.78% Si; 2) 0.2% SL Table 3 also shows that an i n c r e a s e in hardening t e m p e r a t u r e has p r a c t i c a l l y no effect on the c o r r o s i o n rate of t e m p e r e d steel in boiling 65% HNO 3 while the c o r r o s i o n rate of steel containing m o r e than 0.025% St considerably d e c r e a s e s . The r e s u l t s of c o r r o s i o n tests and of the metallographic examination of welded specimens are given in Table 4. The data of Tables 2 and 4 shows that with r e g a r d to c o r r o s i o n l o s s e s in boiling 65% HNO3, the welded specimens differ little f r o m those of hardened base metal, The welded specimens of steel containing 0. 03% C and 0.7~0 Si (second ingot of heat No.7)have p r a c t i c a l l y the same c o r r o s i o n r e s i s t a n c e as hardened specimens and a g r e a t e r c o r r o s i o n r e s i s t a n c e than the t e m p e r e d specimens (Table 3). Metallographic examination under a m i c r o s c o p e established that time to the appearance of ICC in the welding zone d e c r e a s e s 5 times when the silicon content of steel i n c r e a s e s f r o m 0.14 to 0.78% (Table 4). Since experience shows that an equipment working under conditions of intense oxidation usually fails because of ICC, in this investigation we d e t e r m i n e d the c o r r o s i o n r e s i s t a n c e of steel f r o m its r e s i s t a n c e to ICC. Only in the case w h e r e c o r r o s i o n loss was not indicative of the r e a s o n of failure* was the r e s i s t a n c e to ICC determined by the metaUographic or e l e c t r o c h e m i c a l method. If the r e s u l t s of all investigations a r e taken into account, it can be concluded indisputably that in oxidizing media the rate of ICC of hardened 03Khl8Nll steel becomes considerably g r e a t e r (sometimes up to 20 times)with an i n c r e a s i n g silicon content. The type of c o r r o s i o n has a big effect on the i n c r e a s e in the rate of ICC, since with d e c r e a s i n g silicon content the pattern of the i n t e r c r y s t a l l i n e etching of steel begins to change (Fig. 1). These r e s u l t s a r e in full a g r e e m e n t with the results of [1, 2] obtained in the investigation of experimental heats. The investigation of c o r r o s i o n *The results of c o r r o s i o n tests showed that the methods used in determining the tendency to ICC f r o m c o r r o sion l o s s e s are not always sufficiently sensitive. These methods can be used to determine ICC only when the falling-out of grains f r o m the metal is observed where the c o r r o s i v e medium is sufficiently active with r e s p e c t to grain boundaries or when the tests last so long that ICC p e n e t r a t e s to a depth g r e a t e r thanthe size of a grain. 916

3 [Av. corrosion rate (mm/yr)of 03KhlSNll stee~s~fi"2 No. [sitizedat 650"Cfor lh and hardenedat different temp. of [after 5 test e..____yele in boiling ~olutionof 6~toHNOs

TABLE

heat )J -

1osO~

t

1

I I~'O~

0.199 0.276 0.180 0,I67 -0,2~!8 0,~82 6,5~ 16,840

3 4 5 6 7t'

1150~

0,196 0,241 0,170 0, I32 0,293 0.19,5 0,064 4,650 0.172

0,17"/ 0,24I 0,180 O. 130 0.2&~ 0,205 0fi52

1,443

0,I~9

*First ingot. ~'Second ingot.

TABLE

4

[No.of heat

[AV.corrosionrate ofw~-dcd t 03Kh'ISNtt steel inmm/yr lafter 5 test eyct~ ~ a boiling [solution of 65,r~HNOs

7"

Time to appear-~ ance of I C e in weld zone, h

0.249 0.161 0,202 0,217

|

*Second ingot:

r e s i s t a n c e of t e m p e r e d specimens showed that the r e s i s t a n c e of ICC of t e m p e r e d 03KhlSNll steel in oxidizing media is considerably affected not only by silicon but also by carbon. If the steel contains m o r e than 0.025% of carbon then this together with silicon approaching its upper limit according to GOST 5632-72, i n c r e a s e s tens of t i m e s the c o r r o s i o n loss in t e s t s by the method DU. Metallographic investigations w e r e c a r r i e d out to find the causes of the combined negative effect of both e l e m e n t s on the life of grain boundaries of t e m p e r e d steel. T h e s e investigations established that the silicon contained in 03KhlSN11 steel a c c e l e r a t e s the formation of carbides n e a r the grain boundaries during s e n s i t i z ing tempering. I t was found that the carbide phase has a common boundary with and is coherently connected to the austenite l a y e r which has a s m a l l e r lattice p e r i m e t e r as the main p a r t of the austenite grain. A reduction in the lattice p a r a m e t e r of the austenite boundary l a y e r n e a r the carbide phase can be explained by the fact that this l a y e r is saturated by silicon atoms segregating on grain boundaries; these atoms are incorporated. in the austenite lattice by the substitution method and, being s m a l l e r than iron a t o m s , make the austenite l a t tice s m a l l e r . investigation r e s u l t s of r e s i s t a n c e to ICC of hardened and t e m p e r e d c o m m e r c i a l l y produced 03Kh18N11 s t e e l showed that a considerable i n c r e a s e in c o r r o s i o n r e s i s t a n c e of this steel in oxidizing m e d i a can be achieved by reducing the carbon content to --<0.025-% and the silicon content to --<0.3%. Experience shows that ~ o d e r n melting methods of steel can maintain these e l e m e n t s within r e q u i r e d limits. The specification TU 14-1-2450-78 has been issued on steel with this carbon and silicon content. LITERATURE 1.

2.

CITED

I . A . Levin, I. K. B u r t s e v a , L. P. Lozovatskaya, and L. K. Zamiryakin, "Investigation of the effect of low silicon concentrations on i n t e r c r y s t a l l i n e c o r r o s i o n Of 000Khl8N13 steel in an oxidizing m e d i u m , " Zashch. Met., 10, 1, 2-7 (1974). L A. Levin, L. P. Lozovatskaya, I. K. B u r t s e v a , and L. K. Zamiryakin, " I n c r e a s e in the r e s i s t a n c e of 000KhlSNll stainless steel to i n t e r e r y s t a l l l n e c o r r o s i o n through overpassivation by reducing the silicon content in the s t e e l , " in: P r o c e e d i n g s of Scientific-Technical Seminar: C o r r o s i o n - r e s i s t a n t Metallic Structural Materials and T h e i r Use in National Economy, Moscow (1974), pp. 53-57.

917