NEW

MATERIALS

CORROSION

AND

CORROSION

RESISTANCE

OF

CHROMIUM-MANGANESE ACID

CONTROL

AS-43

STEEL

IN

NICKEL-FREE NITRIC

MEDIA A. L. Sotnichenko, and V. S. Yarkovoi

G.

I.

Agapov,

UDC 620.193:669.15.24'74-194

C o r r o s i o n r e s i s t a n c e of c h r o m i u m - m a n g a n e s e stainless steels in many c o r r o s i v e media is v e r y close to that of c h r o m i u m - n i c k e l steels but still c o n s i d e r a b l y lower than, for example, that of Khl8N10T steel. T o r e p l a c e Khl8N10T steel in s o m e types of chemi.cal industry equipment the NIKIMT developed a new AS-43 austenitic c o r r o s i o n - r e s i s t a n t c h r o m i u m - m a n g a n e s e steel in which nickel has been r e p l a c e d by m a n g a n e s e , nitrogen, and other e l e m e n t s [1, 2]. Rolled AS-43 steel is produced by Chelyabinsk Metall u r g i c a l Plant and the Moscow "Serp i Molot" plant. T e c h n i c a l specifications TU/ChMZ-212-71, 213-71, 214-71, and 215-71 have been issued for ingots (weighing up to 4.5 tons), b a r s (diameter o r side of s q u a r e up to 180 ram), sheet (2-20 m m thick, to 1600 m m wide and to 6000 m m long) and tube blanks made f r o m this m a t e r i a l . C o r r o s i o n r e s i s t a n c e of AS-43 steel was investigated in n i t r i c acid as a function of t e m p e r a t u r e and n i t r i c acid c o n c e n t r a t i o n using 40 • 20 • 5 m m s p e c i m e n s . The analysis of investigated steel was as follows (%): 0.05C: 0.36 Si; 18.2 Cr; 16.3 Mn; 0.36 N2; 0.4 Nb; 0.005 ]3 (calculated); 0.05 Ce (calculated); 0.008 S; 0.014 P; and 0.83 Ni. C o m p a r i s o n s p e c i m e n s of TABLE i Kh18N10T steel were cut f r o m plate in the condition of delivery. The s p e c i m e n s of AS 43 s t e e l were cut ~orrosion rate, g/rn z h at a temperaNitric acid cure of ~ C Stee 1 out f r o m plates forged at 1200 and 1000~ (beginning concentragrade and end of forging respectively) and heat t r e a t e d to tion, % 60 ~ loo Iboiling obtain an austenitic s t r u c t u r e (heated to ll00~ held for 2 h, and cooled in water). o,om 0,070 0,20 o,~ AS-43 The results of testing AS-43 and KhlSN10T 45 steels in 45% and 65% n i t r i c acid solutions for 300 h 0,045 0,014 0,062 0,an KhI8NOT (changing the solution a f t e r each 50 h) at t e m p e r a t u r e s f r o m 60~ to boiling point, a r e given in Table 0,028 0,180 0,55 7,0 AS-43 1. It has been seen that a r i s e in solution t e m p e r a ture i n c r e a s e s the c o r r o s i o n rate of c h r o m i u m 0,008 0,080 1 0,30 / 1,4 m a n g a n e s e and Khl8Nt0T s t e e l s . The m o s t intensive c o r r o s i o n has been o b s e r v e d in 45% and 65% solutions at t e m p e r a t u r e s in e x c e s s of 100~ TABLE 2 An i n c r e a s e in the c o r r o s i o n rate of AS-43 Corros_____}on'ratez _g/mfh duri__ngthe testperiod of-hsteel following an i n c r e a s e in n i t r i c acid t e m p e r a Stee 1 60 'i 450 grade ture f r o m 100~ to boiling point is m o s t s t r o n g l y I noticeable in a 65% solution: in the 45% solution a 0,50 0,56 AS-43 OfiO 0,60 1 0,60 t e m p e r a t u r e i n c r e a s e of this magnitude i n c r e a s e s the c o r r o s i o n rate of AS-43 steel 2.5 t i m e s and in 0,40 0,87 0,i2 Kh18N10T 0,22 ' I 0,27 the 65% solution - m o r e than 10 t i m e s . This is a s s o c i a t e d with the fact that in the 45% solution the

1

t

T r a n s l a t e d f r o m Khimicheskoe i Neftyanoe Mashinostroenie, No. 10, pp. 23-24, October, 1974.

9 19 75 Plenum Publishing Corporanon, 227 West 17th Street, New York, N.Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

901

TAB LE 3 Nitric acir concentration,~/o

45

-~Stationary potential%(relative to Steel n. l~ e.) at a term ,erature of, ~ grade 10o Iboiling

~176

A~43

4,1',10 I +1,24

94-1,27 I +1,30


+i,]o I + 1 , ~

-t-t;16 [ +1,29

4..1,32 I 4,,1,34

Khl8N10~ 4-1,16 [ .4-1,18 i

4,,I,18 ] 4-1,22

AS-43

c o r r o s i o n of c h r o m i u m - m a n g a n e s e s t e e l s t a r t s f r o m the p a s s i v e state, while the i n c r e a s e in the c o r r o s i o n r a t e at elevated solution t e m p e r a t u r e s is due to an i n c r e a s i n g dissolution of the p a s s i v e f i l m . T h e c o r r o s i o n m e c h a n i s m of AS-43 s t e e l in the m o r e highly c o n c e n t r a t e d (65%) acid is a different one: s t e e l c o r r o d e s at high r a t e s s t a r t i n g f r o m the o v e r p a s s i v a t i o n condition;

An i n c r e a s e in the c o r r o s i o n r a t e of Khl8N10T s t e e l following an i n c r e a s e in the solution t e m p e r a ture f r o m 100~ to boiling point takes place l e s s 9'Duration of measurements 4 h. s h a r p l y b e c a u s e during the c o r r o s i o n of Khl8N10T s t e e l f r o m the o v e r p a s s i v a t i o n state, the solution r e c e i v e s f r o m the s t e e l m a i n l y iron and c h r o m i u m ions w h e r e a s during the c o r r o s i o n of AS-43 s t e e l the solution r e c e i v e s iron, m a n g a n e s e as ",vell as c h r o m i u m ions. T h e data obtained shows that AS-43 steel, a s s e s s e d by the f i v e : p o i n t c o r r o s i o n r e s i s t a n c e s c a l e [3], t r e a t e d at t e m p e r a t u r e s up to 90~ and up to 60~ in 45% and 65% n i t r i c acid r e s p e c t i v e l y , belongs to stable m a t e r i a l s ( c o r r o s i o n r a t e l e s s than 0.10 g / m 2 'h). At 90~ up to boiling point and f r o m 60~ to 100~ in the 45~c and 65% n i t r i c acid solutions r e s p e c t i v e l y this s t e e l belongs to the stable m a t e r i a l s ( c o r r o s i o n r a t e 0.1-1.0 g / m 2 "h).

65

T a b l e 2 shows the v a r i a t i o n of the c o r r o s i o n r a t e of AS-43 and Kh18N10T s t e e l s in boiling 45% n i t r i c acid solution (change of solution at 50 h intervals) a c c o r d i n g to t e s t duration. It m a y be s e e n that the c o r rosion r a t e of AS-43 s t e e l d e c r e a s e s only insignificantly w h e r e a s the c o r r o s i o n r a t e of KhlSN10T s t e e l in: c r e a s e s , i . e . , the c o r r o s i o n r a t e of both s t e e l s d e t e r m i n e d in prolonged (500 h) c o r r o s i o n t e s t s c o n v e r g e . The t e s t s w e r e a l s o used to study the effect of the s e l f - d i s s o l u t i o n p r o d u c t s of s t e e l s on their c o r r o sion in n i t r i c acid. It was found that the c o r r o s i o n of AS-43 s t e e l kept in boiling 45% nitric acid for 300 h w i t h r e p l a c e m e n t of the acid at 50 h i n t e r v a l s , was 0.60 g / m ~- h while the c o r r e s p o n d i n g Value f o r Khl 8N10T s t e e l was 0.33 g / c m 2 .h; without r e p l a c i n g the solution the c o r r o s i o n r a t e of both s t e e l s i n c r e a s e d noticeably, to 1.9 and 1.25 g / m 2 . h r e s p e c t i v e l y . T h i s is due to the fact that during the c o r r o s i o n of c h r o m i u m m a n g a n e s e s t e e l the solution r e c e i v e d not only c h r o m i u m but also m a n g a n e s e ions, A g r e a t e r saturation of solution by ions of higher valency m e t a l s (Cr 6+, Mn 7+) during the c o r r o s i o n of c h r o m i u m m a n g a n e s e s t e e l shifts its potential into the region of higher p o s i t i v e values than the potential of Khl8N10T s t e e l (Table 3). One of t h e m a i n r e q u i r e m e n t s concerning the c o r r o s i o n r e s i s t a n c e of s t e e l s is the absence of a t e n dency to 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 . The testing of AS-43 s t e e l f o r 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 (ICC) by the AM method (GOST 6032-58) showed the a b s e n c e of a tendency to this type of c o r r o s i o n f a i l u r e a f t e r the austenitic t r e a t m e n t o r a f t e r t h e p r o v i s i o n o f h e a t i n g f o r 0 . 5 - 7 h a t t e m p e r a t u r e s of 600, 650, 700, 750, 800, and 900~ A high stability of c h r o m i u m - m a n g a n e s e s t e e l with r e g a r d to ICC is due to its low (0.05%) c a r b o n content and to the introduction of stabilizing e l e m e n t s (0.3-0.7~c N b a n d 0.005% B) into the steel. It should be noted that the c h r o m i u m - m a n g a n e s e s t e e l alloyed with these e l e m e n t s r e t a i n s p r a c t i c a l l y c o m p l e t e l y its austenitic s t r u c t u r e . F e r r i t e content in the s t r u c t u r e r e m a i n s below 5%. The new s t e e l g r a d e m a y be r e c o m m e n d e d for use as s t r u c t u r a l m a t e r i a l in the m a n u f a c t u r e of p a r t s and s u b - a s s e m b l i e s of c h e m i c a l industry equipment. LITERATURE 1. 2. 3.

902

CITED

A . I . Sotnichenko and V. S. Yarkovoi, A u t h o r s ' C e r t i f i c a t e , No. 212,541, Otkrytiya, Izobreteniya, P r o m y s h t e n n y e O b r a z t s y , T o v a r n y e Znaki, No. 9 (1968). A, L. Sotnichenko and V. S. Yarkovoi, A u t h o r s ' C e r t i f i c a t e No. 331,112, Otkrytiya, Izobreteniya, P r o m y s h l e n n y e Obraztsy, T o v a r n y e Znaki, No. 9 (1972). F . F . Khimushin, Stainless Steels [in Russian], Metallurgiya, Moscow (1967), p. 484.