CORROSION UNDER STRESS INVESTIGATION OF

HIGH-STRENGTH

OF

STRESS-CORROSION

STEELS

M. G. Khitarishvili, A. I. Zyubrik, and

IN CERTAIN

I. I. Dikii, I. I. Vasilenko

CRACKING MEDIA UDC 620.194:621.785.7

One of the f a c t o r s limiting the range of industrial applications of h i g h - s t r e n g t h s t e e l s is t h e i r r e l a tively high s u s c e p t i b i l i t y to s t r e s s - c o r r o s i o n cracking, e s p e c i a l l y in h y d r o g e n - c h a r g i n g media. When a s t r e s s e d m e t a l undergoes c o r r o s i o n in a m e d i u m with hydrogen depolarization, the adsorption of hydrogen ions on cathodic s u r f a c e r e g i o n s leads to hydrogen charging of the m e t a l and p r o d u c e s a s h a r p reduction in i t s strength without a n y noticeable changes in the s p e c i m e n s u r f a c e g e o m e t r y and with only negligible m e t a l weight l o s s e s . No effective method of preventing the r u p t u r e of m e t a l s in c o r r o s i v e m e d i a can be developed without c o r r o s i o n - m e c h a n i c a l failure phenomena including s t r e s s - c o r r o s i o n cracking of m e t a l s . This investigation was c a r r i e d out in continuation of c e r t a i n p r e v i o u s investigations [1-3] of s t r e s s - c o r r o s i o n c r a c k i n g of s t e e l s (quench-hardened for high strength) in acid and neutral media. The e x p e r i m e n t s w e r e c a r r i e d out on fiat (2.3 x 9 x 75 mm) s t e e l U8A s p e c i m e n s quenched f r o m 800~ and t e m p e r e d f o r 2 h at 200~ After the heat t r e a t m e n t the s p e c i m e n s w e r e ground to C l a s s 8 s u r f a c e finish. High r e s i d u a l t e n s i l e s t r e s s e s produced in the m e t a l s u r f a c e l a y e r s during grinding led to spontaneous c r a c k i n g of the s p e c i m e n s in c o r r o s i v e media. In subsequent e x p e r i m e n t s t h e s e r e s i d u a l s t r e s s e s w e r e p a r t l y r e m o v e d by subjecting ground s p e c i m e n s to another t e m p e r i n g t r e a t m e n t (2 h at 200~ surface oxide f i l m s f o r m e d during this t r e a t m e n t w e r e r e m o v e d with e m e r y paper, which produced C l a s s 10 s u r f a c e finish. The s p e c i m e n s w e r e loaded in bending in special fixtures [4]. The kinetics of c r a c k growth w e r e i n v e s t i g a t e d in an a p p a r a t u s which m a d e it p o s s i b l e to fix the m o m e n t of c r a c k nucIeation and to m o n i t o r the c r a c k growth by m e a s u r i n g the s t r e s s relaxation in a given specimen; the s t r e s s was m e a s u r e d a c c u r a t e to 0.2-0.3 k g / m m 2. P a r a l l e l with s t r e s s m e a s u r e m e n t s the magnitude of the s p e c i m e n e l e c t r o d e potential was d e t e r m i n e d with the aid of a s a t u r a t e d c a l o m e l electrode; in t h e s e m e a s u r e m e n t s a potentio m e t e r R-307 and a type N373-1 automatic r e c o r d e r with c o m p e n s a t o r s w e r e used. S t r e s s - c o r r o s i o n t e s t s w e r e c a r r i e d out in the following media: 20% HzSO4 and HC1 solutions; 0.5% n i t r i c acid solutions with and without an addition of 1.5 g / l i t e r thiourea; boiling 50% solutions of v a r i o u s n i t r a t e s (NtItNOs, KNO3, NaNOs, LiNO3, Ca(NO3)z, Zn(NOs)2, Co(NOa)2, Ni(NO3)2). TABLE 1

Time" ~ to-rup~ , ~lture rain

, ~

KNO3 NaNO3 LiNO:, Ca(NQ)~ NH,NO3 Zn(NQ)~ Ni(NOa)~ Co(NO~)2

~,~

6,35 6,1 5,6 4,0 3,45 1,35 1,20 1,20

mm

)

90 78 60 45 26 21 15--18 15--18

T e s t s in a boiling NH4NO3 solution showed that in this c a s e periodic s t r e s s relaxation t a k e s place in time, this c o r r e s p o n d i n g to a discontinuous c r a c k p r o p agation (Fig. 1, c u r v e 2). Each periodic i n c r e a s e in the c r a c k length is a c c o m panied by the e l e c t r o d e potential becoming m o r e negative due to the f o r m a t i o n of f r e s h m e t a l s u r f a c e s (Fig. 2, c u r v e 2, portions a - a ) . In other nitrate solutions (in which the cation NH+ was r e p l a c e d by v a r i o u s m e t a l cations) the t i m e - t o - r u p t u r e of s t r e s s e d s t e e l s p e c i m e n s d e c r e a s e d s h a r p l y with i n c r e a s i n g pH (Table 1). However, the c h a r a c t e r of the v a r i a t i o n in s t r e s s and in the e l e c t r o d e potential was s i m i l a r to that o b s e r v e d f o r s p e c i m e n s t e s t e d in the NH4NO3 solution, which indicates that c r a c k nucleation and growth in s p e c i m e n s tested in v a r i o u s nitrate solutions take place in the s a m e way. When m e t a l cations a r c r e p l a c e d by hydrogen cations, as for instance in t e s t s in n i t r i c acid,

Institute of P h y s i c s and Mechanics, A c a d e m y of Sciences of the Ukrainian SSR, L ' v o v . T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a Mekhanika Matorialov, Vol. 7, No. 4, pp. 19-23, July-August, 1971. Original a r t i c l e s u b m i t t e d F e b r u a r y 7, 1970.

9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 g/est 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.

392

•

T

5

-

.4,.I 0 gO

r

$0

gO

~00

~, kg/mmz Fig. 1. S t r e s s r e laxation in s p e c i mens during t e s t s in the followingmedia: 1) 20% H2SO4; 2) 50%

NH4NO3; 3) 0.5% HN03; 4) 0.5%HNO3+ 1.5 g/liter thiourea.

the s t r e s s d e c r e a s e s gradually throughout the e x p e r i m e n t until the m o m e n t of rupture (Fig. 1, curve 3). The products of c o r r o s i o n of carbon steel in this e l e c t r o l y t e a r e nitrogen oxides so that no hydrogen charging takes place, and the rupture is due only to c o r r o s i o n phenomena [5]. When thiourea is added to n i t r i c acid, the r a t e of dissolution of steel is considerably reduced, but h y d r o gen evolved in this medium e m b r i t t l e s the steel [5, 6]. As a r e s u l t the t i m e t o - r u p t u r e of specimens t e s t e d for s t r e s s - c o r r o s i o n cracking at the same s t r e s s levels in an inhibited nitric acid solution is much s h o r t e r than that r e c o r d e d for specimens t e s t e d in pure HNO3 solutions (Table 2). The surface of specimens t e s t e d in the inhibited solution was p r a c t i c a l l y unchanged a f t e r the tests, and no c o r r o s i o n products were observed; the s t r e s s r e m a i n e d constant until the m o m e n t of rupture (Fig. 1, curve 4). To a s c e r t a i n the role played by hydrogen e m b r i t t l e m e n t of the metal in s t r e s s - c o r r o s i o n cracking, we d e t e r m i n e d the s h o r t - t i m e bending strength of specimens p r e v i o u s l y held (with and without the application of a load) in 0.5T0 HNO3 and 0.5% HNO3+ 1.5 g / l i t e r thiourea solutions. It m a y be expected that holding specimens (load-free o r u n d e r l o a d ) in e l e c t r o l y t e s in which cracking is caused by hydrogen charging will reduce their s h o r t - t i m e strength; heating such specimens (to d e s o r b hydrogen) will a l m o s t c o m p l e t e l y r e s t o r e t h e i r initial strength except in c a s e s of i r r e v e r s i b l e hydrogen e m b r i t t l e m e n t resulting in p e r m a n e n t damage of the metal. Holding l o a d - f r e e specimens in e l e c t r o l y t e s in which cracking is p r o duced by local dissolution of anodic regions will not noticeably r e d u c e t h e i r s h o r t - t i m e strength if no i n t e r g r a n u l a r c o r r o s i o n has taken place. The s h o r t time strength of specimens held under load in such e l e c t r o l y t e s will be r e duced (as a r e s u l t of c r a c k nucleation) and will not be r e s t o r e d by subsequent heating. In our e x p e r i m e n t s the s h o r t - t i m e strength of specimens p r e v i o u s l y held in a 0.5% HNO3 + 1.5 g / l i t e r thiourea solution with and without the application of a load (Fig. 3, c u r v e s 1, 2) was, in fact, lower than that r e c o r d e d for s p e c i mens not exposed to the influence of this medium.* When specimens unbroken during this s e r i e s of e x p e r i m e n t s were subsequently heated (2 h at 100~ t h e i r s h o r t - t i m e strength was a l m o s t completely r e s t o r e d (curve 6). The s h o r t - t i m e strength of specimens held in nit r i c acid without the thiourea addition was also reduced (curves 3, 4) but was not substantially i m p r o v e d by subsequent heating (curve 5), which shows that no hydrogen charging took place in pure n i t r i c acid solution.

+50

~60 #?0 d20-EmV

Fig. 2. Variation in the e l e c t r o d e potential of s t r e s s e d steel U8A specimens d u r ing s t r e s s - c o r r o s i o n cracking t e s t s in i) 20% H2SO4 (a=40 k g / m m 2) and 2) boiling 50% NH4NO~ solution (e= 80 kg/mm2).

When the anion NO3- was replaced by anions SO}- or CI- (HzSO4 or HC1 solutions), the c h a r a c t e r of s t r e s s relaxation of specimens at ~= 80 k g / m m z was the same as in an uninhibited nitric acid solution (Fig. 1, c u r v e 4). At lower s t r e s s levels (or= 40 k g / m m 2) a slight d e c r e a s e in the load was o b s e r v e d i m m e d i a t e l y before r u p t u r e (Fig. 1, curve 1). Comparison of data on s t r e s s relaxation and changes in the e l e c t r o d e potential shows a m a r k e d difference between the m e c h a n i s m s of s t r e s s - c o r rosion cracking of steel in n i t r a t e solutions and in h y d r o g e n - c h a r g i n g media. T e s t r e s u l t s showed that the t i m e - t o - r u p t u r e of high-strength steel in hydrogen-charging media (e.g., 20% H2SO4 solution) is d e t e r m i n e d mainly by the length of the incubation period, since the nucleation of the f i r s t c r a c k is followed almost i m m e d i a t e l y by rupture, which, incidentally, shows that the c r a c k propagation rate is v e r y fast. A different picture is o b s e r v e d in t e s t s on specimens in a boiling ammonium n i t r a t e solution; in this c a s e the p r o p a gation of a c o r r o s i o n c r a c k constitutes a large p a r t of the t i m e - t o - r u p t u r e . * The s h o r t - t i m e strength of specimens in a i r was 210-220 k g / m m 2.

393

r~

220

"e 72~

~TN (

go I 20

I SO

Fig. 3

I 4,0 T, r a i n

i

0

20

i

t

#0 . SQ

I

~0

I

I

700 r, min

Fig. 4

Fig. 3. Variation in the s h o r t - t i m e strength of s p e c i m e n s in a i r in r e lation to t h e i r holding t i m e in the following media: 1) 0.5% HNO3+ 1.5 g / l i t e r thiourea, a = 80 kg/mmZ; 2) the s a m e but without the load; 3) 0.5% HNO3, ~= 80 kg/mmg; 4) the s a m e but without the load; 5) 0.5% HNO3, ~ = 8 0 k g / m m 2, aging; 6) 0.5% HNO3+1.5 g / l i t e r thiourea, ~=80 k g / m m z, aging. Fig. 4. Variation in the s h o r t - t i m e strength of s p e c i m e n s in a i r in r e lation to t h e i r p r e v i o u s holding t i m e in a boiling NH4NOs solution at s t r e s s e s of 1) 25 k g / m m z, 2) 30 k g / m m 2, and 3) 35 k g / m m 2.

Fig. 5. Stages of c r a c k propagation in q u e n c h - h a r d e n e d steel U8A during s t r e s s - c o r r o s i o n c r a c k i n g t e s t s in a boiling 50% NH4NO3 solution. TABLE 2 Time -to -rapture,

212 198 170 142 113 99 85

13 55 80 ~00. 300" 31~0, 300*

0,5 1,5 2,5 8 12 27,5 160

Graphs r e p r e s e n t i n g the dependence of the s h o r t - t i m e strength of s p e c i m e n s * on t h e i r holding t i m e in a boiling 50% NH4NO3 solution at v a r i o u s s t r e s s e s show that the a p p e a r a n c e of a c r a c k a f t e r the incubation p e r i o d leads to a s h a r p d e c r e a s e in the s h o r t - t i m e strength (Fig. 4). Subsequent dwelling of s p e c i m e n s in the m e d i u m p r o d u c e s f i r s t a c e r t a i n i n c r e a s e and then a d e c r e a s e in the s h o r t t i m e strength. Depending on the s t r e s s applied, the m a x i m a and m i n i m a on the g r a p h s in question m a y be r e p e a t e d s e v e r a l t i m e s . T h e r e is a definite c o r r e l a t i o n between the c h a r a c t e r of the v a r i a t i o n in the s h o r t - t i m e strength of s p e c i m e n s and the kinetics of c r a c k propagation: v a r i o u s p o r t i o n s of the s h o r t - t i m e strength c u r v e s c o r r e s p o n d to a p p r o p r i a t e s t a g e s of the c o r r o s i o n c r a c k growth. F r o m the standpoint of the p h y s i c o c h e m i c a l t h e o r y of s t r e s s - c o r r o s i o n cracking these stages of c r a c k propagation can be explained as follows.

* Specimens not broken.

The influence of the applied tensile s t r e s s p r o d u c e s localization of the c o r r o s i o n phenomena in the initial t e s t s t a g e s (the incubation period); the s t r e s s c o n centration gradually i n c r e a s e s , but its magnitude is not sufficient to produce c r a c k nucleation. F u r t h e r localized c o r r o s i o n produces in c e r t a i n regions m i n o r c o r r o s i o n defects (e.g., shallow pits) which a c t as s t r e s s r a i s e r s . The m a x i m u m s t r e s s concentration is at the bottom of such a pit,where the potential is m o r e negative than on the pit walls o r on the unattacked m e t a l s u r f a c e and which t h e r e fore constitutes an anodic region. As a result, c r a c k s a r e f o r m e d in c o r r o s i o n - a f f e c t e d regions; at the * The t e s t s w e r e c a r r i e d out on flat s p e c i m e n s whose s u r f a c e s had been p r e v i o u s l y coated with a grade BF adhesive; before testing, a n a r r o w (0.3-0.6 ram wide) band of the m e t a l s u r f a c e (normal to the longitudinal axis of the specimen) in the m o s t s t r e s s e d c e n t r a l region of the s p e c i m e n was exposed by r e m o v i n g the adhesive coating with a r a z o r edge. 394

initial m o m e n t the c r a c k formation is accompanied by a sharp d e c r e a s e in the s h o r t - t i m e strength of the specimen. The m o s t Hkely cause of the subsequent slight i n c r e a s e in the s h o r t - t i m e strength is a change in the f o r m of the c r a c k root. The periodic variation in the s h o r t - t i m e strength (Fig. 4, c u r v e 1) of s p e c imens also points to a periodic variation in the form of the root of a growing crack. Metallographic m i c r o s c o p e examin~ttion of m i c r o s e c t i o n s r e v e a l e d the following (see Fig. 5). The end of the main c r a c k (a) splits into small c r a c k s in the form of a V-shape fork (b). During the s u b s e quent c r a c k propagation the two small c r a c k s join each other to f o r m a rhomboid loop (c). Some time l a t e r a new main c r a c k (d) s t a r t s growing f r o m the junction of the two small cracks, and the whole cycle is r e peated (e) until rupture takes place as a r e s u l t of a reduction in the effective c r o s s section of the s p e c i men. The s h o r t - t i m e strength of specimens which had c o r r o s i o n c r a c k s with unsplit ends (Fig. 5a, c, d) was in e v e r y c a s e lower than that of specimens having c r a c k s with V-shape split ends (Fig. 5b). This is evidently due to the fact that f r o m the standpoint of strength reduction one c r a c k is m o r e dangerous than s e v e r a l closely situated c r a c k s [7, 8]. LITERATURE

1. 2. 3. 4. 5. 6. 7. 8.

CITED

I . I . Vasilenko, FKhMM, No. 2 (1965). M . G . Khitarishvili, I. I. Vasilenko, and Yu. I. Babel, FKhMM, No. 2 (1967). G . V . Karpenko, M. G. Khitarishvili, I. I. Vasilenko, and Yu. I. Babel, in: Hydrogen Charging of Metals and Combating Hydrogen E m b r i t t l e m e n t [in Russian], MDNTP (1968). F . F . Azhogin, in: C o r r o s i o n and P r o t e c t i o n of Metals [in Russian], Mashgiz (1957). I . N . Putilova, S. A. Balezin, and V. P. Barannik, Metal C o r r o s i o n Inhibitors [in Russian], Goskhimizdat (1958). S . A . Balezin, I. V. Nikol'skii, and G. S. Parfenov, Uchenye ZapisM MGPI ira. Lenina, 76, 29 (1953). U . R . Evans and M. T. Simand, P r o c . Roy. Soc., 188A, 372 (1947). U.R. Evans, Corrosion and Oxidation of Metals, St. Martins Press (1960).

395