Intergranular corrosion behaviour of a new austenitic stainless steel low in nickel

Intergranular corrosion behaviour of a new austenitic stainless steel low in nickel

Cam&n MeraNurgfcaiQuarrer/~, Vol 34. No. 2. pp. 135-141, 1995 Copyright IU 1995 Canadian Institute of Mmng and Metallurgy Prmled III Great Bntam. All ...

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Cam&n MeraNurgfcaiQuarrer/~, Vol 34. No. 2. pp. 135-141, 1995 Copyright IU 1995 Canadian Institute of Mmng and Metallurgy Prmled III Great Bntam. All rights reserved 00084433,f95 69.50+0.00

Pergamon OOOS-4433(94)00024-7

INTERGRANULAR AUSTENITIC

CORROSION BEHAVIOUR OF A NEW STAINLESS STEEL LOW IN NICKEL

E. OTERO,‘f A. PARDO,? E. SAENZ,$ V. UTRILLAt

and F. J. Pl?REZt tDepartamento de Ciencia de Materiales, Facultad de Ciencias Quimicas, Universidad Complutense, 28040-Madrid, Spain fLaboratorio de Corrosion, Pontificia Universidad Catolica de1 Peru, Lima 32, Peru (Receiced

14 December

1993 ; in recisedform

3 October

1994)

Abstract-A comparative study was done on the intergranular corrosion resistance of AISI 304 stainless steel and a new austenitic stainless steel currently being developed, in which the concentration of nickel is less than 2%. The austenitic effect of nickel is replaced by other alloying elements, mainly manganese. The experimental results are discussed according to ASTM A-262, practices C and E (Huey and modified Strauss). A study of both materials under several test conditions was carried out using scanning electron microscopy to analyze the microstructural characteristics and the attack morphology. RBsumGNous avons fait une etude comparative de la resistance a la corrosion intergranulaire d’un acier inoxydable AISI 304 et d’un nouvel acier austcnitique inoxydable en tours de developpement dont la teneur en nickel est infcrieure a 2%. L’effet austcnitique du nickel est remplacc par d’autres tltments de l’alliage, principalement du manganese. Nous parlons des rcsultats experimentaux en accord avec les procedures C et E (Huey et Strauss modific) de la norme ASTM A-262. Une etude des deux mattriaux soumis a plusieurs conditions de tests, au moyen d’un microscope a balayage ilectronique (SEM), pour analyser les caracteristiques microstructurelles et la morphologie d’attaque est effect&e.

INTRODUCTION

section specimens (15 x 15 x 2 mm) were used in the Huey test ; rectangular specimens (75 x 25 x 2 mm) were used for the modified Strauss test. The chemical compositions of both materials are shown in Table I, and their mechanical properties are given in Table 2. (These materials were cold rolled with subsequent annealing and pickling.) The samples were solutionized at 1573 K for 10 min, with subsequent water quenching to homogenize and to prevent chromium carbide precipitation in the steel. To obtain the TTS curves and to study the microstructural influence on the susceptibility to intergranular corrosion, samples were heat treated. The temperatures chosen corresponded to the range of likely chromium-rich carbide precipitation, which is associated with the sensitization phenomenon. Two samples were used in each treatment in order to enhance the reliability of the results. The heat treatments were done in an inert atmosphere (argon), between 723 and 1173 K, for l10 000 min, in a furnace with digital temperature control, followed by water quenching. To quantify intergranular corrosion standard tests, ASTMA262 practices C and E were used. The normalized Huey test (practice C) is recommended for detecting the susceptibility of the austenitic stainless steels to intergranular corrosion. The action of 65% HNO, boiling during five 48-h cycles attacks the region of the grain boundaries where the chromium-rich carbides are found. This attack increases with the increase of chromium-rich carbide precipitates, especially when they form a continuous chain of precipitates. The Huey test indicates not only sensitization (carbide precipitation), but also the presence of impurities and other phases and the character of the protective layer formed on the metallic surface. In the Huey test, the chromium-depleted zones (carbides,

The uncertain cost of austenitic stainless steel, as a result of nickel’s high cost and its unstable international market, makes it worthwhile to fabricate and study a new low-nickel austenitic stainless steel, which maintains the mechanical properties and corrosion behaviour of high-nickel austenitic stainless steel [l31. In this new steel, the lost austenitic balance associated with nickel reduction is compensated by the addition of other alloying elements of gamma character (mainly C, N, Cu and Mn) which do not cause fabrication problems and which confer a stable austenitic structure with cheaper components [4-g]. In the present work, the tendency for sensitization to intergranular corrosion of an austenitic stainless steel with a nickel content lower than 2 wt% is studied. The steel is being developed in Spain by Acerinox under CECA (EC project) financial aid, in collaboration with the Materials Science and Engineering Department of the University Complutense of Madrid. This new stainless steel should compete with the classical AISI 304. The main advantage of the new material would be its lower market cost. In order to study the intergranular corrosion phenomenon in both materials normalized tests were carried out according to ASTM A-262, practices C and E [lo]. From the test results, temperature-time-sensitization (TTS) curves were traced [l lP 151. Simultaneously, a microstructural study relating the corrosion behaviour of the material to its microstructure, derived from the sensitization treatment, was carried out. EXPERIMENTAL

METHOD

Samples of both materials (low-nickel stainless steel and AISI 304) were machined as required by the test procedures : square 135

136

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Table

1. Nominal

STAINLESS

composition

STEEL

of alloys

-

Elements Material

C(%)

Si(%)

AISI 304 Low-nickel stainless steel

0.042 0.079

0.348 0.330

Table

Material

2. Mechanical

Mn(%) 1.39 10.38

properties

Depth (mm)

Situation

Sn(%)

Ni(%)

0.012 0.007

8.05 1.65

of cold-rolled

Hardness (Hv, k,)

Cu(%)

Cr(%)

0.219 2.01

materials

18.30 16.49

P(%)

S(%)

0.028 0.024

0.002 0.003

tested with subsequent

Yield Yield strength strength (0.2%) (MPa)(l.O%) (MPa)

Tensile strength Of Pa)

MO(%)

N,(ppm) -. 600 1421

0.28 0.01

annealing Elongation-l (“/I

and pickling

0, (pw) 88

treatments

r

P

Ar

n

A

Low-nickel stainless steel

0” 45’ 90’

0.71

1X0

357.7 336.1 346.1

401.3 378.8 385.4

707.5 653.2 660.0

41.9 46.1 48.4

1.00 1.30 0.82

1.11

-0.39

0.36 0.36 0.36

0.36

AISI

0” 45” 90”

0.71

168

309.5 290.0 298.1

350.5 327.3 333.1

698.5 641.4 654.1

44.0 49.6 51.5

0.97 1.37 0.84

1.14

-0.46

0.42 0.41 0.42

0.42

P, anisotropy

average

304

t 50 mm gage length.

nitrides pounds

Key:

and the possible chromium-bearing formed in the matrix), phosphide

coefficient;

intermetallic precipitates,

&, planar

RESULTS

average

coefficient;

A, strain

average

coefficient.

comsulfur

and the chromium-depleted matrix are attacked. Consequently, the Huey test is proposed to determine the corrosion behaviour of austenitic stainless steel in the presence of nitric acid. Moreover, the results of this test are quantitative and take into account the weight loss to determine more exactly the TTS curve. The criterion applied in order to determine whether or not the material is sensitized to intergranular corrosion is the corrosion rate, expressed in cm/month. The considered limit of sensitization is a corrosion rate of 3.7 x IO-’ cm/month [16]. In the modified Strauss test, the samples are boiled for 24 h in CuS04-H2S04 (aq) in the presence of chip copper. They are subsequently bent at 180”. The presence of visible cracks (at 40 x magnification) is associated with an intergranular corrosion phenomenon. It is interesting to note that specimens sensitized by the modified Strauss test also displayed sensitization by the Huey test, but the opposite did not necessarily occur. On the other hand, qualitative results are achieved with the Strauss test [17, 181, where the attack is localized only in the chromium-depleted areas. In this case, then, the Strauss test is more specific for the evaluation of the intergranular corrosion. For metallographic analysis, samples were polished with emery paper (120,400 and 600 grit) in a wet medium, and were then finished with 0: and 7 alumina. Metallographic etching was carried out with the reagent glycerregia (three parts HCI, one part HNO, and one part glycerine). Samples were examined with a Jeol-JSM 35C scanning electron microscope. EXPERIMENTAL

anisotropy

Fig.

1. Microstructures treatment.

of the austenitic stainless steels after (a) AISI 304 steel; (b) low-nickel steel.

solution

AND DISCUSSION

Both materials, once in solution, show an austenitic microstructure without precipitates in the grain boundaries (Fig. 1).

The experimental results will be discussed according intergranular corrosion test, including the material structure after sensitization treatment.

to the micro-

E. OTERO

1

r

I

lo

loo Time

Fig. 2. Corrosion

rates as a function obtained from

et ul.: AUSTENITIC

STAINLESS

STEEL

137

1000

(min)

of time for AISI the Huey test

304 stainless steel,

1123

1023 973 923

Fig. 4. Influence of the exposure time on the magnitude of intergranular corrosion attack. in the Huey test, for AISI 304 stainless steel. (a) 1 min at 943 K ; (b) 60 min at 943 K.

073 823 773

Time (min)

Fig. 3. Temperature-time-sensitization the Huey test for AISI

(TTS) diagram obtained 304 stainless steel.

from

The AISI 304 steel, after solution treatment and five cycles of testing, shows a corrosion rate of 2.6 x 1O-’ cm/month. less than the limit of sensitization ; it is therefore classified as not sensitized. Figure 2 shows the variation in the corrosion rate, after the Huey test, as a function of the time of exposure to the tested temperatures. The dashed line shows the maximum acceptable corrosion rate in unsensitized material. Corrosion rates above this line indicate that the steel has been sensitized to intergranular corrosion. With the data from Fig. 2. a TTS diagram was constructed for AISI 304 stainless steel (Fig. 3). In accordance with the results, it was observed that as the time of heat treatment at a given temperature increases, the attack intensifies. the separation between grains becomes more marked. and the corrosion rate increases due to loss of grain cohesion (Fig. 4). The low-nickel steel, after solution treatment and after five cycles of the Huey test, shows a corrosion rate of 7.9 x LO-’

cm/month. This rate is greater than 3.7 x IO-’ cm/month, the figure considered to be thecorrosion limit of sensitization. However. the microstructure does not indicate the existence of intergranular attack. but rather shows signs of general corrosion (Fig. 5). Figure 6 shows the variation of the corrosion rate as a function of time for the different heat treatments performed. In

Fig. 5. Solution-treated generalized

low-nickel steel. after the Huey test. showing corrosion (1: = 0.0079 cm/month).

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138

l.ooo

X723K 0773K . 823K

0.100-

STAINLESS

STEEL

I

AIO73K

o.olo-

2 8

*en -_---__ --__----

_______-___-.

t 0.001 I 1

I lo

1 100

, loo0

loo00

TIME(min) Fig. 6. Corrosion rate as function of time for low-nickel steel from the Huey test.

accordance with the criterion of rate limit of sensitization, the material is sensitized for all heat treatments applied. The metallographic study of the low-nickel steel samples exposed to the Huey test reveals two types of corrosion behaviour for different heat treatments. For treatment times of approximately 15 min, the predominant attack morphology corresponds to a generalized corrosion phenomenon, and the behaviour of the samples is similar to that of the as-received material after solution treatment. For treatment times over 15 min, the prevalence of intergranular corrosion phenomena is observed. These phenomena are associated with the presence of chromium-rich carbides precipitated as a continuous chain in the grain boundaries (Fig. 7). There is a separation and grain dropping which, with the metallography, explains the greater corrosion rates (more than 0.01 cm/month). obtained in these long heat treatments (Fig. 8). It follows that for temperatures lower than 723 K and higher

Fig. 7. The presence of chromium-rich carbides in grain boundaries, in low-nickel steel, heat-treated for 6000 min at 923 K (etched with glycerregia).

Fig. 8. Microstructure

of the low-nickel

steel, after

the Huey

test: (a)

1j min at 923 K showing generalized corrosion : (b) 60 min at 923 K showing intergranular corrosion. than 1173 K. intergranular corrosion was not detected for any treatment time. In all cases. generalized corrosion was observed. The generalized corrosion phenomenon present in the lownickel stainless steel after the Huey tests could be associated with at least partial dissolution of the passive film in the test medium. The substitution of nickel for manganese reduces the transpassivation potential of the steel (Fig. 9) making the corrosion potential greater than the transpassivation potential (Fig. 10). The dissolution of the passive film leads to a generalized and severe attack on the material. For the low-nickel steel, which shows both generalized and intergranular corrosion, it is necessary to establish a new criterion in order to modify the TTS diagram. Theoretically, a calculated magnitude associated with the generalized corrosion phenomenon, upon being subtracted from the corrosion rates obtained from the Huey test. should indicate the magnitude of intergranular corrosion. To explain the above thesis, it must be considered that solutionized AISI 304 does not suffer generalized corrosion. The corrosion rate for low-nickel steel (Vi) should be attributed mainly to generalized corrosion. We propose to subtract V, (corrosion rate of solution-treated AISI 304) from Vi (corrosion rate of solution-treated low-nickel stainless steel) in order to obtain the magnitude of generalized corrosion in the new steel. If this value (V: - V,) is subtracted from the corrosion rates obtained from the Huey test for the heat-treated samples, the

E. OTERO et al. : AUSTENITIC

STAINLESS Loo0

STEEL

139

x 0

723K 7nK l 623K 0 8?3K n 923K

A lO73K A 1123K

-

AISI

304

---

Low

Nickel

0.0 Vsen 1

2 Current

4

3 density

5

( mA/cm2)

Fig. 9. Effect of HNO, solution temperature on corrosion potential for AISI 304 steel and low-nickel steel : (1) 298 K ; (2) 346 K : (3) 373 K : (4) boiling.

H

A AISI 0 Low

(min)

Fig. 11. Corrosion rate [V- (V, - Vi)] as function of time for low-nickel steel, obtained from the Huey test.

304 Nickel

1200

2 i5 s

Time

Boiling

temperature

llOO-

I

lo

ocl

Do0

loooa

TIME Imin)

Fig. 12. Proposed TTS diagram, obtained from the Huey test on lownickel steel.

8009

310

SO Temperature

350

370

390

410

(K)

Fig. 10. Effect of HNO? solution temperature on corrosion potential for AISI 304 steel and low-nickel steel. amount of intergranular attack can be approximated (Fig. 11). The TTS diagram can then be modified (Fig. 12) using the corrected data. TIME (min)

Modified Strauss test The solutionized AISI 304 shows no cracks after the test and is considered unsensitized. Figure 13 shows the results obtained for each heat-treated sample. The “nose” of the curve is at approximately 908 K (temperature) and 45 min (time). The highest temperature associated with sensitization is above 943 K and the corresponding time is between 90 and 120 min. At

Fig. 13. TTS diagram, obtained from the modified Strauss test on AISI 304 stainless steel. the lowest tested temperature (823 K), sensitization takes longer than 420 min. As in the Huey test, it is observed that upon increasing the

140

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STAINLESS

STEEL

determined sensitization temperature leads to a more intense intergranular attack. The attack gives rise to structures with continuous cracks in the direction of the bending axis. which increase with the duration of the test. It is also observed that the cracking is sudden, transforming unsensitized samples (without cracks) to cracked samples in a few minutes. This transformation affirms that the behaviour of the low-nickel steel is similar to that of AISI 304, as verified by a comparison of the corresponding TTS diagrams (Figs 13 and 15, respectively). The low-nickel steel has a high content of Mn. Cr. C. N and Cu, and a chromium content lower than the reference material AISI 304. A material with this composition is expected to have poorer resistance to intergranular corrosion than AISI 304, which has a lower amount of N and C and a greater amount of chromium. When the low-nickel steel is sensitized, a chromium-depleted zone is generated very quickly and is extended to a large zone (reaching the 12% chromium limit faster). This effect can be appreciated in the Strauss test results. since the sensitization times in this case are lower. Moreover, the sensitization temperatures reached are lower than those obtained for the AISI 304 steel at 773 K. Perhaps due to a lower chromium content in the matrix and the size of the chromium-depleted zone in the low-nickel steel, the diffusion of chromium from the matrix to grain boundaries is slower than in the AISI 304 steel. Comparison of results from larger sensitization times indicates that low-nickel steel does

Fig. 14. AISI 304 steel microstructure subjected to modified Strauss test : (a) 60 min at 848 K, unsensitized: (b) 120 min at 848 K, sensitized.

time of treatment, intergranular attack becomes more intense, generating continuous cracks in the direction of bending (Fig. 14). The low-nickel steel, solution treated, after the modified Strauss test, does not present cracks ; it is therefore unsensitized. In accordance with the test, Fig. 15 shows the nominal TTS diagram for the low-nickel steel. The “nose” of the curve is at 923 K and 22 min. The maximum temperature for sensitization is slightly above 998 K. with associated times of between 120 and 1000 min. At the minimal test temperature (773 K). sensitization takes longer than 1200 min. Figure 16 shows that increasing the heat treatment at the

I

K)

lob0

lcim

TIME (mill1

Fig. 15. TTS diagram, obtained from the modified Strauss test on lownickel steel.

Fig. 16. Low-nickel steel microstructure after the modified Strauss test : (a) 120 min at 973 K. sensitized: (b) 300 min at 973 K. sensitized.

E. OTERO

et al. : AUSTENITIC

not show a loss of sensitization for long testing times, while the AISI 304 steel shows the opposite. Moreover. the low-nickel steel has a larger amount of Cr,N and Cr&, precipitates in the grain boundaries, which enlarges the chromium-depleted zone. The TTS diagrams obtained following the ASTM A-262 practice C (Huey test) and practice E (Strauss test) are shown in Figs 17 and 18, respectively.

STAINLESS

304 steels, then an approximate constructed.

Both stainless steels studied are microstructurally similar, with typical austenitic microstructures. After sensitization heat treatments, the AISI 304 steel presents an intergranular corrosion behaviour similar to that of low-nickel steel. The behaviour of the low-nickel stainless steel, heat-treated and tested according to standard ASTM A-262, practice C, shows the following : corrosion phenomenon that is not observed in the AISI 304 stainless steel. (2) A corrosion rate that is always above the sensitization limit. This leads to difficulties in constructing the TTS diagram. It is necessary, therefore, to subtract the loss of material corresponding to generalized corrosion from the overall corrosion rate. (3) If it is assumed that the corrosion rate due to a generalized corrosion phenomenon is constant and equal to the difference between the corrosion rates in these media for the solutionized conditions of the low-nickel and AISI

Km

CQO

mw

TIME hinl

Fig. 17. TTS diagram, obtained from the Huey test on both steels.

TIME

(mid

Fig. 18. TTS diagram, obtained from the modified Strauss test on both steels.

can be

(1) The behaviour

(3)

(4) (5)

(1) A generalized

I

TTS diagram

As for the behaviour of both heat-treated steels, according to ASTM A-262 standard (practice E), the following are observed :

(2)

CONCLUSIONS

141

STEEL

(6)

of the two materials was similar, both qualifying as acceptable with regard to intergranular corrosion. For the low-nickel steel, the sensitization curve is found at shorter times for each temperature than for the AISI 304 stainless steel. For both steels, the “nose” of the curve is at similar temperatures; the low-nickel steel takes less time (approximately half), to reach a comparable level of sensitization. The highest temperature of sensitization for the lownickel steel is higher than that for the AISI 304. Holding the treatment temperatures between 898 and 948 K for long periods of time produces healing in the AISI 304 steel. In the low-nickel steel, longer times are needed to achieve the same healing. After a 180’ bend test, cracks appear “slowly” in the heat-treated AISI 304 steel, and abruptly in the lownickel

steel.

Acknowledgemenr~The authors wish to express Acerinox, S. A.. CDT1 and CICYT (MAT-90-0980-CO financial support of this work.

their

gratitude to 3-01) for their

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