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Volume 1 Paper 9


Stress Corrosion Cracking Resistance of New Austenitic-Ferritic Steels

Halyna Chumalo
Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, Naukova str. 5, 290601 Lviv, Ukraine. E-mail:

Abstract

A tendency of new cast austenitic-ferritic steels to stress corrosion cracking have been investigated in different aggressive environments to determine the regions of their most efficient application. It has been established that investigated austenitic-ferritic steels are the promising corrosion resistant materials to be used in hot concentrated alkaline solutions, in cold and hot tap water, and low concentrated chloride solutions.

Key words: stress corrosion cracking, austenitic-ferritic steels.

1 Introduction

Conventional corrosion-resistant steels - deformed and, in particular, cast - are susceptible to stress corrosion cracking (SCC) in many processing and natural environments, sensitive to intergranular corrosion in sensitizing, and in certain cases to pitting and crevice corrosion.

Scientists have paid special attention during the last decade to determining the regions of efficient application of ferritic and austenitic-ferritic corrosion-resistant steels instead of conventional corrosion resistant chrome-nickel and chrome-nickel-molybdenum austenitic and other steels [1-6].

A number of publications concerned with the austenitic-ferritic corrosion resistant steels are many time smaller than that dealing with the austenitic steels, and there are almost no data on the corrosion resistance and corrosion-mechanical strength of the cast steels.

According to the corrosion behaviour of the austenitic-ferritic steels, they may represent promising materials for production of cast components designed for service in corrosive chloride and alkaline environments.

2 Experimental Procedure

Investigations were carried out on five melts of cast austenitic-ferritic steels. In this paper we present the results of more investigated two steels. More complete data were obtained on 04X25H7AM3 and on 03X23H7M4 steels (50% austenite, 50% ferrite), after quenching from 1050°C (0.5 h) in water. The chemical composition and mechanical properties of test materials are given in Table 1 and Table 2.

Table 1. Chemical composition of steels (wt %)

Steel

C

Cr

Ni

Mo

Cu

Mn

Si

S

P

others

04X25H7AM3

0.04

25.4

7.4

3.1

0.14

0.21

0.10

0.013

0.008

0.2Y
0.2N

03X23H7M4

0.03

23.0

6.9

4.0

0.12

0.16

0.12

0.015

0.007

0.15Al

Table 2. Mechanical properties of steels

Steel

0.2% Yield stress
(MPa)

UTS
(MPa)

Area Reduction
Z %

Elongation
A %

04X25H7AM3

590

745

35

21

03X23H7M4

475

712

66

36

Test environments were represented by solutions used most extensively in industry and nature. With the combination of specific factors, these solutions caused corrosion cracking of corrosion resistant steels in the stressed state:

1. Hot tap water (100°C);

2. Standard NACE solution for evaluating the sulfide cracking susceptibility of steels (5% NaCl solution + 0.5% CH3COOH solution, water, saturation with H2S concentration of ~ 3 kg/liter, pH4.5; 25°C);

3. 3% NaCl solution (100°C) which simulates sea water in laboratory practice;

4. 30% MgCl2 solution (132°C), used in laboratory practice for evaluating the chloride cracking susceptibility of corrosion resistant steels;

5. 30% NaOH solution (105°C) - a standard solution used for evaluating the sensitivity of corrosion resistant steels to alkaline cracking.

The last two solutions (chloride and alkaline) are used most frequently for steel testing in power engineering which requires the simulating of operating conditions for these materials, used in power plant. The surface of individual elements of power plant may be characterized by the concentration of chlorides or alkali (or both) during evaporation of boiler or reactor water, steam condensation, variations of the composition of the heat carrier in dead zone or below deposits on heat-transferring surfaces, and under other conditions causing the increase of the concentrations of these corrosive compounds.

The susceptibility to SCC was estimated on the basis of threshold stress under static tensile loading for 1440 hours and more; on the basis of the conventional stress corrosion cracking threshold Kscc (the level of the stress intensity factor below which cracks do not grow in the aggressive environment). We determined conventional Kscc because in the given specimen types plane strain does not take place. SCC tests were also conducted at a low strain rate of 2x10-6.s-1 to determine the variation of the ductility characteristics and the time to failure of the specimens in a corrosive environment relatively to inert environment [7]. In the first case we used standard specimens with a diameter of 5 mm, in the second case - beam specimens with a cross section of 20x10 mm with an induced fatigue crack whose depth, together with the depth of the mechanical notch, was 4.5 mm, whereas in the third case we used cylindrical specimens with a diameter of 3.5 and 5 mm (according to Procedure Recommendations in [7]).

In electrochemical investigation we measured the corrosion potentials and recorded I-E polarization curves using standard methods [8,9]. The corrosion and electrochemical behaviour of the steels was also interpreted on the basis of the data on the local distribution of alloying elements in the steel obtained using "CAMEBAX-MBX" x-ray spectrum microanalyzer by scanning ten areas 310 mm long.

3 Results and Discussion

The distribution of chrome and molybdenum, i. e., the main elements determining the passivation stability of the steel is relatively uniform, regardless of the dual-phase structure of metal. The chrome content is any of the analysis point did not drop below the critical content (< 12%) at which intergranular corrosion of the steel can develop as a result of depletion of the boundary zones of the metal in chrome with precipitation of high-chrome carbides at the grain boundary [9,10].

Investigations by the AM and B methods in accordance with GOST 6032-84 [11] showed that 04X25H7AM3 and 03X23H7M4 steels are not susceptible to this type of corrosion damage. It appears that these data could indicate the potential resistance of the steel to SCC in solutions with pH lower then 15 if we assume that in the case of the chrome-nickel corrosion-resistant steels the relationship between these two types of damage is unambiguous [12,13].

Long term tests of 04X25H7AM3 and 03X23H7M4 steels in the boiling NaOH solution showed no predisposition to SCC (Fig.1 a, b, curves 3). In the boiling concentrated alkaline solution 04X25H7AM3 steel doesn’t crack even under stresses considerably higher than the yield limit (Fig. 1 a, curve 3).

 

Fig.1. Long-term corrosion strength of 04X25H7AM3 (а) and 03X23H7M4 (b) steels in boiling 30% MgCl2 solution (1), NACE solution (2), boiling 30% NaOH and 3% NaCl solutions and tap water (3). (Note that these images may be viewed at a larger size by clicking on them - use Back on the browser to return to this page).

To verify the possibility of formation of immunity in this case as a result of macroplastic deformation of metal or microplastic deformation in the surface stress raisers, test were also carried out under stress levels of approximately 0.8 s0.2 (Editors note: the symbol to the left of this note should be a Greek sigma - if you see an s, you should upgrade your browser, and ensure that the Symbol font is intalled on your computer) and also under an initial stress lower than the proportionality limit (~0.5 s0.2). SCC or nucleation of surface corrosion defects of the microcrack type was not recorded after exposure of 1440 hours. Only surface scars damaged the macrostructure.

03X23H7M4 steel showed a very high resistance to SCC in boiling 30% NaOH solution too. Even the beam specimens with an induced fatigue crack showed no reduction of the conventional threshold of SCC in the alkaline solution in comparison with Kc determined in air of a period of 720 hour (Kscc and Kc are on the level of 230 MPa ).

In the specimens tested in the alkaline solution there were no traces of extensive local corrosion damage, and fracturing the beam specimens with an induced fatigue crack showed no signs of initiation of the fatigue crack at the tip of the corrosion cracking cracks. The high resistance of the dual-phase steel is determined by its high passivation capacity in the alkaline environment and by its capacity to reduce the surface film in sharp tip concentration of the microcrack type after its destruction.

Since potentiodynamic polarization curves and crack propagation behaviour for both steels are very similar, the data for one steel only are presented (Fig.2,3,4).

Fig.2. Potentiodynamic polarization curves for 03X23H7M4 steel in hot: 30% NaOH solution (1), 30% MgCl2 solution (2) 3% NaCl Solution (3)

a   d

Fig.3. Specimens of 03X23H7M4 steel tested at low strain rate of 2x10-6.s-1 in air (a) and boiling solutions: 3% NaCl (b), 30% MgCl2 (c), 30% NaOH (d)

Fig. 4. Corrosion crack in 03X23H7M4 steel formed under the effect of the boiling 30% MgCl2 solution.

The corrosion potential of 03X23H7M4 steel in 30% NaOH solution is –0.72V (SHE), that is approximately 0.7 V from the pitting region. There is no loop of active dissolution on the polarization curve, but the maximum density of the dissolution current in anodic polarization reaches 0.5 A/m2 (Fig. 2, curve 1). It should be mentioned here that alkaline SCC of the corrosion resistant steels is induced in the course of the active-passive transition in the range of the activity loop potentials.

SCC of the steels takes place under deformation in the concentrated chloride solution (Table 3, Fig. 3). Percent reduction in area and reduction of time to failure of specimens tested in 30% MgCl2 solution at a low strain rate of 2x10-6.s-1 showed high susceptibility to SCC of these materials (Table 3). Transgranular corrosion cracks have developed in austenitic phase (Fig.4).

Table 3. Results of tests at a low strain rate of 2x10-6.s-1

(data from three specimens with a diameter of 3.5 mm)

Steel

Environment

Area Reduction
Z %

Time to failure
t, h

04X25H7AM3

Inert
30% MgCl2
30% NaOH
3% NaCl

22.5
10.8
18.0
21.0

42.5
17.0
30.0
42.0

03X23H7M4

Inert
30% MgCl2
30% NaOH
3% NaCl

66.0
26.0
65.0
61.0

96.0
44.0
95.0
92.0

In a hot 30% MgCl2 solution both steels are susceptible to local depassivation as a result of the establishment of their corrosion potential in the range of the pitting protection and pitting potentials. Corrosion potential for 03X23H7M4 steel is – 0.02V (Fig. 2, curve 2). Since SCC is initiated in corrosion resistant steels of this grade in breakdown of the passivating film at - potentials more positive than the pitting protection potential, steels show susceptibility to SCC in the hot solution of magnesium chloride. However, this sensitivity for 04X25H7AM3 steel is detected (which must be stressed) in plastic deformation of the metal - at stresses higher than 500 MPa (Fig.1, curve 1).

03X23H7M4 steel showed high sensitivity to SCC - threshold stress is 275 MPa in a hot 30% MgCl2. However both steels showed a very high resistance to SCC in the boiling chloride solution with a low concentration (3% NaCl), in the boiling tap water. This behaviour of the austenitic-ferritic steel in the chloride solution with low concentration is explained by the stable passive state in wide potential range. The corrosion potential (0.2V, SHE) of the metal is situated of 100mV from the pitting potential, and from the potential of possible hydrogen embrittlement in cathodic polarization by at least 200mV (Fig. 2, curve 3).

Under the depassivation effect of the chloride solution with low pH (~4,5) and extensive hydrogen charging (NACE solution) both steels become highly sensitive to SCC as a result of high sensitivity of ferritic phase to hydrogen embrittlement.

04X25H7AM3 steel was subjected to sulfide cracking at a low tensile stresses: the conventional threshold stress after testing for 1440 hours was 100 MPa (Fig.1a, curve 2).

Conventional threshold stress at the test base was 245 MPa for 03X23H7M4 steel (Fig. 1 b, curve 2). However, since there is no tendency to the formation of a plateau on the "stress-time" (s - t) curve with increasing test time, the process of SCC of the steels will continue.

4 Conclusion

Thus the investigated cast dual-phase steels are promising material for production of components which work under stress conditions in aggressive environments, including hot environments with high pH. In cold and hot tap water, and sea water as well as alkaline solutions steels can be used without restrictions as regards to stresses. In the conditions with a possible increase of chloride concentration the working stresses can be determined on the basis of the threshold value. Cast 04X25H7AM3 and 03X23H7M4 steels are not suitable for use in hydrogen charging processing environments, i.e., oil and gas, which contain hydrogen sulfide, in acid solutions; in environments causing hydrogen charging under cathodic protection.

5 Acknowledgement.

The author is grateful to Professor R.Melekhov for his competent advice during the course of this work.

6 References

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