Volume 1 Paper 9
Stress Corrosion Cracking Resistance of New Austenitic-Ferritic Steels
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JCSE Volume 1 Paper 9
Submitted 16 December 1998
Stress Corrosion Cracking Resistance of New
Karpenko Physico-Mechanical Institute of the National Academy of Sciences
of Ukraine, Naukova str. 5, 290601 Lviv, Ukraine. E-mail: mailto2('pokhmurs','ah.ipm.lviv.ua')
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.
§2 Key words: stress corrosion cracking, austenitic-ferritic
comment(3) 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.
§4 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].
§5 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.
§6 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
comment(7) 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.
§8 Table 1. Chemical composition of steels (wt %)
§9 Table 2. Mechanical properties of steels
0.2% Yield stress
§10 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
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
4. 30% MgCl2 solution (132°C), used in laboratory
practice for evaluating the chloride cracking susceptibility of corrosion
5. 30% NaOH solution (105°C) - a standard solution used for
evaluating the sensitivity of corrosion resistant steels to alkaline cracking.
§11 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
§12 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 . 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 ).
§13 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.
comment(14) 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].
§15 Investigations by the AM and B methods in accordance with GOST
6032-84  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].
§16 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
§17 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).
§18 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
§19 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
§20 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.
§21 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).
§22 Fig.2. Potentiodynamic polarization curves for
03X23H7M4 steel in hot: 30% NaOH solution (1), 30% MgCl2
solution (2) 3% NaCl Solution (3)
§23 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
§24 Fig. 4. Corrosion crack in 03X23H7M4 steel formed under the
effect of the boiling 30% MgCl2 solution.
§25 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.
§26 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).
§27 Table 3. Results of tests at a low strain rate of
(data from three specimens with a diameter of 3.5 mm)
Time to failure
§28 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).
§29 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).
§30 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.
§31 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).
§32 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
comment(33) 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.
comment(34) 5 Acknowledgement.
The author is grateful to Professor R.Melekhov for his
competent advice during the course of this work.
comment(35) 6 References
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