CORROSIVE INFLUENCE OF MOLTEN MIXTURE OF
NaCl-Na2SO4 ON INTERMETALLIC COMPOUND Ni3Al
IN THE PRESENCE OF MgO, ZnO, PbO, V2O5, MoO 3 AND Fe2O3
Aneta Magdziarza
Zofia Kalicka b
a,b
Faculty of Metallurgy and Materials Science, AGH-UST, Krakow, Poland
a
mail: [email protected]
Abstract
The high- temperature oxidation behaviour of granulated Ni3 Al intermetallic
compound with addition of the mixture of salts NaCl- Na2 SO4 and some metallic oxides has
been studied in air at temperature up to 1100?C. The results showed very slow mass gain
without salts. The addition of salt eutectic significantly accelerated the oxidation of Ni3 Al.
The process of Ni3 Al oxidation was studied by thermal analysis. The thermal effects
(DTA curves) and the change of mass (TG curves) for sample under the condition of
temperature linear rise were recorded. Selected corroded samples were carefully examined by
means of scanning electron microscopy (SEM) and X-ray diffraction (XFD).
1. INTRODUCTION
A new type of structural material based on Fe-Ni, Ni-Al and Ti-Al intermetallic
compounds is being developed for applications in fossile energy systems such as coal gasifirs,
fluidised combustors and turbine engines. These long–range ordered alloys offer special
properties, such as good high- temperature strength, low creep rate, low density and low price
[1]. Furthermore, some recent studies have shown their superior oxidation resistance based on
the formation of protective Al2 O3 scale on aluminides. These advantages make nickel, iron
and titanium aluminides extremely attractive for various high temperature structural
application and be promising candidates as matrix for intermetallic composites.
Intermetallic compounds in the Ni- Al system such as Ni3 Al and NiAl are promising
potential high temperature materials which may take the place of conventional nickiel – base
superalloys. Ni3 Al demonstrates extraordinary properties such as a high melting point
(1668K) positive dependence of strength on temperature, good resistance to creep and
oxidation and relatively low density (7293 kg/m3 ) [2]. On the other hand mono-nickel
aluminide NiAl receives much attention due to its higher melting point, lower density and
better oxidation resistance than Ni3 Al.
Investigations on mechanical and physical properties of intermetallics, in particular the
aluminides have been conducted over a wide temperature range [3]. Little systematic work
has been done on the behaviour of oxidation and corrosion of intermetallics, and also little is
known about their hot corrosion behaviour. Nevertheless, of major importance is the
knowledge of corrosion resistance of intermetallics under actual operating conditions such as
diesel engines, gas turbines etc. in order to their great potential be finally explored.
The oxidation behaviour of Ni3 Al in air has been examined in different ranges of
temperature [4]. Ni3 Al has very high oxidation and corrosive resistance because of its ability
to form ? - Al2 O3 scales that protect the base metal from corrosive attack.
High temperature corrosion in combustion gas with Na2 SO4 and NaCl has been studied
[5]. The results has shown that corrosion rate of Ni3 Al is significantly accelerated by the
1
presence of Na2 SO4 and NaCl salt mixture especially at 600-800?C. The hot corrosion of
Ni3 Al involves different corrosion mechanisms, including scale fluxing, which concerns
mostly NiO at 600?C but both oxides (NiO and Al2 O3 ) at 700-800?C, mixed oxidationsulfidation, and scale penetration by the molten salt.
Influence of other corrodents on Ni3 Al has not been accurately studying yet. In the
present work the high- temperature oxidation in air of Ni3 Al in the presence of some
corrodents was studied. The distintly accelerated high-temperature oxidation named as “hot
corrosion” proceeds in the presence of some salts [6]. Depending on metals and alloys these
are chlorides, sulphates or chlorides with sulphates. Addition of some oxides can reduce or
intensify this process.
2. EXPERIMENTIAL PROCEDURES
Polish Academy of Science (PAN – Krakow) prepared Ni3 Al intermetallic compound.
X-ray diffraction has showed that only compound detected is.
The studies were made on mechanically dispersed Ni3 Al intermetallic. The oxidation
behavior of Ni3 Al samples mixed with solid corrodents was investigated in air. As corrodents:
eutectic mixture NaCl? Na2 SO4 with addition at appropriates metal oxides MgO, V2O5,
MoO3 , Fe2 O3 , PbO and ZnO were chosen. All the samples of Ni3 Al with the agents (30 mass
%) were put into platinum crucibles.
Simultaneous recording of TG (Thermogravimetry), DTG and DTA (Differential
Thermal Analysis) curves for a sample heated in air was performed using a METTLER
termoanalyzer. The thermoanalytical diagrams were recorded for a sample of 65 mg mass,
within the temperature range from 20 to 1100ºC. The experiments were carried out at
10ºC/min heating rate and in air atmosphere flow of 5 dm3 /h. By differential thermal analysis,
the temperature difference between reactive sample and non-reactive reference is determined
as a function of time, providing useful information about the temperatures, thermodynamics
and kinetics of reactions. Al2 O3 was used as non-reactive reference sample.
Selected oxidized samples were investigated by X-ray diffraction analysis for
identification the phase composition. Some reacted specimens were characterized by scanning
electron microscopy (SEM) Hitachi 3500N.
3. RESULTS
The preliminary tests on pure Ni3 Al, Ni3 Al with NaCl, Na2 SO4 and mixtures of NaCl
with Na2 SO4 , in succession were carried out. No oxidation process was observed for pure
Ni3 Al and Ni3 Al with NaCl up to 1100?C. The presence of the sulphate and mixture of
sulphate with chloride caused the corrosive effect on the intermetallic compound. This
process was observed to initiate between 700-800?C.
Laboratory studies of such corrosion behaviour have been made with deposition
of sulphates on metal surfaces. Sulphate – induced hot corrosion of nickel has been the
subject of considerable research as nickel and nickel base alloys are particularly prone of this
type of corrosion [7]. All previous studies of sulphate induced hot corrosion have been made
at temperature above the melting point of pure Na2 SO4 (884?C).
Figure 1a shows the influence of pure Na2 SO4 on oxidation of Ni3 Al. Addition
of 25 mass % NaCl whic h has melting point 798?C (75%Na2 SO4 + 25%NaCl) decreases the
temperature of the oxidation beginning (Figure 1b). The greatest corrosive effect was
observed for the mixture with about 10 mass % NaCl having an eutectic point at 652?C.
The b and c TG curves are very similar but the temperatures of the oxidation beginning are
different. Figure 1 shows the influence of sulphate and mixtures of Na2 SO4 with NaCl on an
acceleration of Ni3 Al oxidation.
2
DTA curves
TG curves
a
a
T
Mass
b
b
c
c
0
200
400
600
800
1000
1200
0
200
400
600
800
1000
1200
o
o
Temperature [ C]
Temperature [ C]
Figure 1. DTA and TG curves for different mixtures:
a) Ni3 Al + Na2 SO4 ;
b) Ni3 Al + (25%NaCl + 75%Na2 SO4 );
c) Ni3 Al + (10%NaCl + 90%Na2 SO4 );
The mixture of Na2 SO4 with 10 mass % NaCl was chosen for the study of the
influence of some metallic oxides on Ni3 Al oxidation. The investigations were done for PbO,
ZnO, MgO, Fe2 O3 , V2 O5 and MoO3 oxides.
No impact of PbO, ZnO and MgO on TG curves was registered. Addition of V2 O5 or
MoO3 involved an increase of the temperature of the beginning of oxidation, nevertheless the
process started and proceeded more rapidly than for pure NaCl- Na2 SO4 . On the contrary,
Fe2 O3 evidently caused a deceleration of the oxidation, without shifting the temperature of
oxidation start. In Fig2 there are presented the results for V2 O5 addition, while in Fig.3 for
Fe2 O3 .
DTA curve
TG curve
90
200
85
Mass [mg]
250
[ V]
150
100
80
75
70
50
65
0
0
200
400
600
800
1000
60
1200
0
-50
200
400
600
800
o
Temperature [o C]
Temperature [ C]
Figure 2. DTA and TG curves for mixture: Ni3 Al + (NaCl + Na2 SO4 + V2O5 )
3
1000
1200
TG curve
DTG curve
250
90
200
85
Mass [mg]
T [ V]
150
100
80
75
70
50
65
0
0
200
400
600
800
1000
60
1200
0
-50
200
400
600
800
1000
o
Temperature [ C]
Temperature [oC]
Figure 3. DTA and TG curves for mixture: Ni3 Al + (NaCl + Na2 SO4 + Fe2 O3 )
X-ray diffraction analyses were made for the samples of Ni3 Al with pure NaClNa2 SO4 and with Fe2 O3 added to NaCl-Na2 SO4 . The heating processes were interrupted at
740, 800 and 1100o C and the samples were cooled. In Fig.4 there are presented X-ray
diffraction patterns for the mixture without Fe2 O3 and in Table1 are summarized the chemical
compounds detected in the all samples. As one can see the oxidation products were mainly
composed of NiO. Pure Al2 O3 was not observed. Sodium aluminate NaAlO 2 was probably
formed at the first stadium of the oxidation. At higher temperatures it was transformed into
spinel NiAl2 O4 . Some amounts of Ni3 S2 were detected at 740 and 800o C for the both kinds of
mixtures. Although Ni3 S2 is unstable above 806o C [7] the traces of it were deteced at 1100o C
but only in the sample with Fe2O3. The presence of nickel sulfide NiS was detected at 740o C
in the sample without Fe2 O3 .
The presence of Ni3 S2 and NaAlO 2 may be explained on the basis of the mechanism of
pure Ni corrosion in air in the presence of Na2 SO4 , assuming the reaction between molten
sulfate and nickel [7]
6.3 Ni(s) + Na2 SO4 (l) = (Ni3.3 -S)liq +NiO + Na2 O
(1)
Thus, sodium oxide Na2 O may form NaAlO 2 .
Table 1. Chemical compounds detected by X-ray diffraction after cooling of the samples
heated up to appropriate temperatures.
Ni3 Al + (NaCl + Na2 SO4 )
740?C
800?C
Ni3 Al + (NaCl + Na2 SO4 + Fe2O3 )
1100?C
740?C
800?C
1100?C
Ni
Ni
NiO
Ni
Ni
NiO
NiO
NiO
NiAl2 O4
NiO
NiO
Ni3 S2
Ni3 S2
Ni3 S2
AlCl3
Ni3 S2
Ni3 S2
NiAl2 O4
NiS
NaAl4 O4 Cl5 ?
Na2 SO4
NaAlO 2
NaAlO 2
Al2 S3
NaAlO 2 ?
Na17 Al5 O16 ?
NaCl
Na2 SO4
NiFe2 O4
FeAl2 O4 ?
Na2 SO4
Na2 SO4
NaCl
Na2 SO4
Na2 SO4
NaCl
NaCl
NaCl
NaCl
4
1200
a
o
o
o
o
b
c
x
x
40
50
60
70
2 Theta
80
90
100
Figure 4. X-ray diffraction patterns for for mixture Ni3 Al + (NaCl + Na2 SO4 ) at different
temperature: a) 740?C, b) 800?C, c) 1100?C; ? ? NiO, ? ? Ni, x ? NiAl2 O4 , o ? Ni3 S2 .
a)
b)
Figure 5. Scanning electron micrograph of the samples oxidized at 1100?C in air:
a) Ni3 Al with mixture (Na2 SO4 + V2O5 ),
b) Ni3 Al with mixture (NaCl + Na2 SO4 + MoO 3 ).
For the samples with V2 O5 and MoO3 oxidized at 1100o C there were performed scanning
electron studies. In Fig. 5a there is presented the sample corroded by molten V2 O5 , in Fig.5b
grains of NiO formed in the corrosion process of Ni3 Al.
5
CONCLUSIONS
1. Pure Ni3 Al displays an oxidation resistance up to 1100o C
2. The mixture of Na2 SO4 with 10 mass % NaCl shows the greatest corrosive effect
on Ni3 Al.
3. Additions of ZnO, PbO, MgO and Fe2 O3 have no influence on acceleration of
Ni3 Al oxidation.
4. Addition of V2 O5 shifts the start of the oxidation to the higher temperature and at
the same time accelerates the oxidation rate.
5. The oxidation products are mainly NiO, nickel sulphide Ni3 S2 and spinel NiAl2 O4 .
At the first stage of the oxidation process there were detected aluminate NaAlO 2
and sulfide NiS.
BIBLIOGRAPHY
[1] Wu W., Niu Y., Guo J., Zhang Y., High Temperature Corrosion of Advanced Materials
And Protective Coating. In the proceeding from The International Symposium on Solid
State Chemistry of Advanced Materials, Tokyo,1992, pages 301-307
[2] Matsuura K., Kitamura T., Kudoh M., Journal of Materials Processing Technology,
1992, number 63, pages 298-302
[3] Stoloff N.S., Sikka V.K., Physical Metallurgy and Processing of Intermetallic
Compounds, New York,1996, 159 p.
[4] Choi S.C., Cho H., J., Kim Y. J., Lee D. B., Oxidation of Metals, 1996, number 46,
pages 51-72
[5] Niu Y., Gesmundo F., Viani F. and Wu W., Oxidation of Metals, 1994, number 42,
pages 265-285
[6] Kofstad P., High temperature corrosion, Elsevier Applied Science, London, New York,
1988, 465 p.
[7] Karl P.L., Kofstad P.,Oxidation of Metals, 1996, number 21, pages 233-270
Acknowledgement
Polish State Committee for Scientific Research (KBN) sponsors this work (Grant No 4
T08B 049 25).
The authors wish to thank dr E. Bielanska for analytical scanning electron microscopy
and S. Kotas for X-ray diffraction.
6