Influence of solid corrodents on high-temperature oxidation of cobalt powder

Article abstract of DOI:10.1023/B:JTAN.0000005206.63933.2f

The oxidation of a fine cobalt powder mixed with solid corrodents was investigated in the air. NaCl, Na₂SO₄, CaCl₂, mixture of NaCl with Na₂SO₄, mixture of NaCl with MgO, and mixtures of Na₂SO₄ with V₂O₅, MoO₂ and Fe₂O₃, respectively, were chosen as corrodents. Thermoanalytical diagrams were recorded within the temperature range from 20 to 1200°C. It was stated that none of the corrodents induced the cobalt oxidation to start at a lower temperature. NaCl and mixture of NaCl with Na₂SO₄ accelerated the oxidation slightly but addition of MgO stopped this effect.

Full text of DOI:10.1023/B:JTAN.0000005206.63933.2f

Journal of Thermal Analysis and Calorimetry, Vol. 74 (2003) 647–651
INFLUENCE OF SOLID CORRODENTS ON
HIGH-TEMPERATURE OXIDATION OF COBALT
POWDER
A. Magdziarz*, I. Suliga and Z. Kalicka
Faculty of Metallurgy and Materials Science, University of Mining and Metallurgy,
al. Mickiewicza 30, 30-059 Cracow, Poland
Abstract
The oxidation of a fine cobalt powder mixed with solid corrodents was investigated in the air. NaCl,
Na2SO4, CaCl2, mixture of NaCl with Na2SO4, mixture of NaCl with MgO, and mixtures of Na2SO4
with V2O5, MoO3 and Fe2O3, respectively, were chosen as corrodents. Thermoanalytical diagrams
were recorded within the temperature range from 20 to 1200ºC. It was stated that none of the
corrodents induced the cobalt oxidation to start at a lower temperature. NaCl and mixture of NaCl
with Na2SO4 accelerated the oxidation slightly but addition of MgO stopped this effect.
Keywords: cobalt powder, DTA, oxidation, solid corrodents, TG
Introduction
Metals and alloys in many industrial high-temperature applications are often exposed
to various corrosive environments.
By complete combustion of coal and petroleum the main reaction products are
CO2 and H2O(g). If the combustion is incomplete, CO is also formed. The gas may
contain some nonreacted hydrocarbons. The combustion products also include ashes
due to impurities in the fuels. These may be deposited on the walls of reactors.
Both coal and fuel oils contain varying amounts of sulphur, which when combusted
yield SO2 followed by partial oxidation to SO3. At low temperatures, SO3 and water
vapour react and form sulfuric acid. NaCl – present either as impurity in the fuel or in the
air – reacts with SO3 and H2O(g) at combustion temperatures to yield Na2SO4.
Thus, metals and alloys in combustion gases may be exposed to various corro-
sive agents depending upon the composition of fuel, temperature and reaction condi-
tions. Beside oxygen gas, important corrosive agents are SO2, SO3, molten salts like
Na2SO4 or sulphate mixtures, sodium vanadates and NaCl deposited on the metal sur-
face. The presence of combustion gases may cause enhanced corrosion. It is very im-
portant to study high temperature corrosion. Some authors [1–7] have studied the cor-
rosion of metals such as nickel, titanium and chromium.The results show that
*
Author for correspondence: E-mail: amagdzia@metal.agh.edu.pl
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

648
MAGDZIARZ et al.: INFLUENCE OF SOLID CORRODENTS
sulphates induce hot corrosion of nickel, while chlorides–hot corrosion of chromium.
The influence of salts on the high-temperature oxidation of cobalt and cobalt alloys
have not yet been studied in detail [1, 8].
The oxidation behaviour of powder metals or oxides can be investigated by em-
ploying the TG, DTA and DSC techniques [9, 10].
In the present work the oxidation of cobalt powder in air with the presence of
chlorides, sulphates and their mixtures with oxides was studied.
Experimental
The study of oxidation behaviour of a fine cobalt powder (made by Fluka) mixed with
solid corrodents was carried out in the air within the temperature range from 20
to 1200ºC. NaCl, Na2SO4, CaCl2 and mixtures of NaCl with Na2SO4, NaCl with MgO,
Na2SO4 with V2O5, MoO3 and Fe2O3, respectively, were chosen as corrodents. All the
samples of the cobalt powder were mixed with the agents (30% mass) and put into plat-
inum crucibles. Mass of each sample was 65 mg.
TG/DTG and DTA curves were recorded using a METTLER Thermoanalyzer 2.
The thermogravimetric investigation was carried out at heating rate of 10ºC min–1
and air flow of 5 dm3 h–1. Al2O3 was used as a reference sample.
Selected oxidized samples, cooled in nitrogen were subjected to X-ray diffrac-
tion analysis for the phase identification. The X-ray diffractograms in the range of 20
to 120° 2θ of the samples were obtained using X-ray diffractometer with gonio-
meter HZG4. Commercial cobalt powder was also investigated by X-ray diffraction
analysis to verify content of cobalt oxides.
Results and discussion
Thermoanalytical curves, DTA and TG, for cobalt and selected mixtures (cobalt with
NaCl, cobalt with NaCl plus Na2SO4) are presented in Fig. 1. For the other samples
DTA curves are given in Fig. 2.
Fig. 1 DTA and TG curves for: a – cobalt; b – cobalt with 30% NaCl; c – mixture of
cobalt with 15% NaCl and 15% Na2SO4
J. Therm. Anal. Cal., 74, 2003

MAGDZIARZ et al.: INFLUENCE OF SOLID CORRODENTS
649
Fig. 2 DTA curves for different mixtures: a – Co+30% Na2SO4; b – Co+30% CaCl2;
c – Co+15% Na2SO4+15% MoO3; d – Co+15% Na2SO4+15% V2O5;
e – Co+15% NaCl+15% MgO; f – Co+15% Na2SO4+15% Fe2O3
The results reveal that none of the investigated corrodents decreases the start tem-
perature cobalt oxidation, although some of them enable cobalt to oxidize slightly
faster. As it can be seen in Fig. 1 the cobalt oxidation in the presence of NaCl and NaCl
with Na2SO4 finished at a lower temperature i.e. proceeded in a smaller temperature in-
terval comprising to pure cobalt. However this effect did not appear when NaCl was
mixed with MgO (curve e in Fig. 2 comprising with curves b and c in Fig. 1). Each
studied sample of pure cobalt or cobalt mixed with corrodents weighted 65 mg, so the
contents of cobalt in the mixture samples were 45 mg. To avoid the influence of the
smaller mass of cobalt on the results presented above, the termoanalytical curves were
obtained for pure cobalt of 45 and 65 mg as well. Both curves were similar.
As it can be seen in Fig. 2 all corrodents affected the shape of DTA curves. Espe-
cially, the shape of the curves for cobalt with CaCl2 was distinct.
The obtained thermal and mass curves indicated oxidation of cobalt to CoO and
Co3O4 at higher temperature, while at the temperature over about 930°C the mass drop
has occurred due to the decomposition of Co3O4 to CoO and oxygen, as it was ex-
pected. To determine the presence of these oxides, the heating processes were inter-
rupted at different temperatures and the samples cooled in nitrogen were subjected to
X-ray diffraction analysis. The results for cobalt samples at 360 and 450°C are given
in Fig. 3 (cases a and b). The presence of Co3O4 and CoO was noticed for the both tem-
peratures with increasing ratio of Co3O4 to CoO content by temperature increase.
The same analyses were carried out for cobalt and a mixture of cobalt with NaCl
after heating to 1030°C. The first sample after cooling in nitrogen did not contain
CoO because of the oxidation of CoO to Co3O4 by the residual oxygen content in ni-
trogen. Nevertheless, the sample of cobalt with NaCl, prepared in the same way, re-
tained some amount of non-oxidized CoO (c and d in Fig. 3).
J. Therm. Anal. Cal., 74, 2003

650
MAGDZIARZ et al.: INFLUENCE OF SOLID CORRODENTS
Fig. 3 X-ray diffraction patterns (1 – Co, 2 – CoO, 3 – Co3O4) for cobalt and a mixture
of cobalt with NaCl: a – cobalt, 360°C; b – cobalt, 450°C; c – cobalt, 1030°C;
d – cobalt with NaCl, 1030°C
X-ray diffraction of commercial cobalt powder showed that there were not any
oxides of cobalt (CoO and Co3O4). Two allotropic forms of cobalt hexagonal and cu-
bic were detected.
Conclusions
The performed analyses indicate that the studied salts are not highly corrosive, yet
they affect the process of cobalt oxidation.
None of the corrodents decrease the start temperature of cobalt oxidation, how-
ever NaCl slightly accelerates the oxidation process.
J. Therm. Anal. Cal., 74, 2003

MAGDZIARZ et al.: INFLUENCE OF SOLID CORRODENTS
651
References
1 P. Kofstad, High temperature corrosion, Elsevier, London, New York 1988.
2 U. K. Chatterjee, S. K. Bose and S. K. Roy, Environmental degradation of metals,
Marcel Dekker Inc., New York 2001.
3 A. S. Khanna, Introduction to high temperature oxidation and corrosion, ASM International,
USA 2002.
4 K. P. Lillerud and P. Kofstad, Oxidation of Metals, 21 (1984) 233.
5 Y. Shinata and Y. Nishi, Oxidation of Metals, 26 (1986) 201.
6 N. V. Russell, F. Wigley and J. Williamson, J. Mater. Sci., 35 (2000) 2131.
7 Y. Shu, F. Wang and W. Wu, Oxidation of Metals, 54 (2000) 457.
8 A Report of NACE Technical Unit Committee T-3B on Corrosion Products, Corrosion,
18 (1962) 421.
9 S. Kameswari, J. Thermal Anal., 36 (1990) 23.
10 J. P. Sanders, P. K. Gallagher, J. Therm. Anal. Cal., 72 (2003) 777.
J. Therm. Anal. Cal., 74, 2003