Thin Solid Fihns, 83 (1981) 437-447 GENERALFILMBEHAVIOUR
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B E H A V I O U R OF C H R O M I U M - C O A T E D STEELS I N S O D I U M POLYSULPHIDE ENVIRONMENTS* A. WICKER, G. DESPLANCHESAND H. SAISSE Laboratoires de Marcoussis, Centre de Recherches de la Compagnie Gkn~rale d'Electricit~, Route de Nozay, 91460 Marcoussis (France)
(Received March 31, 1981 ; accepted April 10, 1981)
Several chromium coatings on mild steels were characterized and placed in media containing sodium polysulphides for corrosion studies. Chemical corrosion was studied up to 440°C and electrochemical corrosion at 350°C. C h r o m i u m plating with subsequent high temperature treatment was found to give the best results but chromized steels were also found to be suitable for use in cells below 350 °C.
1. INTRODUCTION In the beta-battery, the sulphur electrode material consists of molten sodium polysulphides absorbed in graphite felt. This composite is surrounded by the container which acts both as a reservoir and as a current collector. A wide range of materials (metal alloys, refractory compounds and ceramics) have been studied by the different teams working on the beta-battery 1-3. In order to obtain low cost casing materials, the use of chromium-coated mild steels has been advanced 3,4. In this paper we describe the present status of a continuing evaluation of the behaviour of chromium-coated materials in sodium polysulphide environments. 2. EXPERIMENTALDETAILS Three substrate materials were chosen with regard to their carbon contents. Their compositions are given in Table I. 2.1. Characterization o f the coatings
Three types of coating technique were studied: chromizing, electroplating, and electroplating with diffusion treatment. Chromizing is a high temperature treatment in which chromium, arising from the decomposition of chromium halides, diffuses into the base metal. The thickness * Paper presented at the International Conferenceon MetallurgicalCoatings, San Francisco,CA, U.S.A., April 6-10, 1981. 0040-6090/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
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TABLE I C O M P O S I T I O N S OF THE SUBSTRATES
Composition (wt.~,;)
Designation
Iron XC 10 XC 12
C
Si
P
S
Fe
0.0! 0.08 0.2
0.03 0.24 0.16
0.014 0.013 0.013
0.04 0.02 0~03
Balance Balance Balance
and c h r o m i u m content of the coating layer depend on the c a r b o n content of the substrate and the working parameters (temperature and time). The structures obtained with the three substrates and the same working parameters are presented in Figs. 1, 2 and 3. F r o m these figures, it can be seen that a duplex layer is obtained
In)
!! 2S
50
75 ~1Ill
\
(b) Fig. 1. Chromized iron: (a) chromium analysis; (b) iron (©) and chromium (e) profiles.
BEHAVIOUR OF Cr-COATED STEELS IN NaSy ENVIRONMENTS
439
(a)
•s
~
.
6o
~rn
(b) Fig. 2. Chromized XC 10: (a) chromium analysis; (b) iron (©) and chromium (O) profiles.
only with XC10 steel. When the time or temperature of the chromizing process are increased, the thickness of the coating reaches an upper limit which depends on the carbon content: 100-150 ~tm for iron, 60-80 ~m for XC 10 and 30-50 pm for XC 12. With conventional electroplating techniques, coatings up to 200 ktm thick were easily obtained but the chromium layer was always porous and sometimes the deposits exhibited cracks. A typical mic,rostructure is presented in Fig. 4. Samples electroplated with chromium were subjected to diffusion treatment to obtain a diffusion layer between the chromium and the base metal. The treatment was conducted in hydrogenated nitrogen on the iron and XC 12 steel samples. The coatings obtained are presented in Figs. 5 and 6. The coatings showed (1) some cracks in the external layer, although they do not extend into the diffusion layer (Fig. 7), and (2) a porous zone between the external layer and the diffusion zone, which is found to be far more extensive for iron than for XC 12 steel (Figs. 5 and 6).
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A. WICKER, G. DESPLANCHES, H. SAI'SSE
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(b) Fig. 3. Chromized XC 12: (a) chromium analysis; (b) iron ((3) and chromium ( I ) profiles.
Fig. 4. A chromium electroplated layer.
BEHAVIOUR OF Cr-COATED STEELS IN NaxSy ENVIRONMENTS
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(a)
[7
(b) Fig. 5. Iron electroplated with a chromium layer 200 gm thick and subjected to diffusion at 1000°C for 15 h: (a) microstructure; (b) iron (O) and chromium (O) profiles.
TABLE II TEST C O N D I T I O N S IN THE STATIC C O R R O S I O N TESTS
Duration
Melt composition
(months) T = 300 °C
T = 325 °C
T = 350 °C
T = 400 °C
T = 440°C
Na2Ss-S Na2S,, Na2Ss-S Na2S4
Na2Ss-S Na2S, Na2%-S Na2S,, Na2Ss-S Na2S4
Na2Ss-S Na2S4 Na2S5-S Na2S,, Na2S s S Na2S,,
3 6 12
Na2Ss-S Na2S,
Na2Ss-S Na2S,,
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A. WICKER, G. DESPLANCHES, H. SAISSE
(a) W%
loo .
o
(b)
Fig. 6. XC 12 steel electroplated with a chromium layer 200 gm thick and subjected to diffusion at 1000 °C for 15 h: (a) microstructure; (b) iron (O) and chromium (O) profiles. 2.2. Corrosion tests T w o k i n d s of c o r r o s i o n test were c o n d u c t e d : static tests in s o d i u m polysulphides a n d d y n a m i c tests (in-cell studies). The samples used for the static corrosion tests were in the form of plates 40 m m x 10 m m x 2 mm. The test c o n d i t i o n s are given in Table II. The tests were c o n d u c t e d in the s u l p h u r c o m p a r t m e n t of a beta-cell of capacity 180 A h. The samples were placed in the graphite felt as s h o w n o n Fig. 8.
BEHAVIOUR OF Cr-COATED STEELS IN
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Fig. 7. A crack on chromium-plated XC 12 steel after diffusion.
argonne___
samples (40xlOx2ram)
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ampule j " (~60, ~500 ram)'
Fig. 8. Schematic diagram of a static corrosion cell. Fig. 9. The loading tool for the static corrosion cell.
The desired polysulphide compositions were produced by discharging the betacell, thus enabling anhydrous polysulphides to be obtained at the sample. For safety purposes the excess of sodium was removed after discharge. The corrosion cells were then placed in the test furnaces (Fig. 9). For the dynamic corrosion tests we used small beta-cells of capacity 4 A h, the sulphur compartments of which were constructed from the materials to be tested. The test conditions were as follows: temperatures of 300, 350 and 400 °C; current densities in the electrolyte of 50, 100 and 200 mA cm-2. 3. RESULTS Although the tests are not all completed, significant results have already been obtained.
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A. WICKER, G. DESPLANCHES, H. SA'ISSE
3.1. Static corrosion tests
At 440 °C the chromized samples are strongly corroded. Examples for XC 12 steel treated for different times are given in Figs. 10-12. The chromium layers of the electroplated samples exhibit good corrosion resistance; corrosion occurs only
Fig. 10. Micrograph of a chromized XC 12 sample after 73 days at 440 ~C. Fig. 11. Micrograph ofa chromized XC 12 sample after 92 days at 440 'C.
Fig. 12. Micrograph ofa chromized XC 12 sample after 183 days at 440 ~C. Fig. 13. Micrograph of a chromium-plated (100 lain) XC 10 sample after 73 days at 440 ~C.
Fig. 14. Micrograph of a chromium-plated (100 ~tm) XC l0 sample after 92 days at 440 "C. Fig. 15. Micrograph of a chromium-plated (100 lamj XC 10 sample after 183 days at 440 ~C.
BEHAVIOUR OF Cr-COATED STEELS IN Na x Sy ENVIRONMENTS
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when cracks reach the bulk metal. Figures 13-15 show the behaviour of electroplated XC 10 steel. The electroplated and diffused samples exhibit very good corrosion resistance at 440 °C, even when cracks are present. The growth of cracks is limited by the diffusion layer. Figures 16-18 show the behaviour of electroplated and diffused iron.
Fig. 16. Micrograph ofa chromium-plated(100~tm)and diffused iron sample after 73 days at 440°C. Fig. 17. Micrograph of a chromium plated (100 ~m) and diffused iron sample after 92daysat 440°C.
Fig. 18. Micrograph of a chromium plated (100 p-m)and diffused iron sample after 183 days at 440 °C.
(a) (b) Fig. 19. Micrographs ofchromized XC 12 samples after 183 days at 350 °C in (a) NazS 4 and (b) NazSs-S.
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At 350°C the sodium polysulphide composition strongly influences the behaviour of the chromized samples. Na2S 4 is more corrosive than Na2S 5 S. Figures 19 and 20 show the appearance of chromized XC 10 and XC 12 steels after this test. The electroplated and the electroplated and diffused samples exhibit almost the same behaviour as in the 440 °C tests.
(a)
(b)
Fig. 20. Micrographs o f c h r o m i z e d X C l 0 samples after 183 days at 350 cCin (a)Na/S, and (b)Na2S s S.
(a)
(b)
(c) Fig. 21. Chromium analysis of a chromized XC 10 container after cycling at 350°C for (a) 183 days, (b) 244 days and (c) 426 days.
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3.2. Dynamic tests Some preliminary results have been obtained at 350 °C and with a current density of 100 mA cm -2. The sodium polysulphide composition during electrochemical cycling was mostly Na2S 4. Figure 21 shows the chromium components of chromized XC 10 containers after operation for 6, 8 and 14 months. Only the 14 month test leads to degradation of the chromium coating which can be very marked, as shown on Fig. 22 (different area of the same sample as in Fig. 21(c)).
Fig. 22. Micrograph of a corroded chromized XC 10 container after cycling at 350 c'C for 426 days. 4. CONCLUSIONS
The following conclusions can be drawn from our investigations. (1) In static corrosion tests chromium coatings obtained by electroplating and diffusion exhibit good corrosion resistance (even at 440°C) but the presence of cracks might be deleterious for long-term applications. At 350°C chromized samples exhibit good corrosion resistance and XC 10 steel is the most resistant material; this seems to be due to the duplex structure of the coating. NazS 4 is more corrosive than NazSs-S. (2) In the dynamic tests (which represent simulated use in the beta-battery) corrosion effects are less important than in the static tests. ACKNOWLEDGMENT
This work was supported by the Electric Power Research Institute. REFERENCES 1 S.A. Weiner et al., Research on electrode and electrolyte for the Ford sodium-sulfur battery, Ford Annual Rep.for the Period June 30, 1974, to June 29, 1975, 4 July 1975. 2 B. Hartmann, J. Power Sources, 3 (1978) 227. 3 D. Chatterji and S. P. Mitoff, Development of sodium-sulfur batteries for utility applications, EPRI Interim Rep. 128-4for the Period May 1, 1976, to August 31, 1977, 1978 (Electric Power Research Institute, Palo Alto,CA 94303). 4 G. Desplanches, Y. Lazennec and A. Wicker, U.S. Patent 4,037,027, 1977.