The optical properties of some steel surfaces with different surface preparations for high temperature use

Solar Energy Materials 13 (1986) 185-195 North-Holland, Amsterdam

185

T H E OPTICAL P R O P E R T I E S OF S O M E STEEL S U R F A C E S WITH D I F F E R E N T S U R F A C E P R E P A R A T I O N S FOR HIGH T E M P E R A T U R E USE F.S. B O Y D A ( 3 Yddtz (]niversitesi, Fen-Edebiyat Fak~ltesi, Fizik B6l~m~ Ytldtz, lstanbul, Turkey Received 31 March 1983; in revised form 28 September 1985 A selective surface with large absorption for solar radiation and high reflectance for thermal infrared radiation was produced by use of surface oxidation of stainless steel. The surfaces were studied for use with concentrated light in a solar power plant at temperatures of 400°C and higher. In order to investigate the relation between surface treatment and optical properties, stainless steels (AISI 304 and 430) which were submitted to different chemical and mechanical surface treatments, were used. To increase the spectral selectivity, these surfaces were treated in air and in vacuum at different temperatures and times. The optical properties of these films were investigated. Visual and infrared spectral absorptances were measured at room temperature. The thermal hemispherical emittance and absorptance were obtained by a calorimetric method at 200°C. It was noticed that these chemically and mechanically treated stainless steel surfaces have good spectral properties without further oxidations. This is very important for high temperature uses. The best values are found for samples 7 and 8 under vacuum and air. These two samples with mechanically ground surfaces retained their selectivity and specularity after several hours oxidation. One can conclude that the surface ground treatment confers good selectivity on the steel surfaces for use in concentrating solar collectors with a working temperature of 500°C. Sample surfaces were subjected to long temperature ageing tests in order to gain some idea of the thermal stability of the surfaces. The results promise better-performing surfaces and the production of durable selective finishes at, possibly, lower cost than competing processes.

1. Introduction According to Tabor [1], the efficiency of thermal processes for capturing solar energy can be improved by using a spectrally selective surface in which the monochromatic emissivity varies with wavelength. The desired spectral characteristics of the surface must not deteriorate when the temperature of the solar receiver is at working temperature. An explanation of the methods for obtaining selectivity is given by Meinel and Meinel [2]. They may be classified according their work: - Intrinsic materials, - Tandem stacks, - Interference stacks, - Wavefront-discriminating surface roughness, - Dispersion of metal droplets in a metal or dielectric matrix, - Quantum size effects. The difficulty of these methods depends on the layers to be produced.

0165-1633/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

186

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Most metals have an absorptance for solar light which is much higher than their emissivity for infrared radiation. The absorptance of solar light for a clean polished surface is normally less than 0.5 which means that more than 50% of the solar radiation falling on the surface is reflected. Spectral selectivity of stainless steel surfaces may be improved by immersing the metal in a hot solution of molten salt and nitric acid using a commercially available process. If a certain selectivity can be given to these surfaces, the absorptance of solar light and the emittance in the infrared will increase but because of the low emissivity of stainless steel surfaces the total hemispherical emittance will not increase greatly above the normal emissivity. Tandem stacks cover a wide range of selective coatings ranging from the very simple to the very complex. The simplest way of making a coating as a tandem stack is the metal oxide. In this study a technique for obtaining this type of spectrally selective surface on stainless steel, suitable for high temperature use (up to 500°C) for solar radiation is described. The stainless steels AISI 304 and 430 which were submitted to different chemical and mechanical surface treatments, were used. The steels were treated by an industrial firm which produced surfaces with different optical preparations [5]. The samples were subjected to several hours of heat treatment in vacuum and air in order to obtain long-term thermal stability so we could investigate which surfaces are the best for high temperature use and which treatment should be given to these samples to obtain good spectral selectivity. A simple treatment of stainless steel under controlled conditions of temperature and time promises to produce durable selective finishes at a possibly lower cost than competing processes.

2. Experimental procedure Two different types of stainless steel were used in this experiment. The austenitic stainless steel AISI 304 is an 18 C r / 8 Ni weld steel with a percentage by weight composition of 18.00/20.00 Cr, 8.00/12.00 Ni, 0.08 C, 1.00 Si, 2.00 Mn, 0.030S and 0.045 P. Ferritic stainless steel AISI 430 is an 14 Cr steel with a percentage by weight composition of 14.00/18.00 Cr, 0.12 C, 1.00 Si, 1.00 Mn, 0.030 S and 0.040 P. The list of the samples which are used in the experiment are given in table 1. The stainless steels were furnished by an industrial firm [5], were submitted to different chemical and mechanical treatments in order to give the different surface aspects (table 1). All samples were exposed to the following chemical attacks: - immersion for 20-30 s in a molten salt bath (450°C, approximately 95% N a O H ) after washing in very hot water; Approximately 50 s immersion in an electrolytic cap (10 V, 1000/5000 A) which contains a solution of H N O 3 (6:8%) at ambient temperature; - After washing in cold water for approximately 50-60 s they were immersed in a 6% H N O 3 + 1.20% H F solution with a temperature of 60-65°C and were then washed in hot water.

F.S. Boyda~/ Steel surfaces

187

Table 1 List of the samples Sample

Type AISI

3

304

4 5 6 7 8 9 10

430 304 430 304 430 304 430

hot rolled annealed pickled + cold roiled annealed and pickled same as sample 3 same as 3 + skin passed same as sample 5 same as sample 5 + surface ground same as sample 7 same as sample 5 + sand blasted same as sample 5 + sand blasted

14 samples with the d i m e n s i o n s 32 x 32 m m were p r e p a r e d for the e x p e r i m e n t in o r d e r to k n o w which t r e a t m e n t s fit best for high t e m p e r a t u r e use. As a first step 6 of the samples were h e a t e d in air, then in a v a c u u m in a c o n s t a n t t e m p e r a t u r e oven for various times. T h e second group, with 8 samples, was e x p o s e d to air to i m p r o v e their high t h e r m a l stability. A f t e r each t r e a t m e n t the s a m p l e s were allowed to cool to a m b i e n t t e m p e r a t u r e u n d e r n o r m a l a t m o s p h e r i c conditions. T h e n the optical p r o p e r t i e s were m e a s u r e d after each t r e a t m e n t as d e s c r i b e d below.

2.1. Measurement of the optical properties T h e radiative p r o p e r t i e s of the samples were d e t e r m i n e d c a l o r i m e t r i c a l l y b y an a p p a r a t u s c o n s t r u c t e d in the I S P R A l a b o r a t o r y . A precise d e s c r i p t i o n of the m e t h o d is given in ref. [3].

2.2. Microscopy W i t h a small s c a n n i n g electron microscope, the M 7 SEM, m a d e b y ISI, the surfaces were studies at m a g n i f i c a t i o n s of u p to 7000. T h e s a m p l e s r e p r e s e n t four different finishes for the two groups, A I S I 304 a n d 340, as can b e seen in table 1. A t high t e m p e r a t u r e s m o s t of the desired p r o p e r t i e s of an a b s o r b e r surface are difficult to obtain. S o m e of the desired p r o p e r t i e s of the a b s o r b e r surface are [4]: 1. T h e r m a l shock resistance, 2. T h e r m a l stability at o p e r a t i n g t e m p e r a t u r e , 3. O x i d a t i o n resistance, 4. Spectral selectivity. O n e of the m a j o r questions c o n c e r n i n g selective a b s o r b e r s is their o p e r a t i n g lifetime u n d e r high t e m p e r a t u r e conditions. The surfaces m u s t be subjected to t e m p e r a t u r e ageing tests in o r d e r to gain some i d e a o f the thermal stability of the surfaces. This can be d o n e i n v a c u u m u n d e r a p r o t e c t i v e gas or in the air. I n this s t u d y the stainless steel s a m p l e s were tested for high t e m p e r a t u r e use. T h e y were subjected to several t r e a t m e n t s in v a c u u m and in air at different t e m p e r a t u r e s

E = ehern at 1 5 0 ° C .

0.34 0.31 0.27 0.29 0.52 0.54

3 5 7 8 10 9

a)

Original surface

Sample

0.16 0.19 0.18 0.14 0.31 0.40

0.37 0.35 0.36 0.44 0.59

0.17 0.20 0.17 0.15 0.31

2 h oxidation 300°C

0.62

0.40

24 h o x i d a t i o n at 3 0 0 ° C

0.44 0.41 0.42 0.54 0.66 0.65

a

E 0.17 0.19 0.18 0.14 0.30 0.40

ot 0.41 0.39 0.40 0.48 0.63 0.64

100 h

24 h

T r e a t m e n t at 5 0 0 ° C in v a c u u m

Table 2 O p t i c a l p r o p e r t i e s of the 6 stainless steel s u r f a c e s as a f u n c t i o n of e x p o s u r e to v a c u u m at 5 0 0 ° C a)

E 0.17 0.19 0.17 0.15 0.32 0.40

0.51 0.52 0.51 0.54 0.66 0.68

ot

200 h

0.20 0.22 0.20 0.16 0.34 0.44

e~

e~

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189

for several hours in order to gain some idea of the thermal stability of the commercially treated steel surfaces. On the other hand it is known that slight oxidation can improve the optical properties of many metals by increasing their spectral selectivity. The theoretically obtainable c~/e ratio diminishes with increasing temperature, because of the increasing overlapping parts of the curves. Therefore, for our samples this ratio was satisfactory (e.g. more than 4) having an absolute value of a of more than 60%.

3. Results

Hemispherical emittance values measured by a calorimetric method at - 200°C and the absorptivity measured by the spectrophotometer for wavelengths from 0.3 ftm are reported in tables 2-4. The influence of the surface preparation on the optical properties has been investigated after each thermal treatment. As can be seen from table 2, after 200 h of treatment at 500°C in a vacuum, the samples had begun to lose their spectral selective properties and to show a deterioration of their emittance. Therefore, the samples were thermally stable for a 100 h vacuum treatment at 500°C. Figs. 1 and 2 shows the absorption coefficient a as a function of wavelength of samples 7 and 8 after this treatment. Eight of the samples were subjected to oxidation treatments for long periods, in order to investigate their oxidation resistance and thermal stability. Their spectrophotometric measurements were made after each treatment. Samples 7 and 8 were chosen for the calorimetric measurement facilities, starting from 400°C heat treatment. These two samples provide a general idea about the other samples and show very good specularity in the infrared (low absorptance in infrared) for a long period of time. In fig. 3, one can compare the results for "skin passed" and "surface ground"

Table 3 Optical properties of the 8 stainless steel surfaces as a function of oxidation time in air a) Sample

Original surface

3 4 5 6 7 8 9 10

0.32 0.34 0.30 0.33 0.21 0.26 0.55 0.52

a)

e = eh¢ m at

150°C.

8 h ox. at 200°C

42 h ox. at 200°C

15 h ox. at 300°C

35 h ox. at 300°C

E

~

E

~

E

~

E

~

0.14 0.15 0.19 0.13 0.17 0.16 0.39 0.30

0.33 0.31 0.31 0.28 0.25 0.33 0.58 0.54

0.15 0.14 0.22 0.13 0.22 0.17 0.42 0.31

0.34 0.34 0.32 0.28 0.27 0.34 0.58 0.55

0.17 0.15 0.20 0.13 0.17 0.17 0.42 0.34

0.42 0.54 0.40 0.46 0.41 0.52 0.65 0.66

0.16 0.15 0.19 0.14 0.17 0.15 0.42 0.31

0.42 0.55 0.41 0.48 0.41 0.53 0.65 0.66

E

a) e = ehem at 250°C.

0.64 0.64 0.58 0.65 0.51 0.62 0.69 0.71

0.58 0.64 0.53 0.65 0.47 0.60 0.68 0.70

3 4 5 6 7 8 9 10

0.19 0.16 -

35 h ox. at 4 0 0 ° C

15 h ox. at 4 0 0 ° C

Sample

0.15 0.16 -

0.62 0.65 0.64 0.62 0.67 0.67 0.74 0.73

50 h ox. at 5 0 0 ° C

0.20 0.14

g

0.72 0.73

0.33

0.19

0.25

0.73

0.64 0.64 0.69 0.61 0.60 0.67 0.71

Ot

0.63 0.64 0.67 0.61 0.60 0.67 0.71 0.73

~t

E

0.22 0.22 0.32 0.19 0.22 0.18 0.44

~t

0.62 0.64 0.66 0.61 0.63 0.67

200 h ox. at 5 0 0 ° C

35 h ox. at 6 0 0 ° C

15 h ox. at 6 0 0 ° C

Table 4 Optical properties of the 8 stainless steel surfaces as a function of oxidation time in air a)

0.18 0.43 0.32

0.25 0.23 0.46 0.19 0.25

F.S. Boyda~ / Steel surfaces

191

100 u ~

SampLe 9

Samptc

8

0

] ~ 100 hrs 500"C vacuum J

80

20

60

40

cL 40

60

cO

tn ..121

80

I

0:5

i

i

&

I

I

tO0

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2'.5

5.1

Wevelength (pm)

Fig. 1. The absorption coefficient as a function of wavelength.

0

100 D~C,.~./X\

\

8O

J

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/\

.Yx_

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\ ". ~ Q. ~

~

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v-Sampte~ • ......... •

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air

SOhrs 500"C

air

~ ~ Sampte 7 ~ ~ X-- - - ---K

is .~s ~oo'c o~ 50hrs

20

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--- 60 '-.. \

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0.5

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0.8

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215

5"01 O0

Wavetength (Hm) Fig. 2. The absorption coefficient as a function of wavelength.

F.S. Boyda~ / Steel surfaces

192

100

L.

~

Skin i~assed

~ -

1~

1~ hrs

600°C

1/~"~

80

60

SurfcJce ground

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\

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Sampte 7 Sampte 5 rlir

20 40

\ \\\.,.

60 80

01

...... , , , , 0.5 0.8 1.5 2.5 Wavelength (Hm)

5.0

100

Fig. 3. The absorption coefficient curve for different surface preparations.

Fig. 4. SEM pictures of sample 7. (a) 50 h, 500°C, air; (b) 200 h, 500°C, vacuum.

F.S. Boydaf¢ / Steel surfaces

Fig. 5. SEM pictures of sample 8. (a) 50 h, 500°C. air; (b) 200 h. 500°C. vacuum.

Fig. 6. SEM pictures of sample 5. (a) 50 h, 500°C, air; (b) 200 h, 500°C, vacuum.

193

F.S. Boyda~ / Steel surfaces

194

treatments of some samples of AISI 304. One can observe that after several hours oxidation at 600°C the sample, which was exposed to a "surface ground" treatment, retained its infrared specularity while the other showed deterioration. Figs. 4 - 6 show the SEM pictures of 3 samples with two different surface preparations after several hours of treatment in vacuum and air. The grains of sample 5 having a size of approximately 3/~m, remained the same, which is a rather fine structure for the steel surfaces.

4. Discussion and conclusion The two different groups of experiments, one in vacuum and one in air, show the influence of the surface preparations on the spectral selective properties. The optimum values of solar absorptance and thermal hemispherical emittance for samples heated up to 500°C for 100 h under vacuum conditions were found to be a = 0.54; e = 0.15 (table 2, sample 8). The value of a for 0.5/~m was found to be lower at other temperatures. It should be noted that a high absorptance at 0.5/~m does not always mean a high solar absorptance, because the absorptance values are sometimes lower for other wavelengths. After 200 h treatment in vacuum of the six samples at 500°C the absorption values of around 1 - 1 . 2 / ~ m were rather good. It is difficult to explain from the data which mechanisms are responsible for the displacements in fig. 7. The minimum in the absorption curve around 0.5-0.6 #m, where the solar spectrum reaches its maximum is undesirable (fig. 7). This minimum leads to a low solar absorptance. 100

80 o

--- 60 r o

n 40 U1

.o

X

2O

A--A 0

X S(Irnple 7

3

~ Sa~le 8 ~ 200 hrs s~plo s 0

S~'npLe 3

500"C vocuom

J

I

0.4

'

' ' ' I. o . . . W a v e l e n g t h (~jrn)

.

.

.

1.5 . .

.

.

2.o

'

Fig. 7. Absorption coefficient as a function of wavelength for different stainless steel surfaces.

3.0

F S. Boyda~ / Steel surfaces

195

The strong absorption around 1 # m may be explained by the fact that the optical thickness of the oxide layer (which is very important) was suitable for the interferance phenomenon. The oxidation treatments apparently gave the best values for 400°C. After the treatment at 500°C in air, it was observed that the colour of samples 3-6 and 9 changed from dark blue to light gold, leading to a decrease in solar absorptance. From the specular reflectance measurements in the infrared region, it was found that all samples retained their specularity until heat treatment at 500°C. 400°C was the limit for specularity, except in the case of samples 7 and 8. Samples 7 and 8 continued to show good specularity after several hours of oxidation at 600°C. These two samples with surface ground mechanical treatment have a high degree of spectral selectivity and possess an ct/e ratio of about 5. One can conclude that the surface ground treatment gives a good selectivity to the steel surfaces for use in a concentrating solar collector with 500°C working temperature (fig. 2). An important aspect of the future development of spectrally selective surfaces is the cost of manufacture. If row materials are too scarce to manufacture the surfaces, then the conversion scheme is of little interest in the energy economy picture. Therefore, stainless steel represents a good possibility for solar energy applications. The production of selective finishes on different surface preparations by simply heat treatment offers an attractive alternative to the more complex methods. However, more studies will have to be done on different surface preparations in order to find better ones for solar energy applications.

Acknowledgement The author whishes to thank A.M. Schneiders and P.H. Beucherie for many enlightening discussions, and the High Temperature Absorber Materials (JRC) group for their help.

References [1] H. Tabor, Bull. Res. Coun. lsr. 5A (1956) 119. [2] A.B. Meinel and M.P. Meinel, Applied Solar Energy (Addison-Wesley,New York, 1977). [3] ASTM standard E434-71, Standard Method of Test Calorimetric Determination of Hemispherical Emittance and the Ratio of Solar Absorptance to Hemispherical Emittance using Solar Simulation (1972). [4] A.M. Schneiders and P.H. Beucherie, Solar Absorber Surfaces for High Temperatures, Izmir Int. Symp. II on Solar Energy Fundamentals and Applications (1979). [5] Terninoss Acciai lnossidabili, S.p.A., Viale B. Brin 171, Terni.