The solar energy absorption characteristics of several high temperature coatings were determined and effects of heat on these coatings were evaluated. Included in the investigation were an electroplated alloy of black chrome and vanadium, electroplated black chrome, and chemically colored 316 stainless steel. Each of the coatings possessed good selective solar energy absorption properties at laboratory ambient temperature. Measured at a temperature of 700 K (800 F), the emittances of black chrome, black chrome vanadium, and colored stainless steel were 0.11, 0.61, and 0.15, respectively. Black chrome and black chrome vanadium did not degrade optically in the presence of high heat (811 K (1000 F)). Chemically colored stainless steel showed slight optical degradation when exposed to moderately high heat (616 K (650 F)0, but showed more severe degradation at exposure temperatures beyond this level. Each of the coatings showed good corrosion resistance to a salt spray environment

Lowery, J. R.

English

NASA Technical Reports Server

NASA TECHNICAL 
MEMORANDUM 
NASA TM-82414 
SOLAR ENERGY ABSORPT l ON CHARACTER I STICS AND THE 
EFFECTS OF HEAT OM THE OPTICAL PROPERTIES OF 
SEVERAL COATINGS 
By J. R Lowery 
Materials and Processes Laboratory 
April 1381 
NASA 0 
George C. Marshall Space Flight Center 
M n rshall Space Flight Center, Alabama 
\BFC - Form 3 1 9 0  fRrv  June 1971)  
https://ntrs.nasa.gov/search.jsp?R=19810013943 2020-03-21T14:23:08+00:00Z
TECMNlCAL REPORT ST ANOARO TITLE PAGE I I. REPORT NO. 12. COVEANhlOIlT ACCESSIOF MI. ( 3. RECIPIENT'S CATALOG 6 0 .  
I 
8. PERFORMING ORGANI ZATION REPOR T a 
NASA TM-82414 I 
4. TITLE U ~ O  SUBTITLE 
Solar Energy Absorption Characteristics and The Effects 
of Heat On The Optical Properties of Several Coatings 
I 
10.  WORK UNIT NO. 
5 REPORT OAT€ 
April 1981 
6. PERFORM~NG ORGANIZATION CLOE 
George C. Marshall Space Flight Center I Marshall Space Flight Center, Ala. 35812 1 I .  CONTRACT OR GRANT NO. I 
I 4 
(3. 1 VPE OF REPOR'. & PERIOO COVERL. I 
12. SPONSORING AGEWCY NAME AN0 -SS 
I National Aeronautics and Space Administration Washington, D .C . 20546 Technical Memorandum IJ SPONSORIIIC AGENCY CODE I I 
I S .  SL'PPLEMENThRY NOTES 
Prepared by Materials and Processes Laboratory 
t 
! 
This study was conducted to determine the solar energy absorption 1 
characteristics of several high-temperature coatings and to evaluate the effects of 
heat on these coatings. Included in the investigation were an electroplated alloy of 
black chrome and vanadium, electroplated black chrome, and chemically colored 316 
stainless steel. The results of this study showed that each of the coatings possessed ' 
good selective solar energy absorption properties at laboratory ambient temperature. 1 
Measured at a temperature of 700°K (8OO0F). the emittances of black chrome. black 
chrome-vanadium , and colored stainless steel were 0.11, 0.61, and 0.15, respectively . r  
Black chrome and black chrome- vanadium did not degrade optically in the presence ' 
of high heat [ 81 1°K ( 1000°F) 1 . Chemically colored stainless steel showed slight 1 
optical degradation when exposed to moderately high heat (616OK ( 650°F) 1 , but 1 
showed more severe degradation at exposure temperatures beyond this level. Each i 
of the coatings showed good corrosion resistance to a salt-spmy environment. I 
1 
Unclessified- Unlimited 
I 
20.  SECURITY CLASSIF. (d thl* pose) 
I 7  MET WORDS 
I Unclassified I Unclassified I 13 I NTIS 
WIG - l r o m  a299 ( R w  December 1 9 1 1 )  Far ah by NaUonrl Technlc~I intonnrtlon Smdce, Spnolfield. Vlnctnlr 2 2 1 b i 
10. D l S T R l e u T l o ~  STATEMENT 
PREFACE 
This report records the results of the evaluation by hfarshall .Space 
Flight Ccnter of vllrious cantings and sLqnchrd treatments of surfaces being 
designed a s  s o b  r energy collectors. .Although tlre investignt ion included 
both y r o p r i e h ~  and non-proprietary electro-deposited cmtings. this report 
does not constitute an endorsement of one product as opposed to another. 
It n~erely evaluates the properties of these coatings for the singubr purpose 
of s o h r  heating ancl cooling resmrch. 
TABLE OF CONTENTS 
Page 
Introciuction .................................................. 1 
Experimental Procedure ....................................... 1 
l)iscussion and Results ....................................... 2 
Black Chrome ............................................ 2 
Black Chrome - Vanadium. ................................. 4 
Cdored Stainless Steel ................................... 5 
C Cmclusions ................................................... I 
1,lST OF TABLES 
Title 
H i g h  T e n ~ ~ r ~ a t u r e  Emittnnce 70Q°K (800°F). ..... 
Effect of Hent On Solar Energy rZbsorytion 
Chnracteristics. ................................ 
Effect of Salt Spray Environment on Solar 
Energy Absorption Chart~cteristics .............. 
LIST OF IL1,l'STR..\TIONS 
1nfr;ired Spectral HcflWtnncc o f  Solnr Energy 
Absorption Coatings ; k t  'iOO°K ( 8C)0°F). ......... 
Slmt rrll Reflecta~~ce of Soli~r Erlcrb~y Absory tion 
Caitings (Leborntury Ambient Tcnipcrnturc). ... 
SOLAR ENERGY ABSORPTION CHARACTER1 ST1 CS AND THE 
EFFECTS OF HEAT ON THE OPT1 CAL PRO PERTI ES OF 
SEVERAL COATINGS 
INTRODUCTION 
Before solar energy can be directly utilized for heating and cooling 
buildings and heating water, it must be collected to produce a heating 
effect and the heat energy t r ~ n s f e r r e d  to a working fluid. This is 
accomplished through the use of two basic types of collecting devices: 
flat plate and focused (or concentrator). A flat-plate collector consists 
of 3 transparent cover and a receiving surface or absorber. Solar energy 
is transmitted through the cover, absorbed, and the heat energy trans- 
ferred to the working tluid. In the concentrator type. the solar energy 
is concentrated by passing it through a lens o r  reflecting i t  from a 
parabolic surface. The energy is focused on a collector tube (absorber) 
where extremely high temperatures may be produced and the absorbed 
heat energy transferred by the working fluid. Regardless of the type of 
collector. the receiving surface is  a necessary component of the system 
and must absorb a large percentage of the sun's energy if good efficiency 
is to be achieved. These surfaces are  usually prepared by reducing the 
solar energy ~eflectance (increasing the absorptance) of metals while 
maintaining their high infrared ( 1R ) reflectivity (low emissivity ) . To 
produce such an optical change. black coatings are  applied by electro- 
deposition or  surfaces are  blackened or colored by chemical change. The 
surface treatment or coating is  controlled to  a thickness sufficient to 
make the metal surface optically black yet thin enough to retain its high 
I R reflectance. 
Since the receiving surface or  absorber tube in the concentrator 
collector is subject to high heat. the tube material. a s  well a s  the 
absorber coating, must be able to withstand such heat for extended periods 
of time. This report presents the results of an investigation conducted to 
determine the optical effects of high heat on several absorber coatings. 
The plating parameters and optical characteristics of these coatings are 
also presented. Coatings known to have good optical properties but low 
resistance to heat were not included in the study. 
EXPERIMENTAL PROCEDURE 
The coatings were produced on metals and metal alloys by either 
electroplating or chemically treating the metal surface. In same cases. 
,3~rl6:94 ;,:lP:rmc-):rtrc ;-;r?rc: v ~ r ~ e r i  in a n  effort to produce coatings with 
? I  I , , . I  n ; ,  I I : .  After r~;)tiir.idng ;riating 
pnrametcrs. the thernml stability of the coatings wns evaluated by heating 
test pclncls in n furnlicc at vnrying temperc~tures up to 922OK (1200°F). 
Opt icrll pl-operties w c l r  mc~surod before and clfter heating to  cietenaule 
the efl'wt of the hcot. If the  SO:I~I* C I I C ~ ~ ~  nbsorptiotl properties wcre 
still witliin an  acweptable ninbv tr Cter heating. the  corrosion resista~lcc 
of the cocltings was determined. This was done by subjecting coated test 
specinlens to a five-percent salt - spray environment controlled and  operated 
in accorance with Mil -STD - 8 10C. hlethod 509.1. 
All optical measurements were made with ei ther  a Gier  Dunkle Solar 
and IR Reflectonieter , a Beckman DK - 2A Ratio Recording Spectrophotometer , 
o r  a Willey 318s I R  Spectrophotometer. The solar energy absorptances 
w e r e  determined by computing the  difference between the  solar energy 
reflectance and unity ; the  emittances were determined similarly by com- 
puting the  difference between the  1R reflectance and unity. 
DISCUSSION A N D  RESULTS 
Black Chrome 
Bhck chrome is on electroplated coating which is usuttlly ict black 
in nppeurnnce and composed of around 75 percent chromium and 25 
percent chronlium oxide. The plating parameters. corrosion resistance. 
and "as plated" optical characteristics of this coating were presented in 
H previous report 1 11 itnd will not be repeated except to present the  
opticul effects of high heat on the  conting. The emittance a t  700°K 
(800°F). whicl~ i v ~ s  not measured in the previous s tudy.  was 0.11 ss 
shown in Table 1. ' The infrared reflectance curve  used to obttlin this  
value i s  shown in Figure 1 .  
b 
iJlnck Chrome 0.11 
Ulitck Chrome Vnnudiurn 0.61 
Ulackened Stainless Steel 0.15 
The results of hcnt tests  on plittcd low-carbon steel test spccinicns 
showcd tllr~t tcmporrrttrres up to 81 I°K ( 100O0F) ditl not significcantly 
i~t't'ect the cljltic:;tl ~)twpcrt ics of this  coating. liowever . rt shirrp ir\cntr~se 
itr cn~ittance cxcurrcd upon expo5urc to ;I tempcrttturc of 92Z°K ( 1 200°F) 
;IS shown in l ' ;~l>le 2. 'Tcmperittut*cs in the  range of 70O0h: (800°F) to 
81 I0K ( 1000°F) cuused il drop i l l  thv absorptancc but  fur ther  heirting 11t 
922OK ( 1300°1:) c~rused an incwilsc in +his property as also shown in the 
titble. In appcr~rrtnce. test Pilnels changed to o dull black which would 
suggest thc observed increase in both solar energy absorptence and 
emittence . 
-- 
I .  ~ h e  nbsorpt;i6ces und emittctnces shown in Tables 1, 2 .  nnd 3 tire 
rtvurrtge values obtained from dul~licnte test pnncls. 
2' 
BLACK CHROME 
2 BLACK CHROME - VHNADIUM 
I 
O i ~ - - l . - - ~ L  0 
2 2 . 5  3.3 5 10 20 
WAVELENGTH, u M 
Figure 1 .  Infrared spectral reflectance of solar energy 
absorption coatings at 700°1i ( 800°F) 
TABLE 2. EFFECT OF HEAT ON SOLAR ENERGY ABSORPTION 
CHARACTERISTICS 
NOTE: Exposure Time - 6 hours per heat cycle. 
3 
Optical Properties 
Black Chrome 
A bsorptance 
Emittance 
Black Chrome- 
Vandadium 
Absorptance 
Emittance 
Chemically Blackened 
Stainless Steel 
A bsorptance 
Emittance 
Refore 
Exposure 
0.94 
0.09 
0.87 
0.29 
0.90 
0.11 
Exposure Temperature ,KO(FO) 
616OK 
650°F 
0.91 
0.07 
0.90 
0.18 
0.84 
0.12 
811°K 
1000°F 
0.87 
0.06 
0.93 
0.17 
0.68 
0.14 
700°K 
800°F 
0.90 
0.07 
0.89 
0.19 
0.72 
0.12 
92Z°K 
1200°F 
0 .95  
0.64 
0.91 
0.30 
0.75 
0.25 
B b k  Chrome - Vanadium 
This coating is sm ailoy reportedly oollipwi of aypmdmtely 90 
percant chromiunl and 9 percent vam.Hun~ and tiw oxides of tlw two 
metc~lo I 21 . The vunadiunl is yrerrejrt pPi~ulpily at* vanadium triovide 
(V,Og) with only a sarrll portion of the metal pmfent. The chromium is 
prfiarlly present as metal with only a srnaLl percenta(p of chmium wide. 
The coating is prod& by electrodepodtion mQ iE normally jet W k  
in appearance. It mpactedly has good adherence to subetwtee and 
excellent stability at  elevated temperatu-. When plated to convmtiaRal 
thicknesses. the coating exhibited a high thermal edttar)ce and w&w energy 
crbmorptance. In this otudy, cxmting tbicknefm?o umv rwluoeB to lower 
the emttance yet maintain the high aboarptance to furnib optical 
u e k t i  .ity . 
After conventional cleaning, test specimen6 of l o w  capbon &eel w e r e  
first bright nickel plated to a thickness of 0.0127 mm (0.0005 in. ) to 
furnish both corrosion protectian of the substrate and a lowep emittance 
base upon which to electroplate the black chrome-vanadium alloy. The 
black chrome-vanadium was then plated in a bath with the following 
composition : 
Chromic Acid ( C a 3 )  200 g/ l  (3.59 oelgal) 
Ammonium Metavanadate (NH4V03)  10 g/ 1 ( 1.34 oz/gal) 
Glacial Acetic Acid (CH3COOH) 6.5 mlll (0 .84 fluid ozlgal) 
The bath was operpted at a current density of 500 a ~ l c m '  (600 ~ / f t ' )  
and within a temperatupe range of 3 O P K  (O4OF) to 323OK (12Z0F). 
Although plating time was varied in an effort to pmduce a coating 
with the optimum solar absosptim characteristics, times of 3 to 
5 min produced the optimum coating thiclulwses with mgard to optical 
selectivity. Such coatings showed laberatory ambient temperature 
absorptance and emittiuroe values ranging from 0.84 to 0.80 and 0.27 to 0.41 ,  
respectively. The ernittanoe of thif -tin$, metloured at 700°K ( 800°F), 
was 0.61. This value was derived f~ocrl the infrared wflwtance data 
produced by the WiUey Gpectrophotometer and shown in Tgble 1 and 
Figure 1. This emittance was unexpectedly high in view of the fairly low 
value exhibited by  blaek chrome. Apparently the optbal characteristics 
at this temperature were influenced by the p f e w  of the vanadium or 
vanadium oxide in the alloy. 
The reflectance at laborato~y ambient temperature in the shorter 
wavelength (a small portion of the incrared, the visible light, and a srnall 
portion of ultraviolet) region of the energy spectrum is shown in Figure 
2. A t  this temperature, a d  in this re@n, this mating is very similar 
to black chrome. 
Figure 2. Spectral reflectance of solar energy absorption 
coatings (labomtory ambient temperature). 
The results of heat tests showed that high temperatures did not 
significantly affect the optical selectivity of this coating. The absorptances 
and emittances fluctuated somewhat with the haat cycles, but at the end 
of the final heat period these values were only slightly higher than the 
original valuw as shorn in Table 2. N o  significant thane in appeamce 
was observed as a result of the heat exposure. 
Tests conducted to determine the corrosion resis&nce of this a i l q  
showed that the mating had good resistance to a salt-spray environment. 
Test panels showed no significant corrosion when subjected to this environ- 
ment for a period of 576 hr. As shown in Table 3 a slight decrease in 
both solar energy absorptance and emittance was noted which probably 
resulted from minor stains on the panel surfaces. These stains were 
caused by an edge corrosion product running down onto the main test 
surface of the panel. This edge corrosion was initiated at points where 
electrical con tact was made during the electroplating. 
Colored S tainlesa S tee1 
Various colored films (including black) can be produced on the 
surfaces of stainless steel by immersion in mixtures of sulfuric and chro- 
mic acid. The different colors am produced by varying the treatment 
time in the acid mixture. The "as formedw film is relatively soft but 
may be hardened by a process developed, in recent years, by the 
Snternationd Nickel Co., Inc. [31. This process employs the cathodic 
, . 
.---.. ... .. . - ... .. ... I,._. , .. ... -.... ... "I. . , - ..L - ..L ..- .. 
BrrYenl SL4id4m . , . ----- 
NOTE: Exposum finm - #?dl &s. 
t~tmctnt d the mixture. 
Tsst  p m i b a Q S  
(blackened) 801 
Reference 3). T 
listed below: 
coloping Solutian - 
Chromic Acid gfi 4Ba5 m/gzd) 
~ ~ l f ~ r i ~  acid gn 4 e C 7  Q E ~ ~ U  
Chromic Aeiil %89 fgB dW.g Q I B ~ @ ~  
P ~ O S ~ ~ ~ C  A Q ~  8,s gn <.%I ~ e i g a i )  
Operating Tempemturn 
Time I# iPls 
Current Density 2.8 mA/&mg I + $  whg) 
MGFNAL PAGE I8 
OF POOR QUALITY 
F o r t h e p u r g o g e o f W p ~ m , t h g p ~ e d t l g t i w e i n t b g e a d a r ? l r l g  
e d u t i o a n w a ~ m ~ t o p r s d u c % ) a W ~ g w f t h m ~ . 8 w r l a r  
energy abmrptame and asp low a the,- 0111thntm ~ 1 8  padblig. Imnmrcim 
~ ~ ~ f ~ e n r 8 t o 1 % ~ ( n o s i s a n i P J Q s u r t P I l r r r w ~ f ~ d r t r i n g  
the fbst Sndn),but it crgpewdthrat around 8 t o 8  elnh theadd mixture 
p F W 1 & a f i l m w i t h t h e a a o e t ~ a d a r  abaoaptbn pawpertias. 
Panels treated far thts length of tiafe showed h b m t ~  ambient tssopra- 
ture FM&W energy &mrptdllWtOB and thermal emittmmm in the of 0.87 
to 0.m and 0.14 to Q.26, resperctiwdy. The tmittance of the coating, XXUB- 
wed at 700°K (8Q0°F), oolte 0.15 ae shown in Tsrble 1 md Figtam 1. This 
value was almost as low es Wt of black chrome and them WIUI -what 
less fluctuatian in reflectance tiis noted by the two curves. 
At laboratory ambient temperature in a portion of the ultraviolet and 
visible region (wavelength range of 0.27 to 0.35) of the spectrum, the 
reflectance of thie coating was slightly higher than either the black chrome 
or the black chrome-vanadium alloy as shown in Figure 2. However, in 
the outer visible region, the blackened s ~ l e s s  steel showed a sharp 
decrease in reflectance just prior to a sharp increase beginning with and 
continuing into the infrared. These data are shown in Figure 2. 
The results of heat tests showed that the laboratory ambient tem- 
perature optical properties of this coating were degraded by exposure to 
heat. A slight decrease in solar energy absorptance occurred from 
exposure to moderately high heat, but more severe degradation occurred 
with increase in exposure temperatures. These data are shown in Table 
2. The appearance of test panels changed from an initial bluish black 
to a brownish black after the initial heat period. Further heating at 
higher temperatures caused severe fading of the black chemical coating. 
Still further heating caused the surface of the substrate to become re- 
blackened from oxidation in the high-temperature air. This oxidation 
probably caused the observed increase in both the absorptance and 
emittance after the 92PK (1200°F) heat cycle as shown in the table. 
Tests conducted to determine the corrosion resistance of this alloy 
showed that the chemically blackened stainless steel had excellent resist - 
ance to a salt-spray environment. A s  shown in Table 3 test panels had 
no corrosion or change in optical properties after subjection to this 
environment for a period of 576 hr. 
CONCLUSIONS 
The results of this study showed that the laboratory ambient tempera- 
ture solar energy absorption properties of electroplated black chrome and 
black chrome-vanadium alloy were degraded less by exposure to high heat 
than chemically blackened stainless steel, However, the thermal emittance 
of the black chrome-tanadium alloy, measured at 700°K (800°F), was 
consider~bly higher. than that of the black chrome and chemically blackened 

1 Lowery, James R.: Solar Abr~orgtlon Cheracterhtico of Smml 
Catingar PnB Surf' FMhw. George C. YmvhU Spats Flight 
Center, NAbA !I'M X-3580, March lW. 
8. Q u y *  MPrtin F* : tlng B1Polr: Chromium-Vmadi~m 
Coatin- and Manbere Therewith. U d t d  States Patent No. 2, 
$24, 829, Patent& February 85, 1958. 
3. Colored Stainless S W .  Intmuatimal Niobl Co. , Inc. , Products 
Finishing, July 1977, p. 30. 



Solar energy absorption characteristics and the effects of heat on the optical properties of several coatings

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