Method for obtaining a selective surface for collectors of solar and other radiation

The present invention relates to selective surfaces for collectors of solar and other thermally useful radiation. According to the invention, a zinc-coated metal plate having a thickness of between 2.mu. and 30.mu. is immersed in an alkaline bath containing 2 to 30 g/l OH.sup.- and comprising sodium nitrate and sodium hydroxide. Subsequently, the anodized plate is passivated in an acidic bath which contains more than 2 g/l H.sup.+. Preferred constituents in the acidic bath are selected from acidic phosphate salts, oxalic acid, chromium ions, sulfuric acid or mixtures of two or more thereof. The selective thermal radiation collector surfaces obtained possess improved radiation properties, being capable of maintaining them even under corrosive conditions.

The present invention relates to selective surfaces for collectors of solar 
and other thermally useful radiation. More particularly, the invention 
relates to an improved selective surface for such collectors capable of 
withstanding severe corrosive conditions. 
A common type of collector for solar and other radiation, used for heating 
water, comprises an absorber plate, which absorbs the radiation and 
transfers the resulting heat to the water; one or more glass covers to 
prevent the plates being cooled by the wind; and some means of insulation 
at the back of the plate. It is a well known practice to use black 
coloring to enhance the absorption of radiation and to reduce heat 
reflection. As a plate heats up, it loses heat to the surroundings in 
three ways: (a) by thermal radiation, (b) by convection in the air gap 
between the absorber and the glass cover, and (c) by heat loss through the 
back of the absorber. 
Heat loss by radiation constituting a major portion of overall losses, most 
of the research in the field is concentrated on how to reduce this kind of 
heat loss. As is well known, all heated bodies emit thermal radiation, the 
amount of such radiation being a function of a property of the surface 
called emissivity (.epsilon.) and, to a lesser extent, a function of the 
surroundings. A solar collector surface is called selective if its 
adsorption co-efficient (.alpha.) is high while its emission coefficient 
(.epsilon.) is low. 
Blackening a metal surface by dyeing is the least expensive method but its 
principal disadvantage is the poor fastness of the color and particularly 
its low resistance to sunlight. For this reason, electrolytic treatments 
are preferred for the purpose, enabling the thickness and the related 
properties of the coatings to be closely controlled. 
The main requirements for a practical selective surface of a thermal 
radiation collector are: 
(a) High thermal radiation absorptivity (.alpha.); 
(b) Low thermal emittance (.epsilon.); 
(c) Stability against atmospheric corrosion; 
(d) Applicability to given substrate materials; and 
(e) Reasonable cost. 
A selective coating applied to a solar radiation absorber sheet, for 
example, provides the practical advantage of increasing the efficiency of 
the heating process by reducing re-radiation heat losses from the 
collector of which the sheet forms part. The hotter the absorber sheet, 
the more heat will be transferred to the circulating fluid, whereby the 
overall performance of the systems is enhanced. 
In practice it is not easy to obtain high values, since even black paints, 
which in any case are not particularly suitable, rarely have an 
absorptivity, .alpha., greater than 0.95, a value that is moreover liable 
to drop after some time for a variety of reasons, principally the paint's 
fading. Conventional selective coatings are invariably quite thin because, 
if the thickness is increased in an effort to increase .alpha., the 
emissivity, .epsilon., tends to increase appreciably. Interference effects 
in the thin films have been used to reduce reflectivity to near zero at a 
predetermined point of the solar spectrum; and with more complex optical 
multi-layer coatings the near-zero reflectance can be made to be 
applicable to a wide range of the solar spectrum. Such coatings are, 
however, unlikely to be viable economically except, possibly, in 
high-concentration collectors, where the total absorber surface area to be 
treated is small compared with the total collection area. 
One of the problems recognized by all persons skilled in the art of solar 
collectors is the achievement of stability against atmospheric corrosion. 
As will be realized, it is extremely difficult to obtain good stability of 
exposed surfaces also required to have the very specific optical 
properties characterizing a selective surface. It has been stated by 
persons versed in the art that it is almost impossible to leave a 
selective surface without a protective window, but even then, some 
deterioration in the absorptive properties of the selective surface is 
encountered due to the persistent corrosive effects of the environment. 
Four types of selective surfaces are known: (1) multilayer coatings on 
metal substrates, wherein layers of materials of controlled thicknesses 
are deposited in vacuo; (2) semiconductors, which usually are opaque 
materials exhibiting strong absorption for (visible) solar wave lengths 
plus good reflectance toward the infra red; (3) radiation trapping 
surfaces, incorporating porous structures that trap short-wavelength light 
thereby increasing the solar absorptance, and (4) thin films, which rely 
on process control to provide optimum film thickness for selectivity. The 
latter type is the most frequently encountered, being also the most 
economical to produce. 
A simple spectrally selective absorber, described in a recent paper by M. 
van der Leij (J. Electrochem. Soc., Solid-State Science and Technology, 
Vol. 125, August 1978, p. 1361-1364), consists of a metal substrate 
covered with a thin semiconductor film. The semi-conductor must have a 
high absorptivity for wavelengths shorter than 2.2 .mu.m and a high 
transmissivity for wavelengths longer than 2.2 .mu.m. More particularly 
the black oxide films were deposited on a steel surface electroplated with 
zinc using an alternating current in solutions that contain NaNO.sub.3 and 
10-3 g/1 NaOH with or without NaClO.sub.3. The duration of the anodizing 
treatment is mentioned to be between 1 to 3 minutes. The films obtained 
indeed possessed improved radiation characteristics, being in the range, 
0.08 to 0.17. However, the oxide films failed when tested in accelerated 
corrosion conditions, so that the surface ceased to be considered 
selective after only one year. The brief review given above clearly 
explains the existence of a long-felt need for a selective solar radiation 
collector surface that is resistant to corrosion. 
It is accordingly an object of the present invention to provide a method 
for obtaining a selective thermal radiation collector surface possessing 
improved radiation properties and capable of maintaining them even under 
corrosive conditions. It is another object of the present invention to 
provide a simple method for obtaining selective thermal radiation 
collector surfaces with unimpaired radiation properties. 
The invention consists in a method for obtaining a selective surface for 
thermal, and particularly solar, radiation collectors which comprises the 
steps of: 
(a) anodizing by alternating current density in a range of 5-60 A/dm.sup.2 
--a zinc-coated metal plate of a thickness in the range of 2.mu. to 
30.mu., immersed in an alkaline bath containing between 2 to 30 g/l 
OH.sup.-, and 
(b) passivating said anodized plate in an acidic bath containing more than 
2 g/l H.sup.+. 
The selective surface on the metal plate obtained according to the method 
of the present invention is characterized by its outstanding stability 
even under corrosive conditions. Selective surfaces obtained in accordance 
with the present invention did not show any change in their low emissivity 
after 10 hours of salt spray testing, as against a substantial decrease, 
after only one hour's salt spray testing, in that property of selective 
surfaces not treated with the last-named step, viz. passivation. It was, 
in addition, unexpectedly found that the acidic passivation step does not 
affect the absorption coefficient, .alpha., any more than the emissivity 
.epsilon.. 
The step of acidic passivation is very simple and does not materially 
increase the operational cost of obtaining the selective surfaces in 
accordance with the present invention. The step involves the simple 
insertion of the anodized metal-covered metal surface into an acidic bath 
for a short time in the order of seconds. 
In a lecture given by the inventors on the development of a selective 
surface for solar radiation collectors on galvanized steel (Proceedings of 
Materials Engineering Conference, December 1981, Technion, Israel) a 
method was described for obtaining a selective surface which comprises the 
passivation of anodized Zn-coated metal plate in an alkaline potassium 
dichromate solution. While the method suggested has some merits, it 
requires a thickness of at least 10.mu. Zn. As known, to get thickness of 
such order is quite expensive which reduces the actual use of the method. 
Morever, the selective surfaces obtained were shown to possess improved 
radiation properties which were stable under ordinary, but not under 
corrosive, conditions. It is, however, well known that solar collectors, 
being installed in the open air, have to withstand corrosive conditions 
that are often unavoidable. The improved radiation properties of the 
selective surfaces obtained by the method of the present invention are 
illustrated by their basic parameters, viz. absorptivity, .alpha., which 
is in the range, 0.86 to 0.95, and emissivity, .epsilon., which is in the 
range of 0.06 to 0.10. The selective surfaces obtained by the method of 
the present invention are further characterized by the durability of their 
coating, which is an important factor in determing the service life of the 
collector. This should be at least eight years. During that time the 
reduction in the solar radiation gathering performance due to the 
degradation of the coating is avoided. In order to obtain consistent 
selective properties, it is suggested to control the thickness of the 
deposited layer and the conditions of deposition in order to keep them as 
uniform as possible. 
The metal to be utilized as substrate may be selected from any of the 
commercially available varieties of Zn-coated metal, or Zn, or Zn-alloy, 
such as Al.sub.4 Zn.sub.96. Any galvanized or dip-coated plate with a Zn 
coating will also suffice. In a preferred experiment, using Zn-coated 
steel plate as substrate material, the metal plate was heavily 
Zn-electroplated in a cyanide bath prior to anodizing, resulting in a 
zince coating of a thickness in the range of 12 to 20 microns. A thickness 
smaller than about 10 microns would generally reduce corrosion resistance 
of the final product and thus might not be preferred, while any greater 
thickness would produce no added advantage but would naturally be 
considerably more expensive. 
The zinc-coated plate was then cleaned in an alkaline bath, which treatment 
was followed by acidic etching. The actual anodizing step was carried out 
with alternating current in the density range of 5 to 60 A/dm.sup.2 and 
preferably 10-40 A/dm.sup.2 in a solution containing sodium nitrate and 
sodium hydroxide, the latter in a concentration of 2 to 30 g/l OH.sup.- 
and preferably 4.2-17 g/l OH.sup.-. In order to optimize the radiation 
characteristics, the bath composition, current density, and time of 
anodizing may be varied in the manner known to any person skilled in the 
art. Generally speaking, the concentration of NaNO.sub.3 in the anodizing 
bath should be in the ranges of 15 to 30 grams/liter (NaNO.sub.3), and a 
temperature of about 30.degree. C. When the a.c. densi ty is between 15 
and 25 amperes per square decimeter, the time required will be less than 1 
min. After this step the anodized plate will have an absorptivity, 
.alpha., of about 0.95 and an emissivity, .epsilon., of about 0.12 . 
The subsequent treatment of passivation in an acidic bath is a step of 
great importance to the present invention, since it is that treatment 
which imparts to the selective surface its outstanding stability even 
under severely corrosive conditions. It was quite surprisingly found that 
acidic passivation, when following the anodizing by alternating current of 
a Zn-coated plate, neither reduces the absorptivity nor increases the 
emissivity, of the selective surface. After this passivation treatment the 
.alpha. and .epsilon. values remained substantially unchanged. 
There are several acidic compositions capable of imparting anti-corrosive 
stability to the selective surface of the collector. The more important 
groups that are suitable are as follows: acidic phosphate salt solutions, 
oxalic acid, chromic acid, sulfuric acid, or mixtures of two or more of 
these ingredients. A person skilled in the art will be able to select the 
preferred acid-salt compositions to accord with the specific requirements 
and available facilities. In particular, an acid salt solution containing 
potassium dichromate and sulfuric acid with a minimum 2 g/l H.sup.+ was 
found to be most suitable for the present invention, the following 
concentration ranges proving advantageous: 
Sulfuric acid: 2 to 20 ml/l, 
Potassium dichromate: 80 to 200 grams/liter. 
With an acid bath as described the insertion time required for achieving 
sufficient passivation is of the order of a few seconds and generally not 
above 5 seconds. 
The stability of the selective surface was tested in accordance with ASTM 
B.117 by the salt spray method, considered to be the most drastic test for 
this purpose. Even after 10 hours no sign of deterioration could be 
detected, the reflective properties of the selective surface, as 
characterized by the .alpha. and .epsilon. values, having remained 
unaffected. This stability makes it reasonable to predict that the 
selective surface according to the present invention should remain stable 
for at least eight years with the reflective properties unimpaired. 
Ultra-violet radiation is an important component of solar radiation, and 
the anodized and twice passivated plates were accordingly subjected to UV 
rays. A visual inspection of the irradiated plates did not disclose any 
fading or other deterioration of the blacking, and this was corroborated 
by other tests showing that there was no impairment of the absorptivity 
(.alpha.), while emissivity (.epsilon.) increased by a mere 10%. 
Summarizing this test it can be stated that the surface treated in 
accordance with the present invention stands up well to ultra-violet 
radiation and that its selectivity is maintained. Numerically, the results 
were as follows: 
______________________________________ 
Absorptivity Emissivity 
______________________________________ 
Before UV irradiation 0.94 
0.34 
After UV irradiation 0.95 
0.44 
______________________________________ 
Although the acidic bath exemplified is shown to contain chromium ions, it 
should be understood that other acidic baths known for their metal 
passivation properties may be used provided that they serve to keep the 
.alpha. and .epsilon. parameters stable, both during passivation and in 
service. Prima facie it would, however, seem that an acidic bath 
containing chromium ions will be the preferred formulation. A preferred 
acidic bath will contain at least 5 g/l H.sup.+. 
The passivated Zn-coated plate obtained according to the present invention 
was found to possess improved conductivity and can be useful for other 
purposes wherein a persistent black plate is required such as in the 
electronic industry. 
While the invention will in the following Examples be described with the 
aid of certain preferred embodiments, it should be understood that these 
are not intended to limit the invention to the their particular 
conditions. On the contrary it is intended to cover all alternatives, 
modifications and equivalents as may be included within the scope of the 
invention as defined by the appended claims.

Thus, the following Examples which include preferred embodiments will serve 
to illustrate the practice of this invention, it being understood that the 
particulars described are by way of example and for purposes of 
illustrative discussion of preferred embodiments of the present invention 
only. 
EXAMPLE 1 
A metal plate having a zinc coating of a thickness of 13-17.mu. was 
anodized for 1 minute in a bath containing 25 g/l NaOH and 20 g/l 
NaNO.sub.3 at a temperature of about 30.degree. C. The current density was 
20 A/dm.sup.2. 
The anodized plate was subsequently passivated by immersing it for 3 
seconds in an acidic bath containing K.sub.2 CR.sub.2 O.sub.7 130 g/l and 
H.sub.2 SO.sub.4 (conc) 8 ml/l. 
The plate obtained had an absorption coefficient (.alpha.) of 0.86 and 
emission coefficient (.epsilon.) of 0.009. 
EXAMPLE 2 
A metal plate having a coating of 2-2.7.mu. zinc, was anodized for 45 
seconds in a bath having the same composition as in Example 1, at a 
temperature of about 30.degree. C. The passivation was performed as in 
Example 1 under the same conditions. The plate obtained had .alpha. of 
0.85 and .epsilon. of 0.07. After salt spraying test for ten hours, the 
above properties remained substantially unchanged, the values of the above 
coefficient being: .alpha.=0.89 and .epsilon.=0.07.