SUBSTRATE COATED WITH A CORROSION-RESISTANT BLACK LAYER AND PROCESS FOR OBTAINING SUCH A SUBSTRATE

Abstract: Black, selectively absorbing layer consisting of finely comminuted nickel, nickel-zinc or nlckel-lead on a sub­ strate, particularly a solar collector plate, which is protect­ ed from corrosion by a thin passivating chrornic-oxide lay­ er.

Description: "Substrate coated with a corrosion-resistant black layer". The invention relates to a substrate coated with a corrosion-resistant layer, particularly intended for use as a receiver plate for a solar energy collector Such an object is known from the United States Patent Specification 2,917,817. The black layer described in the aforesaid U.S. Specification consists inter alia of a deposit, containing nickel obtained by electroplating, for example a finely comminuted Ni-Zn alloy which probably also contains a quantity of NiS. A suitable electroplating solution for producing this deposit contains per liter: 75 g of nickel sulphate, 28 g of zinc sulphate, 24 g of ammonium sulphate, 17 g of ammonium thiocyanate, 2 g of citric acid and has a pH of approximately 4. The solution is used at a temperature of approximately 30 C and a currentdensity of 2 mA/cmê for 2-4 minutes. Of importance for use in a solar energy collector is the absorption for sun rays in the visible part of the spectrum (A), which must be as high as possible, and the emission (±) in the infrared part, which must be as low as possible. The use of black layers of this type is not limited to solar energy collectors only: such layers are also interesting for all sorts of decorative purposes. However, all applications require a good corrosionresistance of the layers. In this respect the known layers leave much to be desired in an accelerated damp heat test the deep black colour faded to a considerable extent after one day only. The value of away consideraily lowered and the value ofE also became somewhat lower. One object of the invention is to provide a considerable improvement of the corrosion-resistance. According to the invention, a substrate provided with a layer containing a black, selectively absorbing, finely comminuted nickel is characterized in that a thin protecting, passivating layer, predominantly consisting of CrOx, wherein x = 1.5 + 0.1, is present on the black layer. Such a layer, which is only some nm thick does not noticeably affect the appearance and the optical properties of the nickel-containing black layer. It is assumed that, in addition, the CrO layer prevents the finely comminuted metal particles from being sintered at elevated temperature, so that the layer also considerably contributes to an increase of the life. This protective layer can be obtained by immersion in a neutral or weakly alkaline chromate solution, but the layer is preferably deposited by electroplating from a neutral or weakly alkaline chromate solution, the object being alternately the cathode and the anode (the so-called periodical reverse current electrolysis). The switching frequency is preferably of the order of 1-10 Hz, for example 5 Hz. Copper or nickel is preferably used as the substrate to obtain an optimum corrosion-resistance of the assembly. The copper or nickel may be applied in the form of a layer on a steel base. Nickel is used when the emphasis is for corrosion-resistance, whereas copper is preferred for use in vacuum collectors in view of the lower The invention will now further be explained with reference to the following embodiments. 1. Steel plates having a surface area of 0.5 dm2 are provided by means of electroplating with either a 2/um thick layer of copper or a 2/um thick layer of nickel. A black Ni-Zn layer is electrodeposited on these steel plates by means of an electroplating bath having the following composition per liter of solution: 100 g NiCl2 40 g NH4Cl 20 g NaSCN 40 g ZnCl2 The pH of the solution is 4.7. The solution is operated at room temperature and with a current density of 1-1.5 A/dmê. Composition of the deposit: 20 % Ni, 40-50 0A Zn and 30-40 /0 sulphur in the form of sulphide. The following Table 1 shows the influence of the layer thickness on &alpha;(aborption of visible radiation in O/o) and ±(emission in the infrared in %). The number of Coulombs used as representing the thickness of the deposition of the layer are given in order to obtain an indication of the layer thickness of the layer. TABLE 1. Selectivity of black Ni/Zn layers EMI3.1 <tb> Current <SEP> density <SEP> Number <SEP> of <SEP> <SEP> Coulombs/dm2 <SEP> oc <SEP> C <SEP> <tb> <SEP> 1 <SEP> A/dm2 <SEP> 10 <SEP> 0.56 <SEP> 0.03 <tb> <SEP> 20 <SEP> 0.94 <SEP> 0.10 <tb> <SEP> 30 <SEP> o.96 <SEP> 0.16 <tb> <SEP> 40 <SEP> o.96 <SEP> 0.22 <tb> <SEP> 50 <SEP> 0.97 <SEP> 0.25 <tb> <SEP> 1.5 <SEP> A/dmê <SEP> 20 <SEP> 0.92 <SEP> 0.07 <tb> <SEP> 30 <SEP> o.96 <SEP> ; <SEP> 0.18 <tb> <SEP> 40 <SEP> 0.97 <SEP> 0.28 <tb> <SEP> 60 <SEP> 0.98 <SEP> 0.33 <SEP> <tb> <SEP> 100 <SEP> 0.98 <SEP> 0.54 <tb> Plates, having a 2/um thick Ni-layer are plated with a Ni-Zn layer from the above-mentioned bath, which is operated with a current density of 1 A/dmê and a charge of 26 C/dmê. The following Table 2 shows the influence of an accelerated damp heat test on plates which were subjected to various different treatments. The accelerated damp heat test in accordance with IEC 68-2-11 test KA consists of a number of cycles wherein the temperature is increased from 200 to 550C and the relative humidity (RH) is simultaneously changed from 80 to 100. The cycle consists of a period of 2 hours in which the temperature changes from 250 to 550C and RH changes from 80 to 100, a period of16 hours in which the temperature remains 550C and RH 95-100% and a period of 3 hours, wherein the temperature changes from 55 to 25 C and the humidity RH from 100 to 80 %. Plates B1 and were not subjected to a final treatment; B3 and B4 were chemically processed by immersing them in a chromic acid solution containing 10 g of CrO3 per liter (pH = 7). The layers B5-B10 were electrochemically post-treated with an alternately, periodically reversed, cathodic and anodic current: B5 and 3 with a frequency of 0.75 Hz 6 and a current of 100 mA; B7 and B8 with a frequency of 0.1 Hz and 150 mA and B9 and 10 with 5 Hz and 80 mA. The above-mentioned solution was used at room temperature. -TABLE 2 TABLE 2 Slectivity of black Ni/Zn layers after damp-heat test EMI5.1 <tb> <SEP> A <SEP> <tb> Sample <SEP> duration <SEP> of <SEP> duration <SEP> of <tb> <SEP> damp-heat <SEP> test <SEP> damp-heat <SEP> test <tb> <SEP> 0 <SEP> 1 <SEP> day <SEP> 2 <SEP> days <SEP> 3 <SEP> days <SEP> 0 <SEP> 3 <SEP> days <tb> <SEP> 31 <SEP> 0.95 <SEP> 0.91 <SEP> o.64 <SEP> 0.38 <SEP> 0.23 <SEP> 0.20 <tb> <SEP> B2 <SEP> 0.96 <SEP> 0.81 <SEP> 0.49 <SEP> 0.27 <SEP> 0.24 <SEP> 0.11 <tb> <SEP> 33 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> 0.25 <SEP> 0.20 <tb> <SEP> 34 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> o.g6 <SEP> 0.27 <SEP> 0.23 <tb> <SEP> B5 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> 0.31 <SEP> 0.27 <tb> <SEP> 36 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> o.g6 <SEP> 0.34 <SEP> 0.28 <tb> <SEP> 37 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> 0.12 <SEP> 0.10 <tb> <SEP> 38 <SEP> 0.95 <SEP> 0.95 <SEP> 0.95 <SEP> o.s5 <SEP> 0.25 <SEP> 0.21 <tb> <SEP> B9 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> 0.96 <SEP> 0.13 <SEP> 0.12 <tb> <SEP> 310 <SEP> o.g6 <SEP> o,g6 <SEP> o.g6 <SEP> o.g6 <SEP> 0.31 <SEP> 0.27 <tb> 2. A black Ni-Pb layer is deposited on metallized steel plates from a solution having the following composition per liter (pH = 4.3): 100 g NiCl2 40 g NH4Cl 20 g NaSCN 4 g Pb (CH3C00)2.3H 0 (saturation concentration), 2 which is operated for 15 seconds at 3 A/dm2 (45 Coulombs/ dm2) and a bath temperature of 30 C. The value of is 0.96 - 0.97 and of # 0.03 (on a copper base) and 0.12 - 0.13 (on a nickel base). The accelerated damp-heat test caused the non-final treated plates to fade after only 1 day; the final-treated plates behaved in a similar way to those of Table 2. 3. A layer of black nickel is deposited on metallized steel plates by means of a bath having the following composition per liter 12 g NiCl2 20 g NH4Cl 10 g NaCl 20 g boric acid (5.5 4 pH 4 6.5) 150 mg Na-lauryl sulphate the bath being operated at a current density of 30 - 50 A/dmê (total number of Coulombs/dmê 200 to 300). The value of is 0.93 and of ± 0.1. In itself, the black Ni-layer has a corrosion-resistance which is better than that of Ni-Zn or Ni-Pb layer. However, also for these layers the durability is considerably increased when they are post-treated with a chromate solution with an alternately periodically reversed, cathodic and anodic current or without any current.

Claims: CLAmS: 1. A substrate provided with a layer containing a black, selectively absorbing, finely comminuted nickel, characterized in that a thin protective passivating layer predominantly consisting of CrOx, wherein x = 1.5 + 0.1, is present on the black layer. 2. A corrosion-resistant substrate according to Claim 1 for use as a flat plate collector in which the substrate comprises a steel plate with said finely comminuted metallic nickel layer and on top thereof said chromic oxide layer, the whole assembly being incorporated in a glass envelope. 3. A corrosion-resistant substrate according to Claim 1 for use as a vacuum plate collector in which the substrate comprises a steel plate and having a copper layer with said finely comminuted metallic nickel layer and on top thereof said chromic oxide layer, the whole assembly being incorporated in a glass envelope. 4. A method of producing the product as claimed in Claim 1, wherein finely comminuted Ni, Ni-Zn or Ni-Pb is deposited by means of electro-plating on the substrate, characterized in that the product is subjected to a final-treatment in a solution containing chromate. 5. A method as claimed in Claim 4, characterized in that the fiiial-treatment is performed electrolytically, wherein the workpiece is alternately the cathode and the anode, with a switching frequency of the order of 1-10 Hz. 6. A flat plate collector, comprising a steel substrate, provided with a nickel layer on which there are present ill accordance with Claim 1 a selectively aUsolbirlg, finely comminuted metallic nickel layer and oil top thereof a chromic-oxide layer, the whole assembly L,ein incorporated in a glass envelope. 7. A vacuum plate collector, comprising a steel substrate having a copper layer in which there are in accordance with Claim 1 a layer containing a selectively absorbing, finely comminuted metallic nickel and on top thereof a chromic-oxide layer, the whole assembly being incorporated in a glass envelope.

Inventors: VAN DE LEEST, RENAAT EDMOND, VAN DER WERF, BERNARDUS THEODORUS
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1980
Main classification: C23F9
IPC codes: F24J2, C23C22, B32B15, C25D11, B32B9, C25D5