|Title of Invention||
"A PROCESS FOR ELECTROPLATING OF SOLAR SELECTIVE BLACK CHROMIUM ALLOY COATINGS ON SUBSTRATES WITH IMPROVED THARMAL STABILITY"
|Abstract||Solar selective black chromium coating has been widely used for domestic and industrial water heating applications. Black chromium alloy coatings have been successfully plated at room temperature (~ 30°C) from black chromium plating bath (USA Pat. 5019223) by adding alloying elements like Mo, W, Mn and Ag. The long term thermal stability of these alloy coatings has shown improvement in order of 2 to 3 times for the alloys containing Mo, W and Mo + W, Ag. Such coatings find applications in the solar steam generation.|
|Full Text||The present invention relates to a process for manufacturing solar selective black chromium alloy coating with improved thermal stability.
The present invention particularly relates to a process relating to the thermal stability of solar selective black chromium coating by alloy formation.
The present invention more particularly relates to a process for manufacturing solar selective black chromium alloy coating with improved thermal stability, wherein use is made of compounds of elements such as Molybdenum, Tungsten, Manganese and Silver.
In recent years, greater attention is shown in harnessing alternative sources of energy like solar energy in domestic and industrial application to save fossil fuels like petroleum, diesel and coal. Use of flat plate and concentrating type solar collectors for high temperature applications like solar steam generation requires large area of absorbers to run turbines or for industrial applications. Main usage of the present invention is in the field of harnessing solar energy for water heating and steam production.
In the prior art, conventionally black chromium is deposited by electrolyzing an aqueous solution of sulphate-free chromic acid containing certain additives known as 'catalysts' at low temperature and at very high current density.
In our U.S.A patent (5019223), Canada patent (2006130), Australian patent (611957), European patent (033683) and Indian patent (167895), we have described and claimed a room temperature black chromium-plating bath and process for depositing solar selective coatings.
The black chrome coatings deposited from chromic acid bath containing inorganic additives has shown better thermal and UV stability. However, the thermal stability of such coatings is limited to 200°C for long term applications. In solar steam generation where the solar selective coating applied on the absorber tube kept in the focus of the concentrating collectors has to have higher thermal stability. The optical properties, absorptance (a) and emittance (e) change with time at higher temperature. It should be minimal at and above 300°C. These values
decrease steadily with time at higher temperatures resulting in decreased solar selectivity (α/e ratio). It is important to have higher absorptance above 0.90 at higher temperatures.
Black chromium is a graded composite of metallic chromium particles in a dielectric matrix of chromium compounds. The substrate / coating interface is rich in metallic component and the surface is rich in dielectric compounds. Morphology and chemical composition together determine the optical properties of the black chrome coating. So, for the stability of the coating, both these characteristics should not change. Emittance is observed to drop upon initial heating. The evaporation of water vapour from the coatings observed at the early stages of heating has the beneficial effects of reducing emittance. However, degradation does occur by oxidation of metallic particles and by creation of pores in the film. Therefore, retarding the oxidation of chromium particles present in the coating can enhance thermal stability of the black chrome coating and also a modification of the non-metallic part of the film may be beneficial. This could be achieved by making chromium to form a solid solution with another metal of similar nature. The other obvious possibility is to alloy the metal with another element, which has good thermal resistance. Metals, which are suitable for fine particle deposition, must be stable at high temperature. Refractory metals such as Mo, W, Mn could be worthwhile candidates. Chromium is the only refractory metal that can be deposited from aqueous solution. Other refractory metals like Mo, W can only be codeposited.
Reference may be made to Smith and Ignatiev (1981), Solar Energy Materials 4, 119-133, an attempt has been made to codeposit Mo from the proprietary 'Chromonyx' black chrome plating bath. They have suggested additives of 20-75 gpl of Na2MoO4 and plated at 3000 -4000 A/m2 for 4 1/2 to 5 mins. duration in comparison with standard black chrome 'Chromonyx' bath requiring at least 1200 A/m2. The drawback of above said invention lies in high current density of the order of 3000-4000A/m2 for a period of around 5mins. This increase of current density attributed to polarizing effect of molybednum. Thus, use of high current density coupled with long duration results in high cost of production.
Basically, above referred proprietary 'Chromonyx' bath suffers from low temperature operation (18°C) and also contains organic acid additives resulting in a coating with low UV stability.
Recently Lee KD, Jung WC and Kim JH, 2000, Solar Energy Materials & Solar Cells, 63, 125-137, have reported that they have deposited black chrome coating from a bath containing
CrO3: 350-380 gpl, H2SiF6 : 8-12 gpl and Na2MoO4 : 10-15 gpl at 4000 A/m2 for 20 sec . They did not claim any Mo codeposition and any increase in thermal stability of the coating. Further the current density used is high, that of order of 4000A/m2.
Reference may be made to U.S.A. Patents 4174265 where in a conductive body as a cathode is electroplated with chromium in the presence of radioactive rays in a plating bath comprising an aqueous solution of chromium trioxide. In this patent use has been made of radioactive rays such as x-rays which are hazardous. Further the disclosure made was when x-rays were not applied only yellowish chromium coating was obtained, thus defeating the very requirement of black coated service.
Reference may be made to US patent No. 4196063 where in they have described electrodeposition of black chromium using electrolyte solution comprising water, trivalent chromium ions , ions of metal selected from the groups comprising of iron and cobalt , weak complexing agent for the trivalent chromium ions and phosphate ions. The complexing agents are hypophosphite or formate. In the disclosure, there is no mention about solar absorptivity and emissivity and long term thermal stability of the coatings.
Reference may be made to U.S. A. Patent 4894125, which covers optically black pliable foils. This patent discusses black chromium plated pliable foils useful for space environment and close to a temperature of about 450°C. This patent does not discuss about solar steam generation and long term thermal stability. This patent claims that black chromium is plated on a composite ductile metal foil for entirely different application. Solar absorptivity of at least about 0.95 and emissivity of about 0.4 to 0.8 has been obtained.
From the hitherto known prior art as given above, it is seen that there is definite need for providing a process, which can provide solar selective black chromium coatings with improved thermal stability
The main object of the present invention is to provide a process for manufacturing solar selective black chromium alloy coating with improved thermal stability to obviate the drawbacks of prior art processes.
Another objective of the present invention is to provide a coating with improved thermal stability.
Yet another objective of the present invention is to provide a process for the black chrome alloy coating plated from black chromium bath by adding compounds of different metals (Mo, W, Mn and Ag).
Still another object of present invention is to achieve codeposition of Mo, W, Mn and Ag with black chromium, which has higher solar selectivity at temperature at least 300°C.
Still yet another object of this invention is to develop solar selective coatings for steam generation by electroplating route.
Further object of this invention is to standardize the conditions to electrodeposit black chrome alloy coatings suitable for concentrating type solar collectors useful for steam generation.
One more objective of this invention is the development of black chromium alloy coatings suitable for automobile exhausts and silencers.
The process of present invention provides black chromium alloy coatings plated at room temp in a black chromium plating bath provided with alloying compounds of element such as Mo , W, Mn, and Ag. This has resulted in the improvement of the long -term thermal stability of these alloy coatings by a factor of 2 to 3.
Accordingly the present invention provides a process for manufacturing solar selective black chromium alloy coatings with improved thermal stability which comprises, subjecting a substrate to be coated as a cathode in a black chromium electroplating bath characterized in that the said bath being provided with compounds of Mo, W, Mn and Ag, singly or in combination thereof.
In an embodiment of the present invention, the black chromium plating bath is such as an aqueous solution of sulphate free chromium acid containing 2-20 gpl of alkali metal nitrate, 3 -40 gpl of boric acid and 0.2 -5 gpl of fluosilicate ions or 0.2 - 5 gpl of floride ions.
In another embodiment of the present invention, the substrate such as copper, nickel, steel and stainless steel has been coated with black chromium alloy coatings.
Yet another embodiment of the present invention, the compounds of molybdenum is molybdates in the range of 10 -150 gm/l.
Still another embodiment of the present invention, the compounds of tungsten is dodeca tungstophosphoric acid in the range of 10 - 50 gpl.
In still yet another embodiment of the present invention, the compounds of manganese is manganate in the range of 10 - 30 gpl.
In a further embodiment of the present invention, the compound of silver is silver nitrate, less than 10 gpl.
In a still further embodiment of the present invention, the co-deposition of Mo, W, Mn and Ag with black chromium is at room temperature.
In an another embodiment of the present invention, the electroplating is effected at a current density in the range 2000 - 3000 A/m2 for duration in the range of 1 - 5min.
In still another embodiment of the present invention, the black chromium alloy coating has solar selectivity of the order of 2 to 5 or higher at a temperature of at least 300°C.
In the present invention, there is provided a process relating to the improvement of thermal stability of solar selective black chromium coating by alloy formation wherein compounds of Mo, W, Mn and Ag have been added to black chromium plating bath individually or collectively, subsequently the electroplating process being conducted, and the cathode gets the coatings of improved black chromium alloy with improved thermal stability and better service.
This invention relates to a solar water heater for domestic, industrial hot water and steam generation applications and more particularly to electrodeposition of solar selective black chromium alloy coatings and a method to manufacture the same.
Black chrome coatings deposited from the bath (USA Pat. 5,019,223) have many advantages like high solar selectivity at 30-40°C, 1000-3000 A/m2 to get coherent and adherent coatings. This bath also has other advantages like low sensitivity to composition changes and the bath has comparable throwing power to that of a conventional chromium-plating bath. The bath can work with a trivalent concentration as low as 1gpl and can tolerate upto 16 gpl. This bath has an almost infinite life and can be rejuvenated.
Black chrome coatings can be deposited with high absorptance (a > 0.95) and low emittance (s The novelty of the present invention resides in providing improved thermal stability with solar selectivity of the order of 2 to 5 or higher at a temperature of at least 300°C besides the process of present invention is economical. This novelty has been achieved by the non obvious inventive steps of adding compounds of alloying elements such as Mo, W, Mn, and Ag, singly or combination thereof.
The process of the present invention is illustrated by the following examples however these examples should not be construed to limit the scope of present inventions.
Black chromium alloy deposits were plated from the bath the composition is given below
1. Alkalimetal nitrate : 5 gpl
2. Boric Acid : 10 gpl
3. Fluosilicate or fluroide : 0.5 gpl
4. Compounds of alloying elements :
Molybdate : 10-150 gpl
Dodeca Tungstophosphoric acid : 10-50 gpl
Manganate : 10-30 gpl
Silver nitrate : 1-5 gpl
The remaining being an aqueous solution of sulphate-free chromium acid.
The processing sequence is as follows:
Cathodic cleaning, rinse, acid dip, nickelplate from a nickel sulphamate bath, rinse black chromium plate with the above solution at 1000 A/m2 . The duration of the plating was in between 40 sec to 300 sec. The temperature of the bath is between 20°C-40°C. The compounds of alloying element can be added singly or in combination. When copper was used without nickel undercoat, the substrate was dipped in 100 gpl of chromic acid with 10 ml/l of sulphuric acid in the processing sequence before black chromium plating.
Similar experiments were performed using nitric acid, alkalimetal borate or borax, fluosilicic acid or fluoric acid.
The absorptance (a ) and emittance (E) values of the deposits obtained from these baths at 30°C, 3000 A/m2 for a duration of 50 sec are given in Table 1A and 1B.
To one litre of a 400 gms/l aqueous solution of chromic acid, 10 gms of barium carbonate were added. The mixture was the filtered to obtain a sulphate free chromic acid solution. To this solution 5 gms of sodium nitrate, 10 gm of boric acid, 0.15 gm of sodium fluoride were added. To this bath 50 gpl of sodium molybdate was added. Electroplating was carried out at 3000 A/m2 for 60 sec at a bath temperature of 30°C or for 120 sees at a bath temperature of 20 °C. The obtained coating was analysed chemically for molybdenum and it was found out to be around 0.8-1.2 weight % in the coating. This experiment shows that molybdenum has been codeposited along with chromium
To the black chromium prepared as in example2, the electroplating was carried on nickel-plated copper substrate a bath temperature of 20°C after adding varying quantities of Sodium Molybdate, Dodeca Tungsto phosphoric acid, Potassium Permanganate, plating duration was 75-90 secs, the absorptivity P a) of these coatings were tested and given in table 2
For evaluating the thermal stability of the solar selective alloy coatings, the samples were kept inside borosil glass petri dishes and placed in an air-circulated oven maintained at the required temperature with an accuracy of ± 1% of the set temperature.
The optical properties (absorptance and emittance) of the samples were measured periodically with standard instruments procured from M/s Devices & Services, USA.
Na2MoO4 was added in the range of 0-125 gpl and H3PO4.12WO3.xH2O was added in the concentration range of 0 to 50 gpl. The additive effect of addition of Mo and W was done from the black chromium bath containing 50 gpl of H3PO4.12WO3.xH2O and with varying addition of Mo from 10 to 100 gpl. KMnO4 was added between 0 to 30 gpl. Addition of silver nitrate was in the concentration range of 0 to 5 gpl as Ag+ ion.
Typically thermal stability test was conducted at 350°C and the time taken at this temperature for 5% degradation in the absorptance values are given in Table 3. The black chromium alloy plating was done on copper substrate with 12(im nickel undercoat.
The thermal stability of the black chromium alloy coatings containing Mo was carried out at 400°C on electroformed nickel substrate and the effect is given below in table 4.
Similarly the effect of Ag+ ion addition to the black chromium on electroformed nickel substrate was studied at 400°C and the result is given in table 5.
As silver does not belong to refractory metals, the cause of this improvement could be due to the removal of chloride ion or control of grain size or codeposition of silver in the coating.
By simple addition of compounds of Mo, W, Mn and Ag the thermal stability of the black chromium coatings could be improved substantially.
From the examples it can be concluded that the thermal stability has improved by the addition of compounds of Mo, Mn, and Ag individually or collectively significant improvements in thermal stability were observed by the addition of Mo.
The main advantages of the present invention:
1. Substantial improvement with respect to thermal stability. By simply addition of compounds
of Mo, W, Mn and Ag the thermal stability of the black chromium coatings could be
2. Increased service life of the solar collectors of the order of 2 - 3 times.
3. Reliability of the solar selective coatings has been improved.
4. Useful for high temperature applications like solar steam generations.
5. Useful for coatings automobile components like exhaust pipe and silencers.
1. A process for manufacture of solar selective black chromium alloy coatings with improved thermal stability which comprises, subjecting a substrate to be coated as a cathode in a black chromium electroplating bath characterized in that the said bath being provided with compounds of Mo, W, Mn and Ag, singly or in combination thereof.
2. A process as claimed in claim 1, wherein black chromium plating bath is such as an aqueous solution of sulphate free chromium acid containing 2-20 gpl of alkalimetal nitrate, 3-40 gpl of boric acid and 0.2 -5 gpl of fluosilicate ions or 0.5-5 gpl of floride ions.
3. A process as claimed in claim 1-2, wherein the substrate such as copper, nickel, nickel-plated copper or steel, steel and stainless steel has been coated with black chromium alloy coatings.
4. A process as claimed in claim 1-3,wherein the compound of molybdenum is molydates in the range of 10 -150 gpl.
5. A process as claimed in claim 1-4, wherein the compounds of tungsten is Dodeca Tungstophospharic acid in the range of 10 - 50 gpl
6. A process as claimed in claim 1-5,wherein the compounds of manganese are manganate in the range of 10 - 30 gpl.
7. A process as claimed in claim 1-6, wherein the compound of silver is Silver Nitrate, les$ than 10 gpl.
8. A process as claimed in claim 1-7, wherein the co-deposition of Mo, W, Mn and Ag with black chromium is at room temperature.
9. A process as claimed in claim 1-8, wherein the electroplating is effected at a current density in the range 2000 - 3000 A/m2 for a duration in the range of 1 - 5min.
10.A process as claimed in claim 1-9, wherein the black chromium alloy coating has solar selectivity of the order of 2 to 5 or higher at a temperature of at least 300°C .
11. A process for the manufacture of solar selective black chromium alloy coatings with improved thermal stability as herein described with reference to the examples.
|Indian Patent Application Number||743/DEL/2003|
|PG Journal Number||31/2009|
|Date of Filing||29-May-2003|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG NEW DELHI0-110001 INDIA.|
|PCT International Classification Number||C25D 00304|
|PCT International Application Number||N/A|
|PCT International Filing date|