AU611957B2 - Improved black chromium plating bath useful for solar selective coatings - Google Patents

Description

FORM 10 6 1 1 SgU&HRGUSON COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int. Class Complete Specification Lodged: Accepted: Published: 600D tto 0 oo Priority: 0 0 a «o o 0000 0if 00 6- 0 0 o fo 0 0 0 00 0 06 0 00 0 0 0 00 **0 0 0 6 f 4. 1 0

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I C Related Art: Name and Address of Applicant Address for Service: Council of Scientific Industrial Research Rafi Marg, New Delhi-110001, India Spruson Ferguson, Patent Atterneys, Level 33 St Martins Tower, 31 Market Str.eet, Sydney, New South Wales, 2000, Australia Complete Specification for the invention entitled: Improved Black Chromium .Plating Bath Useful for Solar Selective Coatings The following statement is a full description of this invention, including the best method of performing it known to me/us 1 IMPROVED BLACK CHROMIUM PLATING BATH USEFUL FOR SOLAR SELECTIVE COATINGS 000o 90.0 0 0 0 0 oo S 00 a 6 0 00 0 0 0 0 o0 0 0 O0 0 00 o oo i0 I a 00

ABSTRACT

The composition disclosed is an improved black chromium plating bath useful for solar selective coatings. The bath comprises an aqueous solution of sulphate-free chromic acid containing ions of nitrate, borate and fluosilicate. Being a room temperature black chromium bath, good quality black chromium deposit is obtained in the temperature range of 30° to 50°C which is quite coherent and whose selectivity is better than 8.

g o 0 ~-cp IMPROVED BLACK CHROMIUM PLATING BATH USEFUL FOR SOLAR SELECTIVE COATINGS This invention relates to the formulation of a bath for black chromium plating, which is used to produce solar selective coatings of die-cast steel or zinc, instruments and camera parts and consumer goods like spectacle frames, knobs, watches, pens, parts of radio, taperecorder, TV etc.

Black chromuim is deposited by electrolysing an aqueous solution of sulphate-free chromic acid, containing certain additives known 0 0 as 'catalysts' at low temperature and at very high current *o o density. These additives and their effects are described below.

O 0 Acetate or acetic acid has been used as catalyst. The deposit on from this bath is generally powdery and grey at roomtemperature.

0 00 o.Oo Only at a temperature below 20 0 C, the deposit tends to become black. The black colour can be improved by adding boric acid.

0 0 For getting black chrome deposits from acetate based baths, current density has to be greater than 0.5 Amp/Cm 2 (500 Asf).

e 6 oo S* Deposits from these baths have low stability.

o o Fluorides have also been used as catalysts. They give nonuniform grey coatings. Complex fluorides like cryolite, silicofluoride and fluoborates have been used to improve the uniformity. Addition of nitrates and ferric compounds are made to these baths to improve the throwing power. Baths based on fluorides also suffer from the disadvantage of operation at 1 A temperatures less than 20 0 C and at current densities in the range of 400 to 2000 Asf.

Nitrate has been used as a catalyst for producing black chromium deposit at current densities greater than 500 Asf. Mixtures of nitrate and boric acid have been used as a catalyst. Here again a current density of 1000 Asf (1 Amp/Cm 2 is needed.

Catalyst like sulphamic acid, potassium ferro cyanide and o ammonium borate have also been reported. These baths have to be operated at low-temperatures and at current densities greater than 0.5. A/Cm 2 (500 Asf).

0 0 6 0 e1 i t The main deficiencies in the black chromium baths which have been described above are 0 00 ii. the current density needed for plating is very high (greater than 500 Asf); and 6 00 iii. the deposit tends to be powdery with a lower absorptivity.

Since current density is very high, there will be a lot of heating due to passage of current. The increases the load on refrigeration.

Selective black coatings are used on surfaces of devices meant to 2 than 00 As); an .15 151 collect solar energy for its utilisation. Such black coatings are characterised by very high absorptivity in the UV and visible region and very poor absorption in the near IR and IR region.

Black chromium coating is one of the proven selective coating for the collection of solar energy. This bath is useful for plating black chromium useful for collecting solar energy.

Besides black antireflection finishes are required on certain parts of automobiles, optical components, photographic equipment,

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l 0 instruments and on a variety of consumer goods for aesthetic 4 '4 appeal. Black chromium is ideally suited for these applications because of its good corrosion and abrasion resistance.

4 4 The bath described in this invention is useful for black chromium plating solar collectors, automobile parts made.

Thus there is a need for a black chromium bath which can be 4. operated at room temperature i.e. without refrigeration and at normal current densities.

The object of the present invention is to provide an improved plating bath useful for the solar selective coatings which can be operated at a relatingly high temperature and a reasonably low current density. In other words at room temperature and at normal current densities. i Another object of the invention is to provide a black chromium 3 bath which improves the absorptivity and emissivity as well as the coherence of the deposit.

The main finding of the present invention is that the use of a mixture of catalyst consisting of ions of nitrate borate and fluosilicate in combination with an aqueous solution of sulphate free from chromic acid provides black chromium deposites at room temperature and at normal current density.

Accordingly the present invention provides an improved black chromium plating bath useful for solar selective coatings which *e comprises an aqueous solution of sulphate free chromic acid f containing ions of nitrate borate and fluosilicate.

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According to a feature of the invention the ion of nitrate may be I' selected from alkali metal nitrate and nitric acid.

I t According to another feature of the invention the ion of borate I t is selected from alkali metal borate, boric acid and borax.

According to still another feature of the invention the ion of fluosilicate is selected from fluosilicic acid and fluosicate.

According to yet another embodiment of the invention the proportion of the ingredients of the bath may range as follows a) Ion of nitrate 2-20 gpl I b) Ion of borate 3-40 gpl 4 r, c) Ion of fluosilicate 0.2-5 gpl d) the remaining being chromic acid.

The composition is not a mere admixture. The property of the bath is not the sum total of the properties of the components.

Electrolysis of sulphate-free chromic acid produces only hydrogen at the cathode. Neither bright chromium nor black chromium deposits on the cathode. To obtain black chromium deposits of adequate selectivity ratio of absorptivity to emissivity, the bath containing any one of the three catalysts or combinations of any two of them will have to be operated at low 7S temperatures i.e, below 20 0 C and current densities above 40 A/dm2 (400 A/sq.ft.). Baths, known as room temperature baths have to be operated at current densities greater than 100 A/dm 2 (1000 A/sq.ft.) and generally yield brownish deposit which often tends «to powdery. When all the three substances are mixed and used as a 'mixed catalyst', black chromium deposit with high selectivity o7 i.e. is obtained at room temperature (30-40°C) at low current densities (10-30 A/dm2 or 100-300 A/sq.ft.). The deposit is also i coherent and adherent. These results show that the three when present together act synergistically to bring about the deposition of black chromium coating with high selectivity at room temperature and at low current density.

The bath is not very sensitive to composition changes. Throwing power of the bath is comparable to that of chromium plating bath.

The bath can work at trivalent chromium concentration as low as i 7 iT 1 gpl and can tolerate upto 16 gpl. The bath has got an almost infinite shelf life. The bath can be rejuvenated.

The invention is illustrated by the following Example which, however, is not to be construed to limit the scope of the invention Example Black chromium was plated from the bath, the composition of which is given below *tt: St., 1r I *444 4o 4 *4 4 4r 4s 1.

2.

3.

the Alkali metal nitrate 2-20 gpl Boric acid 3-40 gpl Fluosilicate 0.2-5 gpl remaining being sulfate free chromic acid.

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4 4 44 44 iOlfta 4 4 I The preplating sequence is cathodic cleaning, rinse, acid dip, rine, nickel plate from nickel sulphamate bath, rinse black chromium plate at 80 Asf to 300 Asf. The duration of plating is 40 sees to 5 mins. Similar experiments are performed using nitric acid, alkali metal borate or borax and fluosilicic acid.

The absorptivity and emissivity of the black chromium plated at different temperatures and current densities are given in table 1.

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TABLE

TABLE

Absorptivity chromium deposit and Emissivity of black obtained from the new bath Temp.°C Current density Absorptivity Emissivity A/Cm 2 a E 22 0.29 0.97 0.10 30 0.29 0.97 0.12 0.29 0.97 0.14 40 0.29 0.96 0.20 50 0.29 0.97 0.24 30 0.15 0.95 0.12 0 .Q 0 0 0 O O 0 0 00 0 0 o a a a o The solar selectivity of a chromium deposit, the efficiency with which it traps solar energy, is determined by the ratio between the o:oo: absorptivity and the emissivity of the coating as discussed above. The useful range of a values for such coatings is between about 0.9 and 1.0 whereas emissivity values of less than 0.25 are acceptable.

ooi" The preferred emissivity range is 0.1 to 0.15. Coatings are considered to have an excellent selectivity if the ratio of a:E 9. Ratios in the range of 6-8 are considered good, while 4-6 is fair, whereas ratios of less L, than 4 are considered poor in terms of selectivity.

With regard to the present invention, applicants have obtained an improved selectivity for chromium deposits obtained with their baths when the concentration range of the fluosilicate component in the bath is between about 0.5 and 4 gms/l. At concentrations below 0.5 gms/l of this material, the emissivity of the black chromium layer is too high 0.3 to 0.4), while at above 4 gms/l of the fluosilicate, the absorbance ,r'ops to about 0.80. Thus, coatings formed in baths having less than about 0.5 gms/l or more than abnut 4 gms/1 of the fluosilicate component are fit only for decorative use since they are not solar selective.

8 The improved black chromium bath of the present invention has the following advantages: a) It is a room temperature black chromium bath. Good quality black chromium deposit is obtained in the temperature range 30 to b) The operating current density is low. Good quality black chromium deposit can be obtained with a current density in the range 0.08 to 0.3 A/cm 2 (80 to 300 Asf).

c) The deposit is quite coherent and is not powdery.

d) The selectivity i.e. absorptivity and emissivity is better than 8. The absorptivity is better than 0.95.

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Claims (15)

1. A black chromium plating bath which comprises an aqueous solution of sulphate-free chromium containing nitrate, borate and fluosilicate ions in the following proportions: nitrate ions 2 20 g/L borate ions 3 40 g/L fluosilicate ions 0.5 5 g/L remaining volume made up by an aqueous solution of sulphate-free chromic acid.

2. The bath as claimed in claim 1, wherein the nitrate ions are provided by an alkali metal nitrate or nitric acid.

3. The bath as claimed in claim 1 or 2, wherein borate ions are provided by an alkali metal borate, boric acid or borax.

4. The bath as claimed in any one of claims 1 to 3, wherein the platir coatir claime temper per sc coatin forego r' C fluosilicate ions are provided by fluosi

5. The bath as claimed in claim S an absorptivity of at least about 0.9.

6. The bath as claimed in claim is between about 0.9 and

7. The bath as claimed in claim is between about 0.95 and

8. The bath as claimed in claim S an emissivity of less than about 0.25.

9. The bath as claimed in claim between about 0.1 and 0.15. The bath as claimed in claim solar selectivity of at least about 4. C 11. The bath as claimed in claim is between about 8 and cic acid wherein or fluosilicate. the black chromium has Il t 0 a. a a a o a 110 0 a 5, wherein the absorptivity value 5, wherein the absorptivity value 1, wherein the black chromium has 8, wherein the emissivity is 1, wherein the black chromium has a 10, wherein the solar selectivity 0 0 too a a o) d i lola, ari Ir 4 44e

12. A method of black chromium plating, wherein a bath as claimed in any one of the preceding claims is employed.

13. A method as claimed in claim 12, wherein said plating is carried out at a temperature about 20 0 C.

14. A method as claimed in claim 13, wherein such plating is carried out at a temperature of about 30-50*C. A method as claimed in any one of claims 12 to 14, wherein said plating is carried out at a current density of about 0.07 to 0.28 A/cm 2 C1L( _^IYI _lr_~_bX n-?ill~-l lil_ 10

16. A method as claimed in any one of claims 1 to 8, wherein said plating is carried out for about 40 seconds to 5 minutes.

17. A process for the electrolytic deposition of a black chromium coating useful as a solar coating which comprises formulating the bath as claimed in claim 1 and effecting electrolysis of the said bath at a temperature of about 30 to 50 0 C at a current density of about 80 to 300 A per sq.ft. to deposit a black chromium coating upon a substrate.

18. An improved black chromium plating bath useful for selective coatings substantially as hereinbefore described with reference to the foregoing Example. #fi DATED this FOURTH day of APRIL 1991 1 Council of Scientific Industrial Research t* 0 4 ro o Patent Attorneys for the Applicant o 0 I SPRUSON FERGUSON i o 0 0 '31 CO Q o o