|Title of Invention||
A PROCESS FOR ANODIZING AND INORGANIC BLACK COLOURING OF ALUMINIUM AND ITS ALLOY
|Abstract||This invention relates to a process for anodizing and black colour coating aluminium and its alloys. "rhe steps consist of degreasing. descaling. desmutting and then anodizing in a sulphuric acid bath. After anodization the article is immersed in a bath containing an aqueous bath of cobalt acetate and then irl an aqueous bath of yellow ammonium sulphide. These steps are repeated till the desired colour intensity is achieved. The article is then heated in a bailing bath of dimineralised water to seal the colour.|
This invention relates to a process for anodizing and inorganic black colorings on aluminium and its alloys.
Black coated aluminium articles exhibit uniform temperature distribution due to improved radiative coupling. They exhibit high infrared emittance. Black anodized aluminium and its alloys find use in thermal control applications and optical devices where reflections are to be avoided. They also have applications in various engineering industry including space technology.
Conventionally, high emittance black coating on aluminium is obtained by anodizing aluminium in sulphuric acid bath and dyeing or painting the anodized surface. Though colouring with organic dye formulation is economical, the coated articles do not possess the desired colour fastness and as such are found unsuitable for many industrial applications. Painting the anodized surface with thermal control black carbon paints is not only cost prohibitive but also requires meticulous handling of the different process steps. It is also noticed that thermal control paints show a very high percentage of weight loss of the coating in the vacuum atmosphere and as such have restricted applications.
The object of this invention is to develop a simple process for anodizing and black colour coating of aluminium and its alloys which is cost effective. The coating obtained by this method is extremely light fast and provides an infrared emittance as high as 0.90. The coated articles withstand stringent qualification tests for space application.
The process according to this invention involves the following sequence of operation:
5. Inorganic colouring
These steps will now be described in detail:
Degreasing is carried out in tanks preferably in stainless steel tanks. It is preferred to degrease articles in a solvent bath. The bath consists of trichloroethylene or isopropanol. The article is immersed in this bath for a period of 2 - 10 mts and may be agitated either mechanically or ultrasonically.
Descaling Bath consists of an aqueous solution of sodium hydroxide, sodium fluoride, sodium polyphosphate, and wetting agent. Range of these compounds in the bath is given below.
40 - 120 g/L 10 - 30 g/L 1 - 3 g/L 1 - 2 g/L
The article is placed in the above bath and agitated mechanically. The bath temperature may be 40 to 80°c and the time for immersion may range from 2-5 mts. The article is taken out from the bath and rinsed with water.
Desmutting bath for aluminium alloys containing copper consists of dilute nitric acid (70%) in the strength of 250 to 750 ml/L of water. For other alloys the bath used consists of an aqueous solution of the following:
Sulphuric acid (SG 1.84)
10 - 15 ml/L
20 - 30 ml/L
The article from the previous step is immersed in the above bath for 1 to 3 minutes and the temperature of the bath is maintained at 10 to 45°C. After the specified period the article is withdrawn and rinsed with water.
Anodizing is the next step in the sequence and is carried out in a known manner in an aqueous bath containing sulphuric acid. The concentration of sulphuric acid in the bath may be 75 to 125 ml/L of sulphuric acid of 1.84 SG. The bath temperature is maintained between 10 to 45 C and the current density between 12 to 25 A/Ft2. The voltage of the bath ranges from 13 - 25 y and the time for electrolysis from 20 to 60 mts. The bath may be agitated with the help of compressed air. After anodizing the article is rinsed with water.
Inorganic colouring of the anodized aluminium is the next step in the process., The first step for obtaining black colour is treatment of the article in a bath containing an aqueous solution of cobalt acetate. The strength of the solution ranges from 150 - 300 g/L of water. The temperature of the bath is maintained at 10 to 45 C and the treatment is carried out for 5 to 15 mts.
The second step is by immersing the above treated article in an aqueous bath containing ye 1 low ammonium sulphide. The strength of the solution is 20 - 40 gm/L. The bath temperature is in range of 10 to 45°C. The article from the first stage is immersed in this bath with agitation for a period of 5 to 10 mts. This cycle of immersing in cobalt acetate bath followed by yellow ammonium sulphide bath is repeated till the desired colour intensity is achieved. The article is then rinsed with water.
Sealing is the next step which is carried out in a tank with heating facility. The bath consists of dematerialized water at a pH range of 6.8 to 7.0. The temperature of the bath is maintained at its boiling point or above 98 C for a period of 15 to 30 mts after which the article is dried in air.
The process for anodizing and inorganic black colouring of aluminium and it’s alloys according to this invention comprises the steps of degreasing, rescaling and desmutting the said aluminium and its alloys by known methods, subsequently anodizing the same in aqueous sulphuric acid bath and black colour coating the said anodized aluminium by immersing the same first in a bath of aqueous cobalt acetate and then in an aqueous bath of yellow ammonium sulphide.
It is found that by the above described process, black anodic film of infrared emittance of 0.90 and microhardness of 100 VHN may be obtained on most of the aluminium alloys. The coating thus obtained exhibits excellent colour stability and fastness. The coated articles are found to withstand humidity test of RH 95% at 50°C for 100 hrs without degradation. Thermal cycling and thermo vacuum tests in the temperature range of -196 to +120°C and 10 torr at -80 to +80°C of vacuum respectively are also tolerated by the articles. These properties make the black coated anodized article ideally suited for terrestrials and space applications.
It will occur that various modifications to the above disclosed embodiments may be made by persons of reasonable skill in this art. Such modifications or alterations are also intended to be within the scope of the appended claims.
1, A process for anodizing and inorganic black colouring of aluminium and its alloys comprising degreasing, descaling and desmutting the said aluminium and its alloys by known methods, subsequently anodizing the same in aqueous sulphuric acid bath and black colour coating the said anodized aluminium by immersing the same first in a bath of aqueous cobalt acetate and then in an aqueous bath of yellow ammonium sulphide.
1. The process as claimed in claim 1 wherein degreasing is
carried out by immersing in a bath containing trichloroethylene or isopropanol for a period of 2 to 20 mts under agitation.
3. The process as claimed in claim 2 wherein the degreasing
bath is agitated with ultrasonic wave generating transducer.
4, The process as claimed in any of the preceding claims
wherein descaling is effected by treating aluminium and its
alloys from the degreasing step in an aqueous alkaline bath
containing sodium hydroxide, sodium fluoride, sodium
polyphosphate and a known wetting agent.
5. The process as claimed in claim 4 wherein the aqueous
alkaline bath consists of 40 to 120 g/L of sodium hydroxide, 10
to 30 g/L of sodium fluoride, 1-3 g/L of sodium polyphosphate,
and 1-2 g/L of a known wetting agent, the bath temperature is
from 40 to 80°C and the duration of treatment is from 2-5 mts.
6. The process as claimed in claims 1 to 5, wherein desmutting of the said descaled aluminium alloys containing copper as an alloying element is carried out in aqueous nitric acid.
7. The process as claimed in claim 6 wherein the aqueous nitric acid has a strength of 250 - 750 ml/L of nitric acid of 70% strength.
8. The process as claimed in claims 1 to 5 wherein desmutting
is effected for aluminium and its alloys with the exception of
aluminium-copper alloy, by immersing the same in an aqueous bath
containig sulphuric acid, hydrofluoric acid, and nitric acid at a
temperature of 10 to 45°C for a period of 1 to 3 mts.
9. The process as claimed in claim 8 wherein the said aqueous bath consists 5-15 ml/L of sulphuric acid of SG 1.84, 10 - 15 ml/L of hydrofluoric acid of 40% strength, and 20 - 30 ml/L of nitric acid.
10. The process as claimed in claims 1 to 9 wherein the degreased, descaled and desmutted aluminium and its alloys are anodized in an aqueous bath of 75 to 125 ml/L of sulphuric acid of SG 1.84 at a bath temperature of 10 to 45°C, at a current density of 12 to 25A/Ft2, a bath voltage of 13 to 25V for a period of 20 - 60 mts.
11. The process as claimed in claim 10 wherein the anodizing bath is agitated by means of compressed air, and the process is carried out till a thickness of 10 - 30 urn is obtained.
12. The process as claimed in claims 1 to 11 wherein the colour
imparting steps are repeated till the desired colour intensity is
13. The process as claimed in claims 1 to 2 wherein after the step of colour imparting the article is sealed by boiling it in demineralised water for a period of 15 to 30 mts.
14. A process for anodizing and inorganic black colouring of aluminium and its alloys substantially as herein described.
|Indian Patent Application Number||516/MAS/1997|
|PG Journal Number||20/2006|
|Date of Filing||12-Mar-1997|
|Name of Patentee||M/S. INDAIN SPACE RESEARCH ORGANIZATION|
|Applicant Address||DEPARTMENT OF SCIENCE, ANTARIKSH BHAVAN, NEW BEL ROAD, BANGOLRE 560 094|
|PCT International Classification Number||C25D 005/00|
|PCT International Application Number||N/A|
|PCT International Filing date|