Title of Invention

A PROCESS FOR HARD-ANODIZING ALUMINIUM AND ITS ALLOYS

Abstract

This invention relates to a process for hard anodizing aluminium and its alloys containing impurities of iron bearing inter metallics. It is particularly useful in hard anodizing aluminium which contain iron bearing intermetallic impurities. The article to be anodized is first subjected to known steps of degreasing, descaling and desmutting. Anodization is effected in an alectrolytic bath containing 80 - 125 ml/L of sulphuric acid and 1 - 6 ml/L of hydrochloric acid at a temperature ranging from -10 to +5°C at a current density of 15 to 40 A/Ft2. For obtaining an anodic coating of 40 to 100 um anodizing is carried out for 60 to 180 minutes. The process also involves washing, drying and sealing after each stage.

Full Text

This invention relates to a process for hard anodising aluminium and its alloys, and is particularly related to hard anodising aluminium alloys containing impurities of iron bearing inter-metal1ics. Hard anodized metals particularly aluminium and its alloys find use in many industrial applications. Spacecraft deployment mechanism components, aircraft under carriage legs, naval magazine ammunition guides, hydraulic gears, etc. are made with hard anodized aluminium and its alloys. They are also excellent bases for dry lubricants and are used to prevent cold welding in space conditions. Components made with hard anodized aluminium and its alloys exhibit reduced friction on sliding surfaces. The anodic film (alumina) has only about one-tenth of the thermal conductivity of aluminium and therefore can withstand temperatures of upto 2050°C. Components of aluminium which require high resistance to heat are therefore applied with hard anodic coating. Further, the infrared emissivity of hard anodic film above 75 um thickness is about 0.90. Hence hard anodizing of surfaces improves its heat radiation characteristics. It is found that iron bearing impurities in aluminium and aluminium based alloys inhibit the growth of anodic film thereon during conventional anodizing processes. Presence of iron bearing impurities causes steep rise in bath voltage without attaining the required film thickness of 50 to 80 um. it is also noticed that the edges of the components burn, if hard anodization is continued to attain the required thickness, thus damaging the article. An object of this invention is to develop a process for hard anodization where the deleterious effect of iron bearing intermetal1ics is countermanded. Yet another object is for the production of continuous coating or anodization of articles till the desired thickness is achieved. Impurities such as iron bearing intermetallics are removed by dissolution in the electrolyte hath during anodization. The process consists generally of the following steps sequentially: 1. Solvent degreasing 2. Descaling 3. Neutralizing / desmutting 4. Hard anodizing 5. Sealing (optional) The above process steps are described in detail hereinafter: 1. Solvent degreasing is carried out in a known manner in stainless steel tanks and the solvent and the article may be agitated with ultrasonic wave generating transducer. Any suitable degreasing agent or solvent such as trichloroethylene or isopropanol may be used. The article may be immersed in this and agitated either mechanically or ultrasonically for about ten minutes. It is then taken out of the bath and dried. 2. Descaling is also carried out by known methods of deoxidizing or alkaline cleaning. Deoxidizing bath consists of an aqueous solution of sulphuric acid and chromium trioxide. The range preferred is 50 to 200 ml of sulphuric acid and 30 to 100 gms. of chromium trioxide per litre of water. The bath temperature may be 40 to 80 C. The article is immersed in the bath for a period of 2 - 5 minutes to effect descaling. Alkaline mediuffl for descaling which may be used consists of an aqueous solution containing 40 - 120 gms. of sodium hydroxide, 10 - 30 gms. of sodium fluoride, 1-3 gms of sodium polysulphate and 1-2 gms. of any known wetting agent per litre of water. The bath temperature may range from 40 to 80 and the time for descaling is about 2-5 mts. The article may then be removed from the bath and rinsed with water. 3. Neutralizing / Desmutting. The descaled article is then neutralized by immersing it in nitric acid bath which contains 250 - 750 ml of 70% nitric acid per litre of water. Neutralizing or desmutting takes place at 10 - SS"^C and the time taken is approximately 1-3 minutes. The article is then taken out and rinsed with water thoroughly. 4. Hard Anodizing of the article thus processed is carried out in a bath consisting of an aqueous solution of sulphuric acid, hydrochloric acid, aluminium sulphate and glycol or glycerol. The above • compounds may be present in the bath in the range specified below; Sulphuric acid (SG 1.84) ; 80 - 125 ml/L Hydrochloric acid (35-38%) ; 1-6 mi/L Aluminium sulphate : 0-20 g/L Methylene or Ethylene glycol or glycerol : 0-30 ml/L The presence of aluminium sulphate and glycol or glycerol is optional. Aluminium sulphate may be added for artificial ageing of electrolyte and glycol or glycerol for improving the glossiness of hard-anodic coating. The bath temperature may vary from -10 to +5°C. Anodization is carried out under refrigeration 2 conditions and the current density may vary from 15 - 40 A/Ft at a voltage of upto 100 volts. The article is agitated in the bath preferably with compressed air to maintain uniform electrolyte temperature. A coating thickness of 40 to 100 um is achieved during 60 to 180 minutes. The article is thereafter rinsed with water. Sealing of the anodic pores is carried out optionally. It is found that after the sealing step, the corrosion resistance of the film is increased marginally while microhardness is reduced. Hence this sealing step may be carried out only if corrosion resistance is preferred over microhardness of the film. Sealing bath consists of aolution of 0 - 1 gm/litre of ammonium acetate in the pH range of 6,5 to 7.0. The article is placed in a tank containing boiling sealing solution for a period of 1 to 2 hours which is subsequently removed, rinsed with water, and dried. It is observed that an anodic film of upto 120 um can be obtained by this process even in the case of aluminiuai alloy components containing iron based intermetalHc impurities which are difficult to hard anodize by conventional methods. The cathod used in this anodizing step may be lead or carbon. Microhardness value of this coating is above 300 VHN and its infrared emmittance is in the order of 0.90. The coatings were found to withstand normal tests for ground and space applicat ions. The process for hard anodizing aluminium and its alloys containing impurities of iron bearing intermetal1ics according to this invention comprises the steps of degreasing, descaling and desmutting the same in a known manner followed by anodization in an electrolytic bath containing an aqueous solution of 80 125 ml/L of sulphuric acid and 1-6 ml/L of hydrochloric acid at a bath temperature of -10 to 5°C. The article may be washed and dried after the completion of each individual step. It wi11 occur that various modificat ions to the above disclosed embodiments may be made by persons of ordinary skill in this art and that such obvioiis modifications and alterations are intended to be included in the appended claims. We Claim: 1. A process for hard anodizing aluminium and its alloys containing impurities of iron bearing intermetallies comprising the steps of degreasing, descaling and desmutting the same in a known manner, followed by anodization in an electrolytic bath containing an aqueous solution of 80 - 125 ml/L of sulphuric acid and 1 to 6 ml/L of hydrochloric acid at a bath temperature of -10 to +5°C. 2. The process as claimed in claim 1, wherein the electrolytic bath optionally contains upto 20 gms/L of aluminium sulphate and upto 30 ml/L of ethylene glycol or glycerol. 3. The process as claimed in claims 1 and 2 wherein the 2 anodization is carried out at a current density of 15 - 40 A/Ft at a voltage of upto 100 volts. 4. The process as claimed in any of the preceding claims wherein the anodization is carried out for a period of 60 to 180 minutes till a coating thickness of 40 to 100 urn is reached. 1 5. The process as claimed in any of the preceding claims wherein the anodization is carried out under agitation and under bath uniform electrolytic/temperature. 6. The process as claimed in claim 5 wherein the electrolyte is agitated by means of compressed air flow. 7. The process as claimed in any of the preceding claims wherein aluminium and its alloys are degreased by agitating in a bath of trichloroethylene or isopropanol followed by drying. 8. The process as claimed in claims 1 to 7 wherein descaling is effected in a deoxidizing bath containing an aqueous solution of sulphuric acid and chromium trioxide. 9. The process as claimed in claim 8, wherein the aqueous solution has 50 to 200 ml of sulphuric acid and 30 to 100 gm of chromium trioxide per litre of water and the deoxidizing step is carried out at a temperature of 80 to 100°C. 10. The process as claimed in claims 1 to 7 wherein the descaling step is carried out in an alkaline bath consisting of sodium hydroxide, sotjium fluoride and sodium polyphosphate in the presence of a wetting agent. 11. The process as claimed in claim 10 wherein the alkaline bath contains 40 to 120 gms./L of sodium hydroxide, 10 to 30 gms./L of sodium fluoride and 1 to 3 gms./L of sodium polyphosphate and 1 to 2 gms./L of the wetting agent in water. elecxrolyte 12. The process as claimed in claims 10 and 11 wherein the /bath temperature is 40 to 80°C. 13. The process as claimed in any of the claims 1 to 12 wherein desmutting is carried out in an aqueous bath containing 250 to 750 ml of nitric acid/litre of water at a temperature range of 10 to 35°C. 14. The process as claimed in any of the preceding claims wherein the pores of the anodized film are sealed by the immersing the same - in demineralised water or water containing Ig./L of ammonium acetate at a pH of 6.5 to 7 / for f 1 to 2 hour followed by rinsing and drying. 15. A process for hard anodizing aluminium and its alloys containing iron bearing intermetallies, substantially as herein described.

Documents:

515-mas-1997 abstract-duplicate.pdf

515-mas-1997 abstract.pdf

515-mas-1997 claims-duplicate.pdf

515-mas-1997 claims.pdf

515-mas-1997 correspondence-others.pdf

515-mas-1997 correspondence-po.pdf

515-mas-1997 description (complete)-duplicate.pdf

515-mas-1997 description (complete).pdf

515-mas-1997 form-1.pdf

515-mas-1997 form-26.pdf