METHOD OF SURFACE TREATMENT OF ALUMINUM OR ITS ALLOY

Abstract: A method of treating the surface of aluminum or its alloy to give a desired color thereto and, in addition, improve the abrasion and corrosion resistances thereof. An anodic coating formed by the Almite process was disadvantageous in that it was porous, had low abrasion and corrosion resistance and was unsatisfactory in color fastness. The method of the invention is characterized by forming an anodic coating on the surface of aluminum or its alloy by an ordinary process, dipping the product of anodization in a solution of a sulfate or nitrate of a desired metal, and applying an AC voltage of 10 to 30 V thereto to thereby infiltrate the metal into the anodic coating by electrolysis. As a result, the metal is embedded in the pores of the porous anodic coating to thereby improve the abrasion and corrosion resistances, and the embedded metal serves to attain desired coloration.

Description: The present invention relates to an improvement of a process for surface treatment of aluminium or aluminium alloy. It is known as alumite treatment to anodize aluminium or its alloy within an electrolytic solution such as an aqueous solution of nitric acid, sulphuric acid, or chromic acid to form a corrosion resistance oxide film. Such alumite treatment is widely utilized in various fields, for example aircraft, automobiles, marine vessels, optical instruments, instruments for chemical industry, and even daily needs such as pans and teakettles. However, an upper surface of the alumite film is generally porous. Therefore, in order to improve the corrosion resistance of the porous layer, it is required to perform a sealing treatment, e.g. to dip the product within boiling water. Further, an alumite film is generally of a silver white colour. Therefore, when a coloured product such as a building material or daily needs utensil is desired, it is necessary to colour the products with a dye or a pigment which must be impregnated into the porous layer of the alumite film. Further, a process for forming a natural colour anodic oxiation coating by an electrolysis using an electrolyte containing sulphuric acid and sulphosalicylic acid added thereto is also adopted. However, any of the above described processes can colour only a shallow area of the upper layer of the alumite film and thus the coloured area is likely to be subject to wear and discolouration. Thus the alumite film has not necessarily sufficient durability because a deep portion under the shallow area remains porous. It is an object of the present invention to eliminate the above-described disadvantages and to provide a process for the surface treatment of aluminium or aluminium alloy, which is able to colour various articles and does not use a toxic material such as cyanogen and can produce articles having an excellent corrosion resistance and abrasion resistance. According to the invention there is provided a process for the surface treatment of an aluminium or aluminium alloy workpiece, the process comprising: as a first step, passing an electric current through a low temperature electrolyte containing a low grade water soluble acrylate resin compound capable of being polymerized at an anode with the workpiece being the anode, so forming an anodic oxidation coating combined with the acrylate resin compound on the workpiece, and as a second step, applying an alternating voltage of 10 to 30V to the workpiece on which the anodic oxidation coating has been formed, within an electroylyte containing a sulphate or nitrate of a desired metal, so that the metal is electrolytically impregnated into the anodic oxidation coating. In the second step, the electrolyte preferably contains from 10 to 25 g/1 of metallic salts, 25 to 30 g/1 of boric acid, and 0.3 to 0.5 g/1 of sulphuric or nitric acid. Also, preferably, the treatment temperature is within a range of 5 to 20°C and most preferably 10 to 15°C. As metallic salts, silver is most useful. The low grade acrylate resin compound capable of being polymerized at an anode with the work piece being the anode in the process according to the invention are disclosed in Japanese Patent Applications Sho 61-251914 and Sho 63-249147 both of which were filed by the present applicant. According to the above described process, the metal within the electrolyte may enter or penetrate into the porous oxidation coatings formed on the ground metal of aluminium or its alloy to combine with aluminium oxide to thereby form strong and dense composite coatings. Accordingly, weatherability, corrosion resistance, heat resistance and wear resistance of the oxidation coatings are increased and the oxidation coatings can be variously coloured depending upon a kind of metal within the electrolyte and a depth in the coatings into which the metal penetrates. Thus, the process for surface treatment according to the present invention can be successfully utilized in a wide range of fields in order to treat the surface of for example, bearings, gears, a spindles, valves, pistons, fittings, interior and exterior parts, stationery, accessaries, and parts adapted to be contacted with a magnetic tape in computers and video recorders. In the accompanying drawings: Figure 1 is a schematic view showing an embodiment of a device for carrying out a process for surface treatment of aluminium or its alloy not according to the present invention; and Figure 2 is an enlarged sectional view showing a part of coating formed on aluminium or its alloy not according to the process of the present invention. Referring to the drawings, in Figure 1, an electrolytic bath 1 contains an electrolyte 5 containing a desired metal salt. An aluminium member 3 on which an alumite film is to be formed by a conventional manner is immersed in the bath as one electrode and electrodes 4 made from carbon or graphite act as the other electrodes, the electrodes are subject to an AC power supply 2. On the surface of the aluminium member 3 to be treated is formed an alumite film of about 50 to 100um thickness in a conventional manner. If it is desired that the surface of the aluminium member 3 be coloured in a golden colour by a second treatment, a silver salt is used as the metal salt within the electrolyte. In this case, the electrolyte 5, for example, is composed from: Silver sulphate 10-25 g/1 Boric acid 25-29 g/1 Sulphuric acid 0.3-0.5 g/a Balance water Further, it is also preferred to add the following two components to the above electrolyte: D-tartaric acid 15-25 g/1 nickle sulphate 15-25 g/1 The voltage of the AC power 2 is 10 to 30V, preferably 15 to 25V, and the temperature of the electrolyte is 5 to 20°C, preferably 10 to 15°C. The silver ion which decreases in concentration as the treatment advances can be replenished by adding silver sulphate. If the voltage is not more than 10V, treatment efficiency is low. On the other hand, if the voltage is not less than 30V, deposition of metal is made rapidly so that the metal can not be sufficiently impregnated into the porous layer of alumite, being likely to result in uneven colouring of the porous layer and separation of the metal from the porous layer. Similarly, if the temperature of the electrolyte is less than 5 to 10°C, treatment efficiency is low. On the other hand, if the temperature is more than 15 to 20°C, uneven colouring of the porous layer is likely to occur. Boric acid is added to the electrolyte mainly for regulating a conductivity of the electrolyte. Referring to Figure 2 this shows an enlarged sectional view of a skin portion combined anodic oxiation coating based from the second treatment as will be explained hereunder. As shown in Figure 2, a ground metal portion 21 of the aluminium member 3, has anodic oxiation coatings 22 formed by the alumite treatment. These coatings include a barrier layer 23 and a porous portion 24. Metal 25 is impregnated into the porous portion 24 by the second treatment using electrolyte containing the metal salts. Anodic oxidation coatings 22 formed by the alumite treatment consist generally of the barrier layer 23 and the porous portion 24. When the aluminium member, on which such anodic oxidation coatings are formed, is subjected to the above described second electrolytic treatment, metal molecules such as silver within the electrolyte 5 can be deeply impregnated into the porous coatings 24, resulting in strong and dense composite coatings. As metal salts used in the electrolyte 5, other metal salts than the above described silver salt, for example copper salt, iron salt and even gold salt may be utilized. In any case, it is preferred that the electrolyte contains about 15 g/1 of metal salt and other compositions as above described. If silver salt is utilized, coatings of golden colour are formed, and if copper salt is utilized, coatings of a brown or bronze colour are formed. When silver salt is used, in particular, obtained products have many advantages, for example, a low friction coefficient of the surface, a beautiful golden colour, and high wear resistance, and thus the silver salt is most preferably utilized. The brown colour can be varied by changing a kind of metal salt used, its thickness i.e. the thickness of the initial alumite layer, or the duration of electrolysis. As means for forming the anodic oxidation coatings on the surface of the aluminium member prior to the second electrolytic treatment, not only the usual alumite treatment but also and according to the present invention means for forming the anodic oxiation coatings combined with an acrylate resin compound can be utilized, the latter being disclosed in Japanese Patent Applications Sho 61-251914 and Sho 63-249147 both of which were filed by the present applicant. Since the present invention is constructed as described above, according to the present invention, the metal within the electrolyte can be deeply entered into the porous oxidation coatings formed on the ground metal of aluminium or its alloy, being combined with aluminium oxide to form strong and dense composite coatings, so that weatherability, corrosion resistance, heat resistance, and wear resistance are increased, friction coefficient of the surface is decreased, change of colour with the passage of time is reduced, machine work on the product which was not able to performed up to now because the coatings are separated from the ground metal can become possible, and toxic chemicals such as cyanogen need not to be used.

Claims: A process for the surface treatment of an aluminium or aluminium alloy workpiece, the process comprising: as a first step, passing an electric current through a low temperature electrolyte containing a low grade water soluble acrylate resin compound capable of being polymerized at an anode with the workpiece being the anode, so forming an anodic oxidation coating combined with the acrylate resin compound on the workpiece, and as a second step, applying an alternating voltage of 10 to 30V to the workpiece on which the anodic oxidation coating has been formed, within an electrolyte containing a sulphate or nitrate of a desired metal, so that the metal is electrolytically impregnated into the anodic oxidation coating. A process as claimed in Claim 1 in which the electrolyte used in the second step is composed of metallic salts in amounts of from 10 to 25 g/1, boric acid in an amount of from 25 to 30 g/1, and sulphuric acid or nitric acid in an amount of from 0.3 to 0.5 g/1. A process as claimed in Claim 1 or Claim 2 in which the desired metal salt is a silver salt. A process as claimed in any preceding claim in which the treatment temperature in the second step is from 5 to 20°C. A process as claimed in Claim 4 in which the treatment temperature in the second step is from 10 to 15°C.

Inventors: MITANI MINORU, MITANI, MINORU
Applicants: MITANI MINORU, MITANI, MINORU
Application year: 1990
Main classification: C25D11
IPC codes: C25D11