METHOD FOR ELECTROLYTIC COLORING OF ALUMINUM OR ALUMINUM ALLOYS

Abstract: A method for electrolytic coloring of anodized aluminum or aluminum alloys in an electrolyte containing metal salts is disclosed, in which the aluminum or aluminum alloy is preliminarily treated in an electrolyte by applying a voltage of substantially positive voltage wave form and, thereafter, the electrolytic coloring is carried out in the same electrolyte by applying an asymmetrical alternating current voltage that the positive voltage is smaller than the negative voltage.

Description: The present invention relates to a method for electrolytically coloring of an aluminum or aluminum alloy workpiece (hereinafter the term "aluminum" is used to indicate both aluminum and aluminum alloys). More particularly, it is concerned with a method for electrolytically coloring of an aluminum workpiece which comprises preliminarily treating an anodized aluminum workpiece by applying a voltage of positive voltage wave form followed by electrolytically coloring by applying a voltage of asymmetrical alternating current, whereby uniform and good coloring can be efficiently applied onto the aluminum surface. In electrolytic coloring of aluminum, various methods have been proposed to improve the throwing power or coloring ability and coloring speed. For example, for improvement of the throwing power in coloring, the following have been proposed: a method in which a newly developed electrolyte is used (JP-B-11119/1985), a method in which in electrolytic coloring, the voltage is increased or applied in a newly developed manner (US-A-4,070,255, JP-B-46557/1983, JP-A-145798/1984, JP-B-34287/1974, 49408/1977, 27953/1982 and 4503/1978), a method in which prior to electrolytic coloring, specified direct current anodization is applied (JP-B-13859/1979, US-A-4,021,315), JP-B-23664/1979, US-A-4,316,780 and JP-B-39237/1983), and so forth. In addition, to increase the colording speed in electrolytic coloring, a method of modifying an electrolyte (JP-B-11119/1985 and 23663/1979), a method of using special material for counter electrode (Japanese Patent Publication (JP-B-13440/1985), and so forth have been proposed. When practicing these known electrolytic coloring methods and particularly the alternating current electrolytic coloring method, either the throwing power or the coloring speed is improved but the other is improved only insufficiently or rather tends to be reduced. In particular, the method of coloring by controlling the alternating current with a resistor or thyrister as disclosed in JP-B-4503/1978, 34287/1974 and 27953/1982 has a problem in that the coloring effect cannot be sufficiently obtained because the barrier layer is not adjusted. The method in which the barrier layer is previously adjusted and then electrolytic coloring is applied using negative direct current with a positive pulse voltage applied, as disclosed in US-A-4,316,780 has problems in that controlling the current becomes markedly complicated and equipment cost is increased, both of which are disadvantageous from an economic standpoint. Furthermore, a method for forming a colored oxide film on an anodized aluminium surface is known (Patent Abstracts of Japan, Unexamined Applications, C Field, Vol. 4 Number 106, July 30, 1980, 55-65396 A), wherein an aluminium workpiece is subjected to electrolysis under specific conditions in an electrolytic bath containing metal salts in order to obtain a colored oxide film which is free from spalling, and to obtain superior throwing power. To this end, a pulse voltage of trapezoidal wave form is applied to the electrolytic bath in which the absolute value of the negative voltage is higher than that of the positive voltage. This known method is a one stage treatment. Furthermore, it is known for producing anodized colored aluminium articles (US-A 4 414 077) to apply a DC current to an electrolytic bath to which current positiv pulses are superimposed. The present invention overcomes the above problems and has the object to provide an electrolytic coloring method whereby both the throwing power or coloring ability and the coloring speed are increased at the same time. As a result of extensive investigations, it has been found that the object can be attained by first treating the anodized aluminum workpiece by applying an anodic voltage wave form followed by the electrolytic coloring accomplished by applying a specified asymmetrical alternating current voltage. The essential features of the invention are the subject-matter of independant claim 1; preferred features of the invention are the subject-matter of dependant claims 2 - 9. In the drawings Figs. 1 to 7 show examples of preferred voltage wave forms for the asymmetrical alternating current to be used in the electrolytic coloring step of the present invention, Fig. 8 shows the voltage wave form used in the preliminary treatment of Example 1 and Comparative Example 2, Fig. 9 shows a cross-sectional view of the extruded aluminum profile used in Example 1 and Comparative Examples 1 and 2 and Fig. 10 shows a plan view illustrating the arrangement of the apparatus used in Example 2 and Comparative Example 3, and the test plate placed therein. The aluminum workpiece to be colored according to the present invention is an aluminum workpiece the surface of which has been anodized. This anodization can be carried out by methods which have heretofore been widely employed. Usually the anodization is carried out by passing a direct current through an acidic electrolyte containing sulfuric acid, oxalic acid, sulfamic acid or the like, using aluminum as an anode. The surface of the aluminum to be anodized is normally cleaned, etched before the aluminum is introduced into the anodizing bath as the anode. Aluminum, graphite, or other "electrolically conductive" material is used as the cathode. In accordance with the present invention, aluminum which has been anodized is first treated in an electrolyte prior to the electrolytic coloring for modification of the barrier layer. During modification of the oxidic barrier layer formed by anodization, generally, as the thickness of the modified barrier layer gets larger, the resistance of the barrier layer formed at the time of anodization gets uniform and thus uniform electrolytic coloring can be achieved. If, however, the thickness of the barrier layer is too great, a problem arises in that spalling occurs during electrolytic coloring. In the method of the present invention, because a special wave form is employed for the voltage used in the electrolytic coloring (after the preliminary treatment) the extent of modification of the barrier layer in the preliminary treatment is not significant. That is, even if the modified barrier layer is relatively thin, the necessary throwing power for electrolytic coloring can be sufficiently obtained. Similarly, even if the thickness of the modified barrier layer is excessively increased, there is no danger of spalling occurring. For the above reasons, there are no special requirements for the potential to be applied during the preliminary treatment as long as it has a positive voltage wave form. Thus a half-wave rectified current including three phase, a full-wave rectified current including three phase and so forth can be used satisfactorily. The term "substantially positive voltage wave form" is used herein to include not only a wave form which exhibits a positive voltage all over one cycle but also a wave form which has a small negative voltage portion (for example, the ratio of negative voltage/positive voltage=0 to 0.5). Application of a voltage of wave form containing a negative voltage permits increasing the positive voltage to increase the modification effects of the barrier layer. Further, an asymmetrical alternating current voltage with larger positive voltage than negative voltage can be preferably used. It is convenient that the asymmetrical alternating current voltage is reverse to that of the subsequent coloring treatment. Also there are no special limitations to the current application time, the magnitude of positive voltage to be applied, the voltage increasing speed and so forth. These factors can be determined appropriately depending on conditions. Usual and preferred conditions are exemplified below. The time for the preliminary treatment depends on treating conditions. Usually the current density for the preliminary treatment is 0.01 to 2 A/dm² (amperes per square decimeter) and preferably 0.01 to 1 A/dm² indicated by average positive currents; and usually the time for the preliminary treatment, including the time required for increasing the voltage is 5 to 180 seconds and preferably 10 to 90 seconds. These conditions are conveniently attained in the coloring bath electrolyte. If, however, the electrolytic coloring treatment is carried out directly without performing the preliminary treatment, the throwing power is not sufficiently satisfactory and either uniform coloring cannot be obtained or coloring can be attained only with difficulty, depending on the type of the electrolyte. Application of a high voltage at the time of coloring to accelerate coloring speed may cause spalling. The preliminary treatment can be carried out in the electrolyte, the electric conductivity of which is similar to that of an electrolyte used in the subsequent electrolytic coloring treatment. In accordance with the method of the present invention, after the above preliminary treatment is performed in an electrolyte, the electrolytic coloring treatment is applied, usually in the same electrolyte. This electrolytic coloring treatment is basically an alternating current electrolytic coloring treatment. In this electrolytic coloring treatment, it is necessary to use an asymmetrical alternating current voltage where the positive voltage is smaller than the negative voltage. Various types of asymmetrical alternating current voltages can be used in the present invention, including: the usual asymmetrical alternating current wave form as shown in Fig. 1 (in which the times for passing positive and negative voltages are equal but their peak values are different); the asymmetrical alternating current wave form as shown in Fig. 2 in which a sine wave alternating current is controlled at different phase angles for positive and negative waves by the thyrister-control (as a result of which the conduction angle of the negative wave is larger than that of the positive wave); an asymmetrical alternating current wave form as shown in Fig. 3 in which the positive and negative waves of the asymmetrical alternating current shown in Fig. 1 are each doubled; an asymmetrical alternating current wave form as shown in Fig. 4 in which the positive and negative waves of the thyrister-controlled asymmetrical alternating current as shown in Fig. 2 are each doubled; an asymmetrical alternating current wave form as shown in Fig. 5 in which the asymmetrical alternating current wave form shown in Fig. 1 is controlled at the same or different phase angles for the positive and negative waves by the thyrister; an asymmetrical alternating current wave form as shown in Fig. 6 in which the positive and negative waves of the thyrister-controlled asymmetrical alternating current wave form as shown in Fig. 5 are each doubled; and asymmetrical alternating currents in which the positive and negative waves of the asymmetrical alternating current wave forms shown in the above figures are each even numbered like 4, 6, 8 ... instead of double, can be used. In addition, a direct current superimposed alternating current as shown in Fig. 7 can be used. In all the asymmetrical alternating current voltages shown in Figs. 1 to 7, the negative voltage is larger than the positive voltage. In thyrister-controlling therefore, the conduction angles of the positive and negative waves should be controlled so that the negative wave is larger than that of the positive wave. The ratio of the positive voltage to the negative voltage in the asymmetrical alternating current voltage varies with the type of the electrolyte. In general, however, based on an average voltage which is indicated by average value, the ratio of the positive voltage to the negative voltage is 1:1.5 to 1:20 and preferably 1:2 to 1:5. The current for the coloring stage or step of the process is usually from about 0.03 to about 1 A/dm² and preferably 0.05 to 0.3 A/dm² indicated by average negative currents. Although coloring time depends on the required color shade and can be determined by inspection, it is usually 10 sec to 30 min and preferably 30 sec to 20 min. The electrolyte to be used for coloring aliminum according to the present invention contains various metal salts depending on the purpose. Representative examples of the metal salts are the sulfates, nitrates, phosphates, hydrochlorides, oxalates, acetates and tartrates of metals such as nickel, cobalt, copper, salenium, iron, molybdenum and tin. Conditions for the electrolytic coloring treatment, such as the magnitude of voltage to be applied, the electricity passing time and the liquid temperature can be determined appropriately. Since, however, in accordance with the method of the present invention, the coloring treatment can be carried out at a higher voltage (negative voltage) than in the conventional alternating current electrolytic coloring, the coloring speed can be increased and thus the electrolytic coloring can be carried out in a relatively short time. In accordance with the present invention, the barrier layer of the anodic coating on the aluminum surface is modified to a certain extent by application of the preliminary treatment and as the electrolytic coloring step, the specified asymmetrical alternating current is used. Thus even if the electrolytic coloring is carried out at a high voltage, the coloring is achieved rapidly and with a high throwing power without causing problems such as spalling, and a uniform and beautiful electrolytic coloring coating can be formed in a short time. The present invention is described in greater detail with reference to the following examples. Example 1 An electrolyte containing 90 g/l of nickel sulfate 6 hydrate, 100 g/l of magnesium sulfate 7 hydrate, 40 g/l of boric acid and 3 g/l of tartaric acid and having a pH of 5 was placed in a 500-liter electrolytic coloring vessel. Three A-6063-T₅ aluminum extruded profiles, each of which has the cross section shown in Fig. 9 (total length: 500 mm; total depth: 145 mm; total width: 80 mm) were used for coloring. The three test profiles and nickel plates as a counter electrode were placed in the electrolyte. The three test profiles were subjected to the preliminary treatment by increasing a voltage having the wave form shown in fig. 8 (peak voltage at the maximum, Vp = 80 V; the ratio of the positive voltage to the negative voltage is 7:1), at a voltage increasing rate of 1 V (peak value of the positive voltage)/second and passing electricity at the peak value of 50 V for 5 seconds. During this treatment the average voltage increased from 0 to 16 V (volts), and current density measured as average positive current, went from 0 to 0.3 A/dm². After the preliminary treatment, the electrolytic coloring treatment was carried out by passing electricity for 3.5 minutes at an average positive voltage of 3.5 V and an average negative voltage of -10.8 V using an asymmetric alternating current voltage having the wave form shown in Fig. 6 (the positive and negative waves of the voltage wave form shown in Fig. 8 were reversed). During this treatment the current density was 0.18 A/dm² measured as average negative current. As a result, each of portions A, B and C of the test profile shown in Fig. 9 was finished in a uniform bronze color. COMPARATIVE EXAMPLE 1 The procedure of Example 1 was repeated with the exception that the preliminary treatment was not applied. The test profiles were not almost colored. When the electricity passing time was prolonged to 10 min, the test profiles were slightly colored, but spalling occurred. COMPARATIVE EXAMPLE 2 The procedure of Example 1 was repeated with the exception that the conventional alternating current was applied in the electrolytic coloring treatment was carried out under conditions of voltage 27 V (current density: 0.2 A/dm²) and treating time 3.5 minutes. In the test profile shown in Fig. 9, the portion A was in a bronze color and the portions B and C, in a gold color, and the coloring was not uniform. Example 2 An electrolyte containing 80 g/l of cobalt sulfate 6 hydrate, 80 g/l of magnesium sulfate, 30 g/l of boric acid and 2 g/l of citric acid and having a pH of 4.3 was placed in a Hul cell test apparatus as shown in Fig. 10 (a plan view with an upper bottom of 80 mm, a lower bottom of 250 mm and a length of 80 mm; the angle is made sharper than that of the normal Hull cell test apparatus for plating). An anodized test plate (A-1100-H14 aluminum: 100 mm (length) x 180 mm (width) x 1.5 mm (thickness)) as an anode and a carbon rod as a cathode were placed in the electrolyte, and the preliminary treatment was applied by passing a direct current of 30 V (current density: 0.2 A/dm²) for 10 seconds. After the preliminary treatment, the electrolytic coloring treatment was carried out using an asymmetrical alternating current voltage having the wave form shown in Fig. 2 (negative peak voltage at the maximum, Vp = 50 V; conduction angle of the positive voltage: 60°) under conditions of average positive voltage 3.1 V, average negative voltage -8 V (current density as average negative current: 0.2 A/dm²) and treating time 3 minutes. Both the portions D and E were in a darker bronze color, and the whole was finished uniformly. COMPARATIVE EXAMPLE 3 The procedure of Example 2 was repeated with the exception that the conventional alternating current (voltage: 18 V; current density:0.25 A/dm²) was applied in the electrolytic coloring treatment. The portion D of the test plate (a portion near the counter electrode) was in a darker bronze color and the portion E (a portion far from the counter electrode), in a lighter bronze color. This shows that the test plate was colored unevenly.

Claims: Method for electrolytically coloring of an anodized aluminum or aluminium alloy workpiece, wherein said aluminum or aluminum alloy workpiece is preliminarily treated in an electrolyte by applying a voltage of positive voltage wave form and, thereafter, said aluminum or aluminum alloy workpiece is electrolytically coloured in an electrolyte containing metal salts by applying an asymmetrical alternating current voltage in which the absolute value of the negative voltage is higher than that of the positive voltage. Method of Claim 1, characterized in that the asymmentrical alternating current voltage is controlled by electric resistor. Method of Claim 1, characterized in that the asymmetrical alternating current voltage is controlled by thyrister. Method of Claim 1, characterized in that the asymmetrical current voltage is controlled by electric resistor and thyrister. Method of one of Claims 1 - 4, characterized in that the asymmetrical alternating current voltage is formed by a number of half-sine wave. Method of one of Claims 1 - 4, characterized in that the asymmetrical alternating current voltage is formed by superimposing a direct current to an alternating current. Method of Claim 2, characterized in that positive and negative waves of the asymmetrical alternating current voltage are each doubled. Method of Claim 3, characterized in that positive and negative waves of the asymmetrical alternating current voltage are each doubled. Method of Claim 4, characterized in that positive and negative waves of the asymmetrical alternating current voltage are each doubled.

Inventors: HINODA, YUJI C/O FUJISASH COMPANY
Applicants: FUJISASH COMPANY
Application year: 1987
Main classification: C25D11
IPC codes: C25D11