US3915813A

US3915813A - Method for electrolytically coloring aluminum articles

Info

Publication number: US3915813A
Application number: US416809A
Authority: US
Inventors: Takashi Abe; Toshimitsu Uchiyama; Tatsuo Otsuka
Original assignee: Showa Aluminum Corp
Current assignee: Showa Aluminum Can Corp
Priority date: 1972-11-21
Filing date: 1973-11-19
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28
Also published as: AU6270773A; JPS5249408B2; SE409473B; AU471462B2; JPS4975433A; GB1398231A; DE2358089A1

Abstract

An aluminum article is colored by forming an oxidized coating thereon by usual anodizing treatment and subsequently subjecting the treated article to electrolysis in a metal salt-containing electrolytic bath with current having a discontinuous distorted wave. The current includes alternating positive current portions and negative current portions, at least the negative portion having a conduction angle controlled by a thyristor, the positive current portions being thereby rendered larger than the negative current portions.

Description

0 United States Patent 1191 [111 3,

Abe et al. 1 Oct. 28, 1975 METHOD FOR ELECTROLYTICALLY 3,769,180 10/1973 Gedde 204/35 N C I G ALUMINUM ARTICLES 3,787,295 1/1974 Endinger et a1. 1 204/35 N 3,795,590 3/1974 Patric 204/35 N [75] Inventors: Takashi Abe; Toshimitsu Uchiyama;

Tatsuo Otsuka, all of Osaka, Japan [73] Assignee: Showa Aluminium Kabushiki Primary Exammer R' Andrews Kaisha, Osaka, Japan [22] Filed: Nov. 19, 1973 ABSTRACT [21] Appl. No.: 416,809

An aluminum article is colored by forming an oxidized coating thereon by usual anodizing treatment and sub- [301 Foreign Apphcatlon Pnonty Data sequently subjecting the treated article to electrolysis Nov. 21, 1972 Japan 47-1175 61 in a metal salt containing electrolytic h with rent having a discontinuous distorted wave. The cur- U-S- N t i l d lt ti g p iti Current p ti a d [5 ll'llt. Cl. 1 g ti current p ti t l t th g ti ti [58] Fleld of Search 204/228, 35 N, 58 having a Conduction angle Controlled by a thyristor, the positive current portions being thereby rendered [56] References C'ted larger than the negative current portions.

UNITED STATES PATENTS 3,708,407 1/1973 Newman et a1. 204/228 8 Claims, 10 Drawing Figures US. Patent Oct.28, 1975 Sheet10f2 3,915,813

FIG 5 FfG4- US. Patent L value 8- AE FIG Oct. 28, 1975 Sheet 2 of 2 6 FIG: 10

Conduction Angle of negative current portion L value& AE

Conduction Angle of negativ current portion METHOD FOR ELECTROLYTICALLY COLORING ALUMINUM ARTICLES I BACKGROUND OF THE INVENTION The present invention relates to a method for coloring aluminum articles, more particularly to a method for coloring aluminum articles uniformly.

Throughout the specification and claims, the term aluminum refers to pure aluminum, commercial aluminum containing small amounts of impurities and aluminum alloys predominantly composed of aluminum.

As disclosed in U.S. Pat. No. 3,382,160, a process for coloring aluminum articles has already been known which comprises anodizing the article to form an anodic coating thereon and then subjecting the anodized article to electrolytic treatment with alternating current in a metal salt-containing electrolytic bath.

The conventional process has the drawback of being poor in throwing power. More specifically, when treated with the known process, a plate material will be colored deep at its periphery whilst the center portion thereof will exhibit a pale color. With a shaped extrusion, the process fails to produce a satisfactory coloring effect especially on a recessed portion. It is further noted that when a plurality of articles are suspended from the same hanger for electrolytic treatment, there arises a color difference between articles positioned in vertical arrangement or between those arranged on the same level. Moreover, it is difficult to color articles in different colors using the same electrolytic bath.

SUMMARY OF THE INVENTION This invention provides a method for electrolytically coloring aluminum articles free of the foregoing drawbacks. The present method comprises a first step of forming an oxidized coating on an aluminum article by usual anodizing treatment and a second step of subjecting the treated article to electrolysis in a metal salt-containing electrolytic bath using discontinuous distorted wave current. The current has alternating positive current portions and negative current portions, at least the negative portion having a conduction angle controlled by a thyristor, the positive current portions being thereby rendered larger than the negative current portions so as to assure excellent throwing power and to thereby make it possible for the second step to give coatings in different color tones using the same electrolytic bath.

To prepare the electrolytic bath for the first step, acids conventionally used to form anodic coatings are all employable, examples thereof being sulfuric acid, oxalic acid, chromic acid, etc., among which sulfuric acid is generally used. The acid concentration is suitably 100 to 200 g/l. Usually, the electrolysis is preferably conducted with direct current having a current density of 1.0 to 1.5 A/dm The electrolytic bath for the second step contains as a main ingredient a mineral acid or organic acid useful for the formation of anodic coating like the electrolytic bath of the first step and at least one metal salt added to the acid ingredient. Generally, sulfuric acid and boric acid are usable as the mineral acid, and oxalic acid and sulfosalicylic acid as the organic acid, among which sulfuric acid is preferable. It is advantageous to use in the second step an acid such as given above which is capable of forming an anodic coating because the positive current portions of the discontinuous distorted wave current will participate in anodizing the aluminum article. Examples of the metal salt are watersoluble salts of tin, silver copper, selenium, cobalt, nickel and the like. For example, the electrolytic bath may contain 10 to 300 g/l of sulfuric acid and l to 50 g/l of stannous sulfate. When subjected to electrolysis in the bath thus prepared, aluminum articles are colored in tones widely ranging from gold to dark amber. In the case where silver sulfate is used, the bath may contain 10 to 300 g/l of sulfuric acid and 0.5 to 5.0 g/l of silver sulfate. The electrolysis of aluminum in such bath gives a wide variety of colors ranging from gold to dark bronze. If the amount of metal salt is below the lower limit, the electrolytic bath will have a poor coloring ability. The discontinuous distorted wave current is supplied from a single-phase power source or threephase power source. Polyphase power sources in excess of three in phase number involve problems in design and are practically infeasible. Generally, a silicon controlled rectifier (hereinafter referred to as SCR) is used as the thyristor for controlling the current.

The present invention will be described below in greater detail with reference to accompanying drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an electric circuit for electrolytically coloring aluminum articles according to this invention in which a single-phase power source is used, a coloring electrolytic cell being shown in horizontal section;

FIG. 2 is a fragmentary diagram showing an electric circuit including a modification of the single-phase power source of FIG. 1;

FIG. 3 is a fragmentary diagram showing an electric circuit including a three-phase power source connected to the electrolytic cell of FIG. 1;

FIG. 4 shows the waveshape of current obtained using the single-phase power source of FIG. 1;

FIG. 5 shows the waveshape of current obtained using the three-phase power source of FIG. 3;

FIG. 6 is a diagram showing an electric circuit used for testing throwing power achieved with the singlephase power source, the diagram showing an electrolytic cell in horizontal section;

FIG. 7 is a graph showing the test results of throwing power as determined using the single-phase power source of FIG. 6;

FIG. 8 is a graph showing the test results of throwing power as determined using the three-phase power source of FIG. 3 in place of the single-phase power source of FIG. 6;

FIG. 9 is a side elevation showing a number of extruded aluminum shapes as arranged in two rows and suspended in the electrolytic cell of FIG. 1; and

FIG. 10 is a side elevation on an enlarged scale showing a modification of extruded aluminum shape to be colored electrolytically.

DETAILED DESCRIPTION OF THE INVENTION With reference to FIG. 1, there is shown a coloring electrolytic cell 1 containing an electrolytic bath for the second step and accommodating two aluminum articles 2. Each of the aluminum articles 2 opposes a carbon electrode 3 serving as a counter-electrode. The aluminum articles 2 and carbon electrodes 3 are connected to a single-phase power source 4 for electrolysis to form an electric circuit. Connected to the singlephase power source 4 are a silicon rectifier (hereinafter referred to as SR") 5 and an SCR 6 which are arranged in parallel and oriented in opposite directions to each other. Indicated at 7 is the secondary winding of a transformer. Positive current i, flows from the aluminum articles to the carbon electrodes 3, and negative current i flows from the carbon electrodes 3 to the aluminum articles 2. As shown in FIG. 4, the portion of positive current i rectified by the SR 5 has a constant conduction angle a of 180, whilst the portion of negative current i rectified by SCR 6 has a conduction angle B which is variably controlled by the SCR 6.

The portion of positive current i may preferably have a greater conduction angle, but the parallel arrangement of the SR 5 and SCR 6 may possibly involve a trouble in the electric installation. To avoid this, an SCR 8 is substituted for the SR 5 in a modified electric circuit of single-phase power source 9 as partly shown in FIG. 2. Also in this case, the conduction angle a of portion of the positive current i is preferably greater than the conduction angle [3 of portion of negative current FIG. 3 shows a three-phase power source 10 as used for the power source of electrolytic circuit seen in FIG. 1. One phase of the power source 10 incorporates an SCR 11 and the remaining two phases include

SCRs

12 and 13 in parallel. Indicated at 14 is the secondary winding of transformer. Preferably, the conduction angle -y of portion of positive current i is greater than the conduction angle 5 of portion of negative current i and the conduction angle 6 of portion of negative current 1' It will be readily understood from FIGS. 4 and 5 that by virtue of control of SCR the waveshape of current is changed to a discontinuous distorted wave. Varying the porportion of the conduction angle of negative current portion to the conduction angle of positive current portion gives various shapes of discontinuous distorted waves. The proportion between the conduction angle of positive current portion and the conduction angle of negative current portion corresponds to the proportion between the anodizing current and the current depositing metal ions into the anodic coating on the aluminum article.

Inasmuch as the degree of conduction angle of nega tive current portion exerts a great influence on the ability to color the aluminum article, variations in the conduction angle permit the same electrolytic bath to produce coatings which vary in the lightness of its color. Accordingly, the aluminum article can be colored to the desired lightness by suitably determining the conduction angle of the negative current portion. Generally, the greater the conduction angle of negative current portion, the darker will be the resulting color, whereas the smaller the angle, the lighter the color obtained.

FIG. 6 shows an electrolytic electric circuit for testing the throwing power. The drawing shows a coloring electrolytic cell 15 measuring 300 mm in length, 100 mm in width and 150 mm in height. Provided at the midportion of its length is a partition 16 which is centrally formed with a hole 17 having a diameter of 6 mm. On the opposite sides of the partition 16, two aluminum plates 18 and 19 are suspended which are made of AA 1 100 aluminum alloy and each of which measures 50 mm X 100 mm X 1.2 mm. The aluminum plate 18 is positioned at an approximate midportion between the partition 16 and one side wall of the electrolytic cell 15. The other aluminum plate 19 is disposed close to the 4 other side wall of the cell 15. The aluminum plate 18 only is provided with a counter-electrode, i.e. a carbon electrode 20 measuring 50 mm X 50 mm X 10 mm and positioned beside the side wall. The same single-phase power source 4 as in FIG. 1 is used for electrolysis.

The plates 18 and 19 have already been anodized by the first step of electrolytic treatment conducted in a 15% sulfuric acid bath at a bath temperature of 20C with direct current having a current density of 1.5 A/dm for 20 minutes, the plates 18 and 19 therefore having anodic coatings of 911..

The second step of treatment was conducted for 3 minutes to color the plates 18 and 19, using the electric circuit of FIG. 6 and an electrolytic bath containing g/l of sulfuric acid and 20 g/l of stannous sulfate and having a temperature of 25C, the secondary voltage of transformer being 15 V.

FIG. 7 shows the lightnesses of plates 18 and 19 and color differences therebetween. The above-mentioned second step was carried out varying the conduction angle of negative current i to observe changes in the lightness of color L of plate 18 and color L of plate 19 as well as the changes in color difference. In FIG. 7 L values and AE are plotted as ordinate, and the conduction angle as abscissa. The lower the L value, the darker is the color. The higher the AE, the greater is the color difference. At about 0.5, the color difference is indetectable. It is seen from the graph that hardly any coloring takes place if the conduction angle is less than 30, whereas in excess of 150, a marked color difference arises. Accordingly, in the case where a singlephase power source is used, the conduction angle of portion of negative current i should be varied within the range of 30 to 150.

FIG. 8 is a graph corresponding to FIG. 7 and obtained using a three-phase power source in place of the single-phase power source of FIG. 6. The graph indicates that when the three-phase power source is used, the conduction angles of negative currents i and 1' should be determined within the range of 30 to respectively.

EXAMPLE 1 Ten extruded shapes of AA 6063 aluminum alloy were prepared which had sectional forms shown in FIG. 9 and measuring 60 mm in the dimension of a portion, 30 mm in the dimension of b portion and 1,000 mm in length. For pretreatment, the shapes were etched with a 5 percent caustic soda washing solution at 46C for 8 minutes. The shapes were then subjected to two steps of electrolytic treatment as described below. The ten shapes were suspended by hangers in two rows, five shapes in each row, and arranged in an anodizing cell as illustrated in FIG. 9. More specifically, the uppermost shapes 15A and 15F were spaced apart by a distance 0 which was 80 mm, and the

lowermost shapes

15E and 15,] were spaced apart by a distance d that was 40 mm, with each two vertically adjacent shapes spaced apart by a distance e that was 50 mm. The carbon electrodes 21 were located at a distance f of 250 mm from a plane 22 at the middle of the space between the two rows of opposing shapes.

For the first step of treatment, the shapes were subjected to electrolysis in a 15% sulfuric solution bath at a bath temperature of 20C with direct current of 1.5

' A/dm for 20 minutes, whereby the shapes were formed with anodic coatings of about 8 to 9p..

The shapes anodized by the first step of treatment into the coloring electrolytic cell 1. For the second step of treatment, the shapes were subjected to electrolytic treatment at a constant voltage for 3 minutes in a bath containing 80 g/l of sulfuric acid and 22 g/l of stannous sulfate and having a temperature of 26C, using the single-phase power source shown in FIG. 1. The secondary voltage of transformer was V and the conduction angle of negative current portion was controlled to 65. Gold color was obtained. For after-treatment, the shapes were washed with water, dried and then sealed in pure water for minutes. The colors of surface A facing the counter-electrode, recessed portion B and rear surface C, on the opposite side of surface A, of each shape thus treated were measured by I-Iunters 6 treatment was conducted in the same manner as in Example l. Gold color was obtained.

For comparison, the second step was followed with alternating current, using the same shape 16 as above obtained by the first step of electrolytic treatment. Thus the shape 16 was treated for 5 minutes in an electrolytic bath containing g/l of nickelous sulfate, g/l of ammonium sulfate and 30 g/l of orthoboric acid and having a temperature of 25C with alternating current having a current density of 0.3 A/dm and voltage of 15 V.

Table 2 shows the results of coloring treatments achieved in Example 2 and the above-described comparison example, in terms of color difference AE between surfaces D and E illustrated in FIG. 10.

Table 2 Color-Difference Meter. The results are listed in Table 1 below. Note Table l Shapes A B C Measured values Measured values Measured values L a b L a b L a b ISA 45.5 0.4 9.1 47.0 0.4 9.3 46.6 0.4 9.2

As apparent from Table I, each of the shapes was found uniformly colored with hardly any variations in This 0 Botfh suil'facesl Dhand 5 were colored invention .6 uni orm y in ig t am er color. color between its surface A, recessed portion B and Comparison Surface Dwas colored bronze but rear surface C. Moreover, almost no var1at1ons in color example 37.2 surface E was scarcely colored.

were found between the shapes. Thus the present method assures very excellent throwing power. Particularly, it is to be noted that the shape was colored uniformly even in its recessed portion as well as on the rear surface which did not face the counterelectrode.

The second step of treatment was further conducted with varying conduction angles of 30 to 150,whereby it was ascertained that the angle range of 30 to 100 produced gold color, whilst angles ranging from 100 to 150 yielded light amber color.

EXAMPLE 2 An extruded shape 16 of AA 6063 aluminum alloy having a sectional form of FIG. 10- and measuring 25 mm in the dimension of g portion, 30 mm in the dimension of h portion and 1,000 mm in length was subjected to the first step of electrolytic treatment as in Example I to form a anodic coating of 8 to 9p..

For the second step of treatment, the shape 16 was subjected for 3 minutes to electrolytic treatment at a constant voltage in an electrolytic bath containing 80 g/l of sulfuric acid and 10 g/l of stannous sulfate and having a temperature of 25C, using the single-phase power source of FIG. 2 supplying a secondary voltage of 15 V, with the conduction angle of positive current portion controlled to 160 and the conduction angle of negative current portion controlled to 100. After- Table 2 reveals that the method of this invention assures higher throwing power than the comparison example wherein alternating current was used.

The same procedure as in Example 2 was followed with the conduction angle of positive current portion adjusted to 120, which produced variations in color tone.

EXAMPLE 3 In exactly the same manner as in Example 1, the same shapes as used therein were subjected to the first step of electrolytic treatment and then to the second step of electrolytic treatment at a constant voltage for 3 minutes in a bath containing g/l of sulfuric acid and 22 g/l of stannous sulfate and having a temperature of 26C, using the three-phase power source shown in FIG. 3, each secondary voltage of transformer being 20 V. The conduction angle of positive current portion was controlled to and that of each negative current portion, to 93. An after-treatment was conducted in the same manner as in Example 1. The colors of surface A facing the counter-electrode, recessed portion B and rear surface C, on the opposite side of surface A, of each shape thus treated was measured by I-Iunters Color-Difference Meter with the results listed in Table 3.

Table 3 Shapes A B Measured \alues Measured values Measured values a a l. a b

15A 180 -08 5.3 18.3 O.8 54 18.3 O.8 5.4 158 17.5 0.7 5.0 17.6 0.7 5.0 17.5 0.7 5.0 15C 17.7 -08 5.0 17.7 -08 5.0 17.7 -07 5.0 15D 16.6 O.8 4.6 16.) -06 4.6 17.1 O.7 5.8 15E 18.6 0.8 5.4 18.8 0.8 5.4 18.8 O.8 5.4 15F 17.4 O.8 5.0 17.9 "0.8 5.2 17.9 O.8 5.2 15G 18.4 O.8 5.3 18.6 O.8 5.4 18.7 O.8 5.4 15H 17.7 0.8 5.2 18.1 0.8 5.2 18.4 0.6 5.3 151 17.0 0.7 4.7 17.0 0.7 4.7 17.6 O.7 5.0 15J 18.6 O.8 5.3 18.6 0.8 5.3 18.8 0.8 5.3

Table 3 indicates that the use of three-phase power source also entailed hardly any varistions in color between the surface A, recessed portion B and rear sur Table 4 shows the resulting colors in Example 4 and comparison example as determined by Hunters Color- Difference Meter.

face C of each shape as well as between the shapes, hence good throwing power. The fact that the electrolytic treatment conducted for such a short period as 3 minutes yielded deep amber coatings evidences the excellent coloring ability of the present method. With conventional electrolytic coloring methods employing alternating current, deeply colored coatings were obtained by extending the treating time or elevating the voltage, but a prolonged period of electrolysis resulted in a corresponding reduction in the efficiency or an elevated voltage was liable to cause peeling of the coating and like fault.

The second step of treatment was further conducted with varying conduction angles of negative current por tion of 30 to 1 10 to find that the angle range of 30 to 60 produced gold color, angles ranging from 60 to 90 resulted in light amber color and amber color was obtained with angles ranging from 90 to 1 10.

EXAMPLE 4 A bus bar made of AA 6063 aluminum and measuring 50 mm in width, 100 mm in length and 3 mm in thickness was subjected to the first step of electrolytic treatment in the same manner as in Example 1 to form an anodic coating of 8 to 9a.

The anodized bus bar was then subjected to the second step of electrolytic treatment for 2 minutes in a bath of 20 g/l of silver sulfate and 0.5 g/l of silver sulfate with the three-phase power source shown in FIG. 3. The secondary voltage of transformer was 100 V, and the conduction angle of positive current portion was controlled to 93 and the conduction angle of each negative current portion to 45. An aftertreatment was conducted in the same manner as in Example 1. The same procedure as above was followed except that the second treatment was carried out for minutes.

For comparison, the same bus bars as above obtained by the first step of electrolytic treatment were subjected to electrolytic treatment for 3 minutes and 10 minutes, respectively, in the same electrolytic bath with alternating current having a current density of 0.3 A/dm and voltage of V.

It will be seen from Table 4 that the present method has a very high coloring ability to produce deep colors in a short period of time, whereas the conventional method employing alternating current has very poor coloring ability.

The same procedure as in Example 4 was followed with the conduction angle of positive current portion of It was found that partial deposition of metal ions took place to produce an uneven color tone.

The present invention may be practiced in other different modes without departing from the spirit and basic features thereof. Accordingly, the examples herein disclosed are to be construed as being illustrative only but not as limitative in every respect. The scope of this invention is defined by the appended claims rather than by the foregoing deteiled description, and all the modifications and alterations within a scope equivalent to that of the claims are to be covered by the claims.

What is claimed is:

l. A method for electrolytically coloring aluminum articles comprising a first step of forming an oxidized coating on an aluminum article by usual anodizing treatment and a second step of subjecting the treated article to electrolytic coloration in a metal salt-containing electrolytic bath with discontinuous wave current having alternating positive current portions and negative current portions, at least the negative current portion having a conduction angle controlled by a thyristor, the positive current portions being thereby rendered larger than the negative current portions, the said electrolytic bath used for the second step containing as a main ingredient an acid selected from the group consisting of mineral acids and organic acids and the said metal salt being a salt of metal selected from the group consisting of tin, silver, copper, selenium, cobalt and nickel.

2. The method as set forth in claim 1 wherein at least the negative current portion has a conduction angle controlled by a thyristor incorporated in a singlephase power source.

3. The method as set forth in claim 1 wherein at least the negative current portion has a conduction angle controlled by a thyristor incorporated in a three-phase power source.

4. The method as set forth in claim 1 wherein metal salt-containing electrolytic bath contains 10 to 300 g/l of sulfuric acid and l to 50 g/l of stannous sulfate added thereto.

5. The method as set forth in claim 1 wherein metal salt-containing electrolytic bath contains 10 to 300 g/l of sulfuric acid and 0.5 to 5.0 g/l of silver sulfate added thereto.

6. The method as set forth in claim 1 wherein the thyristor is a silicon controlled rectifier.

7. The method as set forth in claim 1 wherein the conduction angle of the negative current portion is 30 to 150 and the conduction angle of the positive current portion is at least 1 30 and is greater than the conduction angle of the negative current portion.

8. The method as set forth in claim 1 wherein the conduction angle of the negative current portion is 30 to 110 and the conduction angle of the positive current portion is at least and is greater than the conduction angle of the negative current portion.

Claims

1. A AMETHOD FOR ELECTROLYTICALLY COLORING ALUMINUM ARTICLES COMPRISING A FIRST STEP OF FORMING AN OXIDIZED COATING ON AN ALUMINUM ARTICLE BY USUAL ANODIZING TREATMENT AND A SECOND STEP OF SUBJECTING THE TREATED ARTICLE TO ELECTROLYTIC COLORATION IN A METAL SALT-CONTAINING ELECTROLYTIC BATH WITH DISCONTINUOUS WAVE CURRENT HAVING ALTERNATING POSITIVE CURRENT PORTIONS AND NEGATIVE CURRENT PORTIONS, AT LEAST THE NEGATIVE CURRENT PORTION HAVINNG A CONDUCTION ANGLE CONTROLLED BY A THYRISTOR, THE POSITIVE CURRENT PORTIONS BEING THEREBY RENDERED LARGER THAN THE NEGATIVE CURRENT PORTIONS, THE SAID ELECTROLYTIC BATH USED FOR THE SECOND STEP CONTAINING AS A MAIN INGREDIENT AN ACID SELECTED FROM THE GROUP CONSISTING OF MINERAL ACIDS AND ORGANIC ACIDS AND THE SAID METAL SALT BEING A SALT OF METAL SELECTED FROM THE GROUP CONSISTING OF TIN, SILVER, COPPER, SELENIUM, COBALT AND NICKEL

2. The method as set forth in claim 1 wherein at least the negative current portion has a conduction angle controlled by a thyristor incorporated in a single-phase power source.

4. The method as set forth in claim 1 wherein metal salt-containing electrolytic bath contains 10 to 300 g/l of sulfuric acid and 1 to 50 g/l of stannous sulfate added thereto.

7. The method as set forth in claim 1 wherein the conduction angle of the negative current portion is 30* to 150* and the conduction angle of the positive current portion is at least 130* and is greater than the conduction angle of the negative current portion.

8. The method as set forth in claim 1 wherein the conduction angle of the negative current portion is 30* to 110* and the conduction angle of the positive current portion is at least 90* and is greater than the conduction angle of the negative current portion.