- Email: [email protected]
Anodic oxidation and coloring of aluminum powders
ALUMINUM FINISHING
Anodic Oxidation and Coloring of aluMinuM Powders by L. Anicai, C. Trifu, and L. Dima Dept. of Electrochemical Technologies and Products, Research and Development Institute for Electrical Engineering, Bucharest, Romania
T
o improve the anticorrosive characteristics of organic coatings (e.g., paints, enamels, etc.), some kinds of pigments are used; among them of special importance are aluminum powder pig• ments, mainly for the automotive industry. They significantly reduce organic solvent emission to the atmosphere during paint application. 1 A problem that is met with aluminum powder pigments is corrosion in alkaline media (pH >8), which favors hydrogen evolution according to the reaction: 2Al + 6H 2 0 -.. 2 Al
(l)
This implicitly leads to color paint degradation. Under these circumstances, diminishing or inhib• iting of the pigment corrosion reaction is required. This can be done through: 1
1. Chemical treatment involving hexavalent chro•
mium or organo-phosphoric compounds but this is not always recommended due to hexavalent chromium toxicity as well as diminishing paint adhesion after a humidity test;2 2. Pigment immersion in some salts or oils at cer• tain temperatures; 3. Procedure based on tannin and basic organic dye• stutT when the powder is colored in blue or violet tones but the resulting product shows a weak chemical resistance in acidic or alkaline media as well as an insufficient stability to solar radia• tion;;) 4. Chemical treatment based on B20 3 , with a good efficiency for reaction inhibition between alumi• num and water, but the resulting film shows no brightness. 4 Other relatively recent research involved the use of some corrosion inhibitors of aluminum pigments based on styrene-maleic acid copolymers with high molecular weight that lead to a protection factor of around 90 to 99%, better than others based on poly• acrylic acids. The inhibition acitivity is supposed to be determined by the polymer absorption on the surface.! It is generally well known that to improve the corrosion resistance of aluminum an anodic or chem• ical oxidation is applied to form an oxide film 5 •6 •7 •8 20
that may be subsequently colored to extend the decorative appearance. 9 •10 The formed oxide layer influences, through its structure, the adherence of subsequent coatings on the base metal. Thus, the film should be continuous and inert against the medium where it is used, with a good mechanical resistance and not favor acceleration of base metal attack. Thus, this article presents some attempts related to a new procedure of aluminum powder pigment anodizing and coloring, to enhance anticorrosive, chemical, and light stability characteristics.
....-MT.
To perform the experiments aluminum powder of minimum 99.5% purity, of maximum 100 ~, was used, having the following distribution: >100 J.Lm - 1-2% 63-100 !Lm - 55-58% 40-63 !Lm - 20-23% <40!Lm - 18-20% The powder was subjected to the process shown in Figure 1. Powder anodizing was performed in two ways, respectively: 1. 15% (wt) sulfuric acid solution, usually recom• mended as the initial stage of anodic films color• ing;4.5 2. 10 M nitric acid solution, recommended to obtain porous anodic films that may offer a better ab• sorption capacity in the coloring phase, according to our own recent research.ll.12.13 The anodizing process was conducted using con• tinuous current against an aluminum counterelec• trode at different values of applied current for vari• ous durations at room temperature. To ensure a uniform current distribution and good electrical contact, two types of anode structure were tested as follows: 1. Aluminum powder onto a titanium anode embed• ded in resin so that the current is forced to pass through the powder mass to facilitate its oxida• tion (see Fig. 2); 2. An aluminum powder anode, made by pressing Metal Rnllhlng
CHEMICAL DECREASING-PICKLING NaOH, 2·3 aIL Na,P040 25-40 gIL N.zC~
25-40 aIL
1 5
6
-
-
TcmpcndW'C. 4O-6
4
- - -- -
- ---------------
HOT WATER RINSING
Tcmpcratun:,6G-70°C
", ••••
I
."
.
.. '
.
'
••
,'"
"
'
..
'
.. '
2 3
POWDER ANODIZING 1S%H,50. 10MHNO.
----l -------! ----
[
COLD WATER RINSING
[
POWDER COLORING
-------l COLD WATER RINSING
[
Figure 2. Cell for anodic oxidetlon of aluminum powder with a titanium anode 11-cel1; Z·titanlum; 3-polymer resin; 4-powder; 5-eathode; 6-eIectroIyte).
Evaluation of the formed anodic oxide amount during the anodizing process was made by selective chemical dissolution in phospho-chromic solution (20 gIL crOs , 35 mIlL HaPO..) at the boiling temper• ature. A ratio R = moxidjmpowder was determined for the experimental conditions. Sealing in hot water for 5 to 30 minutes was applied after the coloring step.
SEALING Tempcratun:, 90_100°; time, 5-30 min. Figure 1. Aluminum powder anodizing and coloring proc... scheme.
the powder in the shape of a disk (26-mm dia. x 7 mm) as shown in Figure 3. The powder was first homogeneously mixed with 10-20% water-solu• ble ligand. An anodic contact aluminum rod is tightly introduced in the electrode mass during pressing to assure electrical contact. This new constructive shape for the aluminum powder an• ode allows its total anodic oxidation; during the process, the electrolyte assures anodic film for• mation, gradually penetrates the powder mass and dissolves the water-soluble ligand, so that the oxidized powder is released and a new quan• tity is anodized until the total disk is processed. The coloring phase involved different inorganic dyestuff solutions, based on different metallic salts. December 2000
5 1
4
2
--------
-------
3
-------Figure 3. cell for anodic oxidation of powder with a pressed disk anode (1-cell body; 2·cathode; 3-electrolyte; 4·dlsk anode of pressed aluminum powder; 5-anode powder contact rod).
21
5
-·-6A -.-SA
4
-&-10A
10
'0 Ii::
'1:» 6 D::
...
\.J. // /
3
..
8
~.
2
1
4
0
2
•
0
10
5
15
25
20
time, min.
4
2
6
8
time, min.
....
Figure 4. Dependence of oxidation ratio, A, on anodizing time for nitric acid electrolyte at different current valu...
1S_.~
Anodic Oxidation of Powder The formation efficiency ofthe anodic oxide film onto the powder depends both on the electrolyte type and construction of the anode. Figure 4 presents the dependence between the formed anodic oxide mass, expressed as oxidation ratio, R, and anodizing time, for the nitric acid electrolyte, at several applied current values in the case of the first anode structure. It may be noticed that an increase of anodizing time determines a higher oxide mass formation, up to a limiting value. This limit time is almost the same, regardless of applied current value. Thus, an anodizing period of 6 to 8 minutes may be considered optimum to anod• ize aluminum powder in nitric acid electrolyte. An increase of applied current density determines a higher anodizing rate, but an increase of operating temperature is not recommended because it facili• tates chemical dissolution of the film in the electro• lyte. A strong stirring of aluminum powder during the anodic process facilitates complete, uniform ox• idation, which determines an adequate dyestuff ab• sorption in the coloring phase. If the second anodic structure is used, i.e., when aluminum powder is pressed as disks a consider• able improvement of oxidation ratio, R, 'was noticed. Figure 5 shows dependence between R and the an• odic process duration for the powder disk anode in nitric acid electrolyte. In this case the R value is about 30 times greater than in the first case and this improvement is subsequently highlighted i~ the coloring phase when covering power of dyestuff was 22
Figure 5. Dependence of oxJdetfon retio, A, on anodizing time for powder disk anode In nitric acid electrolyte (3.5A).
100%. The optimum duration ofthe anodic process is 8 to 10 minutes. To select the optimum thickness of aluminum powder disk, several experiments were done with different values between 5 to 20 mm. For thick• nesses of the anode up to 10 mm, a total oxidation of powder was noticed with good anodic efficiency; an increase of this dimension over 10 mm resulted in nonuniform anodizing and a relatively great number of powder remaining unoxidized. Thus, an alumi• num powder disk anode of maximum 10-mm thick• ness is recommended for use to obtain a total anod• ized powder. Figure 6 presents dependence of oxidation ratio, R, as a function of anodizing duration, for a sulfuric acid electrolyte, at various current values when the first anode type is involved (powder onto titanium anode). The anodic oxide amount increases with anodizing duration up to a critical value and then remains almost constant. The optimum anodic pro-
eo 50
---.-5A
=:=~oAy ~
40 30
20
10 5
10
15 20 tlm_, min.
25
30
e.
Dependence between oxidation ratio, A, end anodiz• Figure Ing time for sulfuric acid electrolyte at different current valu... Metel Finishing
•
Titanium racks for anodizing and electropoli hing are available from a lot of places, for use in your job ilOp, captive operation or the new line you're planning. But while many rack are fabricated, only a few are really de igned. Fact i , a poorly de igned rack costs you more in lost production and reject than it' worth. At Servi-Sure, our engineer know anodizing because the design and fabrication of titanium racks...standard and cu tom...are our only bu iness. That's why, when quality matters, one name keeps rising to the top...Servi-Sure. Our catalog detail a full line of tandard product a well a our cu tom engineering service . Plea e contact Servi-Sure for your copy today.
SERVI·SURE
CORPORATION 2020 W. Rascher Ave., Chicago, IL 60625 Phone: 773-271-5900· Fax: 773271-3777 Web: www.servisure.com Email: [email protected]
Circle 036 on reader information card
5 4
c... IX:
•
•
•
golden•
gray
3 2
bronze•
gray
1 5
15
20
dark brown
5
10
15
20
time, min. Figure 8. Color tone evolution for anodized aluminum powder IUbHquently colored In IOlutlon '1.
cess duration is about 20 to 25 minutes for this kind of electrolyte. Figure 7 shows dependence of R against anodic process duration for aluminum powder disk anodes, and a considerable improvement of anodic oxide formation rate is also noticed, about 10 times greater than in the first variant. According to the obtained results it may be con• sidered that to anodize aluminum powder to form a porous anodic oxide, which is to be subsequently subjected to a coloring phase, the best formation rate is obtained for aluminum powder disk elec· trodes when a better electrical contact is assured and a total powder oxidation takes place. This fact allows a higher efficiency to the coloring phase.
Coloring of Aluminum Powder The anodized powder is then subjected to a coloring phase, using various solutions and operation param• eters. Inorganic coloring was chosen because it of· fers a better stability from anticorrosive, light, and temperature resistance viewpoints. The following coloring solutions were used: 1. Co(CH;)COO)2 - 13-100 gIL KMn0 4 - 6-50 gIL Time - 5-30 min Temperature - 30-60·C 2. K4 [Fe(CN)6) - 1.5-5 gIL FeCI;) - 1.5-5 gIL Time - 10-30 min Temperature - 30-60·C 3. CO(NOa )2 - 5-15 gIL Time - 10-30 min Thermal treatment at 300-350°C for 2 hr . Figu~e 8 .presents color tone evolution against ImmerSIOn time for oxidized powder that was col· ored in solution #1, based on cobalt acetate and 24
permanganate. When more concentrated solutions are used, darker tones are obtained up to dark• brown for 100 gIL Co(CHS COO)2 and 50 gIL KMn04' At relatively short immersion times of about 5 to 6 minutes, a yellow-golden powder was obtained that becomes bronze-golden after 10 minutes and dark brown after 20 minutes. Another aspect that should be mentioned is the influence of the anodic oxidation method on the coloring step. Thus, the use of disk anodes in initial anodizing leads to better oxide film formation that allows 2 to 3 times shorter coloring times than in the case of powder located on the titanium anode, as well as a covering power of 100%. Figure 9 shows color tones evolution versus color• ing time when solution #2, based on ferrocyanide and ferric chloride, is involved. During the coloring process a blue ferrous ferrocyanide precipitate is formed inside the oxide pores. Light shades may be obtained for short immersion times of about 10 min• utes and dark ones when this time is increased
gray
blue•
gray
IIght•
..
blue -- -dark blue-gra'Jl-_,--_ _--r----..----,,---..---, 20 10 15 5
time, min. Figure 9. Color tone evolution for anodized aluminum powder subsequently colored in solution In.
gray• greenish
•
\
\
gray• olive
olive
o
\
"" ',,-------
5
.---.
10
15
Concentration, giL Figure 10. Color ton. evolution v....u. cobliit compound con• centr8tion for enodlzed eluminum powder subsequently col• ored for 10 min.
about 20 min. In all cases covering power of the coloring solution was 100% based on the powder disk anodes. Initial anodizing in sulfuric acid electrolyte facil• itates a metallic appearance of the colored powder, relatively bright, comparatively with that performed in nitric acid, when the final aspect is matte. If the concentration of both components is relatively low, of maximum 2.5 gIL, the final color is light blue• gray, due to a small amount of precipitate incorpo• rated into oxide pores. Higher contents of around 5 gIL lead to darker tones of blue-gray. When solution #3 is used gray-olive tones are obtained. If solution concentration is increased from 5 to 15 gIL, dark shades are formed (Fig. to). After immersion, a thermal treatment is done for 2 hr at 300 to 350·C. In this case it is not necessary to seal in hot water, because chemical bonds with the exis• tent anodic oxide are formed. A covering power of 100% is noticed in this case, too.
COIICLUSIO'"'
Several coloring procedures for aluminum powder pigments were established based on an anodic oxi• dation phase and a subsequent coloring phase.
Anodizing may be performed with good results in nitric or sulfuric acid electroytes using a special construction for the powder anode that improves the formation rate of the anodic oxide and covering power of the inorganic dyestuff. Depending on the inorganic coloring solution used, a relatively large range of colors may be obtained that are attractive from a decorative point of view with a better light resistance. Inclusion of these oxidized and colored powder pigments in paints and enamels offers sev• eral advantages such as a smaller amount of pig• ments in the organic coating composition; darker tones obtained; diminishing of powdery metal reflec• tion level that leads to resin destruction, which is a main component of paint, through a mirror effect. These preliminary experiments proved the possi• bility of aluminum powder coloring even for rela• tively small sizes of metallic particles, and further studies will deal with improvement of charge trans• fer at the electrolyte/metallic particle interface to increase the current efficiency. . . . . .NCD
1. Muller, B. and T. Schmelich, Corrosion Science, 37(6):
877; 1995 2. Besold, R. et aI., Farbe Lacke, 97:311; 1991 3. Orban, N., "Pigmenti Anorganici" ("Inorganic Pig• ments"), Technical Publishing House, Bucharest, Ro• mania; 1974 4. Orban, N., European Coatings Journal, 10:790; 1994 5. Wernick, S. et aI., "The Surface Treatment and Fin• ishing of Aluminum and Its Alloys," 5th Ed., ASM International, Metals Park, Ohio; 1987 6. Thanh, P.T., Electrochim. Acta, 37:847; 1982 7. Sato, S. et aI., Electrochim. Acta, 26:1303; 1981 8. Anicai, L. and C. Trifu, Research Project No. 57/A-19, unpublished; 1996 9. Hsieh, S.A.K., Metal Finishing, 79(10):21; 1981 10. Patrie, J., Galvano-Organo, Traitements de Surface, 613:127; 1981 11. Dima, L. and L. Anicai, Rom. Patent 98,547; 1989 12. Anicai, L. et at., Materials Science Forum, 185-188: 489; 1995 13. Anicai, L. et aI., Metal Finishing, 96(12):10; 1998 MF
Electroplating Engineering Handbook by L.]. Durney 790 pages $220.00 This standard ~dbook .for electroplating en~ineers consists of two pans, as in earlier editions. The first pan provides general processing data such as metal preparation, testing, and troubleshooting. The second pan comprises engineering fundamentals and practice. The handbook is a wonhwhile edition to the finisher's library. Send Orders to:
METAL FINISHING
660 White Plains Rd., Tarrytown NY 10591-5153 For faster service, call (914) 333-2578 or FAX 'yow order to (914) 333-2570 All book orden m... be prepaid. ~ shipped esp_ '0 aU Olber COUll'.....
December 2000
iAdude 55.00 shippiaa aDd baDdJinc for delivery of each book via UPS in d", U.S.• SlO.OO lor ach book shipped "'P..... to Canada; aDd $20.00 for each book
25
Anodic Oxidation and Coloring of aluMinuM Powders by L. Anicai, C. Trifu, and L. Dima Dept. of Electrochemical Technologies and Products, Research and Development Institute for Electrical Engineering, Bucharest, Romania
T
o improve the anticorrosive characteristics of organic coatings (e.g., paints, enamels, etc.), some kinds of pigments are used; among them of special importance are aluminum powder pig• ments, mainly for the automotive industry. They significantly reduce organic solvent emission to the atmosphere during paint application. 1 A problem that is met with aluminum powder pigments is corrosion in alkaline media (pH >8), which favors hydrogen evolution according to the reaction: 2Al + 6H 2 0 -.. 2 Al
(l)
This implicitly leads to color paint degradation. Under these circumstances, diminishing or inhib• iting of the pigment corrosion reaction is required. This can be done through: 1
1. Chemical treatment involving hexavalent chro•
mium or organo-phosphoric compounds but this is not always recommended due to hexavalent chromium toxicity as well as diminishing paint adhesion after a humidity test;2 2. Pigment immersion in some salts or oils at cer• tain temperatures; 3. Procedure based on tannin and basic organic dye• stutT when the powder is colored in blue or violet tones but the resulting product shows a weak chemical resistance in acidic or alkaline media as well as an insufficient stability to solar radia• tion;;) 4. Chemical treatment based on B20 3 , with a good efficiency for reaction inhibition between alumi• num and water, but the resulting film shows no brightness. 4 Other relatively recent research involved the use of some corrosion inhibitors of aluminum pigments based on styrene-maleic acid copolymers with high molecular weight that lead to a protection factor of around 90 to 99%, better than others based on poly• acrylic acids. The inhibition acitivity is supposed to be determined by the polymer absorption on the surface.! It is generally well known that to improve the corrosion resistance of aluminum an anodic or chem• ical oxidation is applied to form an oxide film 5 •6 •7 •8 20
that may be subsequently colored to extend the decorative appearance. 9 •10 The formed oxide layer influences, through its structure, the adherence of subsequent coatings on the base metal. Thus, the film should be continuous and inert against the medium where it is used, with a good mechanical resistance and not favor acceleration of base metal attack. Thus, this article presents some attempts related to a new procedure of aluminum powder pigment anodizing and coloring, to enhance anticorrosive, chemical, and light stability characteristics.
....-MT.
To perform the experiments aluminum powder of minimum 99.5% purity, of maximum 100 ~, was used, having the following distribution: >100 J.Lm - 1-2% 63-100 !Lm - 55-58% 40-63 !Lm - 20-23% <40!Lm - 18-20% The powder was subjected to the process shown in Figure 1. Powder anodizing was performed in two ways, respectively: 1. 15% (wt) sulfuric acid solution, usually recom• mended as the initial stage of anodic films color• ing;4.5 2. 10 M nitric acid solution, recommended to obtain porous anodic films that may offer a better ab• sorption capacity in the coloring phase, according to our own recent research.ll.12.13 The anodizing process was conducted using con• tinuous current against an aluminum counterelec• trode at different values of applied current for vari• ous durations at room temperature. To ensure a uniform current distribution and good electrical contact, two types of anode structure were tested as follows: 1. Aluminum powder onto a titanium anode embed• ded in resin so that the current is forced to pass through the powder mass to facilitate its oxida• tion (see Fig. 2); 2. An aluminum powder anode, made by pressing Metal Rnllhlng
CHEMICAL DECREASING-PICKLING NaOH, 2·3 aIL Na,P040 25-40 gIL N.zC~
25-40 aIL
1 5
6
-
-
TcmpcndW'C. 4O-6
4
- - -- -
- ---------------
HOT WATER RINSING
Tcmpcratun:,6G-70°C
", ••••
I
."
.
.. '
.
'
••
,'"
"
'
..
'
.. '
2 3
POWDER ANODIZING 1S%H,50. 10MHNO.
----l -------! ----
[
COLD WATER RINSING
[
POWDER COLORING
-------l COLD WATER RINSING
[
Figure 2. Cell for anodic oxidetlon of aluminum powder with a titanium anode 11-cel1; Z·titanlum; 3-polymer resin; 4-powder; 5-eathode; 6-eIectroIyte).
Evaluation of the formed anodic oxide amount during the anodizing process was made by selective chemical dissolution in phospho-chromic solution (20 gIL crOs , 35 mIlL HaPO..) at the boiling temper• ature. A ratio R = moxidjmpowder was determined for the experimental conditions. Sealing in hot water for 5 to 30 minutes was applied after the coloring step.
SEALING Tempcratun:, 90_100°; time, 5-30 min. Figure 1. Aluminum powder anodizing and coloring proc... scheme.
the powder in the shape of a disk (26-mm dia. x 7 mm) as shown in Figure 3. The powder was first homogeneously mixed with 10-20% water-solu• ble ligand. An anodic contact aluminum rod is tightly introduced in the electrode mass during pressing to assure electrical contact. This new constructive shape for the aluminum powder an• ode allows its total anodic oxidation; during the process, the electrolyte assures anodic film for• mation, gradually penetrates the powder mass and dissolves the water-soluble ligand, so that the oxidized powder is released and a new quan• tity is anodized until the total disk is processed. The coloring phase involved different inorganic dyestuff solutions, based on different metallic salts. December 2000
5 1
4
2
--------
-------
3
-------Figure 3. cell for anodic oxidation of powder with a pressed disk anode (1-cell body; 2·cathode; 3-electrolyte; 4·dlsk anode of pressed aluminum powder; 5-anode powder contact rod).
21
5
-·-6A -.-SA
4
-&-10A
10
'0 Ii::
'1:» 6 D::
...
\.J. // /
3
..
8
~.
2
1
4
0
2
•
0
10
5
15
25
20
time, min.
4
2
6
8
time, min.
....
Figure 4. Dependence of oxidation ratio, A, on anodizing time for nitric acid electrolyte at different current valu...
1S_.~
Anodic Oxidation of Powder The formation efficiency ofthe anodic oxide film onto the powder depends both on the electrolyte type and construction of the anode. Figure 4 presents the dependence between the formed anodic oxide mass, expressed as oxidation ratio, R, and anodizing time, for the nitric acid electrolyte, at several applied current values in the case of the first anode structure. It may be noticed that an increase of anodizing time determines a higher oxide mass formation, up to a limiting value. This limit time is almost the same, regardless of applied current value. Thus, an anodizing period of 6 to 8 minutes may be considered optimum to anod• ize aluminum powder in nitric acid electrolyte. An increase of applied current density determines a higher anodizing rate, but an increase of operating temperature is not recommended because it facili• tates chemical dissolution of the film in the electro• lyte. A strong stirring of aluminum powder during the anodic process facilitates complete, uniform ox• idation, which determines an adequate dyestuff ab• sorption in the coloring phase. If the second anodic structure is used, i.e., when aluminum powder is pressed as disks a consider• able improvement of oxidation ratio, R, 'was noticed. Figure 5 shows dependence between R and the an• odic process duration for the powder disk anode in nitric acid electrolyte. In this case the R value is about 30 times greater than in the first case and this improvement is subsequently highlighted i~ the coloring phase when covering power of dyestuff was 22
Figure 5. Dependence of oxJdetfon retio, A, on anodizing time for powder disk anode In nitric acid electrolyte (3.5A).
100%. The optimum duration ofthe anodic process is 8 to 10 minutes. To select the optimum thickness of aluminum powder disk, several experiments were done with different values between 5 to 20 mm. For thick• nesses of the anode up to 10 mm, a total oxidation of powder was noticed with good anodic efficiency; an increase of this dimension over 10 mm resulted in nonuniform anodizing and a relatively great number of powder remaining unoxidized. Thus, an alumi• num powder disk anode of maximum 10-mm thick• ness is recommended for use to obtain a total anod• ized powder. Figure 6 presents dependence of oxidation ratio, R, as a function of anodizing duration, for a sulfuric acid electrolyte, at various current values when the first anode type is involved (powder onto titanium anode). The anodic oxide amount increases with anodizing duration up to a critical value and then remains almost constant. The optimum anodic pro-
eo 50
---.-5A
=:=~oAy ~
40 30
20
10 5
10
15 20 tlm_, min.
25
30
e.
Dependence between oxidation ratio, A, end anodiz• Figure Ing time for sulfuric acid electrolyte at different current valu... Metel Finishing
•
Titanium racks for anodizing and electropoli hing are available from a lot of places, for use in your job ilOp, captive operation or the new line you're planning. But while many rack are fabricated, only a few are really de igned. Fact i , a poorly de igned rack costs you more in lost production and reject than it' worth. At Servi-Sure, our engineer know anodizing because the design and fabrication of titanium racks...standard and cu tom...are our only bu iness. That's why, when quality matters, one name keeps rising to the top...Servi-Sure. Our catalog detail a full line of tandard product a well a our cu tom engineering service . Plea e contact Servi-Sure for your copy today.
SERVI·SURE
CORPORATION 2020 W. Rascher Ave., Chicago, IL 60625 Phone: 773-271-5900· Fax: 773271-3777 Web: www.servisure.com Email: [email protected]
Circle 036 on reader information card
5 4
c... IX:
•
•
•
golden•
gray
3 2
bronze•
gray
1 5
15
20
dark brown
5
10
15
20
time, min. Figure 8. Color tone evolution for anodized aluminum powder IUbHquently colored In IOlutlon '1.
cess duration is about 20 to 25 minutes for this kind of electrolyte. Figure 7 shows dependence of R against anodic process duration for aluminum powder disk anodes, and a considerable improvement of anodic oxide formation rate is also noticed, about 10 times greater than in the first variant. According to the obtained results it may be con• sidered that to anodize aluminum powder to form a porous anodic oxide, which is to be subsequently subjected to a coloring phase, the best formation rate is obtained for aluminum powder disk elec· trodes when a better electrical contact is assured and a total powder oxidation takes place. This fact allows a higher efficiency to the coloring phase.
Coloring of Aluminum Powder The anodized powder is then subjected to a coloring phase, using various solutions and operation param• eters. Inorganic coloring was chosen because it of· fers a better stability from anticorrosive, light, and temperature resistance viewpoints. The following coloring solutions were used: 1. Co(CH;)COO)2 - 13-100 gIL KMn0 4 - 6-50 gIL Time - 5-30 min Temperature - 30-60·C 2. K4 [Fe(CN)6) - 1.5-5 gIL FeCI;) - 1.5-5 gIL Time - 10-30 min Temperature - 30-60·C 3. CO(NOa )2 - 5-15 gIL Time - 10-30 min Thermal treatment at 300-350°C for 2 hr . Figu~e 8 .presents color tone evolution against ImmerSIOn time for oxidized powder that was col· ored in solution #1, based on cobalt acetate and 24
permanganate. When more concentrated solutions are used, darker tones are obtained up to dark• brown for 100 gIL Co(CHS COO)2 and 50 gIL KMn04' At relatively short immersion times of about 5 to 6 minutes, a yellow-golden powder was obtained that becomes bronze-golden after 10 minutes and dark brown after 20 minutes. Another aspect that should be mentioned is the influence of the anodic oxidation method on the coloring step. Thus, the use of disk anodes in initial anodizing leads to better oxide film formation that allows 2 to 3 times shorter coloring times than in the case of powder located on the titanium anode, as well as a covering power of 100%. Figure 9 shows color tones evolution versus color• ing time when solution #2, based on ferrocyanide and ferric chloride, is involved. During the coloring process a blue ferrous ferrocyanide precipitate is formed inside the oxide pores. Light shades may be obtained for short immersion times of about 10 min• utes and dark ones when this time is increased
gray
blue•
gray
IIght•
..
blue -- -dark blue-gra'Jl-_,--_ _--r----..----,,---..---, 20 10 15 5
time, min. Figure 9. Color tone evolution for anodized aluminum powder subsequently colored in solution In.
gray• greenish
•
\
\
gray• olive
olive
o
\
"" ',,-------
5
.---.
10
15
Concentration, giL Figure 10. Color ton. evolution v....u. cobliit compound con• centr8tion for enodlzed eluminum powder subsequently col• ored for 10 min.
about 20 min. In all cases covering power of the coloring solution was 100% based on the powder disk anodes. Initial anodizing in sulfuric acid electrolyte facil• itates a metallic appearance of the colored powder, relatively bright, comparatively with that performed in nitric acid, when the final aspect is matte. If the concentration of both components is relatively low, of maximum 2.5 gIL, the final color is light blue• gray, due to a small amount of precipitate incorpo• rated into oxide pores. Higher contents of around 5 gIL lead to darker tones of blue-gray. When solution #3 is used gray-olive tones are obtained. If solution concentration is increased from 5 to 15 gIL, dark shades are formed (Fig. to). After immersion, a thermal treatment is done for 2 hr at 300 to 350·C. In this case it is not necessary to seal in hot water, because chemical bonds with the exis• tent anodic oxide are formed. A covering power of 100% is noticed in this case, too.
COIICLUSIO'"'
Several coloring procedures for aluminum powder pigments were established based on an anodic oxi• dation phase and a subsequent coloring phase.
Anodizing may be performed with good results in nitric or sulfuric acid electroytes using a special construction for the powder anode that improves the formation rate of the anodic oxide and covering power of the inorganic dyestuff. Depending on the inorganic coloring solution used, a relatively large range of colors may be obtained that are attractive from a decorative point of view with a better light resistance. Inclusion of these oxidized and colored powder pigments in paints and enamels offers sev• eral advantages such as a smaller amount of pig• ments in the organic coating composition; darker tones obtained; diminishing of powdery metal reflec• tion level that leads to resin destruction, which is a main component of paint, through a mirror effect. These preliminary experiments proved the possi• bility of aluminum powder coloring even for rela• tively small sizes of metallic particles, and further studies will deal with improvement of charge trans• fer at the electrolyte/metallic particle interface to increase the current efficiency. . . . . .NCD
1. Muller, B. and T. Schmelich, Corrosion Science, 37(6):
877; 1995 2. Besold, R. et aI., Farbe Lacke, 97:311; 1991 3. Orban, N., "Pigmenti Anorganici" ("Inorganic Pig• ments"), Technical Publishing House, Bucharest, Ro• mania; 1974 4. Orban, N., European Coatings Journal, 10:790; 1994 5. Wernick, S. et aI., "The Surface Treatment and Fin• ishing of Aluminum and Its Alloys," 5th Ed., ASM International, Metals Park, Ohio; 1987 6. Thanh, P.T., Electrochim. Acta, 37:847; 1982 7. Sato, S. et aI., Electrochim. Acta, 26:1303; 1981 8. Anicai, L. and C. Trifu, Research Project No. 57/A-19, unpublished; 1996 9. Hsieh, S.A.K., Metal Finishing, 79(10):21; 1981 10. Patrie, J., Galvano-Organo, Traitements de Surface, 613:127; 1981 11. Dima, L. and L. Anicai, Rom. Patent 98,547; 1989 12. Anicai, L. et at., Materials Science Forum, 185-188: 489; 1995 13. Anicai, L. et aI., Metal Finishing, 96(12):10; 1998 MF
Electroplating Engineering Handbook by L.]. Durney 790 pages $220.00 This standard ~dbook .for electroplating en~ineers consists of two pans, as in earlier editions. The first pan provides general processing data such as metal preparation, testing, and troubleshooting. The second pan comprises engineering fundamentals and practice. The handbook is a wonhwhile edition to the finisher's library. Send Orders to:
METAL FINISHING
660 White Plains Rd., Tarrytown NY 10591-5153 For faster service, call (914) 333-2578 or FAX 'yow order to (914) 333-2570 All book orden m... be prepaid. ~ shipped esp_ '0 aU Olber COUll'.....
December 2000
iAdude 55.00 shippiaa aDd baDdJinc for delivery of each book via UPS in d", U.S.• SlO.OO lor ach book shipped "'P..... to Canada; aDd $20.00 for each book
25