Popov K.I., Djokic S.S., Grgur B.N. Fundamental Aspects of Electrometallurgy
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242
Chapter 9
Inorganic coloring techniques are based on immersion of anodized
aluminum into solutions containing ions, which will produce a color. For
example, anodized aluminum is successively immersed into solution
containing cobalt acetate and then a solution of potassium carbonate, at 30 to
50 °C, for blue color. A successive immersion of anodized aluminum, first in
a solution of lead nitrate and then in a solution of potassium chromate, at 40
to 50 °C, is carried out to produce yellow color. A green color is produced by
an immersion of anodized aluminum in a solution containing copper sulfate
and then a solution of ammonium sulfide. Mechanistic aspects of coloring
are not well understood, but results suggest this process is based on
absorption and diffusion of anions and cations into pores.
Among organic compounds dyestuff such as Alizarin Red S, Chrome
Fast Orange R, Aluminum Black MLW, Aluminum Turqoise PLWS etc., are
used for coloring.
Electrolytic coloring is carried out from Ni(II), Cu(II) or Sn(II)
electrolytes. Under the conditions of direct current (d.c.), anodized
aluminum acts as the cathode. However, since d.c. processes are sensitive to
contaminants present in the electrolyte, electrolytic coloring is frequently
carried out under alternating current conditions. The counter electrode
materials include graphite, nickel, stainless steal and lead.
As mentioned earlier, the oxide films on anodized aluminum are porous
and they have very good adsorption properties. In order to improve the
protection properties films are treated with boiling water, chromate, Ni(II)
solutions etc., which causes a further hydration of oxide and sealing of pores.
Sealing is a process in which major pore blockage and an increase in
water content in the anodic films occurs. This preventive method is
practically very important for the improvement of properties of anodized
films.
In the sealing with non-aqueous sealants, organic substances provide a
physical blockage of the coating pores. For this process lubricating oil,
silicone polymers, electrophoretic coatings with pigmented paint etc. are
used.
75
Sealing based on aqueous solutions is more widely used in commercial
applications. In this process, after a careful water-rinse, oxidized parts are
treated in saturated steam at atmospheric pressure or in hot water, nickel
acetate, chromate and various cold sealing solutions.
While “as produced” oxidized aluminum contains about 0.5 %
during sealing the amount of water increases rapidly in the initial stages of
the process, and later much more slowly. The amount of water in the sealed
coating is estimated at approximately 8 to 13 %.
In the sealing process, the porous structure of the anodic films becomes
less distinct and pores are blocked starting at outer surface. The aluminum
9.
Electroplating and Surface Finishing
243
oxide is transformed into various forms of hydrated oxides. The most
important form of these hydrated oxides is böhmite.
Sealing methods used in practice are divided into following groups:
i)
ii)
iii)
water sealing,
steam sealing and
sealing in metal-salt solution.
Sealing in water depends on temperature (usually the working
temperature is 96 to 100 °C), presence of various ions
etc.) and pH (between 5.5 and 6.5).
76
The time of sealing is about 2 to 3
minutes per of coating thickness.
The steam sealing presents considerable engineering difficulties, and
there is no evidence for superiority of this process over hot water sealing. As
mentioned above, the process is carried out with saturated steam and at
atmospheric pressure, although some plants use very large chambers, which
are hermetically sealed and the work is steamed under pressure.
77
Sealing in metal-salt solutions is based on nickel acetate or potassium
dichromate formulations. These processes are carried out at elevated
temperatures (e.g. sealing in the nickel acetate solution at 75 - 80 °C, and
sealing in the chromate solutions between 94 and 98 °C).
There are other sealing systems that were developed in order to reduce
the higher costs associated with hot water, or environmental concerns related
to chromate solutions. Among other systems cold sealing in nickel fluoride
solutions, sealing in sodium silicate solutions, or use of ammonia vapor
under pressure, should be mentioned.
78
9.9 FURTHER READINGS
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2.
3.
4.
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6.
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8.
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Celis J., Roos J. R.. Kinetics of the Deposition of Alumina Particles from Copper Sulfate
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Nowak P., Socha R.P., Kaisheva M., Fransaer J., Celis J. P., Stoinov Z., Electrochemical
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Musiani M., Electrodeposition of Composites: An Expanding Subject in Electrochemical
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Hydrogen at Polypyrrole Film Electrodes Containing Nanodispersed Platinum Particles.
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Alonso-Vante N., Cattarin C., Musiani M. Electrocatalysis of Reduction at
Polyaniline + Molybdenum-Doped Ruthenium Selenide Composite Electrodes. J.
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Beck F., Dahlhaus F., Zahedi N. Anodic Codeposition of Polypyrrole and Dispersed
Electrochim. Acta. 1992; 37:1265.
Beck F., Dahlhaus M. Anodic Formation of Polypyrrole/Tungsten Trioxide Composites.
J. Appl. Electrochem. 1993; 23:781.
Li H.S., Josowicz M., Baer D.R., Engelhard M.H., Janata J. Preparation and
Characterization of Polyaniline – Palladium Composite Films, J. Electrochem. Soc.
1995; 142:798-805.
Hapel M. The Electrolytic Oxidation of Methanol at Finely Dispersed Platinum
Nanoparticles in Polypyrrole Films. J. Electrochem. Soc, 1998; 145:124-134.
Musiani M. Anodic Deposition of Composites and Their Use as Electrodes
for Oxygen Evolution reaction. J. Chem. Soc. Commun. 1996; 21:2403-2404.
Cattarin S., Frateur I., Guerriero P., Musiani M. Electrodeposition of
Composites by Simultaneous Oxidation of and and Their Use as Anodes for
Oxygen Evolution. Electrochim. Acta. 2000; 45:2279-2288.
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Musiani M., Guerriero P. Electrodeposited Composites I: Effect of the
Dispersed Phase on Nucleation and Growth of the Matrix. J. Electrochem. Soc. 1998;
145:549-554.
Musiani M.M., Furlanetto F., Guerriero P. Electrodeposited Composites
II Electrocatalysis of Oxygen Evolution reaction on Composites. J.
Electrochem. Soc. 1998; 145:555-560.
Kulesza P.S., Grzybowska B., Malik M.A., Galkowski M.T. Tungsten Oxides as Active
Supports for Highly Dispersed Platinum Microcenters: Electrocatalytic Reactivity
Toward Reduction of Hydrogen Peroxide and Oxygen. J. Electrochem. Soc. 1997;
144:1911-1917.
Spencer L.F.. Modern Electroforming. I Requirements and Mandrels. Metal Finishing.
1973; February:64-72.
Wearmouth W.R. Applications and Developments in Nickel Electroforming and
Toolmaking. Metal Finishing. 1980; November:35 – 39.
Mehdizadeh S., Dukovic J., Andricacos P.C., Romankiw L.T., Cheh H. Y. The Influence
of Lithographic Patterning on Current Distribution in Electrodeposition: Experimental
Study and Mass-Transfer Effects. J. Electrochem. Soc. 1993; 140:3497-3505.
Romankiw L.T. Electroforming of Electronic Devices. Plat. And Surf. Finish. 1997; 84
(1):10-15.
Brenner A. “Electrolysis of Nonaqueous Systems.” In Advances in Electrochemistry and
Electrochemical Engineering. C.W. Tobias, ed. Vol. 5: pp.205-248. New York:
Interscience Publishers, 1967
Popovych O., Tomkins R.P.T., Nonaqueous Solution Chemistry, New York: John Wiley
& Sons, 1981.
Lowenheim F. A., Modern Electroplating, Third Edition, New York: John Wiley &
Sons, 1974.
Capuano G.A., Davenport W.G. Electrodeposition of Aluminum from Alkylbenzebe
Electrolytes. J. Electrochem. Soc. 1971; 118:1688 – 1695.
Capuano G.A., Davenport W.G. Cathodic Polarization of Aluminum in Alkylbenzene
Electrolytes. J. Electrochem. Soc. 1984; 131:2595 – 2600.
Biallazor S. Lisowska-Oleksiak A. The Modification of Aromatic Electrolytes for
Electrodeposition of Aluminum. J. Appl. Electrochem. 1990; 20:590-595.
Mc Chesney M. Electrodeposited Aluminum. Plating and Surface Finishing. 1995; 82
(10):42.
Safranik W.H., The Properties of Elctrodeposited Metals and Alloys, Second Edition,
Orlando, FL: American Electroplaters and Surface Finishers Society, 1986.
Wilkes J.S., Levisky J.A., Wilson R.A., Hussey C.L. Dialkylimidazolium
Chloroaluminate Melts: A New Class of Room temperature Ionic Liquids for
Electrochemistry, Spectroscopy and Synthesis. Inorg. Chem. 1982; 21:1263-1265.
Lin Y.F., Sun I.W. Electrodeposition of Zinc from a Mixture of ZincChhloride and
Neutral Aluminum-Chloride-1-Methyl-3-Ethylimidazolium Chloride Molten Salt. J.
Electrochem. Soc. 1999; 146:1054-1059.
Moffat T.P. Electrodeposition of Al-Cr Metallic Glasses. J. Electrochem. Soc. 1994;
141:L115-L117.
Mitchell J.A., Pitner W.R., Hussey C.L.. Stafford G.R. Electrodeposition of Cobalt and
Cobalt-Aluminum Alloys from a Room temperature Chloroaluminate Molten Salt. J.
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Pitner W.R., Hussey C.L. Electrodeposition of Zinc from the Lewis Acid Aluminum
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Molten Salt”, J. Electrochem. Soc. 2000; 147:3350 – 3355.
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Chapter 10
METAL DEPOSITION WITHOUT AN EXTERNAL
CURRENT
Deposition of metals and alloys without an external current source is very
important in modern technology, especially in the production of new
materials for applications in electronics, wear and corrosion resistant
materials, medical devices, battery technologies, etc.
These processes supplement and in some cases replace electrodeposition
for several practical reasons. The solutions for deposition of metals without an
external current source have excellent throwing power and allow plating on
articles of very complex shapes and plating through holes. Deposits are denser
and exhibit better properties for corrosion and electronics applications. Other
important advantages of this type of deposition over electrodeposition include
applicability for metallization of non-conductive surfaces (glass, ceramics,
polymers, etc.) and the ability to selectively deposit thin metal films only on
catalyzed areas of the substrate. Finally, for this type of deposition, an external
current source is not needed.
It seems that all metals electrochemically depositable from aqueous
solutions can also be deposited chemically under proper conditions (bath
composition, pH, temperature, and corresponding catalytic surface), using
suitable reducing agents.
Table 10.1 presents a survey of metals and alloys that have been
deposited without an external current source hitherto. In the first group are
listed commonly deposited single metals such as Ni, Co, Cu, Ag, Au and Pd.
Other metals from this group do not have significant applications at the
present, but it should be noted that there are reports on their deposition in the
published literature.
The second group lists elements, which cannot be deposited alone.
However, they can easily be codeposited with nickel or cobalt. Typical
examples are Mo and W. The phenomenon is somehow analogous to
induced electrodeposition as Brenner defined it.
249
250
Chapter 10
The third group represents alloys based on the first and/or second group
of elements. These alloys have been deposited for various applications,
mainly in the electronics industry. There is a high probability that other
alloys, which are commonly electrodeposited, can also be electrolessly
deposited, but there is no published data so far.
10.1 BASIC DEFINITIONS
Brenner and Riddell were the first authors to introduce the term
electroless metal deposition when describing an autocatalytic process of
depositing a metal in the absence of an external source of electrical current.
Since there are other metal depositions from aqueous solutions that are
carried out without an external current, this process can be divided into three
main groups:
1.
2.
3.
displacement deposition
contact deposition
autocatalytic deposition
10.1.1 Displacement Deposition
Displacement deposition is a heterogenous galvanic process in which the
noble metal ions are reduced and deposited at the surface of an active metal,
as a consequence of dissolution of that metal. The process is sometimes
called immersion plating, although this term is not a specific description, and
therefore should be avoided, or cementation. The overall displacement
reaction is quite simple
1
:
and involves the displacement half-reaction of a more active metal
10. Metal Deposition without an External Current
251
by a more noble metal,
Typical cementation systems in practice are Ag/Zn, Ag/Cu, Cu/Zn, Cu/Fe,
Cu/Al, Sn/Cu etc. The displacement reaction stops immediately after the
reduced metal (more positive metal) covers the surface of the immersed
metal (more negative metal). Accordingly, the thickness of the deposited
metal is always limited. The time of immersion is particularly critical for
achieving a uniform coating layer. Very often, the adhesion of the deposited
films is not as good as that of films prepared by electrodeposition or by
autocatalytic deposition. The displacement deposition differs from all other
plating processes from aqueous solutions without an external source of
electrical current, because it does not require a reducing agent. Because of
lower quality and thinner coatings, displacement deposition has found
applications mainly in the refining metals. To a certain extent, however,
there are other applications such as coatings for porcelain enamelling,
zincate coatings, decorative finishing, soldering, purification of electrolytes
before electrowinning, environmental purposes, etc..
2
10.1.2 Contact Deposition
Contact deposition is equivalent to electrochemical deposition with the
exception that the current is derived from the chemical reaction and not from
an outside source. The metal on which deposition takes place, and the
auxiliary metal with which it is in contact, form a galvanic element. In this
galvanic element, the auxiliary metal acts as an anode and dissolves; the
other metal is a cathode. Consequently, the dissolved metal is deposited on
the cathode (metal on which deposition takes place) at a mixed potential.
The importance of contact deposition for industrial applications is
relatively small. Sometimes the process is used with autocatalytic Ni to
initiate Ni deposition on copper and its alloys.
2
This is achieved by coupling
the Cu or Cu-alloys with Al, Fe or Ni. The contact deposition is applicable
only to a limited extent, and uniform thicker deposits cannot be obtained. On
the other hand, the constant increase of dissolved metal concentration in the
solution may cause instability of the solution.
10.1.2 Autocatalytic Deposition
Autocatalytic deposition is the most commonly used chemical method for
the deposition of metallic films from aqueous solution without an external
source of electrical current. The metal films are formed only on catalytically