Theoretical and Practical Aspects of Alloy Plating - The 16th William Blum Lecture - Part 1
This article is the first of three parts of a re-publication of the 16th William Blum Lecture, presented at the 62nd AES Annual Convention in Toronto, Ontario, Canada on June 23, 1975. Dr. Ernst Raub presents a comprehensive treatise on alloy plating.
Forschungsinstitut für Edelmetalle und Metallchemie
Recipient of the 1974 William Blum
AES Scientific Achievement Award
Originally published as Plating & Surface Finishing, 63 (2), 29-37 (1976), this article is the first of three parts of a re-publication of the 16th William Blum Lecture, presented at the 62nd AES Annual Convention in Toronto, Ontario, Canada, on June 23, 1975. A printable PDF version of Part 1 is available by clicking HERE. The printable PDF version of the complete 44-page paper is available HERE
The enormous literature on alloy plating is mostly restricted to alloys in the strict sense which contain metallic components with at least some per cent of a second metal. For practical purposes, codeposition of a second metal and also of non-metallic elements, e.g., sulfur or selenium, in small amounts is often interesting. Hydrogen as an alloying element has been known for a long time, but only incompletely investigated. The same is true for the activity of hydrogen ions in the electrolyte. The influence of electrodepositing conditions, e.g., of specific inhibition, cathodic adsorption and incorporation and of the effects of complexing agents present, is discussed. Properties of electrolytic deposits, their changes by heat treatment, and alloy formation between deposit and underlying metal or between two or more electrolytic layers are mentioned.
Introduction and background
William Blum and George B. Hogaboom1 wrote in 1949, in the third Edition of their book Principles of Electroplating and Electroforming, on alloy deposition, "In a strict sense every plating bath of a single metal is an alloy bath, since both metal and hydrogen may be discharged and the hydrogen may dissolve in or form compounds with the metal deposited."
In the same publication the authors mentioned the existence of an enormous amount of literature on alloy deposits. Abner Brenner2 gives an extensive description on the Electrodeposition of Alloys, which comprises two volumes with 1370 pages. Therefore it is a hazardous enterprise to talk about alloy deposition in a short paper. And yet I will try to do so because the electrodeposition of alloys holds problems of general interest for electroplating, which even today are only partially cleared. Furthermore the practical importance of alloy plating has grown during recent years, especially for various new applications in modern industry.
For a long time brass and gold-alloy deposits have been employed as decorative deposits, but for some decades brass plating has also been used for technical purposes to improve bonding of rubber to steel or to enhance the deep drawing properties of steel sheet. Gold-alloy deposits are used in the modern electronic industry as tarnish resistant surface layers with low contact-resistance. Tin-lead deposits are employed to guarantee good solderability of printed circuits in electronics. For the same purpose nickel-tin deposits are applied.
They also find rising interest in other industries. Tin-lead and to a certain degree tin-indium deposits have been used for many years as linings for friction bearings. At present electrodeposited magnetic alloys, which contain as main components iron and nickel or cobalt with different additional elements, serve as memory elements in the electronics industry.3 Electrolytic alloy plates have been used for many years as wear resistant materials. Nickel-cobalt alloys serve for the production of tools by electroforming. Other metal deposits with small amounts of alloying metals are used for bright and / or hard deposits. Some so called hard-bright silver layers contain antimony. Gold alloys with gold contents as low as possible while still yielding sufficient chemical resistance, are not only deposited in all colors for decorative purposes but also for industrial use to minimize the use of the expensive gold. For the same reason the deposition of palladium alloys, such as palladium-nickel and palladium-copper, find rising interest.
Generally in practice deposits with small amounts of a second metal or of non-metallic elements are no longer considered to be alloys, but they often have such significant properties that it is impossible to neglect them.
It may be mentioned that electrolytically produced compound materials, to which dispersion hardened deposits also belong, are only mixtures of metals and non-metallic substances with quite different properties, and therefore will not be discussed here.
Of special relevance to alloy deposits in the strict sense is the molecular inclusion of non-metallic, organic or inorganic substances. Not only is their influence on the properties of the deposited metal similar to that of a codeposited second metal, but quite often alloy deposition is connected with molecular inclusion. Therefore it is necessary to discuss it with alloy deposition.
The formation of alloys by diffusion between deposit and basis metal or double and triple layers is of practical interest. Occasionally an annealing of alloy deposits is necessary to promote diffusion and to achieve certain properties. Another class of alloy deposits, formed by autocatalytic chemical reduction without an external current source, will not be discussed in this paper.
Since up to now electrolytic deposition of alloys has been carried on for the most part in aqueous solutions, this report will be restricted to the cathodic reactions in aqueous solutions, and the properties of the deposits dependent upon the deposition conditions, including the influence of subsequent heat treatment.
During the passage of current the cathode reacts with all the substances contained in the electrolyte. Besides cathodic reduction, chemical reactions in the electrolyte and physical adsorption on the cathode surface are of importance. Often the adsorbed substances do not exist in the original electrolyte. They are formed by cathodic reduction and subsequent chemical reactions. Cathodic adsorption and inhibited crystallization were investigated by H. Fischer.4-7 For electrodeposition of metals and alloys partial coating by physical adsorption is most important. Chemisorption or chemical reactions with the cathode surface may cause surface layers which exhibit an unfavorable influence.
The cathodic reduction reactions not only include metals and hydrogen, but also involve non-metallic elements which are present as oxides, or as sulfur, selenium or phosphorous compounds. They may be cathodically reduced to negatively charged ions, such as S+4 to S-2 ions. Since the solubility of heavy metal sulfides is small, cathodic adsorption and inclusion takes place readily. Sulfonic acids and other organic sulfur compounds used as addition agents also undergo cathodic reduction and sulfide formation. Adsorption and inclusion of non-metallics as elements or insoluble compounds, e.g., heavy metal sulfides, do not necessarily presume a cathodic reduction of oxidic compounds. They take place too when soluble compounds present as addition agents chemically decompose in the electrolyte, e.g., by hydrolysis, during formation of elements or negative ions.
Cathodic reduction is not limited to simple cations, but also extends to neutral molecules and anions.
Metals which exist in several valences may be reduced under proper conditions by cathodic reduction to a lower valence without metal deposition. When the bath contains a component which forms an insoluble compound with a metal ion of lower valency, this may be of decisive influence on the deposition of alloys.
For the discharge of ions, their migration in the electric field is not critical as is the transport of the dischargeable mass particles to the cathode by diffusion and convection. At sufficiently high concentrations of reducible mass particles in the cathodic diffusion layer, their reduction potentials are the controlling factor for the reactions actually taking place on the cathode surface. Concerning the discharge of metal ions one has to keep in mind that in most practical cases electrolytes are used which contain the metals to be discharged, as complex ions, quite often as anions. Normally in these electrolytes the discharge step does not result from the predominant complex, but from a lower coordinated complex, or from a totally different compound. Our knowledge of the mechanism of discharge from complex cyano ions is due largely to the work of H. Gerischer.8,9 The discharge of zinc in cyanide electrolytes proceeds via Zn(OH)2 and not via the Zn(CN)4-2 complex predominant in the electrolyte. In the cyanide cadmium bath the discharge of the cadmium goes via Cd(CN)3ˉ or Cd(CN)2, dependent on the cyanide concentration of the bath as the discharge controlling step. In fact, especially in cyanide electrolytes mass particles which exist only in small amounts control the cathodic discharge. They are formed by chemical reactions of the predominant complexes. So it is possible to influence the discharge potential of a metal in an extremely wide range by changing the concentration of complexing agents in the electrolyte. The stability of the predominant complex ion in the cyanide solutions of many metals can be changed to a great extent, but differing for every metal, by merely varying the cyanide concentration of the electrolyte. The stability of the predominant complex controls the formation of the lower coordinated dischargeable complex, and thus the discharge potential of the metals. It is possible to achieve similar discharge potentials of metals, or to reverse potentials, by altering the concentration of the complexing agent.10 In cyanide brass-plating, the zinc content of the deposits increases with increasing cyanide content of the electrolyte because the stability of the copper cyanide complex Cu(CN)3-2 or even Cu(CN)4-3 is much stronger at higher cyanide concentration. The discharge of copper in the cyanide bath is controlled by the Cu(CN)2ˉ complex. Its formation depends on the stability of the predominant higher coordinated complex.
Addition of sodium hydroxide to the electrolyte favors codeposition of zinc because it shifts the equilibrium,8
Zn(CN)4-2 + 4OHˉ ⇔ Zn(OH)4-2 + 4CNˉ
to the right, and facilitates the formation of the dischargeable Zn(OH)2. Many other examples of the influence of the complexing agent concentration on alloy plating can be given, especially when several complexing agents are present.
The total quantity of the complexing agents and their ratio to the metal in the electrolyte may be much more decisive for the composition of the deposited alloys than the concentration ratio of the metals to be deposited. This is valid as long the limiting current density of one of the metals is not exceeded.
Furthermore, due to the formation of complexes with low solubility, the discharge of the more noble metal may be inhibited by cathodic adsorption, so that the codeposition of the less noble metal is facilitated.
During electrodeposition a steady state occurs at the cathode, which is controlled by several factors, e.g., the ratio of the dischargeable or reducible mass particles in the cathodic double layer, their average concentration in the electrolyte, the convection and diffusion processes, the number of mass particles reduced per unit of time, and the replacement of substances included during electrodeposition.
As a rule of thumb it may be stated that under normal conditions the discharge of the most noble metal is favored, causing deposits to be richer in this metal than could be expected from its relative concentration in the electrolyte. All changes of electrolysis conditions which increase polarization, such as rising cathodic current density, decreasing temperature, unstirred electrolyte, do shift the composition of the deposit in direction to the less noble metal. All changes of the depositing conditions which diminish the polarization, such as decreasing current density, rising temperature and stirred electrolyte, increase the content of the more noble metal in the alloy. This rule is only applicable as long as the limiting current density is not surpassed. Above the limiting current density the cathodic discharge is diffusion-controlled since the concentration of dischargeable particles at the cathode surface is zero. As soon as the limiting current density for the deposition of one metal is obtained any further increase of the current density has no additional influence on the electrodeposition of this metal. If the current density is higher than the limiting current density for both metals then the ratio of the metals in the deposit will not change with increasing current density. Any current rise will only produce a third cathodic reduction reaction, e.g., hydrogen evolution.
Molecular inclusion of chemical compounds
The influence of the inclusion of non-metallic substances has been investigated long ago, and has been thoroughly discussed by A. Brenner.2 In spite of this, I want to mention the main points. The molecular inclusion of non-metallic substances may cause changes of properties which are more pronounced than those resulting from the codeposition of an alloying metal. Indeed, by inclusion the real properties of the alloy may be masked completely.
Such changes of properties are only proportional to the included substances in the lower range of inclusion. At high inclusion levels, no clear relationship upon the quantity of the included substance exists.
By molecular inclusion the hardness of copper can reach about 300 kp/mm2, of silver nearly 200 kp/mm2 and of iron up to 800 kp/mm2.
Hard iron layers attain the hardness of chromium deposits, and their wear resistance equals that of chromium.
Table 1 shows the abrasion resistance of deposits with different hardnesses.