T H E ELECTROCHEMISTRY OF NICKEL W. A. WESLEY Research Laboratory, International Nickel Co., Inc,, Bayonne, T h e present knowledge, both scientific and practical, of the electrochemical reactions of nickel as they And application in industry today is reviewed. Most industrial electrolytes for the electrodeposition of nickel are based on a nickel sulfate-nickel chlorideboric acid solution which is modified to produce nickel deposits of different mechanical properties, metallographic structure, or surface brightness. Evidence is cited to show that it is possible to electrodeposit sound nickel metal at speeds much higher than have yet been employed commercially. industrial processes discussed include electrorefining, electroforming, electroplating, resizing of mismachined parts, and electropolishing.
chloride. The activity coefficients for aqueous solutions of these salts have been determined and are presented in Table I. The variation of activity coefficient with concentration is similar to that of the sulfates and chlorides of other bivalent heavy metals, with the chloride showing very high activities in concentrated solutions.
Table
95"
I
+
+ 2e = Ni
(2)
In the year 1949, the total amount of nickel won from its ores by the International Nickel Co. of Canada, Ltd., was 209,000,000 pounds. More than half this nickel was refined by undergoing the above reactions. I n addition, these reactions were carried out in a multitude of plants and shops in America and Europe where electrodeposition is practiced and where about 40,000,000 pounds of nickel were deposited during the year. Reaction 1is of industrial significance on the added count that it represents the anodic half of the corrosion reactions of nickel. The subject of corrosion is reserved for another paper in this symposium. The electrochemistry involved in the manufacture and operation of nickel alkaline storage batteries is very interesting, but it too is treated in a separate paper. A reaction of minor importance is the electrochemical displacement of nickel ions by iron, which finds use in pretreating steel shapes to secure good adhesion of vitreous enamels. This is considered under the heading of nickel immersion coatings. FUNDAMENTAL DATA
There is definite proof of the existence of nickel atoms in higher valence states in solid compounds, but only bivalent nickel ions are known in aqueous media. It is very difficult or perhaps impossible t o achieve true equilibrium between massive nickel and the ions. A survey of the literature indicates that the standard electrode potential of nickel can be represented as follows: Ni
Ni++
+ 2e, E" = -0.24
f 0.01 volt a t 25" C.
This corresponds to a free energy of formation of the nickel ion of about - 11,000 calories. Nickel is, therefore, to be classed among the moderately active elements less noble than copper and lead but more noble than iron and ainc. In most environments in which nickel is used as a material of construction it is more or less passive and behaves electrochemically like a more noble metal than its standard potential would indicate. Activity Coefficients of Salts. The most important salts of nickel in its electrochemical applications are the sulfate and May 1952
I. Mean Activity Coefficients in Aqueous Solution at Molality 0.1 0.3 0.5 0.7 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.8
F T H E importance of a chemical reaction can be measured by the tonnage of product made by it, it can be said that the following reactions are most important in the electrochemistry of nickel: Ni = N i + + 2e (1) Ni++
N. 1.
c.
Nickel Sulfate (IS) (0.150) 0.0841 0.0628 0.0516 0.0426 0.0360 0.0343 0.0357
... ... ...
... ...
...
Nickel Chloride (24) 0.522 0.463 0.464 0.482 0.536 0.684 0.906 1.236 1.692 2.26 2.96 3.76 4.69 6.43
Other salts of electrochemical interest are the acetate, formate, borate, fluoborate, sulfamate, phosphate, and a wide variety of nickel salts of sulfonate aryl and alkyl compounds, for none of which are thermodynamic activity data available. Diffusion CoeBcients. Diffusion coefficients of ions and salts are important factors in relation to limiting current densities and polarographic analysis. Here again available data are meager. The coefficients for nickel chloride and sulfate have been measured by one worker a t only one temperature, as shown in Table 11,but are not yet available for other salts.
Table
II. Diffusion Coefficients in Aqueous Solution at PO" C. (8) Normality, N 0.1 0.25 0.5 1.0 2.0 3.0 4.0
D,Sq. Cm./Seo. X 106 NiSOa 0.555 0.516 0.503 0.460 0.410 0.370
...
NiClr 0.920 0.911 0.891 0.887 0.888 0:894
The diffusion coefficient of the nickel ion a t infinite dilution a t 25" C. is 0.69 X 10-6 sq. cm. set.-' as calculated from its equivalent conductance of 52 ohm-' cm.2 (7). Specific Conductance and Transference Numbers. The literature includes a goodly mass of data on conductances of solutions of nickel salts. Solutions of the chloride, perchlorate, nitrate, and fluoborate have relatively high conductances, those of the sulfate, sulfamate, and acetate have intermediate values, while phosphate solutions are of low conductivity, But few determinations of the transference number of nickel ion in these salts have yet been made. Zitek and McDonald (10) found the value for nickel ion in 0.1 N nickel sulfate to be 0.366 a t 40" C.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
957
NICKEL-META L, OXIDE deposits. The ingredients of a typical modern Watts bath are given below. Where other reagents are added in commercial electrolytes, this is done for very specific purposes discussed later. Solution Composition (Grams per Liter) Nickel sulfate, NiSOa.7HgO Siokel chloride. KiC12.6H20 Boric acid, HaBOa
300 60 38
Common Operating Conditions Temperature,
PH
O
C.
Current density. amperes per sq. foot Rate of deposition, inches per hour
Figure 1.
Microstructure of Watts Nickel Deposit
Magnification Tensile strength Elongation in 2 inches Hardness (Vickers)
500X 51,000 Ib./sq. inch 30% 150
ELECTRODEPOSITION OF NICKEL
The electrodeposition of nickel, Reaction 2, is a principal step in the application of nickel in various industrial processes called electroplating, electrorefining, electroforming, and electrosalvaging, and in the preparation of electrotypes and stereotypes The electrical circuit involved is simple: The positive pole of a source of low voltage direct current is connected to a set of anodes immersed in a solution of nickel salts in a plating tank while the negative pole is connected to the cathode of the plating cell, the cathode being the object on whose surface a nickel deposit is desired. By suitable regulation of the current flow through the cell, nickel is deposited by Reaction 2 on the cathode at a rate which can be very accurately controlled. The industrial importance of nickel deposition is based upon these characteristics of the process: In providing a surface finish of pleasing appearance and reasonable permanence on articles of manufacture, electrodeposited nickel offers a unique combination of corrosion resistance, good physical properties, ease of deposition, and compatibility with a chromium surface layer, together with a reasonable cost. The fact that electrodeposited nickel can be made free of nonmetallic inclusions makes possible the development of a mirror finish by polishing and buffing more nearly flawless than can be produced commercially on wrought metals. Sound nickel metal can be deposited where it is wanted, in a layer of controlled thickness from a few millionths to 0.5 inch, yet IXithout exposing the object being treated to temperatures in excess of 60” C.; in other words, this is a method of forming metal which might be called “cold casting.” By suitable choice of electrolyte composition and plating conditions, the structure of nickel deposits can be varied from that of coarse-grained soft metal to that of very hard, bright metal of submicroscopic grain size. A correspondingly wide choice of physical properties is afforded, ranging from a tensile strength of 50,000 pounds per square inch with 30% elongation and a Vickers hardness number of 140 to a tensile strength of 150,000 pounds per square inch a t a hardness level of 450. The corrosion resistance of pure nickel is obtained in heavy deposits for use in lining process equipment. Electrolytes. T o understand modern nickel plating it is necessary to become familiar with the Watts bath and the effects of all the controllable variables, such as temperature, current density, pH, and agitation, upon the properties of the
958
50 t o 60 2 to 4 . 5 30 t o 75 0 0015 t o 0 . 0 0 3 3
Each ingredient of the electrolyte has a definite function. Nickel sulfate is used to provide the major part of the nickol ion content because it is the least expensive salt of nickel with a stable anion which is not oxidized a t the anode, reduced a t the cathode, or volatilized. The chloride is introduced chiefly to improve anode behavior by reducing polarization, but high chloride contents also permit higher rates of deposition, tend to form smoobher, finer-grained deposits, and increase uniformity of metal distribution on the cathode. While sodium and potassium chlorides are used in nickel plating baths abroad, the use of nickel chloride is slowly becoming universal. This trend results in simpler composition and greater ease of control. In the absence of a buffer nickel deposits are prone to be hard, cracked, and pitted. Reaction 2 is always accompanied by hydrogen ion discharge which tends to make the pH of the cathode film rise. Boric acid is the preferred buffering agent in general use because it is obtainable in a very pure inexpensive form and is st’able.
COURTESY ALUMINUM CO. OF A M E R I C A , PITTSBURGH, P A .
Figure I. Household Utensils with Bright Nickel-Chromium Coating on Aluminum Base
Certain impurities may have a pronounced deleterious effect upon the quality of nickel deposits and rigid control of these is a principal concern of the operator. Tolerance limits vary somewhat with the nature of the bath and operating conditions chosen, but an indication of their magnitude can be had from typical limits for the Watts bath of 150 p.p.m. of iron, 40 p.p.m. of copper, 50 p.p.m. of zinc, 2 p.p.m. of lead, and 10 p.p.m. of hexavalent chromium. Organic impurities can be troublesome as well and are commonly removed by adsorption on activated carbon. A tendency for development of pits in the depositing
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 44, No. 5
NICKEL-METAL, OXIDEnickel, due to continued formation of hydrogen bubbles at active points on the cathode surface, is counteracted by maintaining a small concentration of hydrogen peroxide in the bath or by adding a surface active agent to reduce the surface tension. The microstructure of a typical soft Watts nickel deposit is shown in Figure 1. The grain size is large, the structure is columnar, and the metal is relatively soft and ductile. Bright Nickel. The development of tarnish-resistant chromium coatings led to the modern tremendous popularity of mirror-bright metal finishes. Because of the extreme hardness, brittleness, and internal stress in heavy chromium deposits it is customary in producing decorative coatings to provide nickel or nickel and copper layers on the base metal for corrosion resistance and then apply a thin chromium film (only 0.00001 to 0.00002 inch thick) to prevent tarnishing of the nickel. Experience and research have shown that nickel is the most suitable metal for this undercoat. Parts intended for severe outdoor service require 0.001 to 0.003 inch of nickel if the base is steel or zinc or 0.0005 inch on brass. Such layers of nickel if deposited from a Watts bath are dull gray in appearance and must be polished and buffed before chromium plating to secure a bright finish. These mechanical operations became so costly that they accounted for a major fraction of the cost of fabricating many articles. The invention of bright and semibright nickel plating processes has reduced and in some cases eliminated mechanical finishing. I n addition, they reduce the loss of nickel metal entailed in the polishing process. The Watts bath is the base of most bright nickel electrolytes. A multitude of addition agents have been discovered which cause nickel to deposit with a smooth or even mirror-bright surface. Most of these are useless commercially because no way has been found to avoid excessive internal stresses and brittleness of the bright product. However, a number of useful combinations of reagents have been found. These fall into several fairly distinct classes (IO). One group consists of organic compounds having in
I
the molecule the group (=C-SO-). If used alone, these would produce a rather bright deposit upon a buffed base metal surface but not upon a moderately polished base. Although the electrochemistry of the brightening action of cobalt compounds must be different from that of the above organic compounds, cobalt ions must be included in this fist class of brighteners for their general hehavior.
Figure 4.
Microstructure of Chloride Nickel Deposit
Magnification Tensile strength Elongation in P inches Hardness (Vickers)
May 1952
Microstructure of Bright Nickel Deposit (500X)
w
230
The second class of brighteners differs from the first because its members produce a very brilliant result but cause excessive brittleness and stress in the nickel deposits. Used in conjunction with the first class these defects are overcome and the modern bright nickel results. I n the second class are ions of metal u hich show high hydrogen overvoltage, such as zinc, cadmium, and lead; certain compounds of sulfur, selenium, tellurium, and arsenic; and organic compounds containing unsaturated C=O, C=N, C=C, N=N, and N=O groups. Hydrogen peroxide rannot be employed in the organic bright nickel baths to prevent pitting because it affects most brighteners. All the proprietary bright nickel electrolytes therefore, except the cobalt type of bath, contain wetting agents. The actual mechanism of brightening is not understood. The deposits are of submicroscopic grain size and show a type of rhythmic banding when examined in metallographic cross section. They contain small amounts of elements from the brightener compounds used, such as sulfur, zinc, carbon, or cobalt. The mystery of brightening action is deepened by the now undisputed evidence that the degree of brightness is not determined by orientation of the grain structure. Another unexplained phenomenon is that called “leveling action,” which means a gradual reduction in surface roughnesP as the deposit thickens. Some plating baths show this effect in a strong poaitive degree while with others, producing coatings just as bright, the readings of a surface roughness instrument do not decrease with increasing thickness. I n Figure 2 are shown some articles coated with a bright nickel decorative finish; the microstructure of a heavy layer of an organic bright nickel deposit is reproduced in Figure 3. The grain size is too small to resolve, the structure is laminated, and the deposit is very hard with only a small degree of ductility. All-Chloride Bath. Figure 4 is a metallographic section of a heavy layer of nickel deposited from an all-chloride bath. The conditions were as follows: Solution, grams per liter Niokel chloride, NiClz.0H10 Borio aoid PH Temperature! a C. Current density, amperes/sq. foot
Figure 3.
500X 98,900 1b.h. inch 21
300 30 2.0 60 50
Deposits from this electrolyte are finer-grained, harder, smoother, stronger, but less ductile than Watts nickel. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
959
-NICKEL-METAL,
OXIDE
chloride bath permits use of lower voltage and a 50% savings in power consumption, has a wide plating range and lower tendency to pit or form nodular growths, and is easier to control. I t s chief disadvantages are a high internal stress in the deposits and a greater corrosivity of the electrolyte, which limits the choice of materials of construction of plating equipment.
I
I
500c
0
example of a way in which such information can be of use may be quoted a recent study (19) aimed a t determining the upper limits of current density, information desired in planning new developments involving high speed nickel deposition. Such possible applications include plating on continuous steel strip and wire and the electroforming of continuous wire mesh. The theoretical limiting current density, I , for a given cathode reaction can be estimated from the following expression derived from the classical diffusion laws (4):
-
I=--- DnF 6(1
;
c 3ooc w
5
s3 LL
20(t 100
0
CURRENT DENSITY
Figure 5.
-
Limiting Current Density vs. Rate Electrolyte (5)
A.s.f
of Flow of
Chloride baih pH 2.0, temp. 71 O C.
With the addition of a proprietary stress-reducing agent this electrolyte has permitted electroforming of spectacularly large heavy-walled objects like that shown in Figure 10. It is also fundamentally the best type of solution where unusually high rates of deposition are required. The reasons for this are explained in discussing limiting current densities below. Some plating engineers believe that the advantages of the chloride bath can be substantially achieved without its disadvantages by simply raising the ratio of chloride to sulfate in the Watts bath. A gradual trend in this direction has indeed been under way for years. Hard Nickel. When nickel plating is employed for salvaging worn or mismachined parts or in protecting parts against severe erosion, it is often desirable to produce a deposit of much higher hardness and tensile strength. This is done by incorporating some ammonium chloride in a dilute nickel sulfate-boric acid electrolyte and operating it a t a very high pH between 5.6 and 5.9. Typical physical properties are: Vickers hardness Tensile strength lb./sq. inch Elongation in 2 jnohes, %
formation a t high p H and the dispersion throughout the nickel layer of hydrated basic compounds of nickel of a critical, submicroscopic size, These tend to cause a form of dispersion hardening akin to precipitation hardening in metallurgy. Other Electrolytes. A wide variety of solution compositions are in use for special purposes such as electrotyping, plating on plastics, etc. I n addition, there have been introduced recently baths unrelated to the standard electrolytes in which the principal ingredients are nickel fluoborate and nickel sulfamate. These are said to yield deposits of low internal stress and to be especially adapted to electroforming a t rapid rates. Limiting Current Density. Fundamental electrochemical data for solutions of nickel salts are rather scarce. As an
960
(3)
a)
where n is the number of electrons involved in the discharge process, F is the Faraday constant, 6 is the thickness of the cathode film, CY is the transport number of the discharged ion, and D is the diffusion coefficient of the salt furnishing the dischargeable ion. D is the coefficient related to salt of concentration c For concentrated solution of strong electrolytes such as are used in nickel plating, the theory has not yet been extended far enough to show a form of Equation 3, based on activities instead of concentrations. Equation 3 shows in x-hat directions one must move to achieve a maximum limiting current density in nickel plating: A salt with large D should be selected and the solution maintained a t a high temperature; c should be as large as is compatible with reasonable viscosity and dragout losses; maximum agitation or movement of the electrolyte with respect t o the cathode surface should be employed to reduce 6 to a minimum; and the transport number, a,should be kept high by forcing the nickel ion to carry a large share of the current, in other words, by not having other cations present. Estimates of theoretical limiting current densities for nickel plating from electrolytes containing nickel sulfate or nickel chloride as the principal conducting salt led t o a value of 4000 amperes per square foot for a hot concentrated nickel sulfate solution with violent agitation as compared with 8400 amperes per square foot for an equivalent nickel chloride electrolyte. In actual laboratory tests it was found possible to deposit sound nickel layers at cathode current densities as high as 4260 amperes per square foot from the chloride bath (see Figure 5 ) . At this level nickel was deposited a t the rate of 0.001 inch in 16 seconds, a speed amply high for contemplated commercial processes. Nickel Anode Behavior. Nearly all nickel deposition takes place with the aid of soluble nickel anodes, so that Reaction 1
I3 12 I1 10-
425 152,000 6
It is believed that the cause of this high hardness lies in the
-
9-
8m
50 7 '6-
54-
POLARIZATION cathode
AMP. PER SQ. FT.
Figure 6.
I
Typical Relationshi of Polarization to Cell Voltage in Watts NicEel Plating Tank
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
NICKEL-METAL, OXIDF tends to take place a t the same rate as Reaction 2 and the nickel content and pH oi the electrolyte do not change rapidly. Although pure nickel would remain rather passive in sulfate solutions low in chloride content, it dissolves anodically with 100% efficiency in electrolyte containing more than about 4 grams per liter of chloride ion a t ordinary current densities, pH, and temperatures. The uniformity of corrosion and the tendency to release particles of nickel and to break off large pieces from the anode vary amazingly with the chloride content, the p H of the bath, and other plating conditions. Years ago it was observed that impure nickel tended t o dissolve more uniformly and smoothly in the low-chloride baths then in vogue and it was common to employ cast nickel anodes containing only 90 to 94% nickel with several per cent of iron, carbon, and silicon. With the introduction of the Watts bath requiring freedom from dissolved iron, purer anodes came into use. Their behavior was improved by an increase in chloride content and lowering of the p H of nickel baths. At present smoothness of corrosion of pure nickel anodes is obtained in two ways: by incorporating small amounts of nickel oxide to form the so-called “depolarized” wrought anodes widely used in plating baths operated a t p H above 4!5, and by incorporating several tenths per cent carbon in cast or wrought anodes together with a little silicon to provide optimum behavior in low p H baths. It is customary t o enclose both types of anode in cotton bags to make sure that no loose particles of nickel, carbon, or oxide reach the cathode surface where they might cause roughness of the deposit. If the pH of the plating bath is very low and its chloride content high, it is possible t o use anodes of the highest commercial purityLe., electrolytic nickel-without excessive disintegration and scrap losses. Plating with Insoluble Anodes. For certain industrial processes the use of insoluble anodes of lead in the nickel plating
’-
I
.-
.
A -Hard b a t h -Watts pH 5 5 C -Walls pH 2.0 D -Chlortde bath
CATHODE CURRENT EFFlCl E NCY
? I c
0
1
2
3
Current Figure 7.
4 Density
5 in
I
6 7 8 Amp. per Sq. Dm.
Variation of Cathode Efficiency with Plating Conditions
(18) Temperature 54.4O C.
May 1952
I
cell with replenishment of the depleted nickel content of chloridefree electrolyte in a separate vessel offers distinct advantages. Examples of such processes are plating the inside wall of tubes and plating on rolls and on moving steel strip and wire. The advantages are: the elimination of anode bags, permanent anode shapes permitting close and unchanging anode to cathode spacing with low I R drop, reduction of scrap losses, elimination of addition agents normally required to prevent pitting, evolution of oxygen a t the anode which provides additional agitation of aid in reaching high current densities, and possible beneficial effect in oxidizing organic bath contaminants a t the anode. The over-all plating cell reaction with insoluble anodes is:
This means that the electrolyte becomes depleted of nickel ion and enriched in sulfuric acid with consequent drop in pH. In the past the depleted electrolyte has been regenerated by adding nickel carbonate or hydrate ( 5 ) which reacted with the sulfuric acid. An interesting new development (16) involves replenishment of the nickel content by electrolysis of the bath in a separate cell using nickel electrodes. Conditions are selected so as to give a low cathode efficiency, thus discharging hydrogen ions and raising pH. At the same time the anode efficiency is kept high by periodically reversing the current so that nickel dissolves t o replenish the electrolyte. Electrode Polarization. Figure 6 was drawn to depict the relationship of anode and cathode polarization to the total cell voltage in a typical commercial nickel plating operation with soluble anodes. The anode-to-cathode spacing was 16.5 cm. and the specific conductance of a Watts electrolyte a t 54” C. was taken as 0.091 ohm-’ cm.-1 Although nickel deposition has been considered to be a process showing high polarization, the major portion of the energy expended in nickel plating is dissipated in overcoming the resistance of the electrolyte. This is an important reason for the modern trend toward high chloride concentrations. Electrochemical Efficiency. It was noted above that the efficiency of Reaction 1 a t nickel anodes in the presence of chlorides is approximately 100%. The cathode efficiency is not so nearly ideal, as there is a tendency for part of the current to be diverted to the discharge of hydrogen ions with consequent rise in pH. This reaction varies mith electrolyte pH, composition, agitation, and the cathode current density in understandable ways. Typical ‘values for commercially important conditions are shown in Figure 7. Under normal operating conditions the slight excess of anode efficiency over cathode efficiency leads to a gradual rise in pH and nickel content which tells the operator all is well. Porosity of Nickel Deposits. Extremely thin layers of electrodeposited metals do not fully protect the base metal when exposed to corrosive environments but act as if there were tiny holes in the coating. The nature of porosity is being studied intensively by N. Thon and his associates a t Princeton University under an American Electroplaters’ Society fellowship (15 ) . At the present time it appears that thin coatings below 1 mil (0.001 inch) thick of most metals are more or less permeable to gases but in the absence of gross defects are not permeable to liquids. Upon exposure to corrosive media the permeability t o gases increases until visible perforations occur, a t which point liquids can pass through as well. Regardless of theory, which is in a state of rapid flux, application of nickel coatings can be based on the practical rules that: Porosity whether of the permeability type or gross defects tends to disappear with increasing thickness as shown in Figure 8; and heavy nickel deposita above 4 or 5 mils thick show the chemical and corrosion resistance of pure nickel. Nickel Plating on Bases Other Than Steel or Copper Alloys. Articles of steel, iron, copper, brass, and lead alloys are immersed
INDUSTRIAL AND ENGINEERING CHEMISTRY
96 1
-NICKEL-METAL,
OXIDE
directly into the nickel plating bath after suitable cleaning and etching of the surface. With some of the other base metals it is necessary to coat the object with a preliminary thin electrodeposit to ensure good adhesion of the nickel layer. Zinc-base die castings are commonly first coated with a copper film, while aluminum and magnesium alloys require a few atom-layers of TH I C KN ESS , M IC RON5
0
5
IO
15
20
25
30
35
40
45
50
Military applications of nickel electroforming nnclude Pitot tubes for air speed indicators, precision tubing of rwtangular cross section and fittings for radar v-ave guides, molds for plastics parts for aircraft (Figure lo), computing cams, and large searchlight reflectors. The electrolyte used and plating conditions are selected t o confer upon the nickel the desired physical and mechanical properties in accordance with the principles discus,& earlier. Resizing. The salvaging of worn and overmachined metal parts by depositing a heavy layer of nickel is a British practice of Iong standing. I n this country this is done only on a modest scale, particularly in connection with the maintenance of trucks, buses, and airplanes. Here again the plating baths are selected to give the desired mechanical properties in the deposiks. Where heavy deposits with an extremely hard surface ale needed, it is considered good practice to make up the major portion of the desired thickness with nickel plated st a rapid rate and then follow this with a &ish coating of chromium several thousandths of an inch thick. Of interest to the chemical engineer is the treatment of paper and plastics processing rolls in this manner, which experience has shon-n to be very effective (Figure
11). DEPOSITION T H I C K N E S S , INCH
Figure 8.
x io3
Conventional Porosity Tests of Nickel Coatings on Steel Most wblirhed mearurements l i e wlthin shaded area
zinc, followed by a copper film before nickel plating. The reason for this is that these electrochemically very reactive metals tend to displace nickel from the Watts bath in a .powdery, nonadherent form. Stainless steel is nickel-plated directly after a suitable activation treatment, which may or may not involve deposition of a few atomic layers of nickel in a special acid nickel chloride bath. Nickel plating on wax, plastics, wood, glass, and other nonconducting bases is practiced commercially. The surface is first rendered conductive by silvering or treatment with metallic powders or graphite.
OF NICKEL A L L O Y S
The physical and corrosion-resisting properties of pure metals are limited; hence there has always been a keen interest in forming alloys b y electrodeposition. On a laboratory scale the deposition of alloys of nickel with each of the following metals has been reported in the literature with a t least fairly convincing substantiating evidence: cadmium, cobalt, copper, gold, iron, lead, manganese, molybdenum, phosphorus, rhenium, selenium, silver, sulfur, tantalum, tellurium, tin, tungsten, and zinc. Unfortunately, it is usually much easier t o deposit a thin film of an alloy than to build up a useful deposit 1 or more mils thick. With some combinations there are fundamental reasons why the desired elements cannot be codeposited. This appears to be the case with nickel-chromium alloys. It is believed that the optimum condit,ions required in the cathode film for formation of sound chromium metal are incompatible with those necessary for nickel or iron deposition, and that a principal difference is the tolerable pH level.
A P P L I C A T I O N S OF H E A V Y NICKEL DEPOSiTS
In the processing industries nickel coatings with or without a chromium finish are finding application, particularly in the caustic soda, paper and pulp, plastics, and food processing groups. Steel pipe nickel plated internally is available in sizes from 2 inches upward. Nickel-lined kettles and nickel-coated paper and film processing rolls are in service in the largest commercial sizes (Figure 9). Thickness of such coatings ranges from 0.003 to 0.030 inch. Electroforming. Electroforming offers inherent advantages over other methods of fabrication for parts which require a very high surface finish, especially on internal contours, require high precision in certain dimensions, incorporate intricate details, or are needed in quantities too small for die-casting runs. Copper is the most favored metal in quantity of product, but nickel is prominent wherever its greater strength, toughness, hardness, and corrosion resistance are needed. Thicknesses of electroformed or “cold-cast” nickel parts can cover the entire range from the 0.003-inch wall of hypodermic needles to the solid nickel mold 3/s inch thick illustrated in Figure 10. Even where the bulk of the electroformed part is made of copper, it is a common practice t o apply a layer of nickel to the surface of the mold in starting electroforming. This layer is then in a position to supply wear and corrosion resistance plus some stiffening. This is done, for example, in making electrotypes, phonograph record matrices, and stampers and metal molds for plastics.
962
Figure 9.
Vacuum Pan for Food Processing Coated with Nickel Deposit 0.010 Inch Thick
The commercially significant nickel alloy deposits are fen-namely, cobalt-nickel, zinc-nickel, nickel-iron, gold-nickel, sulfurzinc-nickel (or black nickel), and, potentially, phosphorusnickel and nickel-tin, Nickel-Cobalt. Kickel and cobalt are so nearly alike in electrochemical behavior that it is small wonder their alloys can be readily deposited from solutions similar to the Watts bath in any desired ratio of nickel to cobalt. One of the popular proprietary bright nickel processes involved deposition of an 18%
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
NICKEL-METAL, OXIDE used either with alloy anodes or with separately controlled tin and nickel anodes. The composition of the deposit is largely i n d e pendent of variations in current density and temperature and lies close t o 65% tin. The crystal structure seems t o be that of a single metastable phase, a hard intermetallic compound not far from NiSn in composition. The tin-nickel deposits are bright with a pleasing appearance and are very resistant t o atmospheric tarnishing, so that there is no need t o apply a chromium finish. Thicknesses required t o protect brass and,steel are comparable to those used with nickelchromium duplex coatings. In the long run the fact that the entire alloy deposit is tarnish-resistant may prove to be a significant advantage over the usual nickel-chromium finishes. Miscellaneous Alloys. Black deposits are used for decorative h i s h e s only. They are only partly metallic and contain nickel, zinc, and sulfur. A very recent development is that of nickel deposits containing 1 to 15% phosphorus made in a dilute Watts bath t o which phosphorus acid is added. The 15% alloy has a hardness of over 600 Vickers as deposited and can be heat treated to much higher hardness numbers. These alloys have been described by Brenner, Couch, and Williams (a). For alloys which can retain a high hardness a t elevated temperatures for certain engineering applications, a process of depositing heavy layers of nickel containing up t o 35% tungsten has been disclosed ( 1 ) . ELECTROREFINING
The process of electrorefining nickel has recently been described in detail by Queneau (If) and Renzoni (la). The electroahemistry of the process is similar t o that of nickel plating from
Figure 10.
Electroformed Nickel M o l d for Aircraft Radome
Made by Bone Engineering Corp., Glendale, Calif.
cobalt-nickel alloy, but the amount of cobalt has been successively reduced until now the deposit contains only 1% cobalt. The anodes are of nickel containing the same proportion of cobalt as is desired in the deposit. Recently, a continuous wire plating process has been developed in which a magnetic 20% nickel80% cobalt alloy is deposited on a silicon bronze wire t o be used for sound-recording equipment (6). Zinc-Nickel. Some of the proprietary bright nickel deposits depend upon the addition of zinc salts as one of the brightening agents. This leads to a measurable amount of zinc in the product. Nickel-zinc alloys containing up to 50% nickel find steady commercial application on steel for providing resistance t o atmospheric corrosion, particularly on insect screen mesh. I n this case however, the difficulty of control of alloy plating led t o a process in which the wire mesh is coated first with nickel, then with zinc by electrodeposition, and the duplex coating is alloyed by heating. Nickel-Iron. Nickel-iron alloys of a wide range of composition have been produced on a commercial scale. The basic baths contain nickel and ferrous sulfates and boric acid, to which are added chlorides or fluorides plus small amounts of an addition agent which assists in maintaining a low ratio of ferric to ferrous ion in the bath. Among commercial uses of iron-nickel alloy deposits are wire coatings of high magnetic permeability (20 to Myo iron) for communications equipment, coatings for steel which promote adhesion of ceramic enamels (1 t o 2% iron), and heavy industrial coatings which can be carburized like steel (3 t o 5 % iron). Bright Nickel-Tin. A new process in which bright coatings of nickel-th alloy are deposited is arousing considerable interest in England (9). An electrolyte containing stannous chloride, nickel chloride, sodium fluoride, and ammonium bifluoride is May 1952
COURTESY UNSON SCREEN PLATE 00.. LENNOXVILLE, OUE.
Figure
11.
Sweat Dryer Roll 13 Feet Long Being Resurfaced by Heavy Nickel Deposit
the Watts bath, but the chemistry of the continuous removal of .copper, iron, arsenic, and lead from the circulating electrolyte and the recovery of precision metals is complex and is not dealt with here. PREPARATION OF SPECTROSCOPICALLY PURE NICKEL
I n order t o provide a small amount of highly purified nickel for spectrographic standards and scientific use, batches of such metal are occasionally prepared a t the Research Laboratory, The International Nickel Co., Inc., as follows:
As starting material nickel pellets made commercially by the nickel carbonyl process of the Mond Nickel Co. are used. This type of nickel offers the advantage of very low cobalt content,
INDUSTRIAL AND ENGINEERING CHEMISTRY
963
NICKEL-METAL,
OXIDE
even though the iron, carbon, and silicon contents are higher than those of electrolytic nickel. The pellets are dissolved in hydrochloric acid, the iron, copper, and are removed from solution by treatment with freshly prepared nickel peroxide, the last traces of cop er are removed with hydrogen sulfide, pure boric acid is addex to serve as a buffer, and the solution is electrolyzed using platinum-iridium anodes and a cathode of Inconel of the desired shape. The cell reaction is: KClz
+Ni
+ C1,
This reaction does not proceed homogeneously but only on certain surfaces iron, gold? Or Palladium. Fortunately, nickel itself is a good catalyst or provides a favorable surface, SO that the process is not self-stifling a8 is the displacement method. Layers of nickel several mile thick can be deposited readily. The electrochemistry of the principal reaction is probably as follows:
(5)
and because the chlorine gas is evolved as the nickel is deposited, there is no accumulation of acid and the p H of the electrolyte remains steady a t a value of about 1.0. The temperature is maintained at 55" C. and the current density at 40 amperes per square foot. As the electrolyte becomes depleted of salt, small amounts of concentrated nickel chloride solution prepared as above are added from time to time. Typical analysis of the product is:
Anode reaction: R3P0z + H20
H3P03
+ 2H+ + 2e
Eo =
(9)
Cathode reaction: 2e + Ni++ = Ni + X i + + + H20 = Ni + H3PO8+ 2H+
H3P02
E" = +0.24 volt (2)
E o = -0.35 volt (10)
0.0002
0.0011 0.0004 0.0009
99.997
This is equivalent to a decrease in free energy for Reaction 10 of 16,200 calories. The process is not efficient because there is a high loss of the reducing agent by oxidation of water:
(by difference)
NICKEL IMMERSION COATINGS
The reduction of nickel salts in solution t o metal without the use of electrodes and electrolysis is generally considered a chemical reaction. As it is not known to take place as a homogeneous reaction nor a t nonconducting surfaces, it is probably electrochemical in nature. Two processes have been developed, Displacement of Nickel by Iron. When well cleaned steel is immersed in a solution of nickel salt there is a tendency for deposition of a nickel layer by electrochemical displacement as follon-s: Fe
+ N i + + +F e + + + Xi
(6)
The energetics of this reaction are as follow: Anode reaction:
Fe = F e + +
+ 2e
E o = -0.44 volt
(7)
Cathode reaction: 2e Fe
+ N i + + = Xi + Ni++ = F e + + + Ni
E" = +0.24 E" = -0.20 volt
(2) MISCELLANEOUS APPLICATIONS
Therefore, a t unit activities of nickel and ferrous ions Reaction 6 proceeds with a decrease in free energy of 9200 calories. With a low ferrous ion concentration in the solution this value is, of course, enhanced. Under favorable conditions it is possible to deposit an immersion coating of nickel on steel from a hot concentrated solution of nickel chloride of 0.00002-inch thickness in 10 minutes (17). Such deposits are very porous and must be heat treated t o decrease porosity by lateral diffusion and to increase adhesion. This process shows promise for making an inexpensive nickelized steel superior to mild steel as a base for other coatings such as ceramic enamels, organic and conversion coatings, and outer layers of tin or zinc. Much thinner immersion coatings of nickel made in this way have long been used in pretreating steel prior to application of ceramic enamels. I t is believed that a nickel-iron alloy interface layer is formed during the firing of the enamel, which reacts with the enamel t o give a good bond to the base steel. Reduction of Nickel Salts by Hypophosphites. A process for controllable catalytic reduction of nickel to form adherent nickel deposits was developed recently at the National Bureau of Standards (@. I n this method sodium hypophosphite is added t o a hot buffered solution of nickel chloride or sulfate. The overall reaction is expressed as follows: NiClz 964
+ NaHzP02+ H20 = Ni + 2HC1 + NaHzPOa
The nickel deposit is not pure nickel, but contains 3 to 12% phosphorus, probably as nickel phosphide. The deposit is hard, with a microhardness number of about 500 and it can be further hardened t o as high as 800 b y heat treatment. I t is expected that this chemical reduction method, although expensive] will find special applications. It offers the advantage of uniform metal deposit thickness regardless of shape of the part being treated. This can be important in coating the interior of parts and in covering intricate small parts such as instrument gears, pinions, and bearings which cannot be uniformly coated in any other way and where the high hardness of the nickel deposit is desirable. I n special circumstances it may be desirable to use this process for coating parts in the field or in a plant not equipped for electroplating when the ability to deposit nickel without purchase of current generators, control equipment, racks, and purifying tanks might be most economical.
(8)
Electropolishing Nickel. The finishing of wrought nickel and of nickel electrodeposits electrolytically to produce a brilliant luster has been practised commercially on a modest scale. The successful processes are proprietary and employ either strong sulfuric acid or mixed sulfuric-phosphoric acid viscous electrolytes. As with electropolishing of other metals, the object to be polished is made anode a t a high current density of several hundred amperes per square foot. Gravimetric Analysis for Nickel. The standard electrolytic method for nickel is probably the oldest practical application of the electrochemistry of nickel. Deposition of nickel with insoluble precious metal electrodes from a strongly ammoniacal solution a t a current density of about 0.2 to 0.3 ampere per sqdm. has been practiced by analytical chemists since 1864, whereas the first commercial nickel plating occurred in the following year. The procedure has changed but little since t h e date of its inception. Ammoniacal baths have not found favor in nickel plating because the solubility of nickel salts is low in such a medium, the conductivity is poor, and, most important, the nickel deposits are so highly stressed as to crack spontaneously if an attempt is made to prepare thick coatings. One commercial ammoniacal process is in use in some plants for depositing nickel directly upon zinc alloys, but this is limited to a thin film serving as an undercoat for a heavy bright nickel deposit.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
NICKEL-METAL, O X I D F LITERATURE CITED
Queneau, P. E., Trans. Can. Inst. Mining Met., 51, 356-67 (1948).
Brenner, A . , Burkhead, P., and Seegmiller, S., J . Research
Renzoni, L. S. (to International Nickel Co., Inc.), U. S. Patent 2,394,874 (Feb. 12, 1946). Robinson, R. A., and Jones, R. S., J . Am. Chem. SOC.,58,
Natl. Bur. Standards, 39, 351-83 (1947). Brenner, A., Couch, D. E., and Williams, E., Plating, 37, 36, 161 (1950).
959-61 (1936).
Brenner, A., and Riddell, G. E., J . Research Nall. Bur. Stand-
Stokes, R. H., Trans. Faraday SOC.,44, 295-307 (1948). Wesley, W. A., Plating, 37, 949-53 (1950). Wesley, W. A., Cam, D. S., and Roehl, E. J., Ibid., 38, 1243-60
ards, 39, 385-95 (1947). Butler, J. A. V., “Electrocapillarity,” p. 161, London, Methuen and Co., 1940. Hothersall, A. W., and Gardam, G. E., J . Electrodepositors, Tech. Soc., 27, advance copy (1951).
(1951).
Wesley, W. A., and Copson, H. R., J . Electrochem.
No. 23, 49 (1949). Kolthoff, I. M.,and Lingane, J. J., “Polarography,” p. 45, New York, Interscience Publishers, 1941. Oholm, L. W., Finska Kemistsamfundets Medd., 45, 122-8 I r o n Age, 163,
Wesley, W. A., and Roehl, E. J., Trans. EEectrochem.
Soc., 86,
79 (1944).
Wesley, W. A., Sellers, W. W., and Roehl, E. J., Proc. A m .
(1936).
Parkinson, W . , J . Electrodepositors’ Tech. Soc., 27, advance copy (1951). Pinner, W. L.. Soderberg. G.. and Baker. E. M.. Trans. Electrochem. Soc., 80,539-68 (1941).
Soc., 94,
20-3 1 ( 1948).
Electroplnlers’ Soc., 36, 79-92 (1949).
Zitek, C., and McDonald, H., Trans. Electrochem.
Soc., 89,
433-41 (1946). EECEIVED for review October 17, 1861.
.4CCsPTED
Eebruary 25, 1952.
CORROSION-RESISTING NICKEL ALLOYS AND CHEMICAL PROGRESS W. Z. FRIEND
AND
F.
L. LAQUE
The lnternationel Nickel Co., Inc., New York, period of great development of the chemical and process industries during the past quarter century has coincided with the period of commercial development of most of the corrosion-resisting metals and alloys for process equipment. M a n y of these alloys contain nickel as an essential alloying element, not only because of the corrosion-resisting properties of the metal itself but also its metallurgical compatibility with a good many other metals. Following a summary of the corrosion-resistingcharacteristics of the principal nickel-containing alloys, examples are given of specific applications of these alloys in the manufacture of a number of chemical products, including products involving halogens such as chlorine, bromine, fluorine, and hydrogen fluoride; synthetic textiles such as viscose rayon, cellulose acetate rayon, and nylon) dyestuff manufacture and textile dyeing and Rnishing, synthetic plastics such as phenolics, alkyds, polystyrene, and organic chloride polymers) antibiotics such as penicillin, streptomycin, and chlorumycetin; fatty acid products; and corn products.
NY study of the progress of the chemical and chemical proo-
A
ess industries during the past quarter-century and of trhe rapid development of new processes to the continuous large scale production of many useful products, should give recognition to the part played by the availability of suitable corrosion-resisting metals and alloys for the construction of process equipment. It probably is not a matter of coincidence but of some direct relationship that this period of great chemical activity has coincided with the period of development of many af the corrosion-resisting materials which now are considered indispensable for the large volume economical manufacture of chemical products of good quality. From a historical standpoint i t might be said that our greatest period of chemical development in this country began around 1920 with the commercial production of such materials as the rayons, phenol-formaldehyde resins, and new synthetic dyes and dye intermediates. Our greatest period of activity in the de-
May 1952
N. Y.
velopment of corrosion-resistant metals and alloys began at about the same time with the commercial production of rolled nickel and Monel in 1921, stainless steels about 1923, and some aluminum alloys. Since that time we have seen the production of a host of metallic corrosion-resistant materials including Haatelloy alloys A, B, C, and D; Ni-Resist alloys; Worthite; Illium; Durichlor ; cupronickel alloys; Chlorimet alloys; Inconel and other nickel-chromium alloys; tantalum; magnesium; agehardenable aluminum alloys; rolled aluminum bronze alloys, nickel steels, and a variety of new stainless steel products including such highly alloyed materials as Durimet 20, Aloyco 20, and Carpenter 20. Recently we have seen the commercial introduction of wrought titanium (7), zirconium ( 7 ) , and molybdenum (77), which give great promise as corrosion-resisting metals of the future. A considerable number of the materials listed above contain nickel as an essential alloying element. This is due not only to the corrosion-resisting properties of the metal itself but also to the fact that it is metallurgically compatible with a good many other metals. It is significant that many of these materiah, except for the few made only as castings, are available in a variety of wrought forms and in a range of sizes. They have high mechanical strength and can be readily fabricated by welding, machining, bending, rolling, and other common means. This, almost as much as corrosion resistance, has contributed to their value as chemical engineering materials. NICKEL ALLOYS
Nickel and High-Nickel Alloys. The approximate compositions of the principal high-nickel alloys used for corrosion-resisting applications are given in Table I. Although commercial nickel has useful resistance t o corrosion by a large number of chemicals and chemical solutions (8,6f, 77), a large portion of the present use of this material for chemical process equipment is based upon its particular resistance to the following corrosives:
INDUSTRIAL AND ENGINEERING CHEMISTRY
965
