cyanide zinc plating baths

A paper 2resented at the Eightieth General Meeting, held at Chicago, II1., October 3, 7947, H, J. Read ~residlnq.

C Y A N I D E Z I N C P L A T I N G B A T H S t

By R. O. HUt.L 2 AND C. J. WERNLUNI~

A B S T R A C T

On account of their high throwing power and their capacity to produce an attractive bright plate, the new zinc cyanide baths introduced in 1935 have made rapid strides in the past several years and their place is now thoroughly established in industry. The protection of steel against corrosion continues to be the main purpose of zinc plate. Detailed formulas and standard zinc plating procedures are presented and discuss.ed at length. Bright zinc plating, to be commercially successful, requires that the plating bath be free from heavy metal impurities, and that the ratio of zinc to cyanide in solution be maintained within close limits. Finally, addition agent selection and control of its concentration are essential. Numerous literature references are included.

I. I N T R O D U C T I O N

Zinc has for many years been one of the most widely eleetrodeposited or electroplated metals, the chief function of the zinc plate being the protection of iron and steel against rust. For this purpose, zinc has long remained the preeminent low-cost metal coating, whether applied by electroplating, from acid or cyanide baths, or by hot dipping, sherardizing or spraying.

Most of the tonnage of zinc 'consumed in electroplating is through the medium of plating baths of the acid type in the application to steel strip, wire, wire cloth and similar products. For such purposes, a white matte deposit is produced, since appearance is usually secondary in importance to thickness, adherence and ductility of coating.

Cyanide zin'c plating baths possess several inherent advantages over the acid baths, e.g., high throwing power, and more recently the capacity for production of truly bright deposits which make them applicable to plating of finished articles. For such purposes, the cyanide baths have made rapid forward strides in the past several years, until their place is now thoroughly established in industry.

Most of the development of cyanide zinc baths has taken place since 1919, although as early as 1907 R. C. Snowden 4 deposited zinc from

x Manuscript received August 18, 1941. Electroplating Div., E. I. duPont de Nemours 8: Co., Cleveland, Ohio.

a Eleetroplating Div., E. I. duPont de Nemours & Co., Niagara Falls, N. Y. Trans. Eleetroehem. Soe. 11, 131-33 (1907).

407

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 132.203.227.61Downloaded on 2014-07-06 to IP

http://ecsdl.org/site/terms_use

408 R.O. HULL AND C. I. WERNLUND

a zinc sulfate solution containing enough potassium cyanide to redis- solve the precipitate. C. H. Froctor ~ was among the first to recommend cyanide zinc baths, giving three formulas for specific uses. T. C. Eichstaedt 6 described a solution composition for zinc-tin deposits. A few other investigators reported on cyanide formulas at about the same time.

Probably the first authoritative investigation of cyanide zinc baths was that of Blum, Liscomb and Carson 7 who developed the pertinent fundamental chemistry and gave definite recommendations with respect to bath composition and operating conditions. Their bath remains one of the best from the standpoint of operating characteristics, although it is not readily amenable to production of bright deposits.

One of the authors s reported the results of an extended study of addition agents for cyanide zinc baths and recommended a formula that produces deposits at high throwing power. Horsch and Fuwa 9 made an intensive study of throwing power and 'current efficiencies of zinc plating solutions, concluding that the cyanide type of bath is su- perior in these respects to the acid bath.

One of the most important contributions to improvement of cyanide zinc plating was the study of alloy deposition of zinc with other metals, using alloyed zinc anodes. The first successful alloying metal used for this purpose was mercury. ~~ Aluminum is also used in anodes as an alloying metal with zinc or with added mercury, n Aluminum, when present in the zinc anodes, offers the advantage of uniform anode corrosion and minimum sludge formation, but aluminum does not co- deposit with the zinc. Anodes made of alloys of zinc with nickel and with mercury or aluminum 12 are said to possess a fine-grained structure and are capable of electrolysis at high anode current densities. A still later improvement in zinc anodes was the alloying of the zinc with magnesium or calcium 1~ for the purposes of reducing the anode current efficiency to approximate the cathode current efficiency and of improving uniformity of corrosion.

The m~st recent important line of development in cyanide zinc baths has been the introduction of several bright plate processes that give deposits ranging from a bright luster to a brilliant surface resembling chromium. This development has resulted in new applications for zinc plating in which good appearance must be combined with excellent rust protective qualities.

It is expected that cyanide zinc plating will continue to expand in the future, as the advantages of the cyanide bath are more fully appre- ciated. Cyanide baths are now used to a limited extent for continuous

6Metal Ind. N. Y. IS, 152 (1917); 18, 478 (1920); 19, 199 (1921). Metal Ind. N. Y. 17, 22.24 (1919).

7 "Zinc Cyanide Plating Solutions," Nail. Bur. Standards Technol. Paper ~195 (Aug. 17, 1921). a "l'r~ma. ]~Ie~trochera. Soc. 40, 257-85 (I921). a Trans . ]~le.c.trochem. Soc. 41, 363-388 (1922). l a A b r a h a m V a n Winkle, U. S. Pat. 901,758; Proctor and Wernlund, U. S. Pat. 1,435,875;

J. Haas, Jr., U. S. Pat. 1,451,543 and 1,497,265. a G . B. Hogaboom, U. S. Pat. 1,887,841; A. K. Graham, U. S. Pat. 1,888,202; Trans.

Electroehem. Soe. 63, 121-133 (1933). 2a G. B. Hogaboom, U. S. Pat. 2,064,307. 3aR. O. Hull, Cony. Proc. Am. Electroplaters' Soe. (1940); U. S. Pat. 2,214,33I; Ferm.

U. S. Pat. 2,243,696.



CVANI1)E ZINC PI,ATIN(" IIA'|'t:IS 4 0 9

steel strip plating, m!d this use should continue to grow. Bright zinc plating exclusively from cyanide baths has filled an important industrial need as a companion process to bright cadmium plating, which it has in some instances replaced to good advantage. Many more metal parts are now plated than ever before, and the rust protective value of zinc combined with the added excellent appearance of bright zinc offer an e!ectroplated finish of wide application.

II. PRINCIPLES The cyanide zinc bath has the following advantages and disadwm-

tages as compared with the acid type of zinc bath :

Advantages of Cyanide Type. 1. Ability to plate semi-bright to

mirror-bright deposits. 2. Exceptionally high throwing power

of cyanide baths resulting in deposits of uniform thickness.

3. Adaptability to low-cost equipment; steel tanks may be used.

Disadvantages of Cyanide Type. 1. Slower possible speed of deposition. 2. Higher initial solution cost and

maintenance. 3. Not applicable for coating certain

types of basis metals, e.g., malleable and cast iron.

Cyanide zinc baths in present successful use may be divided into three general classes: (1) the straight cyanide bath; (2) the zinc-mercury bath; and (3) the bright zinc baths.

The straight cyanide bath is used to advantage for applications in which rust protective value is of first importance, and appearance of deposit is of little consequence. This type of bath offers the highest cathode efficiency when properly used, although this desirable condition can be maintained only by close chemical control. The high-efficiency, straight cyanide bath is usually not capable of producing deposits of good appearance and with this bath it is particularly difficult to apply zinc coatings to malleable or cast iron having a low hydrogen overvoltage. For general application, the operable current densities can be varied quite widely, and bath composition can be modified to maintain good cathode efficiencies up to 100 amp./sq, ft. (11 amp./dm.2), at which rate deposits are produced approximately 0.005 to 0.006 in. (0.13 to 0.15 mm.) in thickness per hour. This speed of deposition is not as great as is possible from the acid type of zinc bath but, for any except rapid travelling steel strip or wire plating, the above plating speed usually is ample for production requirements.

The zinc-mercury type of bath is used because of its versatility in plating over a large variety of basis metals and its ability to produce uniformly white deposits over a wide range of current densities. This bath has excellent covering power at low current densities and hence plates into deep recesses, although this bath is not usually recommended for zinc plating over cast iron or malleable iron. Its operation is not difficult, bath maintenance being required with respect to only the essential zinc, mercury, cyanide and caustic soda contents. Excessive mercury must be avoided as it results in so-called "mercury spots" in the plate which may develop only after a period of time following plating.

The bright zinc baths are essentially variations in solution composition of the straight cyanide bath, with the additional requirements of



410 ~. o . H U L L A N D C. J . W E R N L U N D

brightening and rapid-covering agents. Maintenance of tile bright type of bath must be carefully controlled in order to insure good cathode efficiency combined with operable bright current density ranges. High purity materials must be used exclusively, since the bright bath is ex- tremely susceptible to exceedingly small amounts of heavy metals or other impurities. The applications and value of bright zinc result from its combination of bright appearance and rust protective value. In many cases bright zinc is now applied, in large part, even to metal articles not requiring a bright finish but whose sales appeal is thereby augmented. In recent years, the control of the bright zinc plating baths has been simplified, and ample experience now enables accurate pre- dictions as to their applicability in the case of almost all industrial metal articles.

III. FUNCTIONS OF CONSTITUENTS OF BATHS

The fundamental chemistry of cyanide zinc baths has been discussed by Blum, Liscomb and Carson, I~ and by Blum and Hogaboom. ~5 Their concept was that, since (from cyanide solutions) zinc can be deposited satisfactorily only from a mixture of sodium zincate and sodium zinc cyanide, with an excess of sodium hydroxide or sodium cyanide or both, all of the following components may be considered present in such solutions: sodium zinc cyanide [Na2Zn(CN)4], sodium zincate (Na2ZnO2), sodium cyanide, sodium hydroxide, and more or less sodium carbonate resulting from cyanide decomposition.

As it is impossible to determine the exact proportions of zinc in the double cyanide and in the zincate complexes, the zinc content may arbitrarily be calculated as sodium zinc cyanide, Na2Zn(CN)4, up to the limit of the sodium cyanide present, and the remainder of the zinc calculated as sodium zincate. On this basis, free sodium cyanide and sodium zincate do not both appear in expressing the composition of the bath. It is obvious, moreover, that zinc ions are furnished in a solution by dissociation of both sodium zincate and sodium zinc cyanide as follows :

(1) Na2Zn(CN)4 + 4NaOH = Na2ZnO-_, + 4NaCN + 2H20

(2) Na,_,ZnO2 = 2Na + q- ZnO2 ZnO2-- + 2H20 = Zn +§ + 4(OH)-

(3) Na._,Zn(CN)4 ---= 2Na + + Zn(CN)4- Zn(CN)~-- = Zn ~+ -1- 4(CN)-

There are definite indications (from unpublished conductivity and pH data) that in the normal composition of bright cyanide zinc baths about 75 to 90% of the zinc is present as sodium zincate and the remainder as sodium zinc cyanide. This probability is also substantiated by cathode current efficiency measurements.

It is evident from the above that the cyanide zinc plating solution is very complex, since the various solution components are in equilibrium

14 See footnote 7. io-"Principles of Electroplating and Electroformlng," 2nd Ed. (1930).



C Y A N I D E Z I N C P L A T I N G b ' A T H S 411

with o'ne another, and the states of equilibrium are dependent upon the respective concentrations of these components. While few studies have been made of the equilibria involved in cyanide zinc plating baths and would be essential to a fundamental, theoretical study of such plating systems, it is possible empirically to control solution constituents for which chemical analytical determinations can be made with sufficient facility to insure continuously satisfactory bath operation. These constituents are: (1) sodium cyanide; (2) zinc metal; (3) sodium hydroxide or caustic soda; and (4) inorganic metal addition agents. The first three of these are interdependent, and hence Graham 16 proposed the expression for the ratio R:

NaCN (normality) q- NaOH (normality) R =

Zn(CN)o normality

This ratio was then specified to be within definite limits for proper operation.

It is possible to simplify the expression for composition of solution still further, as developed later for bright zinc baths, by specifying a definite range of concentration for sodium hydroxide, together with a prescribed range of the arbitrary ratio M of total sodium cyanide (which equals the sodium cyanide plus the sodium cyanide equivalent of zinc cyanide) to zinc metal. Experience has shown that all zinc cyanide baths, expecially the bright baths, can best be controlled by this means. A few fundamental facts may be considered and associated with solution control as determined by these conditions.

It is possible to deposit zinc from a sodium zincate-sodium hydroxide solution in the absence of cyanide, provided a sufficient excess of sodium hydroxide is present both to maintain the zinc in solution and to counter- act the strong tendency toward hydrolysis of sodium zincate. Such a solution used as a plating bath produces zinc deposits at high current efficiencies (above 95%) at low current densities, but the deposits tend to be spongy and of poor quality. It is likewise possible to deposit zinc from a zinc cyanide-sodium cyanide solution (which must necessarily contain some sodium zincate), but such deposits are white matte in color and are produced at low current efficiencies, usually under 15%. It is therefore essential for all three primary constituents, i.e., zinc, sodium cyanide and sodium hydroxide, to be present to produce good-quality deposits at high cathode efficiency. The functions of the sodium cyanide are: (1) to combine with zinc compounds to form soluble complexes; and (2) to provide a control medium for appearance of deposit. Since current efficiency is also closely dependent upon the sodium cyanide concentration, careful regulation of the sodium cyanide content is necessary to combine good deposit quality with high current efficiency.

The principal function of sodiunl hydroxide is to provide the primary source of zinc ions and to permit operation at high current efficiency. Its concentration is easily controlled because sodium hydroxide parallds

i~ Trans. Electrochern. Soe. 63, I29 (1933).



412 it. o. IIULL AND C. J. WERNLUND

and is roughly proportional to the zinc metal content as it may change due to differences between anode and cathode efficiencies. Caustic soda increases or decreases to the extent that anode and cathode efficiencies are not equal. Thus, if the anode efficiency exceeds the cathode efficiency, hydrogen is liberated, hydroxyl ions are formed by resulting dissociation of water, and the sodium hydroxide concentration and pi t increase. Both sodium cyanide (total) and sodium hydroxide are readily determined by direct analysis, detailed hereinafter.

Gray 17 utilized the high alkaline type glass electrode in studying the effect of pH and alkalinity on the operation of cyanide zinc plating baths, and the results showed that the operation of the bath with too high a pH value from caustic additions invariably gave a white matte deposit, while the use of low pH values gave poor anode corrosion. The best range for operation of cyanide zinc plating baths was found to be represented by the fairly well defined pH region intermediate between these two limiting conditions, which in general is a pH range of 12.2 to 12.7. Measurement and control of pH may ultimately prove to be an important factor in the operation of all cyanide zinc plating baths.

The zinc metal concentration must be controlled in cyanide zinc baths, since this factor not only establishes the rate of deposition and current efficiency, but the two complexes that zinc forms determine the concentration of the cyanide and hydroxide components of the bath. Thus experience has shown that the ratio M of total sodium cyanide to zinc is of greater significance than the actual zinc metal content, assuming the customary sodium hydroxide limits of concentration. It is therefore apparent that cyanide zinc baths can best be controlled by main- taining a definite range of sodium hydroxide concentration which is not critical within that range, together with a definite ratio M which is quite critical, as will be shown later.

In cyanide zinc baths few additions of salts are used other than the primary ones mentioned, and even these few are little used at present. Proposals have been made to add small amounts of sodium fluoride or of aluminum salts to whiten the deposit; however, the beneficial effects so obtained are of minor degree in comparison with the effects of mercury compounds or of other inorganic and organic brighteners.

Because a high concentration of sodium hydroxide is essential to good cathode efficiency, no other salts need be added to improve bath conductivity or to buffer the solution with respect to pH changes.

IV. P L A T I N G BATH FORMULAS, OPERATING CONDITIONS

AND MAINTENANCE

The formulas recommended for cyanide zinc plating baths may conveniently be divided into: (1) the conventional cyanide, as described by Blum and HogaboomlS; (2) tbe zinc-mercury process; and (3) the bright zinc processes.

17Proe. Am. Electroplaters' Soe. (1941:). 18 See footnote 15.



CYAN[I)I,: ZINC. I ' I , A T I N G b A T I I S 413

1. d'yanide Bath withoul .,ldditio, .4d~'uls.

a. 5'olution Coenposition: ~./I . . . . z./g,:tl. Zinc cyanide, Zn (CN),~ . . . . 60 8 Sodium cyanide, NaCN . . . . 23 3 Sodium hydroxide, NaOH

(caustic soda) . . . . . . . . . 53 7

Operatin 9 Conditions: Temperature . . . . . . . . . . . . 400-50 ~ C. (104~ ~ F.) Current density . . . . . . . . . . 1-2 amp./dm. 2 (9.3-19 amp./sq, ft.) Cathode current efficiency.90-95% Anodes--High Grade . . . . . 99.75% @ zinc

The solution composition should be maintained within • 10%, and the ratio M of total sodium 'cyanide to zinc should be within the range of 2.0 to 2.5. The throwing power and covering power are good, as the result of the low ratio M. The bath is used for production of zinc primarily for rust protection and serves admirably for this purpose. '['he zinc coatings are of good quality and of high ductility.

To improve the whiteness and stain resistance of deposits from this type of bath, gum arabic with sodium fluoride and lead salt have been patented 19 as additions to the bath.

The above cyanide bath, without addition agents, has recently been modified to permit rapid deposition up to 13.5 amp./dm3 (125 amp./sq. ft.) at high cathode current efficiency. The composition recommended for this purpose is as follows:

b. Solution Composition: g./L. oz./aal. Zinc cyanide, Zn(CN)2 . . . . 90 12 Sodium cyanide, NaCN . . . . 37.5 5 Sodium hydroxide, NaOH. . 90 12

(caustic soda)

Operating Conditions: Temperature . . . . . . . . . . . . 400-700 C. (104~ ~ F.) Current density . . . . . . . . . . 6-12 amp./dm? (55-110 amp./sq, ft.) Current efficiency . . . . . . . . 85-95%

(cathode) Anodes--High Grade . . . . . . . 99.75% q- zinc

As in the lower speed bath, composition ( la) , constituents should be maintained within • 10% of the amounts given, and the ratio M of total sodium cyanide to zinc should be within the limits of 2.2 to 2.6.

It must be emphasized that adequate control of these baths is no less important than for the bright zinc baths described hereinafter, if bath operation is expected at high efficiency within the prescribed current density ranges.

~J Mason, U. S. Pat. 2,136,629.



414 R . o . I t I J L L ANt ) C. J . W E R N L U N D

2. Zinc Mercury Process.

The zinc-mercury process ~~ operates on the principle of production of a zinc-mercury alloy deposit which is smooth, white and more pleas- ing in appearance than the straight cyanide bath deposits, comparing favorably with acid zinc in this respect.

Solution Composition

Zinc cyanide, Zn(CN) . . . . . Sodium cyanide. NaCN . . . . Caustic soda, NaOH . . . . . . . Mercury salt* . . . . . . . . . . . . .

g./L.

37.5 22.5 30.0 0.25

Still

oz./gal.

5 3 4 0.03

g./L.

60 30 45 0.25

Barrel

oz./gal.

8 4 6 0.03

Any mercuric salt such as tlgCl=, or }lgO is dissolved in sodium cyanide.

Operating Conditions. Temperature . . . . . . . . . . . . . . . . . . . 30~ ~ C. (86~ ~ F.),

optimum 40 ~ C. = 104 ~ F. Anodes (Intermediate or Prime

Western) . . . . . . . . . . . . . . . . . . . . 98.25 zinc plus 0.5-1% mercury Anode current density . . . . . . . . . . . 1.1 to 1.6 amp./dm3

(10 to 15 amp./sq, ft.) Ratio anode to cathode surface . . . 2 : 1 Cathode current density . . . . . . . . . . up to 4.3 amp./dm3

(40 amp./sq, ft.) Voltage . . . . . . . . . . . . . . . . . . . . . . 4-6 v. at tank bus bars

Cathode current efficiency, 2.7 amp./dm3 (25 amp./sq, f t . ) . . 9 0 %

Throwing power . . . . . . . . . . . . . . . . 35-40%

Bath Maintenance and Control. For good anode corrosion, not less than 4 v. must be maintained across the plating tank bus bars and an anode current density of 10 amp./sq, ft. (1.1 amp./dm. ~) should be used. If, under normal load, the tank voltage is less than 4, anodes are removed until the desired voltage is secured.

When properly operated, the solution contains from 0.1 to 0.2 g./L. (0.013 to 0.027 oz./gal.) of mercury cyanide. During weekend or other longer shutdowns, most of this mercury deposits out on the anodes (by displacement), making it necessary to replenish the mercury in the bath by adding mercury salts before operation is resumed. During electrolysis mercury codeposits with the zinc. The mercury in the anode forms amalgams with the metallic impurities present. These dis- solve anodically and quantitatively with the zinc without appreciable sludge formation. Accordingly, either low grade or high purity zinc can be used as anode without danger of anode polarization or objectionable bath contamination. I f the bath builds up in zinc, about one-third of the zinc anodes may be replaced by steel anodes, preferably case-hardened. Under normal operation, a brown film covers the zincl anodes, and uniform corrosion is maintained. The chief precaution to

See footnote 10.



CYANIDI,; ZINC PLATING BATHS 415

observe so as to ensure efficient anode performance is to avoid low anode current densities or excessive mercury salts in the bath, as either may result in mercury spots in the plate.

Since the bath is "self-purifying," only intermittent filtration of the solution is required to remove ferric hydroxide and other insoluble salts. As in the case of the plain cyanide bath, all constituents of the bath should be maintained within ___ 10% of the recommended concentrations.

The zinc-mercury bath possesses not only good throwing power, but good covering power or ability to plate readily at low current densities. The rust protective value of the alloy deposit is about the same as that of an unalloyed zinc deposit.

3. Bright Plating Baths. The bright cyanide baths were introduced to the plating industry in

1935 and came into widespread use shortly thereafter, until at the present time they may be 'considered among our essential plating processes. The immediate popularity of bright zinc resulted largely from an acute shortage of cadmium which originated from its use for auto- mobile bearings. With this impetus, bright zinc assumed its place perhaps more rapidly than any other new plating process or principle, because it is the obvious substitute for bright cadmium for rustproofing purposes. The basic principles of bright cyanide zinc were developed as the result of extensive research work, several years after preliminary indications of the possibility of producing bright zinc deposits were observed.

The first principle in bright zinc plating is the use of a plating bath virtually free from heavy metal impurities and employing high purity zinc anodes31 The fact has long been recognized that even minute amounts of some metals affect the physical and chemical properties of zinc metal to a marked degree, and this was likewise found true in bright zinc plating, in which minute amounts of heavy metals markedly affect the color and brightness of deposits. Thus it is believed that molybdenum, used as an inorganic brightener in zin'c baths, may exercise its effectiveness in sequestering small amounts of lead by combining with it to form a complex compound.

There are two methods of purification in general use: (1) addition of 0.25 to 2.5 g./L. (0.033 to 0.33 oz./gal.) sodium sulfide in the form of a concentrated solution (sodium bisulfite or sodium thiosulfate may be used instead of or with sulfide), which precipitates lead but not copper; or (2) addition of zinc dust, 0.25 to 2.5 g./L. (0.033 to 0.33 oz./gal.), followed by thorough stirring and filtering.

The second principle in bright zinc plating is rigorous control of solution composition, the most critical factor being the ratio M of total sodium cyanide to zinc metal. Table I gives experimental results of this factor as related to bright plate current density range and cathode efficiency and shows in a 'concrete manner the narrow M range within which practical current densities can be used with high cathode efficiencies.

~UOplinger, U. S. Pat. 2,075,623; 2,146,439.



416 R . O . HULL AND C. J . WERNLUND

From Table I it is clear that M must be between 2.5 and 2.7 for operation with high efficiency at normal current densities and room temperatures.

In general, a low M value necessitates using a high current density, and increasing M lowers the bright plate current density range.

The ratio M is also dependent upon the temperature of the bath, if all of the operating factors are to be maintained at their optimum value. Experience has shown that the values of M, as given in Table II, must be used for various operating temperatures.

TABLE I

Total NaCN Effect of Ratio M -= on Bright Plate Current Density

Zn (metal)

Range and Efficiency.

Approx. Bright Plate Current Density Range Ratio ]VI

A m p . / d m . 2 Amp./sq. ft.

2.25 4-10 37-93 92% 2.5 2-10 18-93 88% 2.7 1-10 9-93 82% 3.0 0.5-8.0 4.7-75 69% 3.2 0.3-8.0 2.8-75 54%

TABLE II Optimum M Values vs. Temperatures.

Ratio M Temperature Optimum

Cathode Efficiency at 35 amp./sq, ft. (3.8 amp./ din)) and 85 ~ F. (29 ~ C.)

2.6 2.8 3.0

~

27.8-30.6 31.1-33.3 33.9-37.8

~

82-87 88-92 93-100

The caustic soda concentration is less critical than the M ratio, but a minimum concentration is no less important. A minimum concentration of 60 g./L. (8 oz./gal.) of caustic soda is essential, not only for brightness but for good efficiency, and bright deposits can be obtained with concentrations up to 150 g./L. (20 oz./gal.). The optimum values (given below) in the solution compositions insure a sufficiently high concentration and are well within the operable limits. The concentration of zinc metal is not critical within the range of 30 g./L. (4.0 oz./gal.) to 45 g./L. (6.0 oz./gal.) provided it is maintained in the proper ratio to total sodium cyanide.

The third principle of bright plating applicable to most processes is the use of a dilute oxidizing acid bright dip after plating to remove the brown surface film that appears almost inevitably upon removal of the work from the plating bath. This film, however, does not occur with some brightening agents, in which case bright dipping is optional. A



C Y A N I D E Z I N C P L A T I N G B A T I t S 417

second desirable effect of a bright dip after plating is to improve resistance of the zinc plate to tarnish and finger staining, although zinc deposits produced from old baths free from heavy metals possess, in general, excellent resistance to staining.

Until the above three principles are recognized and followed religi- ously, bright zinc plating baths may be found difficult to maintain and to operate satisfactorily as compared with the simpler cyanide cadmium plating baths. This fact is probably largely responsible for the con- tinued wide use of the latter at a higher metal cost. However, when the operating principles of bright zinc plating are understood and practiced, the process is no more troublesome than most of the other plating processes.

The result of improper bath maintenance in bright zinc plating is either lack of brightness, or low cathode efficiency, or both, and these effects do not develop gradually but are noted immediately; whereas with dull zinc plating baths, the prin'cipal effect of improper bath main- te~ance is low cathode efficiency, apparent only upon determining zinc deposit thickness. Hence, proper operation of any cyanide zinc bath, whether bright or dull, necessitates analysis and adjustment of the bath composition at frequent intervals.

The several bright zinc processes are the subject of numerous patents covering addition agents, purifying agents and alloy anodes. However, the fundamental bath compositions are approximately the same and fall within the following ranges of concentration:

Ranges of Bath Composition for Bright Zinc Plate.

Zinc cyanide, Zn(CN) . . . . . . . . . . . Sodium cyanide, NaCN . . . . . . . . . Sodium hydroxide

(caustic soda, NaOH) . . . . . . . . . Corresponding zinc metal . . . . . . . . ~ Corresponding total sodium cyanide

for StilI Plat ing

g. / L. oz./gal.

60-82 8.0-11 18.7-64 2.5-8.5

75-112 10-15 34-45 4.5-6.0 68-135 9-18

for Barrel Plat ing

g . /L . oz./gal.

60-82 8.0-11 34-75 4.5-10

75-97 10-13 34-45 4.5-6.0 68-135 11-19

Total NaCN |2.0-3.0 (still) Ratio M --

Zn =(2.5-3.2 (barrel) Use low ratio for low temp., high ratio for high temp.

As discussed heretofore, the exact concentrations of bath components are not critical and may be varied quite widely, provided they are maintained in the proper balance.

Operating Conditio~zs The operating conditions for the various bright zinc processes are

likewise quite similar and can be conveniently summarized as follows:

Temperature . . . . . . . . . . . . . . 200-45 ~ C. (68~ ~ F.) extreme 27.80-37.8 ~ C. (82~ ~ F.) preferred (see second principle for values of M for different temperatures)



418 R. 0. H U L L AND C. J . W E R N L U N D

Cathode current density . . . . . 0.2-10.7 anap./dm. 2 (2-100 amp./sq, ft.) extreme limits 1.07-5.4 amp./dm? (10-50 amp./sq, ft.) average limits

Tank voltage . . . . . . . . . . . . . . 1.5 to 6 v. (still) 10 to 15 v. (barrel)

Cathode current efficiency . . .75 to 95% Throwing power :

Haring cell, 5:1 ratio . . . . 35 to 55% Solution conductivity . . . . . . . 4.5-5.5 ohm-cm. Anode current density . . . . . . 1.07-3.2 amp./dm. 2 (10-30 amp./sq, ft.)

t Special High Grade zinc ball anodes of high purity 22 Anodes or High Grade 99.8% -t- zinc bar anodes

zinc-aluminum also with mercury ~3 zinc-calcimn or zinc-magnesium ~4

Addition Agents The proper operation of bright zinc baths depends in large part upon

addition agents which are effective in one or more of several categories: (1) brightening effect upon deposits; (2) improvement in covering power at low current density, particularly for barrel plating; (3) lowering of the current density limit at which bright deposits are ob- tainable, thus permitting a low ratio of M and increasing the cathode efficiency; and finally (4) permitting production of bright deposits without bright dipping. Table III summarizes the effects of those addition agents that are in common use, all of which are the subject of patents as indicated.

The rates of depletion of the above addition agents vary with the conditions obtaining in each plating plant. The inorganic addition salts, such as molybdic oxide, can be determined by chemical analysis, whereas there are no published methods for determining the concentration of organic brightening agents or of the covering agents other than by direct plating tests and comparing for brightness range or for degree of brilliance. Optimum addition agent concentrations are usually different for still and for barrel plating. Some prepared addition agents differ in composition, depending upon the specific application. When the plating bath is in operation, the rate of depletion of addition agent can readily be established in each instance and thereafter readily maintained on the basis of the weight of zinc deposited or number of ampere-hours passed through the bath. The selection of addition agents (and hence the particular process) depends in large part upon the degree of brightness desired, whether or not bright dipping is feasible, economy of operation, and various other factors.

Operation and Maintenance of Bright Zinc Plating Baths In general, bright zinc coatings are obtained by depositing very pure

zinc from a cyanide bath of controlled composition at a cathode efficiency somewhat below 100%, giving either a smooth, bright surface

~r~ Stewart and Urban, U. S. Pat. Reissue 19,328; DubpernelI, U. S. Pat. 1,868,052. m See footnote 11. e* See footnote 13.



CYANII~I,; Z I N C I ' I , A ' f l N G I ;ATIIS 419

directly from the bath (c,mlaining prnpcr additi,m agt:ms) or a smooth, clark surface which c;m be bri~zhteucd by ,lipping for 5 to 30 seconds in 0.25 t~, ().5~S, 11itric acid ,,r acidiIh.d ]ly,ll~ogeu :u peroxide (0.25% H2SO.t, 4'/~ tf.~O2).

An important point in the operation of bright zinc plating is the necessity for adequate rinsing of work before and af ter plating and after bright dipping, the final steps consisting of at least two cold water rinses and one hot water rinse.

For operation in barrel plating, in which as much as 1 amp. /L .

TABLE I I I Effects of Addition Agents in Bri, lht Zinc Baths.

Addition Agents Coneelltration

g . /L .

0.5-1.0

10 cc./L.

25 cc./L. 2.5-7.5

3.0

1.0

7.5

oz./gal.

0.06-0.13

1.3 ft. oz./gal.

3.3 ft. oz./gal. 0.33-1.0

0.4

0.13

1.0

1. Ammonium thiocyanate- formaldehyde resin ~ Phenol-thiocyanate '-'~ Thiourea-formaldehyde resin ~

2. Methyl ethyl ketone (conditioned with H~SO,) ~ ; or not conditioned ~

3. Molybdic oxide and similar compounds ~

4. Piperonal or other methylene dioxyphenyl compound ~

5. Gelatin with certain oxyheterocyclic compounds ~

6. Thiourea with metal brighteners 3~

Purpose or Effect

Improves br ightness; increases tolerance of Cu, Ni, Pb and Cd in bath.

Improves brightness.

Produces bright deposits directly from bath.

Improves brightness, especially with molybdenum, at low current density.

Improves rate of covering.

I Improves brightness. I

(5 amp./gal .) may be passed, cooling coils must be used to provide proper temperature 'control. Operation below 21 ~ C. (70 ~ F.) may polarize the anodes excessively and prevent passage of current sufficient for normal rates of production.

In the usual operation of bright zinc baths, approximately one part by weight of sodium cyanide and one-third of caustic soda are consumed for each three parts of zinc metal deposited. Organic addition agents are added from time to time as determined by appearance of deposits. The concentrations of inorgani'c metals, such as mercury, aluminum and molybdenum, are readily determined by chemical analysis and fresh quantities are added to the bath as required.

U. S. Pat. 2,101,580; 2,101,581 (Henricks) to Odyllte Corp. U. S. Pat. 2,109,887 (Mattaeotti) to Hanson-Van Winkle-Munnlng Co. U. S. Pat. 2,080,520 (Westbrook) to E. I. duPont de Nemours & Co. U. N. Pat . 2,218,734; 2,233,500 (Westbrook) to E. I. duPont de Nemours & Co. U. S. Pat. 2,196,588 (Hull) to E. I. duPont de Nemours & Co.

*~ S. Pat. 2,080,479 (Hoff) ; 2,080,483 (Hul l ) ; 2,080,520 (Westbrook) to 1~. I. duPont de Nemours & Co.



4,90 ~. o . t I U L I , A N D C. J . W E R N L U N D

The necessity for proper control of the ratio M of total sodium cyanide to zinc metal should be emphasized and, once the proper rati~ is established for a given bath temperature range, it should be maintained rigorously at that value. Two general rules are:

1. A low M value gives a high current density range; increasing M lowers the bright plate current density range and cathode efficiency, giving more uniform deposits in re'cesses.

2. Increasing the temperature raises the current density range for bright plating and also increases the cathode efficiency by about 2.7% per degree Centigrade (1 .5~ per degree F.) at current densities above 3.2 amp./dm. 2 (30 amp./sq, ft.).

Addition agents are quite indispensable in improving the luster of zinc deposit; in extending the limits of bright current density range; or in improving the covering power at low current densities, particularly for barrel plating. Traces of foreign metals have been found to lower the rate of covering in barrel plating, and therefore these traces must be eliminated for proper operation. A simple way to detect certain metal impurities in the zinc plate is by a bright dip, which, for example, dulls rather than brightens the zinc deposit if copper is present, but not if small amounts of lead are present. Impurities such as these, when present in the bath, are removed by zinc dust, by sodium sulfide or by electrolysis.

In order to check the plating range of a cyanide zinc bath, a plating test may be made, using a small volume of solution. Any additions that may be indicated for the large volume of solution can be checked on a small scale to verify its effect. Such tests have been described by Hull a2 and by Pinner and Baker. aa These small-scale plating tests, aside from visual observation of the deposits from the regular plating tanks, offer the only practical means for determining concentrations of the majority of organic addition agents in use in cyanide zinc baths today, and for determining the amounts of these agents required for restoring an inferior bath to its optimum operating condition.

The carbonate content of cyanide zinc baths is not critical and seldom exceeds 30 g./L. (4 oz./gal.). However, if carbonate is present in excess of 75 g./L. (10 oz./gal.), it can be reduced to 60 g./L. (8 oz./gal.) by cooling to - - 5 ~ C. (23 ~ F.), and decanting or filtering, or by pre- cipitation with specially prepared gypsum, a4

Analytical Methods Determination of Zinc in the Bath.

Reagents: Solution--contains exactly 33.0 g./L. K,Fe(CN)~.3H._,O and 6 g./L. Na~SO~.

Indicator--saturated solution of uranium acetate. 1. Under a hood, to exactly 5 ml. of cyanide zinc solution in a 250 ml.

beaker, add 5 ml. 1:1 HNO3 and 5 ml. 1:1 H2SO4. Boil until dense white fumes are liberated. Add about 1 g. of (NH4),.,SO4 and fume about 5 rain. more.

sl Hull, U. S. Pat. 2,154,451; OpIinger, U. S. Pat. 2,154,469. ~a Cony. Proz. Am. Electroplaters ' Soc. (1939); U. S. Pat. 2,149,344.

Trans. Electrochern. Soc. 55, 315 (1929). R. O. Hull, Cony. Proc. Am. E1ectroplaters' Soc. (1937); U. S. Pat. 2,164,924.



CYANIDE ZINC I,I,ATING BATIIS 421

2. Let cool and cautiously add 50 to 75 nal. distilled water, dissolving precipitate completely.

3. Make ammoniacal (to litmus), add an excess of 5 ml. concd. NH4OH, filter and wash precipitate with hot distilled water containing 10 ml./L, concd. NH4OH.

4. Make filtrate neutral to litmus with HC1, and 5 ml. coned. HCt in excess.

5. Add 15 g. NH4C1 and 25 ml. H2S saturated distilled water. 6. Heat to 80 to 85 ~ C. and bring volume to 300 ml. 7. Titrate with potassium ferrocyanide solution, nsing uranium acetate

on a spot plate as an outside indicator, to the first light red-brown tinge.

Number of ml. of standard potassium ferrocyanide required - - 5 = ~z./gal. of zinc.

7'otal 5odium Cyanide. Reagents: Solution--contains exactly 26.0 g./L. C.P. AgNO~ plus

12.5 ml./L. C.P. HNO3 solution. Indicator--dry mixture of 95% NaHCO3 and 5% K2Cr~OT.

1. Dilute exactly 1 ml. of plating solution sample to about 150 ml. 2. Add 5 g. indicator and stir. 3. Titrate, stirring constantly, with silver nitrate solution to the first

permanent brick-red color. Number of ml. of standard silver nitrate solution required equals

number of oz./gal, of total sodium cyanide.

Sodium Hydroxide (Caustic Soda). Reagents: Solution--H~SO~ 0.940 N.

Indicator--LaMotte sulfo-orange solution. 1. To exactly 5 ml. of plating solution add 1 g. of NaCN, 10 ml. of

water, and 10 drops of indicator solution. 2. Titrate with the standard sulfuric acid solution to a color change

from orange to yellow, or titrate to a pH of 11.0, using the high alkaline type glass electrode as described by Gray? 5

Nmnber of ml. of standard sulfuric acid required ~ number of oz./gal, of caustic soda.

Sodium Carbonate. Reagents: Solution--BaC12 10%.

Solution--HCl 0.710 N. Indicator--Methyl orange solution.

1. Dilute exactly 5 ml. of plating solution to about 100 ml. with hot distilled water, in a 500 rot. erlenmeyer flask.

2. Add 15 ml. barium chloride solution, heat to boiling and allow to settle on the steam plate for 30 min. after closing the mouth of the flask with a wad of cotton.

3. Filter through No. 30 Whatman, washing the flask and residue on the paper with hot distilled water.

4. Return paper and precipitate to the flask, add 100 ml. hot distilled water, three drops of methyl orange, and shake to pulp the paper.

e"' Cony. Proc. Am. Electroplaters' Soc. (1941).



422 ~. o. I IULL AND C. J-. WERNLUND

5. Titrate with standard hydrochloric acid to the first permanent color change of methyl orange.

Number of ml. standard hydrochloric acid solution required = oz./gal, of sodium carbonate.

Molybdenum. Reagents: Solution--KMnO4 0.117 N.

Saturate solution of ferric alum 2 parts by volume + phosphoric acid C.P. 85% 1 part by volume.

Jones reductor--See Scott's Analysis, Vol. I, or Treadwell and Hall, Vol. II, for description.

1. Under a hood, to exactly 5 ml. of zinc solution in a 250 ml. beaker, add 5 ml. 1:1 HNOa and 5 ml. 1:1 H~SO4. Boil until dense white fumes are liberated. Add about 1 g. of (NH4)2SO4 and fume about 5 minutes more.

2. Let cool and cautiously add 50 to 75 ml. distilled water, dissolving precipitate completely.

3. Make ammoniacal (to litmus), add an excess of 5 ml. concd. NH4OH, filter and wash precipitate with hot distilled water containing 10 ml./L, concd. NH4OH.

4. Make filtrate acid to litmus with H2SO4; add 2 to 3 ml. concd. H~SO4 in excess. Pass slowly through Jones reductor into 15 ml. ferric alum-phosphoric acid solution.

5. Wash reductor with 100 ml. 2.5% H2SO4 and titrate with potassium permanganate solution to a permanent faint pink.

Number of ml. standard potassium permanganate solution -- 10 = oz./gal, of molybdenum.

Mercury in Solution. Reagents : So lu t ion - -KI 8.300 g. /L.

Indicator--KNO~ C.P. Starch 1% (freshly prepared solution).

1. Under a hood, acidify exactly 50 ml. of plating solution in a 250 ml. beaker with HNOs, boil until all cyanides are removed. Cool.

2. Neutralize with NH4OH, then add 2 ml. concd. HNOs. 3. Cool to 20 ~ C., add a small crystal of KNO2. 4. Titrate with standard KI solution, using 1% starch solution Oll a

spot-plate as an outside indicator. M1. standard potassium iodide solution required x 0.0134 -- oz./gal.

Hg.

V. ANODES

A wide variety of shapes and composition of the zinc anodes is used in cyanide zinc plating, depending upon the type of bath and the kind of operation. The two main types are the ball type and the customary bar type anodes, cast from slab zinc or suitable alloys. Each of the two shapes has several claimed advantages.

High purity zinc anodes such as Special tligh Grade or High Grade :~'; ~For analysis see Am. Soc. for Testing Materials Designation B6-37, ASTM Standards

1940, Part I, p. 680.



C Y A N I D E ZINC'. I 'I ,A'I 'IN(2, B A T t l S 423

are recommended for the straight cyanide bath, since usually no addition agents are present and no provision is made for removing impurities from the bath. However, alloy anodes may also be used for this bath provided they contain no heavy metals that will deposit out with the zinc.

For the zinc-mercury process, either Intermediate or Prime Western zinc alloyed with 0.5 to 1 ~'o mercury, as mentioned abovey may be used.

The bright zinc baths necessitate high purity zinc such as Special ltigh Grade or High Grade anodes free from objectionable metals, unless recourse is had to purification of the bath at frequent intervals to remove these heavy metal impurities. If alloys are used in which metals such as aluminum, calcium, or magnesium (which do not plate out) are present, these do not influence the character of the zinc deposit adversely but, on the contrary, may be advantageous in improving the corrosion characteristics of the anodes. Experience has shown that control of zinc metal content in the bath is somewhat difficult and pre- sents a problem in bright zinc plating, since variations in this bath constituent influence the current density range and efficiency of bright zinc baths. Zinc passes into solution both electrochemically and by chemical attack, and hence it usually tends to accumulate in the bath to an excessive degree. Aluminum, when present in zinc anodes, increases the rate of anode corrosion, while mercury together with aluminum as alloyed with the zinc tend to reduce the chemical attack and to improve anode corrosion characteristics. Magnesium or calcium a9 alloyed with zinc is effective in reducing anode efficiency to any predetermined value. For this purpose, 0.18% magnesium is used in zinc anodes for still plating and 0.05% magnesium for barrel plating to reduce the anode efficiency to about 88% which is about equal to the average cathode efficiency.

With other than specific alloy anodes, the zinc content of the bath can be controlled, although with difficulty, by using from 25 3 to 50% inert steel anodes together with pure zinc anodes. However, this scheme results in a serious complication: The zinc and iron in the alkaline solution produce an electrolytic or voltaic couple and consequent electro- chemical corrosion of the zinc when the bath is idle. There is then a rapid decrease in the zinc content of the bath when in regular operation, due to the steel anodes. If zinc anodes do not corrode fast enough under steady operating conditions, it is common practice to use a few steel anodes, e.g., 5 to I0 N of the effective zinc anode surface, to accelerate chemical attack.

VI. PREPARATION OF BASIS METALS

No special treatment of basis metal is necessary for plating cyanide zinc. For steel, the usual alkaline cleaner and acid pickle are used, and care must be exercised only to prevent overpickling. Zinc plating on malleable or cast iron is not recommended.

87 See footnote 10. ** See footnote 11. a~ See footnote 13.



424 R . o . t I U L L AND C. J . \ V E R N L U N D

vu. CESTS of" D>:posn's

The usual thicknesses of zinc deposits recommended are those specified in "Tentative Specifications for Electrodeposited Coatings of Zinc on Steel, ''4~ prepared jointly by the American Electroplaters' Society, the National Bureau of Standards, and the American Society for Testing Materials. Three types of coatings are specified as follows, depending upon service and severity of exposure:

(a) Type O.S. minimum thickness 0.001 in. (25 t*) on significant surfaces ;

(b) Type L.S. minimum thickness 0.0005 in. (13/*) on significant surfaces ;

(c) Type R.S. minimum thickness 0.00015 in. (3.8 I*) on significant surfaces.

"Significant surfaces" are those surfaces that are visible and subjected to wear or corrosion, or both.

Other tests of the zinc plate, such as the Preece test and salt spray test, are quite widely used but results of these tests give only an approximate idea of the corrosion resistance and expected life of zinc coatings. The Farnsworth-Hocker 41 intermittent immersion test con- sists of immersing the zinc plated article in an ammonium chloride solution and then allowing to dry in air, then immersing again, etc., at regular intervals. The test is not sufficiently rapid for shop control purposes.

The Brenner magnetic test 4= is utilized for determining the thickness of zinc over steel. The mechanical force required to separate a permanent magnet from the surface is determined by a sensitive hair-spring attached to a calibrated scale, and zinc or other non-magnetic metal thickness is determined by reference to a suitable curve supplied with each instrument.

The "dropping test" is widely used for determining thickness of zinc plate. As originally proposed by S. G. Clarke, 4a a solution of iodine in potassium iodide was used. This test was later modified by Hull and Strausser, 44 who recommend a solution of 100 g./L. (13.3 oz./gal.) ammonium nitrate and 55 ml./L. (7.3 ft. oz./gal.) of nitric acid C. P., sp. gr. 1.42. This solution is dropped at the rate of 100 __+ 10 drops per minute on the zinc plated specimen until the basis metal appears. The rate of stripping is 10 seconds for each 0.0001 inch (2.5/~). To facilitate the detection of the end-point in the case of zinc plated steel, the solution freshly prepared may also contain 3 g./L. (0.4 oz./gal.) potassium ferricyanide which will give a characteristic blue color as soon as the steel is exposed. A dropping solution for the same test was more recently proposed by Brenner, 45 consisting of 200 g./L. (27 oz./gal.) chromic acid and 50 g./L. (6.7 oz./gal.) sulfuric acid which

~o Am. Soc. Testing Materials Designation A164-40-T, Revised June, 1940, Am. Soe. Testing Materials Standards 1940, Supplement Part I, p. 300.

~. Trans. ]~lectrochem. Soc. 45, 281-293 (1924). ~aJ'. Research Natl. Bur. Standards 18, $6S (1937); 20, 357 (1938). ~J ' . Electrodepositlon Tech. Soc. 8, No. 11 (1933). u Monthly Rev. Am. Electroplaters' Soc. 22 (March, 1935).

J. Research Natl. Bur. Standards 23, 387 (1939).



CYANIDE ZINC PLATING BATIIS 425

is dropped at the rate of 100 • 5 drops per minute. The rate of stripping depends upon the temperature, as follows:

Rate of Penetration in Temperature 10 see.

21 ~ C. ( 7 0 ~ F.) 2.5~ (0.000098 in.) 28 ~ C. (82 ~ F.) 2.8~ (0.00011 in.) 35 ~ C. (95 ~ F.) 3.0~ (0.00012 in.)

VIII. PIIYSICAL CHARACTERISTICS OF DEPOSIT

Zinc electrodeposits from cyanide baths are somewhat less ductile than from the sulfate bath. However, very ductile zinc deposits may be produced from the warm, concentrated cyanide bath without addition agents. Deposits from the cyanide bath appear to be dense and rela- tively free from pores. Coarse-grained deposits, such as those from straight acid zinc baths, are very rare from 'cyanide baths. Generally speaking, electrodeposited zinc coatings whether plated from cyanide or from acid sulfate baths are of excellent physical quality and are less brittle than the hot dipped coatings that contain appreciable thicknesses of zinc-iron alloy layers underneath the outer zinc coat.

Resumen del artlculo: "Bafios de Cianuro para el Cincado Electrolltico."

La aplicacidn principal del cincado es la proteccidn del acero contra la eorrosidn. Las bafios moderno~ producen un depdsito relativamente brillante, y tienen buen arrojo, pero el coste es mayor que de un bafio Acido, la velocidad de deposicidn es menor, y no se puede planehear el hierro colado.

Para los Anodos se alea el Zn con A1, Hg, o Ni para poder emplear una densidad de corriente anddica elevada, y con Mg o Ca para dis- minuir la eficieneia anddica al nivel de la eatddica. Se debe mantener constante la razdn de la normalidad total de eianuro e hidrdxido sddicos a la del ciamlro de cinc. Hay muchos agentes que mejoran el briilo (Tabla I I I ) . Se describen las condiciones de operacidn y m&odos de an,~lisis.

DISCUSSION G. SOnF, R~Em;I~: 1 am very pleased to have been asked to discuss this excellen~

paper. My comments will be in the nature of minor additions and. here and there, from a somewhat different viewpoint.

Our tests with aluminum-zinc anodes indicated that, while almninum assists in producing smooth zinc castings, it has no effect on the amount of sludge %treed. This depends entirely on the purity of the zinc and the anode current density em- ployed, high current density making it possible for heavy metal impurities to dis- solve in the bath.

While malleable iron ordinarily cannot be plated successfully, recesses not being covered, one occasionally encounters a foundry which produces metal which can be plated. A study of the size and distribution of the constituents of malleable iron may explain this anomaly. Similar difficulties are encountered with shot- blasted steel having a surface with high carbon content. A change in the heat- treating cycle permitting some deearburization or the use of sand for blasting often remedies the trouble.

~e Technical Director, Udylite Corp., Detroit, Mich.



426 DISCUSSION

The mercury-zinc type of bath must not be used for plating brass parts or steel parts which come into contact with brass. The small amount of mercury present is capable of causing season-cracking of the brass. Expensive and dangerous fail- ures are on record. On the whole, this type of bath may be considered obsolete in view of the bright zinc developments.

Our tests show that about 4 tools N a O H are necessary to keep 1 tool of the Na2ZnO2 in aqueous solution. I f this is true in the cyanide solution, a bath containing 4.5 oz./gal. Zn (about 0.5 M ) would require 16 oz./gal. N a O H (3.0 M) for 100% solution, and a bath containing 10 oz./gal. N a O H could hold only about 63% of the zinc as zincate. The hydrolysis of NaCN may contribute to the amount of N a O H present.

The parallelism of the N a O H content and the Zn content should be watched in practice. Drag-in of pickling acid (and perhaps absorption of COs from the air) lowers the N a O H content and makes additions necessary.

The statement that "for good anode corrosion, not less than 4 volts must be maintained across the plating tank bus bars and an anode current density of 10 amp./sq, ft. should be used" may be misleading. Tank voltage is a complex factor and varies too much with the shape of the parts being plated, the anode-to- cathode distance, contact resistances, etc., to be used for plating control. Similar criticism may be leveled at the expression "ratio anode-to-cathode surface." It is well to consider anode and cathode current densities separately.

The rust-protective value of the mercury-zinc deposit is very low if the mercury content of the plate goes too high, something which has often happened in practice.

In the zinc analysis we prefer to use diphenylbenzidine (1 g. in 100 ml. coned. H~SO0 as an inside indicator instead of uranium acetate as an outside indicator. The color change from blue to pale-yellowish green is satisfactory, especially if one drop of 10 g./L. FeC13 is also added. We do not use Na~SO3 or H~S and have found no need for a correction factor with this method.

The standard AgNO3 titration with KI as indicator is satisfactory for determination of total sodium cyanide if N a O H is added to the sample to take up the zinc.

In practice, the anode efficiency should be 5 to 10% higher than the cathode efficiency in order that zinc salt additions be unnecessary to replace the zinc lost by dragout.

The ferricyanide addition to the Hull & Strausser dropping test delays the appearance of the end-point 1 to 5 seconds when the basis metal is steel, and 10 to 15 seconds when it is cast or malleable iron.

As pointed out in Lyon's paper on acid zinc plating, the Preece test is unsuitable for any testing of zinc coatings.

R. O. HULL: As far as aluminum-zinc anodes are concerned, I have no infor- mation to indicate that Mr. Soderberg is not perfectly correct in that aluminum assists in producing smooth zinc coatings and has no effect on the amount of sludge formed. Perhaps Messrs. Graham and Hogaboom can enlighten us on the subject.

The second point made is the plating of malleable iron and cast iron. If, by using sand-blasted cast iron, or by using cast iron which has been specially heat- treated, we can consistently and satisfactorily zinc-plate from a cyanide hath. we will have made another step forward.

Next is the observation, or perhaps difference in opinion, as to the presence of zinc in a cyanide bath as sodium zincate or sodium zinc cyanide. All the data we have point to the presence of a large percentage of zinc as sodium zincate in the bright zinc bath. First, we know from experience that zinc does deposit from sodium zincate at a very high current efficiency, and from zinc cyanide at a low current efficiency. Since our customary production baths operate at high current efficiency, we can assume, at least by analogy, that zinc is present chiefly as sodium zincate.

I am glad Mr. Soderberg pointed out the hydrolysis of sodium cyanide to form sodium hydroxide, because the hydrolysis of sodium cyanide to the extent of 20 to 25% would account for the difference between the 63% that he mentioned as sodium zincate, and the 75 or 80% that we have proposed.



CYANII,I,; ZI NC I'[,ATING IIATllS 427

Vhla]!y, ~c ha~c c~mducllvi~5 d;Ll;~ ,m zim' c~auhte baths. Without g,,mg inh, details we can bri~fb" q~te that a zi,!c cyanide, plating bath has considerably less eonductiviW than w,m!d be indicated hy a similar so!ution in which there is a high concentration of frec sodimu hydroxide. Therefore we can assume that a great deal of caustic soda is tied up in the sodium zincate complex, and little caustic soda is free in the solution.

The next point Mr. Soderberg makes is the parallelism of the caustic soda content and the zinc content, which does not call for discussion. His observations on analytical procedures and improvement are well worth while.

The point made relative to anode efficiency (which should be 5 to 10% higher than the cathode efficiency) is, perhaps, well taken, but we feel that we accomplish the same result by the normal chemical solution of zinc when the bath is standing idle. Therefore we attempt to have about the same anode efficiencies as cathode efficiencies during the plating operation. Experience has shown that the zinc metal content under normal plating conditions remains re- markably constant.

As far as the dropping test is concerned, I expect to check Mr. Soderberg's observations as to the delayed end-point in cases of additions of ferricyanide indicator.

C. J. WERNLUND: Mr. Soderberg's suggestion concerning closer definition of tank voltages is well taken. We should have said, "Use 4 to 5 volts at the tank bus bars, depending on the distance between the work and the anodes." A rela- tively high tank voltage is essential in the "duozinc" process to insure uniform and complete solution of the mercury in the anodes and to prevent mercury-spotting of the plated work.

Mr. Soderberg's remarks concerning the possibility of season-cracking 9f "duozine" plated brass should not be overlooked, but in practice this situation has caused serious trouble only when inexperienced or careless operators added excessive amounts of mercury salts to the solution at one time, with the result that abnormally high percentages of mercury were thus secured in the coating, which affected the brass base adversely.

G. B. HOGABOOM, SRY: Mr. Soderberg is correct in his statement that mercury- zinc deposits on brass tend to cause season-cracking due to the absorption of some of the mercury by the brass.

I t is well known that aluminum added to zinc materially decreases its chemical solubility. A. K. Graham and I have presented papers before this Society which bear out that point. I t is known that aluminum salts have been added to zinc solutions to obtain a finer crystal structure of the deposit. Aluminum is not added to the anode to decrease the grain size or to refine the cathode crystals; it is added to the anode to prevent a chemical attack of the anode when the current is not flowing. The advantage of this is borne out by the fact that it is quite possible to maintain constant pH and a fairly definite anode area by using a ball anode (composed of zinc plus a little calcium or magnesium) in a spiral wire cage in a cyanide zinc solution. If there is a protective film of calcium or magnesium oxide on the zinc to prevent a rapid attack of the anode by the solution, then that film or blanket is going to increase in thickness as the anode is dissolved, and will eventually almost completely insulate one zinc ball f rom another. I f the zinc balls are clean, free from insulating films, due to there being no chemical attack by the solution, then it is quite possible to use ball anodes, because each ball is in perfect contact with the other.

Dr. Lyons ~ says that the acid zinc solution was improved in 1910. The improvement was really made early in 1900.

An acid zinc solution was developed by the U. S. Galvanizing Company, and also by the Hanson-Van Winkle Company, which solution was a subject of litiga- tion. Dr. C. F. Burgess, a past president of this Society, who is here today, was the expert for the Hanson-Van Winkle Company. The case was carried to the Supreme Court, and the Hanson-Van Winkle Company was rendered a favorable decision. Tha t solution contained an aluminum salt.

R. O. HULL: I should like to correct Mr. Hogaboom in one respect--we do not add magnesium to the zinc anode to prevent chemical attack by the plating solu-

~7 Research Engineer, Hanson-Van Winkle-Munning Co., Matawan, N. J. ,t'~ See page 389.



428 DISCUSSION

tion. And, as far as we know. magncsimn has no intluence on chemical attack. It has an influence only on the relative rate at which zinc goes into solution upnn passage of the current. Observation shows that the anode efficiency is about 85 to 88% when 0.187,, magnesium is added to high-purity zinc. Aluminum is similarly effective, but to a far lesser extent. Mercury in the anode, however, pre- vents solution attack. I believe mercury has little influence upon reduction of anode efficiency.

COLIN G. FiNK~": Mr. Soderberg brought up this matter of cast iron. There is no trick in plating cast iron if you remember that the material you have to remove from the surface is graphite and slag. Many cast irons today are prac- tically free from graphite and slag. If you remove graphite and slag out of the surface (and there are various ways of doing it), you will have no trouble in getting good adherence. Many of the men here I know will agree with me.

R. O. HULL: I agree with you. With certain types of cast iron there is no difficulty in plating with cyanide zinc, but with other types of cast iron a most difficult situation arises in plating with zinc, no matter what the preliminary treatment has been.

COLIN G. FINK: The more you pickle ordinary cast iron, the worse the ad- hesion. Slag and graphite are not removed in pickling.

C. B. F. YOONGS~ Hydrogen embrittlement has not been mentioned in regard to acid and cyanide baths. I think it has been said that acid zinc produces less hydrogen embrittlement than does the ordinary cyanide zinc, and the latter produces less hydrogen embrittlement than the bright zinc solutions. Am I correct in this, Mr. Hull?

R. O. HULL : I would not say that bright zinc shows more hydrogen embrittlement than dull zinc.

49 Head, Division of Electrochemistry, Columbia University, New York City. r~ Electrometallurgist, P. O. Box 292, Flushing, N. Y.



cyanide zinc plating baths

Documents