Organic additives in zinc electroplating

Electrodeposition and Surface Treatment, 3 (1975) 77 - 95 0 Elsevier Sequoia S. A., Lausanne - Printed in Switzerland

ORGANIC

ADDITIVES

77

IN ZINC ELECTROPLATING

K. BOTO Department (Australia)

of Defence,

Materials Research

Laboratories,

Ascot

Vale, Vie. 3032

(Received March 5, 1975)

Summary The use of organic additives in zinc electroplating is reviewed with particular emphasis on the types of compounds used, the bath type for which they are most suited, and their function with respect to brightening and levelling action. Fundamental studies of the mode of action of these additives are also reviewed and comparisons are made with similar studies of nickel and copper electrodeposition. It is concluded that much more fundamental studies are required to fully explain the effects of additives. Further, the effect of codeposition of organic impurities on the corrosion resistance of electrodeposited zinc requires urgent study.

Introduction The use of organic compounds as zinc plating bath additives can be traced back as far as 1907, when Snowden [l] reported that addition of formaldehyde in small amounts to acid plating baths had the effect of reducing the grain size of the deposit. Today, a very large variety of organic additives is used in zinc electroplating, their purpose being (a) to improve the appearance and properties of the deposit and/or (b) to improve the operating performance of the plating bath. In the first category, such factors as brightness, grain size and deposit stress are important and in the second category, anodic depolarisation, current efficiency and throwing power can sometimes be improved by use of suitable addition agents. The purpose of this review is to provide a brief history of the development of the usage of these additives and to give an outline of the types of compounds used and the bath type (e.g. cyanide, acid, etc.) for which they are most suited. The emphasis will be mainly on those additives employed in improving the plate appearance, although the other type (category (b) above) will be discussed briefly. Compounds used in category (b) are usually added in large quantities, e.g. 20 g/l, and their mode of action is more obvious and better understood than the more subtle effects produced by the addition of small amounts, e.g. 0.1 g/l, of category (a) compounds, that is, the brightening and levelling agents.

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The subject of zinc “electrowinning” will not be discussed in this review which is intended only to deal with electroplating, that is, the deposition of a thin metal layer on a substrate consisting of a different metal. Despite the voluminous literature available on the formulation and composition of additives, relatively few fundamental studies appear to have been made concerning the mechanism by which the addition of these compounds affects the deposit structure and appearance. Excellent reviews by Bockris [2] and by Despic and Popov [3] outlining the basic theory of electrodeposition of metals are available. The article by Despic and Popov does include a brief discussion of the application of modem electrochemical theory to explain phenomena such as electropolishing and levelling, for example. However, it is generally fair to state that the application of basic principles to explain the effects of organic additives in electrodeposition is lacking. The reasons for this will be discussed in more detail below. Also, relatively little attention has been paid to the very important question as to whether the possible occlusion of the compounds in the plate has any effect on the corrosion resistance of the zinc plate. Such fundamental studies as have been made will also be briefly discussed in this review as well as some speculation as to the direction of future research into this long-neglected aspect of zinc electroplating. History and development of organic additives (i) Acid baths Despite a fairly large amount of very early work concerning the addition to acid baths of “colloids” such as gum arabic [4] , licorice [ 51 and grape sugar [6], little commercial interest was shown until 1921 when Classen [ 71 patented an acid zinc plating bath containing small amounts of such additives as starch, albumin and gelatin (as a mixture). As in most of the earlier work, it was claimed that such additives gave white, shiny and finely crystalline deposits. Most of the early work in this field appears to have been concerned with additives for use in acid baths. This is possibly because of the poorer quality deposit generally obtained from such baths without additives, as compared with the much more popular alkaline cyanide baths from which quite reasonable plate can often be obtained without the use of additives. Also in the early work, little attention was paid to the effect of these additives on other plate properties such as ductility or hardness. As a further improvement in acid plating (usually of higher current efficiency than cyanide plating), Marino [8] in 1928 suggested the use of alkali metal salts of borobenzoates to prevent anode passivation which is a common problem in plating. The addition of the usual brightening agents such as glycerol and dextrin was also recommended. As the acid bath formulation became more complex and the range of “brightening” and “depolarising” additives increased, a number of workers began to investigate the effect of such additives on the throwing power and current efficiency of

79

the baths and hence make some attempt at classification of the advantages and disadvantages of particular additives. Sato [9] studied a number of compounds and classified them according to their effect on deposit quality and current efficiency. Those compounds giving good quality deposits often had an adverse effect on the current efficiency. Sato came to the general conclusion that amyloses were good addition agents while some alkaloids and proteins were poor additives. Rice [lo] made some interesting comparisons on the performance of acid baths with additive (glucose) and alkaline cyanide baths without additive. In general, it was found that the cyanide baths were superior in throwing power but expensive to maintain and that the acid bath with additive gave a better coloured deposit than the cyanide bath. Pan [ll] carried out a study of the effect of various acid sulphate bath formulations on the throwing power, concluding that the addition of pyridine was effective in increasing the throwing power of such baths. Similarly, Cambi and Devoto [12] also found that pyridine was effective in increasing the current efficiency of acid sulphate baths. Gockel [ 131 stated that the use of thiourea or methylthiourea as an additive in sulphate baths gave similar results to those obtained with a cyanide bath, thus avoiding the use of cyanide baths. Lapin et al. [14] carried out an extensive study of various acid baths and compared the performance of a cyanide bath. They concluded that the acid bath containing licorice as an additive was comparable in quality and throwing power with a cyanide bath without additive. (ii) Cyanide

baths

The above studies demonstrated the beneficial effects of certain organic additives in acid baths and were also beneficial in detailing the effects, beneficial or otherwise, that these compounds had on other properties of the acid baths. In 1936, Mattacotti [15] patented an alkaline cyanide bath containing small amounts of organic additives such as acetone, methyl ethyl ketone, quinone and others. About the same time, Calef [16] also described the use of gum arabic in cyanide baths. Also, Higgins [ 171 claimed that improved deposits could be obtained by the addition of corn syrup to cyanide solutions. Soon after this, numerous patents and publications appeared describing the use of such additives as thioureas or furfural [ 181, phenylthiourea [19], thiocyanate-formaldehyde [20], dextrin or glue [21], heterocyclics such as coumarin, furan and morpholine [22] and gelatin [23] to list just a few. Most of the compounds used to this date were low molecular weight species or natural polymers. However, in 1938, Barrett and Wernlund [24] patented the use of polyvinyl alcohol and other polymers as brightening and levelling agents. Artificial polymers of this type are used extensively today in proprietary mixtures. Because of the inherently better properties of the cyanide baths, little attention has been paid to the effect of additives on throwing power and anode depolarisation, unlike acid baths where these properties could be drastically altered by suitable addition agents. Another point that was recognised fairly quickly was that, in general, the brightening agents were

80

more efficient in cyanide baths, again because of the inherently better qualities of this type of bath as compared with non-cyanide baths [25]. Thus, less additives were necessary and hence the possibility of obtaining brittle plates, through over-use of addition agents, was reduced. Palatnik et al. [ 261 pointed out that compressive stresses were produced in zinc plate deposited from plating solutions containing additives such as glue and sulphided castor oil. Kudryatsev and Nikiforova [27] describe an acid sulphate bath containing sulphonic acid derivatives added to give bright coats of better ductility than usually obtained. This and later work by Hothersall [28], who measured the compressive stress produced in zinc plate produced under different conditions, serve as reminders that additives can have detrimental effects on the deposit if not carefully controlled. From the 1940’s to the present, an enormous amount of literature has been published in which a very large number of different types of organic compounds have been proposed as brightening and levelling agents in cyanide baths. Besides those compounds previously mentioned, other types of compounds extensively used are aromatic aldehydes [29] , proteins [ 301, various types of polymers [31, 321 and polyfunctional compounds [33, 341. It has been found, in many instances, that the use of a mixture of compounds, classified as “primary” and “secondary” brighteners, is necessary to obtain suitably smooth and bright plate. It is claimed by some authors that the components exert a synergistic effect on each other. This effect will be discussed in more detail below. Also, as will be seen from the discussion of the fundamental studies that have been published, it is difficult to evaluate the need for the very complex mixtures of additives used in some proprietary mixtures. In addition, varying claims are made concerning the effectiveness of certain brighteners, some authors claiming to achieve excellent results with such simple additives as thiourea or a single aldehyde such as vanillin. A further complicating feature of the use of addition agents is that the longterm oxidation, reduction, or decomposition products of these additives may in fact be the actual brightening agents or may influence the action of the additive. It is well known that, in many cases, maximum brightness is not achieved until the bath (plus additives) has undergone quite long periods of electrolysis. (iii) Modern non- and low-cyanide baths Although cyanide baths are still widely used and still generally accepted as the easiest type of bath from which good quality deposits can be obtained, the toxic properties of cyanide and the growing concern about pollution and waste disposal problems have led to the growing interest in non- or low-cyanide solutions in recent years. In all of these, the addition of brighteners, levellers, and complexing agents is essential if high quality plate is to be obtained. Also it is generally accepted that these baths need to be more stringently controlled as far as the composition of the additives is concerned. Many patents and other publications are now available detailing suitable non-cyanide bath compositions. Rama Char and Shivaraman [ 351

81

advocate the use of a fluoroborate bath containing 1 g/l of p-naphthol. A patent by Chester [ 361 describes an acid bath containing a fairly complex additive mixture of sulphonated lignin (a natural phenolic resin), molasses and trifluoracetic acid. The last-named compound probably acts as an efficient anode depolariser. Kappana and Deoras [37] describe the use of a formate bath containing thiourea while Chester and Main [381 have patented the use of a gluconate-based acid bath containing thiourea and a peptone as additives. The gluconate functions as. a suitable complexant and anode depolariser. Additives employed in alkaline non-cyanide baths include such mixtures as aromatic and heterocyclic aldehydes, polymers (polyethylene glycol) and triethanolamine [ 391, furfural, lignin sulphonate and gelatin [40], while a patent for an alkaline low-cyanide bath claims that a mixture of y-picoline, 1,4 diaminobenzene, formaldehyde and epichlorohydrin 1411 is successful. In these baths, amine-type compounds are often used as complexing agents to replace the cyanide. Acid chloride baths have also been shown to produce good quality deposits by the use of such addition agents mixture as imidazoline derivatives, 0-chlorobenzaldehyde and ethylene oxide-nonylphenol condensate [42] . Another patent [43] claims that the use of sodium dodecyl ether sulphate in an acid chloride solution produces a bright plate. Additives that have been used in pyrophosphate baths include aldehydes, ketones and sulphonic acids [44]. A paper by Domnikov [45] describes the use of a pyrophosphate bath containing triammonium citrate as anode depolariser and glue and thiourea as brightening agents. This bath is claimed to produce zinc plate of very high quality, corrosion resistance and hardness. Compounds such as saccharin, in conjunction with alcohols ( butynediol or glycerol) and anisaldehyde, are also claimed [46] to be very effective additives to pyrophosphate zinc plating solutions and the author also claims that such a bath runs at a much greater current efficiency than the usual alkaline cyanide baths. Many of the alkaline non-cyanide and phosphate baths present their own special problems in waste disposal, but their ingredients are considered to be far less toxic than cyanide, which is difficult to remove chemically, e.g., by oxidation. A representative listing of the more modern types of baths and additives employed can be found in Table 1. Obviously, this list represents only a sample of the many different formulations available. Also included is a description of the probable role of the various additives where this was stated in the original literature or where it could be inferred by comparison with other bath formulations. The distinction between brighteners (“primary” brighteners) and levelling agents (“secondary” brighteners) is somewhat artifical as many of the compounds listed can have both functions. However, the distinction is made here to indicate the most probable major function of each component and to serve as a reminder that good levelling agents do not necessarily produce a very bright finish and, conversely, that some good brightening agents show little or no levelling action. These points will be discussed

32 TABLE

1

Some typical organic zinc electroplating

additives

Type of bath

Additives used

Purpose of additive

Ref.

Alkaline cyanide

(a) Quinone derivatives (b) Aromatic aldehydes (c) Water-soluble polymers e.g. polyvinyl alcohol

(a) Broadens current density range for bright plating (b) “Primary” brightener (c) “Optional” brightener (probably acts as a levelling agent)

32

Alkaline cyanide

(a) Polymer (gelatin reacted with epoxides) (b) Saturated mono-halogen aliphatic acid or its salt with a tertiary base (e.g. 2 chlorobutyric acid)

(a) Levelling agent (b) Brighteners

34

Alkaline cyanide

(a) Polyvinyl alcohol (b) Gelatin (c) Aromatic amides

(a) Levelling agent (b) Levelling agent (c) Brightener

48

Acid sulphate

(a) Thiourea

(a) Brightener

49

Acid sulphate

(a) Sodium succinate (b) Nicotinamide and/or ethoxylated naphthosulphonic acid

(a) and (b) Brightening and levelling agents

50

Weak acid sulphate

3-carbamoyl-1-( carbamoyl methyl) pyridinium chloride and other pyridinium compounds

Brighteners

51

Weak acid sulphate

Cetyl trimethyl ammohium bromide

Brightener

15

Acid chloride

(a) Citric acid (b) Nonylphenoxypoly (oxyethylene) ethanol (c) Reaction product of aromatic aldehydes with acetylenic alcohols

(a) Anode depolariser (b) Levelling agent (c) Brighteners

52

Weak acid chloride

(a) Polyhydroxide compound (b) Aromatic aldehyde (c) High molecular weight base

(a) Complexing agent (b) Anode depolariser (c) Levelling agent

53

Neutral chloride

(a) (b) (c) (d)

(a) and (c) Brighteners (b) Anode depolariser (d) Levelling agent

42

Alkaline -non cyanide

(a) Aromatic or heterocyclic aldehydes (b) Polymers e.g. polyethylene glycol (c) Triethanolamine

(a) Brightener (b) Levelling agent (c) Complexing agent to replace cyanide

39

Imidazoline derivatives Citric acid O-Chlorobenzaldehyde Ethylene oxide-nonyl phenyl condensate

83 TABLE 1 (continued) Type of bath

Additives used

Purpose of additive

Ref.

Alkaline -non cyanide

(a) Anisaldehyde bisulphite (b) Gelatin (c) EDTA salt

(a) Brighteners (b) Levelling agent (c) Complexing agent

92

Fluoroborate

(a) OP-10 (Polyalcohol) (b) Urea or thiourea

(a) Leveller and to increase throwing power (b) Brightener and grain refining agent

54,55

Fluoroborate

(a) Polyvinylpyrrolidine (b) Naphthalene sulphuric acid or thiourea (c) Sulphosuccinic esters

(a) Levelling agent (b) Brightener (c) Surfactant to aid in solution stabilisation

56

Pyrophosphate

(a) Triammonium citrate (b) Glue (c) Thiourea

(a) Anode depolariser (b) and (c) Brighteners

45

Pyrophosphate

(a) Saccharin (b) Naphthalene sulphonate (c) Gelatin

(a), (b) and (c) Brighteners

57

Iodide (ZnI/NH41)

(a) Sodium citrate (b) Polyether (c) Citral (an aldehyde)

(a) Anode depolariser (b) Levelling agent (c) Brightener

58

Acid sulphamate

(a) Fatty alcohol sulphate (b) Sulphur-containing acid or thienylidene derivatives

(a) Surfactant (b) Brightener

59

further below. Kardos’ [61] classification of brightening agents into those (i) which will produce a bright finish on an initially dull substrate or (ii) which maintain brightness on an initially bright substrate, is fairly closely related to the distinction made between levellers and brighteners in this article. Despite the complexity of most of these mixtures, it can be seen that the most widely used brighteners and levellers are aromatic aldehydes, sulphur-containing compounds, alkyl and aryl ammonium salts and various natural and artificial polymers (e.g. gelatin, polyvinyl alcohol). In some instances, multi-functional additives are prepared by condensation reactions between simpler and more common additives. Most modem formulations also necessarily contain suitable detergent additives to maintain the organic compounds in solution, as many of these species have limited solubility in water. Fundamental studies The primary aim of this section is to discuss briefly the theories that have been proposed concerning the mechanism by which additives affect the plate morphology and appearance. Also, studies that have been made

84

concerning codeposition of organic material and the subsequent effect on plate properties such as internal stress, hardness and corrosion resistance will be reviewed. This discussion will be limited to those compounds denoted as brightening, levelling, or grain-refining agents. A detailed literature search indicated that basic research into the mechanism of brightening and levelling action is lacking for zinc electrodeposition, whereas a number of workers have studied these aspects in relation to nickel or copper electrodeposition. Therefore, in the following discussion, reference will be made to these studies where the results may be applicable to zinc plating. It is also useful to compare these studies with similar studies that have been carried out for zinc in order to obtain some insight into the effect of the different chemical nature of the metals on the mechanism of brightening and/or levelling action.

Levelling Levelling, or the micro-throwing power of an electrolyte solution, can be improved by suitable additives, for example polyvinyl alcohol in zinc plating baths [60] or butyne-diol in nickel baths [61] . Some very interesting studies on the mechanism of levelling action have been made, especially in nickel and copper plating. A recent series of articles by Kardos [61] gives an excellent and up-to-date review of the “diffusional” theory of levelling. Basically, the theory states that the adsorption of the organic compound on the electrode surface inhibits the metal deposition reaction simply by exerting a “blocking” effect, that is, the electrode reaction cannot occur on the sites occupied by the organic molecule. If the adsorption kinetics of the inhibitor are controlled by diffusion of the molecule to the electrode surface, then, given the variation of the diffusion layer thickness over the microprofile, the adsorption of the inhibitor will be greater on the high points. At these points the diffusion layer thickness is small and hence the transport of the inhibitor to the electrode surface is faster. Conversely, much less adsorption will occur in the recesses and hence the metal tends to be preferentially deposited in the recess. A typical microprofile is of such dimensions as to satisfy the criteria: 1. The electrode potential does not vary over the microprofile; 2. The diffusion layer thickness varies over the microprofile. For example, a typical groove depth of 60 pm and enclosed angle of 90” would be considered to satisfy these criteria. Another criterion that must be satisfied is that the metal deposition reaction is not diffusion controlled; however this is the case in most of the electroplating systems used, where the metal concentration is very high and hence diffusion of the metal ion to the electrode surface is not the rate-controlling step in the electrochemical reduction reaction. A much more detailed explanation of this theory is given in Kardos’ articles. The diffusional theory of levelling was derived, although by different approaches, by Watson and Edwards [62] and also by Kardos and coworkers. An earlier paper by Leidheiser [63] outlining the beneficial effects

85

of agitation on levelling action no doubt provided the groundwork for the formulation of the theory. Many experiments using rotating disk electrodes have been reported (for example the work of Kruglikov et al. [64] , Rogers and Taylor [65] and also by Kardos and co-workers [61] ) which support this theory. The theory is still only semi-quantitative in nature but at least it has provided a good scientific basis to explain the phenomenon of levelling and explains such effects as: (i) The increase of levelling action with agitation, (ii) The concentration range of inhibitor over which effective levelling is achieved and, (iii) The maxima observed in levelling action us. current density curves.

Brightening The theory of levelling, while derived from experiments performed with nickel and copper baths, would probably be equally applicable to the case of zinc plating, the nature of the metal not being of importance in the theory. This however, is probably not correct as far as the theory of brightening action is concerned. Here the different chemical nature of the metal being deposited could play a large part in determining the effectiveness of a given compound in altering the crystal orientation texture, for example. At this stage, no definite and generally acceptable mechanism has been devised to explain brightening action. Those theories that have been proposed are highly conjectural and do not have the strong experimental backing as does the levelling theory. Brightness is of course related to the absence of roughness on a very small scale, that is, on the order of the wavelength of the reflected light. The major question is - is this lack of “sub-micro” surface roughness (groove depths < 0.4 pm) due to a very efficient levelling action (on a much smaller scale) operating under a similar mechanism to that given above, or is it connected with a more favoured crystal orientation induced by adsorption on selected sites? Kardos [ 611 appears to favour the “diffusion-controlled levelling” explanation, although he does make a distinction between brightening of a matte surface and the maintenance of brightness on an already bright substrate. In the latter, a “selective adsorption” of inhibitor on certain growth sites appears to be the favoured approach, such inhibition not being necessarily diffusion controlled (see Kardos’ articles for more details). Neither of these theories appears to account satisfactorily for the possible effect of the chemical nature of the metal being deposited. This aspect will be discussed in more detail below. Edwards and co-workers [67] have published many notable papers in the field of nickel electrodeposition. These workers have carried out extensive radio-tracer studies on the extent of codeposition of organic additives and their decomposition products and the subsequent effect on various plate properties, including brightness. They have also demonstrated a synergistic action exerted by mixtures of additives where enhanced adsorption (and incorporation) of each additive and other additives in

86

solution can take place. An example of additives which show this behaviour can be found with quinoline methiodide and succinimide [67] where addition of both of these compounds to a Watts nickel bath produces definite enhancement of the adsorption of each species. This work is useful in explaining the greater effectiveness of some additive mixtures as compared with the addition of the individual components of the mixture. The effect of thiourea and other sulphur-containing additives in nickel and copper electrodeposition has been studied by the above authors as well as by other workers [66,68, 691. It has been found that these compounds appear to be strongly adsorbed at the electrode, as evidenced, for example, in double layer capacitance measurements [69], and are completely decomposed leading to incorporation of fairly large amounts of sulphide in the deposit. Kruglikov and co-workers, for example, showed that up to 1.5% and up to 5% by weight, respectively, of sulphide was found in electrodeposited copper and nickel obtained in the presence of these compounds. Turner and Johnson postulated that (for copper) the formation of copper sulphide modifies the normal crystal growth of the plate. In most instances, the substances known to act as brighteners have been shown to exhibit inhibiting action on the cathode reaction. Thus, for example, Raub et al. [69] made a detailed study of the effects of various acetylenic and sulphur-containing compounds in nickel electrodeposition using a variety of techniques, including polarisation (i-E) curves, and the rise-time of initial current transients. They point out that both brightening and levelling agents inhibit the cathodic metal deposition reaction, but that the levelling action is diffusion controlled, whereas brighteners do not necessarily show this diffusional behaviour. Another important point they make is that some compounds can have both brightening and levelling action and that the synergistic effect of additive mixtures (see above) can complicate experimental work directed towards solving this problem. They observed that the crystal structure of the bright deposits is not necessarily more ordered than for dull deposits. Mirror brightness could be obtained in deposits with completely random crystal orientation as much as in deposits with more ordered structures. Moreover, the same orientation may be shown by both matte and bright deposits. These observations are of interest for comparison with the results that have been obtained in similar studies in zinc electrodeposition. These will be discussed below. Hardness

and in ternal stress

The secondary effects of these addition agents, that is the effects on hardness and internal stress of the deposit, have also been more widely studied for nickel and hence some of the results will be discussed here also for comparison with the zinc studies. Raub and co-workers [69] and others [70, 711 have demonstrated the marked effect that occlusion of various organic additives and/or their decomposition products can have on hardness and internal stress. The tensile stress is usually increased by such occlusion; however, the addition of certain sulphur-containing compounds has been

87

shown to reduce the internal stress and compounds such as thiourea or saccharin have been used as stress-relieving agents in nickel plating. Simultaneously, the occlusion of sulphide in the deposit has been shown to increase the hardness of the deposit by up to 50% [69]. For a more detailed account of the additive effects on such properties for various metals, the reader is referred to references 70 and 71. General remarks

In general, it appears that the effective concentration range of most brightening, levelling or stress-relieving agents in metal deposition is of the order of lo-* to 10e2 M. That these effects can be achieved by the addition of such small amounts of these compounds is a further indication that they are strongly adsorbed at the cathode-solution interface during electrodeposition. The fact that neutral organic compounds do not adsorb as strongly at more negative cathode potentials [ 721 explains such facts as the upper limits of current density for bright plating and the sometimes rapid drop-off in amount of codeposited material when plating at higher current densities. Such data as the zero charge potentials of metals and adsorption-potential curves for various additives would be invaluable in predicting the effectiveness of these additives for the electrodeposition of a given metal. However, such data are usually obtained for “ideal” systems where no charge-transfer reactions are taking place and where the adsorption of the organic compound is considered to have reached steady-state conditions. The results obtained for “ideally polarised” [ 721 electrodes may not be applicable to electroplating where very large currents are flowing and where completely non-equilibrium conditions exist. Even if these theoretical considerations can be overcome, the experimental difficulties in measuring such quantities as the double-layer capacitance in the presence of the large complex impedance arising from the electron-transfer reaction are considerable. Because of the difficulty in obtaining these data for such complex and dynamic systems, it is likely that the trial and error method will remain as the only effective method of selecting suitable additives in the near future. Zinc elec trodeposi tion -

brightness

and other properties

The results that have been obtained for zinc electrodeposition do show some similarities to the results obtained with other metals. Various workers [ 73 - 751 have shown that brightening and levelling agents employed in zinc electroplating baths increase the cathode polarization to varying extents depending on the nature of the additive and the type of bath used. Vagramyan and Titova [ 761 demonstrated that thiourea addition to acid sulphate baths gave an increase in cathode polarization and also resulted in the cathode potential being independent of pH, as opposed to when thiourea is not present. Also, an increase in temperature lowered the effect of thiourea at a given concentration. These results indicated that adsorption of thiourea was dominant in controlling the deposition kinetics. The strong electro-adsorption behaviour of aromatic aldehydes and

88

polyepoxyamines in alkaline baths was demonstrated by Bukaveckas and co-workers [77]. They showed that the addition of these compounds markedly decreased the double layer capacitance at the cathode, a result indicative of adsorption of neutral organic molecules. Dubina and co-workers [78] have studied a variety of organic additives and found that the most effective additives (e.g. phenolsulphuric acid in an acid sulphate bath) were those that gave the largest increase in the zinc deposition activation energy. In all of these studies, the cathode deposition reaction appeared to be inhibited by adsorption of the organic species and a much finer grained deposit usually resulted, depending on the degree of inhibition. These findings are similar to the results obtained for the electrodeposition of other metals as discussed above. The synergistic effect of additives, as was discussed earlier for nickel plating, has also been found to occur for zinc and various patent mixtures claim this effect [ 79, 801. Loshkarev and co-workers [ 55, 811 investigated the combined effects of thiourea and OP-10 (a polymeric species) and showed that the additives have a much greater effect when used in combination than when added singly. As mentioned above, while the diffusional theory of levelling action is probably valid for zinc, the mechanism of brightening may be dependent on the chemical properties of a given metal. This is most evidenced in the work of Gorbunova and Sutiagina [68] who studied the nature and amounts of co-deposited material in nickel and zinc electrodeposits, using sulphurcontaining compounds as additives. They found that much less sulphur is incorporated into the zinc plate (for a given additive concentration) than in the nickel plate, typical values being of the order of 0.05 to 0.27% weight of total sulphur in zinc and 0.08 to 5.26% total sulphur in the nickel plate (the values quoted are minimum and maximum values, which are dependent on current density, additive concentration, etc.). Another important feature of these results is that the additives appear to be completely decomposed at the cathode during nickel electrodeposition with all of the sulphur appearing in the deposit as sulphide. This is in direct contrast to zinc, where, from the small amounts of sulphide found and the carbon to sulphur ratio in the deposit, it was concluded that the compounds were incorporated in unchanged form. These results are in agreement with results obtained recently by the author (unpublished data), where it was found that addition of thiourea to zinc cyanide baths resulted in only very small amounts of codeposited sulphur (up to 100 ppm) with the total sulphur:sulphide ratio being 2:l. A similar study of the effect of thiourea on nickel, copper and zinc plating was made by Baraboshkina et al. [ 821, these authors concluding that the nature of the metal plays a major role in determining the effectiveness of thiourea as a brightening agent. They state that nickel > copper > zinc in terms of the effect upon each by addition of thiourea to the plating solutions. There do not appear to be other similar comparative studies made concerning additives used in nickel and zinc electrodeposition. The situation is complicated, however, by the fact that, in general, quite different classes

89

of compounds are used for each metal, although patents outlining common additive mixtures are available [ 831. It would probably be worthwhile to study the codeposition of such compounds in different metal deposition systems to obtain further information on the role of the nature of the metal. As for nickel electrodeposition, there appears to be no generally accepted theory to explain brightening action in zinc plating. Early studies of this phenomenon appeared to indicate a connection between brightness and specific crystal orientation and size [84, 85, 27, 861. For e=mpk Palatnik [84], using X-ray methods, studied zinc and cadmium deposits obtained in the presence of certain colloidal additives and found that brightness increased with the degree of orientation of the crystallites. Also Usikov [86], in a similar study, found that the brightest deposits were those of the type with (001) as the predominant structural axis in the crystal structure. Later work, however, contradicts this theory. For example, Gorbunova et al. [ 871, who examined nickel and zinc deposits from various baths, claimed that the brightness of electrodeposits is primarily due to levelling of surface unevenness without any obligatory decrease in dimensions or change of texture. This view was supported by Evans [88] who examined nickel deposits from various baths and concluded that brightness was not due to small grain size but merely attributable to the free surface of the grains being flat and parallel to the general surface direction of the specimen. The fact that the deposits could show a high degree of orientation was considered to be incidental and not the cause of the brightness. A similar theory to explain brightness in zinc deposits has been advanced by Schmellenmeier [89] Neither theory is considered to be really satisfactory, as it is well known that substances which act as very efficient levelling agents do not necessarily act as brightening agents and vice uersa [69, 74, 901. The only generally accepted common point in modern theories of brightening is that the adsorption of the organic additive on the electrode plays a determining role, as discussed above. The subsequent codeposition of impurities may not play a major role in the brightening mechanism, although it is generally believed that codeposition will affect the crystal structure of the deposit. This is evidenced by the correlation of the amount of codeposited material with other plate properties as was discussed for nickel and as will be discussed later for zinc electrodeposition. Recent work by Frumkin and co-workers 1911 has shown that both neutral species, e.g. hexyl alcohol, and cationic surfactants such as tetrabutylammonium iodide show different electroadsorption characteristics on different zinc crystal faces. It was shown by the concentration dependence of double layer capacitance “desorption” peak heights that the (0001) face is a more favourable adsorption site than is polycrystalline zinc. This could indicate different adsorption energies for each different crystal face and hence, during the plating process, may result in the enhanced growth of certain crystal faces with resulting changes in the crystal structure of the deposit. Much more work will need to be done before mY connection between this effect and brightness can be established, but studies such as these may eventually soIve this complex problem.

90

A much better correlation between codeposition of organic compounds and other plate properties such as hardness and stress has been established, as was also found for other metals. The property of internal stress of electrodeposited zinc is not of major importance, the stress usually being much less than is found in other metals and the effect of additives not as great [70, 711. Zinc does have the unusual property that the stresses are invariably compressive, rather than tensile [26, 281. In general, the stresses in zinc deposits have been shown to vary between 0 to 8 kg/mm2, depending on the additive concentration and type, such additives as gum arabic and thiourea having large effects. Microhardness usually increases as the compressive stress increases. As the absolute values of the stresses in typical deposits are so small (and usually requiring fairly large concentrations of additive to produce stresses of any magnitude), it seems unlikely that the brittleness of the zinc plate would be greatly affected if the use of additives is reasonably controlled. Indeed, as pointed out by Fedot’ev [71], the phenomenon of hydrogen embrittlement would appear to be a more important factor in zinc plating. Nevertheless, some additives have been used specifically as stress-reducing agents in some baths. Examples of these are the use of formaldehyde-naphthalene sulphonic acid condensate [93] and cu-keto acids [94] . It is difficult to fully explain the development of stresses in electrodeposited metals, however, it is obvious that there is a correlation between such properties as stress and hardness and the amount of co-deposited material. Details of how the crystal structure (and hence these properties) is affected is still a matter of some controversy. A recent paper by Kushner [108] discusses this particular aspect of plating in some detail and throws some new light onto an old problem. Corrosion

resistance

of electrodeposited

zinc

A factor which has received even less consideration is the corrosion resistance of “bright” zinc versus “dull” zinc. Does the presence of occluded material, especially decomposition products such as sulphide, play any part in either the inhibition or the acceleration of the corrosion of the zinc? There are various patents and other publications which claim to give zinc plate of enhanced corrosion resistance [94 - 971, however, there is a notable absence of any publications describing any detailed studies of this aspect of zinc plating. An early paper by Schmellenmeier [98] suggests that the addition of certain “colloid” substances to zinc plating baths gives brighter plate of superior corrosion resistance. Kohler [99] made a study of zinc plate obtained from (i) a “bright” acid bath, (ii) a “matte” acid bath and (iii) a “bright” cyanide bath. He found that plate obtained from (i) and (ii) showed different structural characteristics from those from (iii) and that the latter had a much lower dissolution rate in 1N sulphuric acid solutions. Schlotter and Schmellenmeier [loo] had earlier reported a wide variation in the acid and alkaline dissolution rates for zinc deposits obtained from various “bright” and ‘dull” baths. Kuzub and Gru [loll have also claimed that bright plates obtained from acid sulphate baths containing gelatin show

91

more resistance to attack in sodium chloride solutions than plates obtained from other baths. Pakhamova [102] similarly claimed that addition of glue to a plating bath decreased the corrosion rate of the zinc plate by “refining the crystal structure and promoting crystal orientation with high resistance”. The addition of glue and thiourea as brightening agents in a pyrophosphate bath has been claimed by Kalyuzhnaya and Pimenova [103], and also by Domnikov [45], to give bright plate with three to four times the corrosion resistance (in sodium chloride solution) of matte plate obtained from the same bath without the additives. All of these studies indicate that the presence of certain additives in zinc plating baths can give deposits having superior corrosion resistance either through a structural change or a chemical effect (i.e. codeposition). Other studies suggest otherwise. Golubev and Kovarskii [ 1041 have studied the corrosion rate of copper and zinc deposits obtained under various conditions and have concluded that the corrosion rate was independent of plating conditions and was related only to the micro-geometry of the surface They state that the corrosion rate increased with Ra (the arithmetic mean of the deviation from smoothness over a given area). Belilos [105] , who carried out salt-spray tests on nickel, zinc, cadmium and copper coatings, reports no difference in corrosion rate for dull and bright zinc. In an earlier paper, Schmidt [106] also claims (from the results of extensive tests on plates from a number of bath types) that there is no difference in the corrosion rate between dull and bright plate but that only factors such as plate thickness and uniformity are important. Therefore, as the few studies that have been reported lead to such contradictory results, one can reach no definite conclusions about the effect of organic additives on the corrosion resistance of electroplated zinc. Recent studies at these Laboratories [ 1071 have shown that there can often be a large variation (up to 100%) in the corrosion rates of chromate passivated commercial bright zinc plate. The differences in the corrosion rates could not be definitely attributed to the more obvious causes such as lack of leachable chromate in the passivation coating, plate thickness or amount of copper impurity. As all plates involved used the same chromate-passivation treatment, it seems likely that the actual properties of the zinc plate itself could be the most important factor. As different proprietary brightening additives were used with different coatings, this may point to some relationship between the additive used and the corrosion resistance of the plate. Further work would need to be done to establish such a relationship. Conclusions Despite the widespread use of many different types of organic additives in zinc electroplating, very few conclusions can be made as to the mechanism of their action and their subsequent effect on the properties of the plate. It is generally recognised that these substances are strongly adsorbed at the electrode-solution interface and inhibit the cathodic reaction

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to varying degrees. It is also known that codeposition of fairly large amounts of organic material can occur, the amount depending on the electrolyte composition, current density and temperature. Despite some detailed studies of this effect, especially for copper and nickel electrodeposition, no definite conclusions can be made as to how this codeposition affects the crystal structure of the plate, and how in turn the changes in crystal orientation texture that can occur are related to brightness, hardness or internal stress. Electrodeposition of metals is a very complex system and it is difficult, from a fundamental viewpoint, to devise meaningful experiments in which all the variables are controlled if, indeed, all the variables have been recognised. In view of the difficulty involved, it would appear that a generally accepted theory of brightening, based on known electrochemical and crystallographic knowledge, is still a long way off. The effect of these additives on the corrosion resistance, a very important factor in zinc plating in particular, seems to be a subject requiring urgent investigation even if only to determine whether any correlation does exist. The few studies of this effect to date are contradictory and therefore this would certainly be a subject in which rapid progress could be made in the near future.

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