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A new cyanide-free zinc electrolyte
Electrodeposition and Surface Treatment Elsevier Sequoia S.A., Lausanne - Printed in Switzerland
A NEW CYANIDE-FREE
269
ZINC ELECTROLYTE
M. J. REIDT and C. A. BOOSE Metal Research Institute TNO, Department for Metal Finishing and Corrosion, Postbox 52, Delft (The Netherlands) (Received July 27, 1972)
SUMMARY*
A new cyanide-free zinc bath for barrel plating has been developed by TNO. By adjusting the brightener composition and bath composition it is also possible to use the solution for still vat plating. The zinc plating is carried out in a weak acid or neutral solution; the zinc content may vary from 10 up to 50 g/l. The lower zinc concentrations have advantages in waste water treatment. The temperature range for zinc plating is from 20 to 50°C; the high temperature in particular is attractive in the application of closed loop systems. The throwing power of this bath is very good as compared with other acid zinc-plating baths. The brightener consumption is very low and the ductile zinc deposit is easily passivated in the normal chromating solutions. The zinc-plating process is protected in several countries by means of patents.
INTRODUCTION
Up to the present, zinc plating has been mainly carried out in cyanide zinc baths. Together with the cyanide bath, the acid zinc bath is used especially for the zinc plating of wire and strip material. The cyanide zinc bath, which has already been in existence for a long time, is frequently employed on account of its reliability and the simplicity of correcting possible faults. Regulations concerning the disposal of industrial waste water, now in force in many countries, have compelled the users of the cyanide-containing baths to purchase expensive installations for cyanide destruction. * RCsumC en francais ii la fin de l’article. Deutsche Zusammenfassung Electrodepos. Surface Treat., 1 (1972173)
am Schluss des Artikels.
270
M. J. REIDT.
C. A. BOOS
Many efforts have been made in the past to replace the cyanide zinc bath, either by alkaline zincate baths, with small quantities of cyanide or other complexing agents, or by acid or weak acid cyanide-free zinc baths. In spite of a certain amount of success, however, this development is still proceeding but slowly, and the big break concerning a rejection of cyanide zinc baths is not yet in sight. There are two possible explanations for this state of affairs. On the one hand, a discussion is still going on as to how far the cyanide free zinc baths have to be cleansed of the complexing agents in the waste water treatment. On the other hand, as the result of many practical tests on newly developed baths, there is perceived after a time some deterioration in the quality of the zinc bath, which raises some doubt concerning the durability of the new type zinc bath. The users are therefore rapidly becoming inclined to revert back to their well-known cyanide zinc bath, with the already proved, but expensive, waste water treatment installation. During recent years there has been a tendency to replace the cyanide-containing baths by weak acid or neutral zinc baths, all cyanide-free. On the one hand the waste water treatment costs are low and on the other it is possible to obtain a reasonable throwing and covering power in this pH area, so that by using the right complexing agents and brighteners the bath does not have to be inferior to the cyanide zinc bath and, in respect of the above-mentioned important point, is even better than the acid zinc baths. In the following section, the characteristics of the new type zinc bath are formulated. BATH
FORMULATION
The bath composition
is as follows:
Zinc (metal) Complexing agents Boric acid Potassium chloride PH Brightener 1 Brightener 2
20 120
g/l g/l
60 100 6 0.2 0.4
g/l g/l g/l g/l
Limits 10 -50 60 -200 30 - 90 50 -150 57 O.l0.3 0.21
g/t g/l g/l g/l g/l p/I
1. The zinc content The zinc content in the bath may vary between wide limits. For the make-up of the new bath zinc sulphate, zinc chloride, zinc oxide or other soluble zinc salts may be used. Contaminating metals, such as lead and cadmium, influence the quality of the zinc deposit; therefore it is preferable to use a good quality of a specified zinc salt. A bath with a low zinc content (e.g. 10 g/l) has the advantage of low cost Electrodepos.
Surface
Treat.,
I (1972173)
A
NEW
CYANIDE-FREE
ZINC
271
2
1 Current
ELECTROLYTE
density
(A/dm’)
Fig. I. The influence of the zinc content on the cathodic efficiency for barrel conditions; m5 screw bolts, temperature 5O”C, bright zinc, pH 6.0.
articles
in the waste water treatment and has also a very good throwing power. A high zinc content, on the other hand, increases the cathodic current efficiency (Fig. 1); meanwhile, the influence of the metal contaminations is relatively small. By using only a weak complexing agent for zinc, the zinc ions are mainly present as free zinc ions in the bath. 2. The complexing agent and boric acid By using a weak complexing agent for the zinc, a polyhydroxide acid, only a small portion of the zinc ions is complexed. The favourable influence of the complexing agent may be ascribed to its influence in the cathodic film during zinc plating. The addition of boric acid also has a favourable influence upon the deposit. The boric acid forms with the polyhydroxide acid a complexing agent for zinc with better properties than those of the polyhydroxide acid only. An extra amount of “free” boric acid is used to improve bright plating current density limits. In connection with this, we presume that there has to be a coupling between the concentration of the polyhydroxide acid and the concentration of the boric acid to obtain optimum results. 3. Potassium chloride The potassium chloride is used to improve the conductivity of the electrolyte. When the potassium chloride content is too high, there may be burning effects on the articles, while the conductivity hardly increases after the concentration has reached 100 g/l. The experiments were mostly carried out with a content of 100 g/l KCl. It is also possible to use sodium sulphate. The chloride bath appears to have a somewhat higher cathodic efficiency than that of a zinc bath made up with sodium sulphate. In earlier experiments we used sodium chloride as conductivity salt; however, in practice KC1 turned out to have better properties regarding brightening range and conductivity. Electrodepos.
Surface
Treat.,
1 (1972/73)
272
M. J.
REIDT.
C. A. BOOS
4. The influence of the pH The pH value of the bath has a great influence on the complexing of the zinc and is therefore important for the various bath properties such as cathodic efficiency, brightness area, throwing power and quality of zinc deposit obtained. A low pH value (e.g. 4) has the consequence that more free zinc ions are present and the bath properties come close to those of the acid zinc bath; higher cathodic efficiency, larger crystals, loss of brightness, less throwing and covering power. A high pH value (e.g. 7) leads to a stronger complexing of the zinc so that the cathodic efficiency is reduced; however, a better throwing power is obtained. The applied brightener is at high pH less effective, so that only semi-bright zinc deposits are obtained. The optimum pH value is around pH 6. In Fig. 2 the cathodic efficiency for barrel conditions is shown as a function of the current density for a weak acid zinc bath at three various pH values of the bath. At pH 6.5 a large full bright deposit is obtained, with a reasonable current efficiency at 50” C. Depending on base material and shape of the articles, optimum results can be obtained at the right pH; generally pH 6 will be a convenient average.
D”ll Brlgnt
SDrnl
bright
Dull Bright hma
PH 4.5
right
PH 6.5 %
PH 8 1
Current
2 donslty
(A/d&)
Fig. 2. The influence of the pH on the cathodic efficiency and the brightness for barrel conditions; articles m5 screw bolts, temperature 5O”C, Zn 20 g/l, bright zinc.
5. The brighteners The brightener consists of two components, one an aromatic aldehyde to achieve a high brightness and the so-called base brightener, which brings about a semi-bright deposit, when used separately. The base brightener (high molecular) prevents the “burning” effects in the high current density area, while the aromatic aldehyde produces the levelling and high brightness, especially in the low current density area. The brighteners have a great influence on the properties of the bath (e.g. current efficiency) and the structure of the zinc deposit. Electrodepos. Surfuce Treut., 1 (1972/73)
A
NEW CYANIDE-FREE
ZINC ELECTROLYTE
6. The structure of the zinc To get an idea about the structure
273
of the zinc, a steel panel was zinc plated
to a sufficient thickness (about 30 microns); from this panel cross-sections were made, polished and etched. For purposes of comparison, the same procedure was followed for an acid zinc bath and a cyanide bright zinc bath. The current density in all cases was 2 A/dms. In Fig. 3 is shown a cross-section of a zinc deposit from a conventional type acid zinc bath. The photograph shows very large crystals of the base reproducing type (B.R.) which are already visible with the naked eye. In Fig. 4 the surface appearance of the acid zinc (10 x enlarged) is shown and the separate zinc crystals are very clear.
Fig. 3. Cross-section
of acid zinc (400 x
Fig. 4. Surface photograph
).
of acid zinc (10 x).
Figure 5 shows the surface appearance of the zinc, from dull cyanide-free weak acid zinc plating solution. The separate crystals are hardly visible (enlargement 10x). In spite of the fact that no brighteners are present, there appears a crystal refinement. In the photograph pitting effects are apparent but no brighteners of surface-active components were applied. The cross-section (Fig. 6) of this dull zinc shows clearly the smaller zinc crystals with a field-orientated structure (column). Electrodepos.
Surfhce Treat.,
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M. J. REIDT, C. A. BOOSE
Fig. 5. Surface photograph
of cyanide-free dull zinc (10 X )
Fig. 6. Cross-section
of cyanide-free dull zinc (400 x )
Fig. 7. Cross-section
of cyanide-free
bright zinc (400 x
J.
Figure 7 shows the cross-section of the zinc deposit from the weak acid bright zinc bath. It is no longer possible to see the separate crystals. There are two structures at the same time, the field-orientated structure (column) and a laminated structure, which is also characteristic for the bright and semi-bright nickel-plating deposits. Figure 7 also shows the levelling of the zinc bath as a result of the applied brighteners. For comparison, Fig. 8 shows a cross-section of the cyanide bright zinc. The resemblance to Fig. 7 is evident; a column structure together with the laminates Electrodepos.
Surface
Treat.,
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A NEW
CYANIDE-FREE
Fig. 8. Cross-section
ZINC
275
ELECTROLYTE
of cyanide bright zinc (400 x
).
Fig. 9. Surface photograph
of cyanide-free bright zinc (10 x
Fig. 10. Surface photograph
of cyanide bright zinc (10x).
).
of very fine zinc crystals. The surface appearances of weak acid bright zinc and cyanide bright zinc show the same great similarity (Figs. 9 and 10). 7. ThroMYng pow-er With the Haring-Blum-cell, the throwing power was measured (in %) of the weak acid zinc bath for still vat and barrel zinc plating, together with the acid and cyanide zinc bath. The average results at different current densities are given in Fig. 1 I. From this it is seen that the throwing power of the weak acid zinc bath lies between the acid and the cyanide bath. Electrodepos.
Surface
Treat.,
I (1972/733
276
M. J. REIDT, C.
A. BOOSE
0 -10 1
I-
I
2
Current
3
denshy
(A/dm’)
Fig. 11. The throwing power as a function of the current density of the different zinc baths; cell ratio, 1: 2. For barrel plating conditions (below 2 A/dm2) the difference in throwing power is not significant. For still vat conditions the throwing power of the weak acid bath approximates to that of the cyanide bath. For the still vat zinc plating a higher base brightener concentration is used; the base brightener concentration has an influence upon the cathodic polarization and increases the throwing power. Together with cathodic polarization there is an influence of diffusion polarization in still vat plating; however, the latter decreases when air-agitation or cathodic movement is used. For barrel plating the diffusion polarization is small because the articles are moving and therefore the throwing power decreases in the lower current densities. INFLUENCE OF THE TEMPERATURE AND BRIGHTENER CONCENTRATION ON THE CATHODIC EFFICIENCY
In Fig. 12 the cathodic efficiency for barrel plating conditions is given as a function of the current density at variable conditions with a constant zinc content of 20 g/l. The highest efficiency is obtained in a bath without brighteners at 5O”C, while the lowest efficiency is obtained in a bright bath at 25°C. Increasing the
70. -z w : 5 60. v -___\;\\ 1_
2 I3 4. 1
Current
2
donsit y (A/dm*)
Fig. 12. The influence of the temperature and the brightener concentration efficiency for barrel conditions; articles m5 screw bolts, Zn 20 g/l, pH 6. Electrodepos.
Surface
Treat.,
I (1972/73)
on the cathodic
A NEW
CYANIDE-FREE
brightener
ZINC
concentration
277
ELECTROLYTE
as well as the temperature
efficiency. The decrease in efficiency by addition can be fully compensated (50” C).
by the increase
reduction
of brighteners
decreases the current at room temperature
in efficiency at higher bath temperatures
Hull-cell experiments and bath contaminations The condition of the bath may be judged from the appearance
of the deposit
and by the use of the Hull-cell test. The Hull-cell test is generally carried out at 50°C with 1 amp. cell current during 10 minutes. From a normal weak acid bath in good condition, it is possible to obtain a fully bright Hull-cell panel. To imitate more or less the barrel conditions a glass rod is moved along the Hull-cell panel during the test. By using a moderately rough (base) material as a Hull-cell panel, it is possible to estimate the current density area in which levelling occurs. When metal contaminations are present in the bath, this appears in different ways upon the Hullcell panel. Oxidation products, such as Cr VI, nitrite and nitrate, cause a decrease in the covering power; other contaminations cause a dark colouring of the zinc deposit along a great part of the Hull-cell panel (e.g. Fe) or particularly in the lower current density area e.g. Cu and Cd (dullness). Especially after passivation of the zinc the contaminations become clearly visible. In the following table, the values are given at which the contaminations interfere, at a zinc concentration of 20 g/l. Pb 5 ppm. Cd 5 ppm. cu 10 ppm. Fe 11 200 ppm. Fe III Ni co Cr VII
200 ppm. 1200 ppm. 600 ppm. 80 ppm. As shown in the table, lead, cadmium and copper interfere very severely. In practice most of the contaminations will not interfere during zinc plating. A contamination of lead, mostly present in zinc anodes, appears to be plated out very quickly as well as cadmium. For the make-up of the new bath the quality of the zinc salt must be checked and when necessary a zinc dust treatment is advised; another possibility is to shut down the bath for a short period and the contaminations are rapidly plated out in the low current density area. The lost articles on the bottom of the bath may cause an increase in the iron content of the bath after some time. When an iron contamination becomes too high, iron may be complexed and afterwards precipitated. Electrodepos.
Surface
Treat.,
1 (1972/73)
278 APPLICATION
M. J. REIDT,
C. A. BOOSE
POSSIBILITIES
Compared with the cyanide zinc bath, the weak acid bath has more possibilities by a favourable pH of the bath and the weakly bonded zinc ions make it possible to plate directly on to tempered steel and cast iron base materials. A second advantage is that the zinc deposit causes less hydrogen embrittlement on the steel than does corresponding plating from a cyanide bath. Further, it is an important advantage that this new bath has a good throwing power and levelling, compared with the acid and many weak acid baths. The levelling is only present in a very small current density area. Figure 13 shows the top of a screw thread on the cross-section of a screw bolt. On the top of the screw thread levelling is obtained (laminated structure) while the lowest current densities produce the column structure again. For technical reasons a small piece of polished zinc was plated under barrel plating conditions. The structure of the zinc could be better observed. In Fig. 14 the laminated structure of the bright electrolytic zinc deposit is very clear. By changing the pH of the bath, the optimum conditions may be chosen for plating different articles. Less profiled articles may be plated at high current densities at pH 5, while complicated profiled articles may be plated at pH 6.5 to obtain a better throwing
Fig. 13. Cross-section
of screw thread
Fig. 14. Cross-section
of barrel
Electroriq~os.
Treat..
Surface
(IO )I ).
zinc on Zn base (400 x ).
I ( 1972173)
A NEW CYANIDE-FREE ZINC ELECTROLYTE
279
Fig. 15. Articles from the bright zinc barrel plating solutions.
Fig. 16. Different cyanide-free bright zinc plated articles.
power. In the last case the zinc content of the bath may rise because of the deer measing current efficiency at higher pH. By replacing a number of zinc anodes by/ socalled insoluble anodes it is possible to regulate the zinc concentration. For this purpose the expensive platinized titanium anodes can be used, and also some special degrees of graphite anodes, covered in anode bags. We oursz Aves Eleclrodepos.
Swface
Treat.,
I
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280
M.
had good results with the types YAX and AGLR with carbon anodes from “Deutsche Carbone”.
J. REIDT,
C. A. BOOSE
of the Union Carbide and also From these insoluble anodes
there is some evolution of chlorine gas; only a small quantity escapes into the air, most of it remaining in the bath. During nearly half a year the 200 1 barrel plating bath has been operatedwith a combination of zinc and graphite anodes, without adverse influence upon the quality of the zinc deposits. Herewith the zinc content remains at a constant value of 15 g/l. A disadvantage of using the insoluble anode is a somewhat higher consumption of the complexing agent and the brightener. To give an impression of the articles that may be zinc plated both barrel and still vat, a number of articles is shown in Figs. 15 and 16. DISADVANTAGES
The weak acid zinc bath requires a good quality of chemicals and equipment. Rubber-lined tanks and filter pump are a necessity to prevent metal contamination. Concerning this, the solution shows a strong similarity to the nickel solution. The anodes have to be of a high quality of zinc and their hooks must be covered to prevent any iron contamination. In consequence of this the initial cost of the bath will be higher than that of the cyanide bath. ADVANTAGES
The weak acid zinc bath has a very high brightness, which can be compared with that of the nickel-chromium finish. The throwing power is better than that of most of the acid or weak acid baths, with a very high cathodic efficiency. For this new bath the cathodic efficiency is as high as that of the cyanide bath. Another important advantage is the wide temperature range over which plating can be carried out (2O”C-50°C). At the higher temperatures, around 50°C of this bath there is the opportunity to install an economical closed loop system whereby evaporation costs can be cut down to a minimum. In general, the cooling of the bath will not be necessary. By the use of a reserve tank for the return of the dragout, the waste water can be reduced The bath components are not ment are only a fraction of those by to be precipitated by neutralisation. advantage with regard to equipment
RELEVANT
to a minimum. poisonous; the costs of the waste water treatcyanide zinc plating, because the zinc has only The absence of any ammonia salts is also an corrosion and waste water treatment.
LITERATURE
H. Fischer, Elektrolytische Abscheidung und Electrokristallisation Berlin, 1954. W. Immel, Galvanotechnik, 57 (1966) 79. R. Sieburg, Galvanotechnik, 59 (1968) 951. Electrodepos.
Surface
Treat.,
I (1972173)
von Metallen,
Springer Verlag,
A NEW
CYANIDE-FREE
ZINC
ELECTROLYTE
281
F. Hasko, Gulvunotechnik, 60 (1969) 433. K. P. Bellinger, Plating, 56 (1969) 1135. R. Ostrow, Plating, 57 (1970) 354. E. M. Blaunt, Electroplating and Metal Finishing 23 (1970) 27. W. Immel, Galvanotechnik,
61 (1970) 377.
Un nouvel klectrolyte exempt de cyanure TN0 a mis au point un nouveau bain de zinc exempt de cyanure pour le depot Clectrolytique au tonneau. En adaptant la composition du brillanteur et celle du bain, il est Cgalement possible d’utiliser cette solution pour le depot au montage. Le zingage est effect& dans une solution legerement acide ou neutre; la teneur en zinc peut varier de 10 a 50 g 1-l. Les concentrations faibles de zinc sont plus avantageuses du point de vue du traitement des eaux u&es. Le domaine de temperature pour le zingage s’ttend de 20 a 50°C; en particulier, la temperature plus Clevee est interessante pour l’application dans des systemes a circuit ferme. Le pouvoir de penetration de ce bain est tres bon, compare avec celui d’autres bains de zinc acides. La consommation de brillanteur est trb faible et le depot de zinc ductile est facilement passive dans des solutions normales de chromatation. Le procede est brevete dans plusieurs pays. Neuer zyanidfreier Zinkelektrolyt TN0 hat ein neues zyanidfreies Zinkbad fur die Galvanisierung melverfahren entwickelt. Durch Abstimmung der Zusammensetzung mittels und derjenigen des Bades ist es such miiglich, die Liisung fur sierung im ruhenden Bad anzuwenden. Die Verzinkung wird in einer schwach sauren oder neutralen geftihrt, deren Zinkinhalt von 10 bis zu 50 g 1-t variieren kann. Die Zinkkonzentrationen sind vom Gesichtspunkt der Abwasserreinigung Der Temperaturbereich fur Verzinkung erstreckt sich von 20
im Tromdes Glanzdie GalvaniLiisung ausschwacheren gunstiger. bis zu 5O”C,
wobei der hiihere Temperaturwert besonders fur Anwendung in geschlossenen Kreislauf-Systemen interessant ist. Die Streufahigkeit dieses Bades ist sehr gut im Verhaltnis zu anderen sauren Verzinkungsbldern. Der Glanzmittelverbrauch ist gering und der duktile Zinktiberzug wird in normalen Chromatierungslosungen leicht passiviert. Das Verzinkungsverfahren ist in mehreren Landern durch Patente geschtitzt.
Electrodepos.
Surface
Treat.,
I
(I 972173)
A NEW CYANIDE-FREE
269
ZINC ELECTROLYTE
M. J. REIDT and C. A. BOOSE Metal Research Institute TNO, Department for Metal Finishing and Corrosion, Postbox 52, Delft (The Netherlands) (Received July 27, 1972)
SUMMARY*
A new cyanide-free zinc bath for barrel plating has been developed by TNO. By adjusting the brightener composition and bath composition it is also possible to use the solution for still vat plating. The zinc plating is carried out in a weak acid or neutral solution; the zinc content may vary from 10 up to 50 g/l. The lower zinc concentrations have advantages in waste water treatment. The temperature range for zinc plating is from 20 to 50°C; the high temperature in particular is attractive in the application of closed loop systems. The throwing power of this bath is very good as compared with other acid zinc-plating baths. The brightener consumption is very low and the ductile zinc deposit is easily passivated in the normal chromating solutions. The zinc-plating process is protected in several countries by means of patents.
INTRODUCTION
Up to the present, zinc plating has been mainly carried out in cyanide zinc baths. Together with the cyanide bath, the acid zinc bath is used especially for the zinc plating of wire and strip material. The cyanide zinc bath, which has already been in existence for a long time, is frequently employed on account of its reliability and the simplicity of correcting possible faults. Regulations concerning the disposal of industrial waste water, now in force in many countries, have compelled the users of the cyanide-containing baths to purchase expensive installations for cyanide destruction. * RCsumC en francais ii la fin de l’article. Deutsche Zusammenfassung Electrodepos. Surface Treat., 1 (1972173)
am Schluss des Artikels.
270
M. J. REIDT.
C. A. BOOS
Many efforts have been made in the past to replace the cyanide zinc bath, either by alkaline zincate baths, with small quantities of cyanide or other complexing agents, or by acid or weak acid cyanide-free zinc baths. In spite of a certain amount of success, however, this development is still proceeding but slowly, and the big break concerning a rejection of cyanide zinc baths is not yet in sight. There are two possible explanations for this state of affairs. On the one hand, a discussion is still going on as to how far the cyanide free zinc baths have to be cleansed of the complexing agents in the waste water treatment. On the other hand, as the result of many practical tests on newly developed baths, there is perceived after a time some deterioration in the quality of the zinc bath, which raises some doubt concerning the durability of the new type zinc bath. The users are therefore rapidly becoming inclined to revert back to their well-known cyanide zinc bath, with the already proved, but expensive, waste water treatment installation. During recent years there has been a tendency to replace the cyanide-containing baths by weak acid or neutral zinc baths, all cyanide-free. On the one hand the waste water treatment costs are low and on the other it is possible to obtain a reasonable throwing and covering power in this pH area, so that by using the right complexing agents and brighteners the bath does not have to be inferior to the cyanide zinc bath and, in respect of the above-mentioned important point, is even better than the acid zinc baths. In the following section, the characteristics of the new type zinc bath are formulated. BATH
FORMULATION
The bath composition
is as follows:
Zinc (metal) Complexing agents Boric acid Potassium chloride PH Brightener 1 Brightener 2
20 120
g/l g/l
60 100 6 0.2 0.4
g/l g/l g/l g/l
Limits 10 -50 60 -200 30 - 90 50 -150 57 O.l0.3 0.21
g/t g/l g/l g/l g/l p/I
1. The zinc content The zinc content in the bath may vary between wide limits. For the make-up of the new bath zinc sulphate, zinc chloride, zinc oxide or other soluble zinc salts may be used. Contaminating metals, such as lead and cadmium, influence the quality of the zinc deposit; therefore it is preferable to use a good quality of a specified zinc salt. A bath with a low zinc content (e.g. 10 g/l) has the advantage of low cost Electrodepos.
Surface
Treat.,
I (1972173)
A
NEW
CYANIDE-FREE
ZINC
271
2
1 Current
ELECTROLYTE
density
(A/dm’)
Fig. I. The influence of the zinc content on the cathodic efficiency for barrel conditions; m5 screw bolts, temperature 5O”C, bright zinc, pH 6.0.
articles
in the waste water treatment and has also a very good throwing power. A high zinc content, on the other hand, increases the cathodic current efficiency (Fig. 1); meanwhile, the influence of the metal contaminations is relatively small. By using only a weak complexing agent for zinc, the zinc ions are mainly present as free zinc ions in the bath. 2. The complexing agent and boric acid By using a weak complexing agent for the zinc, a polyhydroxide acid, only a small portion of the zinc ions is complexed. The favourable influence of the complexing agent may be ascribed to its influence in the cathodic film during zinc plating. The addition of boric acid also has a favourable influence upon the deposit. The boric acid forms with the polyhydroxide acid a complexing agent for zinc with better properties than those of the polyhydroxide acid only. An extra amount of “free” boric acid is used to improve bright plating current density limits. In connection with this, we presume that there has to be a coupling between the concentration of the polyhydroxide acid and the concentration of the boric acid to obtain optimum results. 3. Potassium chloride The potassium chloride is used to improve the conductivity of the electrolyte. When the potassium chloride content is too high, there may be burning effects on the articles, while the conductivity hardly increases after the concentration has reached 100 g/l. The experiments were mostly carried out with a content of 100 g/l KCl. It is also possible to use sodium sulphate. The chloride bath appears to have a somewhat higher cathodic efficiency than that of a zinc bath made up with sodium sulphate. In earlier experiments we used sodium chloride as conductivity salt; however, in practice KC1 turned out to have better properties regarding brightening range and conductivity. Electrodepos.
Surface
Treat.,
1 (1972/73)
272
M. J.
REIDT.
C. A. BOOS
4. The influence of the pH The pH value of the bath has a great influence on the complexing of the zinc and is therefore important for the various bath properties such as cathodic efficiency, brightness area, throwing power and quality of zinc deposit obtained. A low pH value (e.g. 4) has the consequence that more free zinc ions are present and the bath properties come close to those of the acid zinc bath; higher cathodic efficiency, larger crystals, loss of brightness, less throwing and covering power. A high pH value (e.g. 7) leads to a stronger complexing of the zinc so that the cathodic efficiency is reduced; however, a better throwing power is obtained. The applied brightener is at high pH less effective, so that only semi-bright zinc deposits are obtained. The optimum pH value is around pH 6. In Fig. 2 the cathodic efficiency for barrel conditions is shown as a function of the current density for a weak acid zinc bath at three various pH values of the bath. At pH 6.5 a large full bright deposit is obtained, with a reasonable current efficiency at 50” C. Depending on base material and shape of the articles, optimum results can be obtained at the right pH; generally pH 6 will be a convenient average.
D”ll Brlgnt
SDrnl
bright
Dull Bright hma
PH 4.5
right
PH 6.5 %
PH 8 1
Current
2 donslty
(A/d&)
Fig. 2. The influence of the pH on the cathodic efficiency and the brightness for barrel conditions; articles m5 screw bolts, temperature 5O”C, Zn 20 g/l, bright zinc.
5. The brighteners The brightener consists of two components, one an aromatic aldehyde to achieve a high brightness and the so-called base brightener, which brings about a semi-bright deposit, when used separately. The base brightener (high molecular) prevents the “burning” effects in the high current density area, while the aromatic aldehyde produces the levelling and high brightness, especially in the low current density area. The brighteners have a great influence on the properties of the bath (e.g. current efficiency) and the structure of the zinc deposit. Electrodepos. Surfuce Treut., 1 (1972/73)
A
NEW CYANIDE-FREE
ZINC ELECTROLYTE
6. The structure of the zinc To get an idea about the structure
273
of the zinc, a steel panel was zinc plated
to a sufficient thickness (about 30 microns); from this panel cross-sections were made, polished and etched. For purposes of comparison, the same procedure was followed for an acid zinc bath and a cyanide bright zinc bath. The current density in all cases was 2 A/dms. In Fig. 3 is shown a cross-section of a zinc deposit from a conventional type acid zinc bath. The photograph shows very large crystals of the base reproducing type (B.R.) which are already visible with the naked eye. In Fig. 4 the surface appearance of the acid zinc (10 x enlarged) is shown and the separate zinc crystals are very clear.
Fig. 3. Cross-section
of acid zinc (400 x
Fig. 4. Surface photograph
).
of acid zinc (10 x).
Figure 5 shows the surface appearance of the zinc, from dull cyanide-free weak acid zinc plating solution. The separate crystals are hardly visible (enlargement 10x). In spite of the fact that no brighteners are present, there appears a crystal refinement. In the photograph pitting effects are apparent but no brighteners of surface-active components were applied. The cross-section (Fig. 6) of this dull zinc shows clearly the smaller zinc crystals with a field-orientated structure (column). Electrodepos.
Surfhce Treat.,
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M. J. REIDT, C. A. BOOSE
Fig. 5. Surface photograph
of cyanide-free dull zinc (10 X )
Fig. 6. Cross-section
of cyanide-free dull zinc (400 x )
Fig. 7. Cross-section
of cyanide-free
bright zinc (400 x
J.
Figure 7 shows the cross-section of the zinc deposit from the weak acid bright zinc bath. It is no longer possible to see the separate crystals. There are two structures at the same time, the field-orientated structure (column) and a laminated structure, which is also characteristic for the bright and semi-bright nickel-plating deposits. Figure 7 also shows the levelling of the zinc bath as a result of the applied brighteners. For comparison, Fig. 8 shows a cross-section of the cyanide bright zinc. The resemblance to Fig. 7 is evident; a column structure together with the laminates Electrodepos.
Surface
Treat.,
I (I 972173)
A NEW
CYANIDE-FREE
Fig. 8. Cross-section
ZINC
275
ELECTROLYTE
of cyanide bright zinc (400 x
).
Fig. 9. Surface photograph
of cyanide-free bright zinc (10 x
Fig. 10. Surface photograph
of cyanide bright zinc (10x).
).
of very fine zinc crystals. The surface appearances of weak acid bright zinc and cyanide bright zinc show the same great similarity (Figs. 9 and 10). 7. ThroMYng pow-er With the Haring-Blum-cell, the throwing power was measured (in %) of the weak acid zinc bath for still vat and barrel zinc plating, together with the acid and cyanide zinc bath. The average results at different current densities are given in Fig. 1 I. From this it is seen that the throwing power of the weak acid zinc bath lies between the acid and the cyanide bath. Electrodepos.
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Treat.,
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276
M. J. REIDT, C.
A. BOOSE
0 -10 1
I-
I
2
Current
3
denshy
(A/dm’)
Fig. 11. The throwing power as a function of the current density of the different zinc baths; cell ratio, 1: 2. For barrel plating conditions (below 2 A/dm2) the difference in throwing power is not significant. For still vat conditions the throwing power of the weak acid bath approximates to that of the cyanide bath. For the still vat zinc plating a higher base brightener concentration is used; the base brightener concentration has an influence upon the cathodic polarization and increases the throwing power. Together with cathodic polarization there is an influence of diffusion polarization in still vat plating; however, the latter decreases when air-agitation or cathodic movement is used. For barrel plating the diffusion polarization is small because the articles are moving and therefore the throwing power decreases in the lower current densities. INFLUENCE OF THE TEMPERATURE AND BRIGHTENER CONCENTRATION ON THE CATHODIC EFFICIENCY
In Fig. 12 the cathodic efficiency for barrel plating conditions is given as a function of the current density at variable conditions with a constant zinc content of 20 g/l. The highest efficiency is obtained in a bath without brighteners at 5O”C, while the lowest efficiency is obtained in a bright bath at 25°C. Increasing the
70. -z w : 5 60. v -___\;\\ 1_
2 I3 4. 1
Current
2
donsit y (A/dm*)
Fig. 12. The influence of the temperature and the brightener concentration efficiency for barrel conditions; articles m5 screw bolts, Zn 20 g/l, pH 6. Electrodepos.
Surface
Treat.,
I (1972/73)
on the cathodic
A NEW
CYANIDE-FREE
brightener
ZINC
concentration
277
ELECTROLYTE
as well as the temperature
efficiency. The decrease in efficiency by addition can be fully compensated (50” C).
by the increase
reduction
of brighteners
decreases the current at room temperature
in efficiency at higher bath temperatures
Hull-cell experiments and bath contaminations The condition of the bath may be judged from the appearance
of the deposit
and by the use of the Hull-cell test. The Hull-cell test is generally carried out at 50°C with 1 amp. cell current during 10 minutes. From a normal weak acid bath in good condition, it is possible to obtain a fully bright Hull-cell panel. To imitate more or less the barrel conditions a glass rod is moved along the Hull-cell panel during the test. By using a moderately rough (base) material as a Hull-cell panel, it is possible to estimate the current density area in which levelling occurs. When metal contaminations are present in the bath, this appears in different ways upon the Hullcell panel. Oxidation products, such as Cr VI, nitrite and nitrate, cause a decrease in the covering power; other contaminations cause a dark colouring of the zinc deposit along a great part of the Hull-cell panel (e.g. Fe) or particularly in the lower current density area e.g. Cu and Cd (dullness). Especially after passivation of the zinc the contaminations become clearly visible. In the following table, the values are given at which the contaminations interfere, at a zinc concentration of 20 g/l. Pb 5 ppm. Cd 5 ppm. cu 10 ppm. Fe 11 200 ppm. Fe III Ni co Cr VII
200 ppm. 1200 ppm. 600 ppm. 80 ppm. As shown in the table, lead, cadmium and copper interfere very severely. In practice most of the contaminations will not interfere during zinc plating. A contamination of lead, mostly present in zinc anodes, appears to be plated out very quickly as well as cadmium. For the make-up of the new bath the quality of the zinc salt must be checked and when necessary a zinc dust treatment is advised; another possibility is to shut down the bath for a short period and the contaminations are rapidly plated out in the low current density area. The lost articles on the bottom of the bath may cause an increase in the iron content of the bath after some time. When an iron contamination becomes too high, iron may be complexed and afterwards precipitated. Electrodepos.
Surface
Treat.,
1 (1972/73)
278 APPLICATION
M. J. REIDT,
C. A. BOOSE
POSSIBILITIES
Compared with the cyanide zinc bath, the weak acid bath has more possibilities by a favourable pH of the bath and the weakly bonded zinc ions make it possible to plate directly on to tempered steel and cast iron base materials. A second advantage is that the zinc deposit causes less hydrogen embrittlement on the steel than does corresponding plating from a cyanide bath. Further, it is an important advantage that this new bath has a good throwing power and levelling, compared with the acid and many weak acid baths. The levelling is only present in a very small current density area. Figure 13 shows the top of a screw thread on the cross-section of a screw bolt. On the top of the screw thread levelling is obtained (laminated structure) while the lowest current densities produce the column structure again. For technical reasons a small piece of polished zinc was plated under barrel plating conditions. The structure of the zinc could be better observed. In Fig. 14 the laminated structure of the bright electrolytic zinc deposit is very clear. By changing the pH of the bath, the optimum conditions may be chosen for plating different articles. Less profiled articles may be plated at high current densities at pH 5, while complicated profiled articles may be plated at pH 6.5 to obtain a better throwing
Fig. 13. Cross-section
of screw thread
Fig. 14. Cross-section
of barrel
Electroriq~os.
Treat..
Surface
(IO )I ).
zinc on Zn base (400 x ).
I ( 1972173)
A NEW CYANIDE-FREE ZINC ELECTROLYTE
279
Fig. 15. Articles from the bright zinc barrel plating solutions.
Fig. 16. Different cyanide-free bright zinc plated articles.
power. In the last case the zinc content of the bath may rise because of the deer measing current efficiency at higher pH. By replacing a number of zinc anodes by/ socalled insoluble anodes it is possible to regulate the zinc concentration. For this purpose the expensive platinized titanium anodes can be used, and also some special degrees of graphite anodes, covered in anode bags. We oursz Aves Eleclrodepos.
Swface
Treat.,
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(I 972173)
280
M.
had good results with the types YAX and AGLR with carbon anodes from “Deutsche Carbone”.
J. REIDT,
C. A. BOOSE
of the Union Carbide and also From these insoluble anodes
there is some evolution of chlorine gas; only a small quantity escapes into the air, most of it remaining in the bath. During nearly half a year the 200 1 barrel plating bath has been operatedwith a combination of zinc and graphite anodes, without adverse influence upon the quality of the zinc deposits. Herewith the zinc content remains at a constant value of 15 g/l. A disadvantage of using the insoluble anode is a somewhat higher consumption of the complexing agent and the brightener. To give an impression of the articles that may be zinc plated both barrel and still vat, a number of articles is shown in Figs. 15 and 16. DISADVANTAGES
The weak acid zinc bath requires a good quality of chemicals and equipment. Rubber-lined tanks and filter pump are a necessity to prevent metal contamination. Concerning this, the solution shows a strong similarity to the nickel solution. The anodes have to be of a high quality of zinc and their hooks must be covered to prevent any iron contamination. In consequence of this the initial cost of the bath will be higher than that of the cyanide bath. ADVANTAGES
The weak acid zinc bath has a very high brightness, which can be compared with that of the nickel-chromium finish. The throwing power is better than that of most of the acid or weak acid baths, with a very high cathodic efficiency. For this new bath the cathodic efficiency is as high as that of the cyanide bath. Another important advantage is the wide temperature range over which plating can be carried out (2O”C-50°C). At the higher temperatures, around 50°C of this bath there is the opportunity to install an economical closed loop system whereby evaporation costs can be cut down to a minimum. In general, the cooling of the bath will not be necessary. By the use of a reserve tank for the return of the dragout, the waste water can be reduced The bath components are not ment are only a fraction of those by to be precipitated by neutralisation. advantage with regard to equipment
RELEVANT
to a minimum. poisonous; the costs of the waste water treatcyanide zinc plating, because the zinc has only The absence of any ammonia salts is also an corrosion and waste water treatment.
LITERATURE
H. Fischer, Elektrolytische Abscheidung und Electrokristallisation Berlin, 1954. W. Immel, Galvanotechnik, 57 (1966) 79. R. Sieburg, Galvanotechnik, 59 (1968) 951. Electrodepos.
Surface
Treat.,
I (1972173)
von Metallen,
Springer Verlag,
A NEW
CYANIDE-FREE
ZINC
ELECTROLYTE
281
F. Hasko, Gulvunotechnik, 60 (1969) 433. K. P. Bellinger, Plating, 56 (1969) 1135. R. Ostrow, Plating, 57 (1970) 354. E. M. Blaunt, Electroplating and Metal Finishing 23 (1970) 27. W. Immel, Galvanotechnik,
61 (1970) 377.
Un nouvel klectrolyte exempt de cyanure TN0 a mis au point un nouveau bain de zinc exempt de cyanure pour le depot Clectrolytique au tonneau. En adaptant la composition du brillanteur et celle du bain, il est Cgalement possible d’utiliser cette solution pour le depot au montage. Le zingage est effect& dans une solution legerement acide ou neutre; la teneur en zinc peut varier de 10 a 50 g 1-l. Les concentrations faibles de zinc sont plus avantageuses du point de vue du traitement des eaux u&es. Le domaine de temperature pour le zingage s’ttend de 20 a 50°C; en particulier, la temperature plus Clevee est interessante pour l’application dans des systemes a circuit ferme. Le pouvoir de penetration de ce bain est tres bon, compare avec celui d’autres bains de zinc acides. La consommation de brillanteur est trb faible et le depot de zinc ductile est facilement passive dans des solutions normales de chromatation. Le procede est brevete dans plusieurs pays. Neuer zyanidfreier Zinkelektrolyt TN0 hat ein neues zyanidfreies Zinkbad fur die Galvanisierung melverfahren entwickelt. Durch Abstimmung der Zusammensetzung mittels und derjenigen des Bades ist es such miiglich, die Liisung fur sierung im ruhenden Bad anzuwenden. Die Verzinkung wird in einer schwach sauren oder neutralen geftihrt, deren Zinkinhalt von 10 bis zu 50 g 1-t variieren kann. Die Zinkkonzentrationen sind vom Gesichtspunkt der Abwasserreinigung Der Temperaturbereich fur Verzinkung erstreckt sich von 20
im Tromdes Glanzdie GalvaniLiisung ausschwacheren gunstiger. bis zu 5O”C,
wobei der hiihere Temperaturwert besonders fur Anwendung in geschlossenen Kreislauf-Systemen interessant ist. Die Streufahigkeit dieses Bades ist sehr gut im Verhaltnis zu anderen sauren Verzinkungsbldern. Der Glanzmittelverbrauch ist gering und der duktile Zinktiberzug wird in normalen Chromatierungslosungen leicht passiviert. Das Verzinkungsverfahren ist in mehreren Landern durch Patente geschtitzt.
Electrodepos.
Surface
Treat.,
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(I 972173)