Electrodeposition of zinc from acid solutions (chloride and sulphate baths)
now constitutes about 45-50% of all zinc baths, particularly in the developed
nations. Zinc is very electronegative and it provides sacrificial protection
for steel substrates. It is very economical to deposit and worldwide supplies
are high (Loto et al., 1991). Zinc is non-toxic
and it is safe in contact with food. Other employed depositing solutions, apart
from acid solutions, are those based on cyanide baths and to a lesser extent
fluoborate, alkaline zincate and pyrophosphate (Fraunhofer,
Cyanide zinc baths are the most widely used, have high throwing power, which
is one of the important factors in zinc plating. However, due to their toxicity
and the stringent regulations against water pollution and costly effluent disposal,
non-cyanide and low-cyanide baths have been investigated and used for commercial
plating (Schlesinger and Paunovic, 2000; Vagramyan
et al., 1979). The trend of replacing the cyanide with non-cyanide
baths has been steady for the past fifteen years, however, it has accelerated
greatly in the last six years. Some recent surveys have shown that the non-cyanide
zinc baths now out-number their cyanide counterparts by more than two to one.
Improvements in alkaline zincate, acid sulphate and acid chloride baths have
been reported in recent years (Geduld, 1998; Bapu
et al., 1998; Saubestre and Hadju, 1974; Pushpavanam
et al., 1981). A typical constituent concentration of cyanide baths
(Fraunhofer, 1976; Lowenheim, 1963;
Gabe, 1978) consists of zinc (as metal), 25-60 g L-1,
sodium cyanide 10-150 g L-1, sodium hydroxide 25-140 g L-1
and operating temperature of 15-60°C.
Non-cyanide zinc plating solutions can be divided into two types, mildly acid
solutions (using chloride or sulphate anions) and alkaline-zincate solutions
(Darken, 1979). The mild baths generally consist of
zinc chloride dissolved in a solution of excess ammonium chloride (more recently
potassium chloride processes, which are far less corrosive have been marketed
and ammonia free formulation is now the most popular in production (Marcos
and Bertazzoli, 1986). Zincate baths consist solely of a small concentration
of zinc metal dissolved in approximately 100 g L-1 sodium hydroxide
solution (Darken, 1979). Chloride zinc solution does
not only eliminate cyanide in plating, it also gives improved bath efficiency
and exceptional brightness. Acid zinc baths are used where it is desirable to
have a high plating rate and low cost.
Chloride zinc plating offers considerable advantages over cyanide based systems,
although, it is not without its share of routine operating problems (Schneider,
1987). Use of the acid zinc sulphate process is increasing due to its relatively
low cost, safety features and pollution control characteristics, but poor throwing
power and insufficient brightness from acid sulphate bath are disadvantages
(Vagramyan et al., 1979).
This study examines the recent progress made in understanding and improving bright deposition from acid chloride and sulphate solutions. It covers areas such as bath solution composition, effect and types of addition agents/brightness, practical aspects of acid based solutions-cathode efficiency, throwing power (deposits from acid solutions and operating problems) and corrosion resistance of the electrodeposited zinc coatings.
The unique effectiveness of addition agents obtained locally from the juices of cassava tuber and sugar cane is also experimentally presented.
THE ACID ZINC PLATING SOLUTIONS
Acid chloride zinc baths currently in use are principally of two types (Geduld,
1982): Those based on ammonium chloride and those based on potassium chloride.
The ammonium-based baths were the first to be developed. They can be operated
at higher current densities than potassium baths.
Marcos and Bertazzoli (1986) has described three types
of contemporary chloride baths. These are listed in Table 1.
The ammonia-free (potassium) referred to as Type I bath has been described as
the most popular in production.
The use of low (or mini) ammonia bath comes next. The ammonium chloride replaces
boric acid at a concentration of about 30 g L-1. these baths offer
the advantage of improved solubility of ammonium vs. boric acid, a wider higher-current-density
plating range at a low metallic zinc levels and high temperature capabilities
(Marcos and Bertazzoli, 1986). The full-ammonia bath
which author has described as Type 3, finds little use and most have been converted
to the low-ammonia or ammonia-free variety.
In the low ammonia and ammonia free baths, there is limited use of sodium chloride in place of potassium chloride. This is because the sodium chloride electrolyte has reduced cathode efficiency and is more corrosive when compared with potassium chloride. Also, the solubility of proprietary addition agents is reduced in the sodium chloride solution.
The concentration of the constituents for chloride zinc baths also varies (Preiksaite
and Sarmaitis, 1981) as indicated in Table 2. All bright
acid chloride processes are proprietary and some degree of incompatibility may
be encountered between them.
Zinc sulphate is used as a source of metal, ammonium chloride increases the conductivity, sodium acetate acts as a buffer and glucose acts as an addition agent.
|| Chloride bath types
|| Concentration of constituents of chloride zinc baths
|*Proprietary additive for zinc plating with the trade name
Limeda, Institute of Chemistry and Chemical Techn., Academy of Sciences
of Lithuania SSR, Vilnius, USSR (Preiksaite and Sarmaitis,
|| Varieties of acid zinc sulphate
Table 3 illustrates the different varieties of acid zinc sulphate as reported in different relevant literature. However, the coating is not bright, but mat and hence different additives are required to improve the throwing power of acid zinc sulphate baths.
A typical old formula for acid zinc sulphate bath (Blum and
Hogaboom, 1930) is indicated in Table 4.
|| A typical old formula for acid zinc sulphate bath
Addition agents for acid zinc chloride bath have been mainly proprietary. In
his review, Marcos and Bertazzoli (1986) described most
secondary brighteners as consisting of aldehydes or unsaturated ketones. These
active ingredients were usually made soluble with alcohol or other solvents.
Bisulphite addition products which introduced sulphur compounds to the bath
were employed in an early attempt to formulate water-soluble brighteners. These
compounds were often detrimental especially in rack plating. Microemulsion technology,
today, produces completely water soluble brighteners without unnecessary side
products. It is also known that solublization by this method increases brightener
mileage with the same level of active ingredients. In the bath, the brightener
is more quickly activated and as it was with the older solvent systems, automatic
feed hardware is not attacked (Marcos and Bertazzoli, 1986).
A detailed discussion of how aldehydes and other brighteners react at the zinc
cathode during deposition was given (Defonte, 1978).
The development of different organic addition agents has enabled bright zinc
electrodeposits to be produced from acid zinc sulphate baths. In a study (Venkatesha
et al., 1987) to find a new brightener for the acid sulphate bath,
an addition of furfural gave satisfactory mirror bright zinc deposits. Deposit
from standard solution with 1 mL L-1 of furfural, i.e., the bath
that contained both dextrin have a fine-grained structure and bright deposits
were obtained over a wide range of current density.
Glycine and thiourea have also been used as brighteners in acid zinc sulphate
baths (Venkatesha et al., 1987). The concentrations
of brighteners were determined during electrolysis by colorimetric (Hiremath
and Mayanna, 1984) and volumetric (Uma and Mayanna,
1980) procedures using tri-beta hydrindene hydrate (ninhydrin) reagents
and chloramine-B, respectively. The uniformity of zinc deposits with a fine
grain size was improved by carrying out Hull Cell experiments using the standard
bath (Table 5), plus various organic addition agents such
as glucose (0.5 to 5.0 g L-1), starch (0.5 to 5.0 g L-1),
gelatin (0.1 to 2.0 g L-1) and dextrin (0.5 to 5.0 g L-1).
A white, uniform, fine-grained zinc deposit was produced over a wide current
density range using 3 g L-1 dextrin. The other addition agents produced
New zinc brighteners for the acid sulphate bath was found by using several oxygen-, nitrogen-and sulphur-containing organic compounds in the bath and a Hull Cell zinc pattern was examined in each case. Glycine in combination with thiourea was found to be an effective brightener over a fairly broad range of current densities. The optimum bath composition and operating conditions used are indicated in Table 6.
The synergistic effect of additives on bright nanocrystalline zinc electrodeposition
was investigated (Nayana et al., 2011). The authors
studied the influence of additives like cetyltrimethylammonium bromide (CTAB)
and Ethyl Vanillin (EV) on zinc electrodeposition from acid sulphate bath by
scanning electron microscopy, X-ray diffraction and voltammetric techniques.
Their result showed the existence of interaction between CTAB and EV.
|| Standard bath composition and operating* conditions (Venkatesha
et al., 1987)
|*Plated in hull cell for 5 min at 2A using air-agitated bath
at 30°C and pH 2.5, a mild steel cathode and an anode of 99.9% zinc
||SEM images of deposit obtained from bath (a) I, (b) II, (c)
III and (d) IV (Nayana et al., 2011)
They exhibited synergistic effect to produce bright nanocrystalline zinc coating
on steel surface. The nano-sized bright crystalline zinc deposit was obtained
from sulfate electrolyte containing both ethyl vanillin and cetyltrimethylammonium
bromide. The deposit transformed from dull appearance (without additives) to
smooth fully bright appearance in the presence of both additives (Fig.
1). The bath composition used for zinc electrodeposition is given in Table
PRACTICAL ASPECTS OF ACID ZINC BASED SOLUTIONS
Cathode efficiency: The high cathode current efficiencies exhibited
by chloride zinc baths are one of the most important properties of these baths
(Geduld, 1982). The average cathode current efficiency
for these baths is approximately 95 to 98% over the entire range of operable
current densities. There is no any other zinc plating system that approaches
this extremely high efficiency at higher current densities. The high cathode
efficiency combined with the quick brightening action provide for faster plating
and increased productivity (Marcos and Bertazzoli, 1986).
The high efficiency can lead to productivity increases of 15 to 50% over cyanide
As shown in Fig. 2, the alkaline non-cyanide and cyanide
zinc baths give respectable cathode efficiencies at low current densities. As
the current density increases, a dramatic drop off is indicated. On the other
hand, chloride zinc maintains very high cathode efficiency across the entire
current density range, so the high-current-density areas of parts are subject
to very high deposition rates. This results into much more mental being plated
in the high-than in the low-current density regions or, simply put, the bath
displays poor throwing power (Marcos and Bertazzoli, 1986).
Throwing power: The throwing power on all the different types of zinc
electroplating solutions have been measured (Paatsch and Hogaboom,
1980). It has been noted that throwing power is dependent on the both primary
and secondary current distributions, the former emanating from work piece shape,
arrangement of electrodes and dimensions of the bath. Todt has quoted the following
throwing power values determined in the Haring-Blum Cell:
The above clearly shows the inferior power of acid zinc solutions. The work
undertaken by Blum and Paatsch on the depth of penetration of a zinc deposit
into a steel tube, with the open end directly facing the anode, has been described
(Darken, 1979). Throwing power under a standard condition
was of the following order:
High cyanide>Low>Acid>Alkaline cyanide free
|| Comparison of cathode current efficiencies of bright zinc
plating electrolytes (Geduld, 1982)
An alternative method of comparing deposit distribution from various zinc plating
solutions by determining the thickness deposited during Hull Cell plating tests
at various distances from the high current density edge was indicated (Darken,
1979). It was indicated from the results obtained that acid zinc solution
gave a deposit with least favourable thickness distribution.
Several investigations (Preiksaite and Sarmaitis, 1981;
Pushpavanam, 1986; Budman, 1995;
Beltowska-Lehman et al., 2002; Adaniya
et al., 1980, 1981; Leidheiser
and Suzuki, 1981) have been carried out to improve the corrosion resistance
of acid sulphate bath electrodeposited zinc. Corrosion resistance, twice as
high as pure zinc, was obtained in the deposits by the additions of cobalt and
chromium. Extensive investigations into the corrosion resistance of chromate
and uncoated zinc electrodeposits were carried out (Preiksaite
and Sarmaitis, 1981). Salt spray tests for 9 μm coated samples showed
that the service life decreased in the following order: bright cyanide>neutral
matt>matt cyanide>bright-zincate>bright weak acid, ammonium chloride>bright,
weak acid, ammonium free. It has been shown that zinc coatings deposited from
baths of different composition are not alike; they differ in porosity, structure
and other characteristics (Todt, 1998). These in turn,
should affect the corrosion resistance of the coatings.
The effect of MoS2 on the deposition properties, morphology, crystallographic
orientation and corrosion behaviour of the electrodeposited Zn-MoS2
nanocomposite coatings on mild steel from zinc sulphate-chloride bath containing
uniformly dispersed MoS2 nanoparticles were studied (Kanagalarasa
and Venkatesha, 2011) (Fig. 3).
It was shown that the addition of MoS2 to the electrolyte significantly
changed the microstructure and crystallographic orientation of the zinc deposits
and enhanced the corrosion resistance of the coatings. The morphological and
electrochemical properties of the zinc coatings were observed to be significantly
affected by the incorporation of MoS2 particles into the zinc matrix.
Zemanova (2009) investigated the corrosion resistance
of zinc coatings in an accelerated corrosion test in a condensation chamber.
Zinc was electrodeposited from alkaline and acidic electrolytes using direct
current DC or Pulse Current (PC). The zinc coating was subsequently protected
against corrosion with a chrome (III) layer.
Morphology and structure of the coatings was investigated using Scanning Electron
Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-ray diffraction
analysis (XRD) before and after the corrosion test. Corrosion resistance of
alkaline zinc coatings electrodeposited with DC and PC under test conditions
was found to be comparable. The corrosion resistance of zinc coatings deposited
from acidic electrolytes with PC was lower in comparison with corrosion resistance
of zinc coatings deposited using DC. Excellent corrosion resistance of the tested
coatings in a condensation chamber was achieved for samples PC-electrodeposited
from both zinc acidic and alkaline electrolyte with the frequency ratio of current-on
to current-off time equal to 1 at the electrodeposition time of 10 min. The
samples were treated with trivalent chromium. The quality of PC-electrodeposited
samples corrosion resistance was found to be comparable to that of samples obtained
by the DC electrodeposition using commercial electrolytes. However, the PC electrodeposited
coatings are not bright enough due to frequency modification, as it is typical
for DC electrodeposition due to the presence of organic additives. The author
was of the opinion that inhomogeneous character of the samples morphology, in
the sense of sheet or leaf-like grains obtained using PC in the frequency range
of 22-90 Hz, could be the reason for the corrosion attack to the white rust.
It was observed, in general, that zinc coatings obtained by PC electrodeposition
from alkaline electrolytes provide more corrosion-resistant coatings in comparison
to those PC-electrodeposited from acidic electrolytes under the test conditions.
The authors results and observations
were supported with SEM micrographs as presented in Fig. 4.
||SEM micrographs of electrodeposited zinc from acidic and alkaline
electrolytes using (a) DC, (b) PC 50 Hz and (c) PC 22 Hz (Zemanova,
DEVELOPMENT OF NEW ADDITION AGENTS
The effects of thiourea, dextrin and glycine additives, as organic addition
agents, on the surface characteristics of zinc electrodeposition on mild steel
in acid chloride solution at different pH levels, varying combinations of the
addition agents, at varying and constant time, had been studied by the Scanning
Electron Microscopy (SEM) examinations (Loto and Olefjord,
1992). The overall results show a good zinc electrodeposition on mild steel,
though the obtained unique crystal structures were characteristic of the different
pH level, varying time and different combinations of addition agents. The above
mentioned addition agents have been previously used in the acid sulphate baths.
The `green addition agents-experimental: This last part of the
paper reviews the results of a study of the performance of cassava and sugar
cane juice extracts as addition agents in acid chloride solution. The detailed
experimental study was reported in a previous publication (Loto,
Flat mild steel-SIS 141147, 0.1 cm thick, with a nominal composition of 0.038% C, 0.19% Mn and the rest Fe, was cut into several test specimens of 10.0 cm long and 1.0 cm wide. A portion of 1.0 cm in length was marked off at one end for the electrodeposition of zinc.
The test specimens were degreased ultrasonically for 5 min with an alkaline degreasing chemical, code named Henkel VR 63 62-1 and then removed from the solution rinsed in distilled water, immersed in methanol and air dried. The specimens were, in turns etched for 50 sec in 3 M HCl, rinsed in distilled water, immersed in methanol, air dried and stored in a desiccators for further experimental process. The acid chloride solution for the electrodeposition consists of ZnCl (71 g L-1), KCl (207 g L-1) and H3BO3 (35 g L-1). Extracted cassava and sugar cane juices, each (25-30 mL L-1 of acid chloride, solution) were used as the addition agents.
Electrodeposition of zinc on steel was performed by partially immersing the
steel specimen and the zinc electrodes in the plating solution (20 mm deep)
through the rectangular hole made on a perspex plastic cover for the 250 mL
beaker used as the plating bath. The steel specimen was connected to the negative
side of a DC supplier while the zinc electrodes were also connected with a wire.
The plating solutions were put, in turns, into the beaker (bath) and their respective
pH was obtained by adjusting the original solution with potassium hydroxide.
Four different plating baths were used. These consist of:
||Plating from the acid solution without the addition agents
||Plating from the solution with cane sugar juice only (in 25 mL L-1)
as addition agent.
||Plating from solution with cassava juice (in 30 mL L-1) alone
as the addition agent and,
||Plating from solution with cassava and sugar cane juices (30 mL each)
as the addition agent
The operating conditions were:
|pH of the solution
||3.0 mA cm-1
A gentle stirring was used during the plating operations.
After each zinc electrodeposition, the plated specimen was taken out, rinsed in distilled water, immersed in methanol and quickly air-dried before the surface photograph was taken (Fig. 5). The unavailability of scanning electron microscope prevented the plated specimen surface from being characterized. The adhesion of the zinc coating to the steel substrate was tested by using a cellotape fastened to the surface and later pulled off and visually observed for any zinc stripping from the plated steels surface. The very acidic plating medium could easily destroy the unstable surface film of the metal specimen and rendered the steels surface bare to serve as a good zinc electrodeposition substrate.
Photomacrograph of the as-received unplated specimen is presented in Fig. 5a. The plated specimen without any of the addition agents-cassava and sugar cane juices. Fig. 5b, did not show bright plating; the plating was dull and though, some how effective as expected. In Fig. 5c, which shows the plated specimen with the cassava juice as addition agent in the plating medium is effective to some extent. Similar result was obtained for the addition of sugar cane juice (Fig. 5d). The effectiveness of these two additives was actually due to their chemistry.
Cassava juice is known (Coursey, 1983; Conn,
1983; Nastey, 1983) to contain about 93% of the
cyanogenic glycoside linamarin-2 (β d-glucopyranosyloxy) isobutyronitrile
with about 7% of the closely related lotaustralin-2 (β-d-glucopyranosyloxy)
2-methyl butyronitrile. During fermentation, these substances hydrolyse under
the influence of the endogeneous enzyme linamarase to liberate hydrogen cyanide.
Cyanide ions in the cyanide plating bath is known to give the brightest zinc
Though making an effective contribution, the very low percentage of cyanide
ion (CN-) in cassava juice, however, did not allow it to give the
very bright plating characteristic associated with the cyanide bath. Sugar cane
juice consists of sucrose, a non-reducing sugar. Sucrose has been known to be
α-D-glucopyranosyl-β-D fructose (Finar, 1969).
Sucrose can be hydrolysed by dilute acids such as in this acid zinc bath or
by the enzyme invertase to an equimolecular mixture of D (-) fructose.
||Photomacrograph of the unplated and plated surfaces of the
mild steel specimen: (a) unplated mild steel specimen, (b) plated mild steel
without addition agent, (c) plated mild steel with cassava juice as addition
agent, (d) mild steel with sugar cane juice as addition agent and (e) plated
mild steel with cassava and sugar cane juices as addition agent (Loto,
The two monosaccharide molecules are linked through their reducing groups.
Glucose has been previously used and is still being used as addition agent in
The synergistic effect of the combined use of cassava and sugar cane juices
is apparent as presented in Fig. 5e. The chemistry of the
cassava juice combined with that of the sugarcane juice would have given a chemical
complex that became very effective in giving a good zinc electrodeposition on
The continuing development of acid zinc plating baths based on zinc chloride has radically altered the technology of zinc plating which now constitutes about 50% of all zinc baths worldwide. It is the fastest growing baths throughout the world. A lot of development has been made in improving the brightness of the acid sulphate bath and also in improving the corrosion resistance of electrodeposits from acid baths. The inherent major advantages have contributed to the recent trend of growth in the acid baths. A good zinc electrodeposition on mild steel surface could be obtained in the acid zinc chloride solution using either the cassava juice or the sugar cane juice extract alone. However, the combination of the two gives a far better synergistic result.