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ESCORT
ACID ZINC PROCESS - 238
This escort acid zinc process-238 is our recent development and
considered to be the best among present day available bright
acid zinc plating processes. It produces fast brilliant bright,
levelled, uniform, highly corrosion resistant and pleasing
bludish-white deposits over a wide range of current densities in
both rack and barrel plating operations. The process used for
plating high tensile steel, without causing hydrogen
embrittlement, and also for cast iron, malleable iron,
carbonitrided steel etc. Due to its improved and high throwing
power it can be used for plating eliminates costly waste water
treatment in comparison to cyanide zinc carrier-0144 m and
brightner-0144r. Both the addition agents are requried for
initial bath make up and for repleanishing during
working of the bath solution. Zinc deposits produced from
the system, readily accept any conventional chromate passivation
treatment from blue, irridescent golden, olive green or black
chrome passivation treatment from blue, irridescent golden,
olive green or black chrome passivation for detailed operation
of passivation treatment see our zinchromate 161, (l), 162(p),
164 and 171 pamphlets.
The escort acid zinc process
produces an acceptable finish from a wide range of bath
compositions. It is advisable to maintain the bath within
recommended limits in order to minimize the chemical consumption
and to get consistent results. The recommended bath make up,
working conditions, maintenance and consumption of the bath
constituents are given as under.
Bath
make up :-
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Escort acid zinc
liquid
|
241
a
|
100 ml/litre
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Escort acid zinc
salt
|
241
b
(rack)
|
225 gm/litre.
|
|
|
|
(barrel)
250 gm/litre.
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|
Escort acid zinc carrier
|
0144
m
|
40 ml/litre.
|
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Escort acid zinc
brightner
|
0144 r
|
0.8 ml/litre.
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|
|
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Working
conditions :-
range
optimum
Density
obe
14.0 ญ 18.0
16.5
Temperature
0c
20.0 ญ 45.0
33.0
Cathode
current density
Vat
amp/sq.dm
0.1 ญ 5.0 4.0
Barrel
amp/sq.dm
0.1 ญ 1.5 1.0
Anode
current density 0.5 ญ
3.0
2.0
Voltage
ญ vat
2.0 ญ 4.0
3.0
Voltage
ญ barrel
4.0 ญ 9.0
6.0
Filtration
continuous continuous
Ph
ศ
4.0 ญ 5.0
4.8
Anodes
pure zinc (99.99%) pure
zinc (99.99%)
Agitation
air agitation
air agitation
or
cathode
rod movement
Rate of deposition
At
2 amp/sq.dm per minute 0.51
microns
0.54 microns
Bath
maintenance :-
Zinc
metal vat
24.0
ญ 40.0
30.0
Total
chloride vat
100.0 -155.0 125.0
barrel
110.0 -170.0
137.0
Boric
acid
27.0 ญ 33.0
30.0
Consumption
:
Carrier
0144 m per kilo amp-hr
in ml
100 + 25
Brightner
0144 r per kilo amp-hr
in ml
200 + 50
Liquid
241 a per kilo amp-hr
in ml
20o
50
Hydrogen
peroxide (30% 10o
volume per day)
for 1000 litre bath
100
25
Note
:- to be addee
per day or weekly basis.
to increase 1.0 gm/litre
of zinc metal add 3.4
litre
of 241a liquid for a
1000 litre bath.
Note
:- after addition, air agitate
the
bath for half an hour and
do filtration priod to start
plating work
on next day so that the bath will be free from iron
impurities
Bath
solution preparation :
The recommended solution
preparation procedure is given as under :-
|
1. |
The
plating tank must be leached, cleaned and rinsed
thoroughly prior to solution preparation. |
|
2. |
Fill
the cleaned and rinsed plating tank about three quarter
full of softened or demineral ise water. Heat to a
temperature of 55-60oc. |
|
3. |
Dissolve
required quantity of acid zinc salt 241 b, stir vigorously
with air and a plastic paddle until all the salts are
completely dissolved. |
|
4. |
Add
required quantity of liquid 241 a with stirring. |
|
5. |
Check
the ph of the solution with a ph meter or ph paper. Adjust
if necessary to 4.8 to 5.0 with chemically pure grade
hydrochloric acid to lower the ph or with 20% solution of
caustic potash to raise the ph. |
|
6. |
Filter
the solution till free from any suspended matter. Clean
the filter cartridges and cloths. Repack with activated
carbon and cyn zinc purifier 214 s. Filter the solution. |
|
7. |
Add
required quantity of carrier 0144m and brightner 0144 r
with stirring. |
|
8. |
Add
sufficient water to bring solution level to final
operating volume. |
|
9. |
Put
necessary number of anodes and use the bath for the
plating. |
Plating
sequence:-
A suitable sequence for the preparation of parts for acid zinc
plating is as follows:-
|
1. |
Hot
soak clean at 70oc using 70 gm/litre of cleaner
ms-120 r for 3 to 5 minutes. |
|
2. |
hot
soak clean at 70oc using 50 gm/ltr of cleaner
ms-112, at 4-6
volt for 2 to to 3 minutes (do if required) |
|
3. |
Swil
in gold running water. |
|
4. |
pickel
in 50% v/v hydrochloric acid. |
|
5. |
water
rince(twice) do not keep cleaned parts/components in rinse
after for a longer time. |
|
6. |
Do
escort acid zinc plating for a required time. Insure that
parts taken for plating are completely cleaned and not
resrusted other wise parts after plating and subsequent
passivation will be hazy and not fully bright. |
|
7. |
Swil
in running water. |
|
8. |
Passivate
as desired. |
|
9. |
Swill
in cold running water. |
|
10. |
dry
and pack. |
Note:
the preferred, conventration of chromate solutions are usually
more dilute than required for cyanide zinc plating.
Equipments:-
Tank :-
Mild steel tanks lined with pvc or polypropylene are
suitable. The tank must be thoroughly cleaned before
final use. The tank should be fitted withtwin row plastic
air agitation pipes. As an alternative to air
agitation,mechanical cathode rod movement may be employed.
Where high current density loading are used e.g. in barrel
plating, solution cooling will be necessary and titanium cooling
coils will need to be fitted.
Filtration:-
Both
barrel and rack plating installations require continuous
filtration to remove iron sludge. The filtration system should
have sufficient capacity to filter the plating solution two
times per hour. All the inside parts of the filter unit should
be made up of approved quality hard rubber.
Aoitation:-
Low
pressure or oil free air equipment is recommended for air
agitation of the bath solution, compressed air coming from a
conventional air compressor should not be used.
Anodes:-
Anodes
used are of 99.99% pure special high grade zinc. Flat top, ball,
oval or slab anodes may be used. Ball anodes in titanium baskets
may be used. It is essential to keep the anode baskets filled
with zinc balls to avoid corrosion of titanium baskets.
Anode
area must be sufficient to keep anode current density below 3
amp/sq.dm. Do not use any inert anodes.
It
is most desirable to use anode
bags of polypropylene for rack plating. If anodes are not
bagged properly then roughness on plated surfaces will be noted.
Anode bages should be cleaned periodically.
Power supply:-
For
rack plating 4-6 volt rectifier is adequate. While 6-9 volt
source is required for barrle plating. Rectifiers must have low
ripple(5% maximum) factor.
Bus
bars:-
All
copper bus bars used over the tank should be either dull nickel
plated with a thickness of 50-100 microns or insulated by taping
them with plastic shields to prevent corrosion and subsequent
contamination of the solution.
Role
of bath constituents:-
Escort acid zinc liquid 241 a:-
Provides
all the zinc ions for make up 3.3 ml/litre of 241 a will be
required to increase 1.0 gm/litre of zinc metal and 1.0 gm/litre
of total chloride. By increasing the concentration of 241
increases the permissible current density.
Escort acid zinc salt 241 b:-
It
is a main source of chloride ions and boric acid. It increases
the conductivity. Covering and throwing power of the bath. It
provides 42% of total chloride and 12% of boric acid.
Escort acid zinc carrier 0238m m:-
Carrier
0144m is our recently developed new formulation, required for
intital make up and maintaining the bath alongwith brightner
0144r. The main functions of carrier 0144 m are to act
synergistically with brightner
0144 r to maintain maximum grain refining, leveling and
brightness, as well as low current density brightness and
coverage. If its concentration decreases below the recommended
level, it leads to hazy dull deposits or roughness in the high
current density are and poor low current density coverage. It is
consumed by electrolysis and removed by drag out losses.
Ecort acid zinc brightner 0238r:-
It
is used to maintian maximum rate of leveling and to get extrmely
bright finish. A low concentration will results in overall loss
of brightness, leveling and performance. Factor influencing
brightner consumption are drag out losses, temperature and
current density. Frequent small additions are more economical
then large occasional ones. This assures a concentration in the
tank at all times and provides the most economical, trouble free
method of operation.
Effluent treatment:-
Detoxification
of escort acid zinc process 0144 m/r solution is very simple, as
compare to cyanide zinc process. Collect the waste water in a
container, neutralised it to a ph 7.5 to 8.5 by adding 25%
solution of caustic sode with enough stirring. This will
precipitate the zinc as hydroxide sludge which can be removed by
filtering the solution of by allowing the sludge to settle and
decanting the solution. Local regulations regarding sludge
disposal should be followed or it should be sold to zinc
recovery plant. Clear liquid can be discharged into
a sewage system, assuring there are no other pollutants
present.
specification requirement and rate of deposition
r a c k
plating:
Rate
of deposition at various current densities and
voltage to achieve
Minimum
and average thickness in minutes.
Class
current voltage
cathode minimum
average time
density
efficiency thickness thickness required
.dm.
(%)
microns microns in
minute
C
1
1.5
98 5.0
6.0
22
C
2
2.0 97
5.0
6.0 12
C
3
2.75
95 5.0
7.0
9
C
4
3.5
92 5.0
7.0
7
B
2
1
1.5
98 7.5
9.0
32
B
2
2
2.0
97 7.5
10.0
18
B
2
3
2.75
95 7.5
10.5
13
B
2
4
3.50
92 7.5
11.2
11
B
1
1
1.50
98
10.0
11.5
41
B
1
2
2.00
97 10.0
12.0
22
B
1
3
2.75
95 10.0
12.5
15
B
1
4
3.5
92 10.0
13.0
12
A
1
1.5
98
25.0 29.0
104
A
2
2.0
97 25.0
30.5
55
A
3
2.75
95 25.0
32.0
39
A
4
3.5
92 25.0
34.0
32
This is in consideration to general observations, but actual
variation in thickness is depending upon the shape and dimension
of the components.
Depending on optimum conditions of the bath.
Barrel
Plating.
Rate
of deposition on threaded parts e.g screws, bolts, nuts ect.
Considering 85% cathode efficiency.
ญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญ-----
no.
Thread
current
volt
minimum
average time
diameter
density
thickness thickness
required
in mm. Amp./sg.dm.
Microns
microns
minutes.
ญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญญ--
1.
0.0-3.0 0.75
6 5.00
5.75
32
1.
0.0-3.0 1.25
7
5.00
6.00
20
1.
0.0-3.0 1.75
8
5.00
6.00
15
2.
3.0-5.0 0.75
6
6.00
6.90
38
2.
3.0-5.0 1.25
7
6.00
7.20
24
2.
3.0-5.0 1.75
8
6.00
7.50
18
3.
5.0-12.5 0.75
6
7.00
8.00
44
3.
5.0-12.5 1.25
7
7.00
8.40
28
3.
5.0-12.5 1.75
8
7.00
8.75
21
4.
12.5-20.0 0.75
6
8.00
9.20
51
4.
12.5-20.0 1.25
7
8.00
9.60
32
4.
12.5-20.0 1.75
8
8.00
10.00
24
Depending
on optimum conditions of the bath.
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