Page 429 - Industrial Power Engineering and Applications Handbook

Switchgear and controlgear assemblies I 3/403

Table A133 Pickling and de-rusting process in mildly scaled and mildly rusted surfaces
Description Pickling-cum-de-rusting tank De-rusting tank (phosphoric
(Hot-rolled or cold-rolled acid-based rust solvent)
sheets with mild scaling and rust)
1 2
Heating type solvent Cold type solvent
Hot-rolled sheets with Cold-rolled sheets
mild scaling
Concentration 10-15% 25-30% 15-20% 15-20%
Temperature 60 f 5°C 4045°C 60-70°C 4045°C
(M. 80°C)
Approx. time of picklin; 5-10 min. 10-15 Min. 10-15 min. 3-5 min.
and de-rusting
Inhibitor 0.25-0.5% 0.2545% Not required
by weight by weight
Checking the By pH paper: should not be more than 3.5 aSee footnote
concentration
Water rinsing Only one water rinsing, with a few extra dips, As noted in steps 1-8
is adequate

a The phosphoric acid-based solvent used for de-rusting can be checked as for degreasing but titration will now be carried out against an
N/10 NaOH solution until the colour changes to green. A burette reading will indicate the actual pointage of the bath compared to the
standard (almost 4 for a concentration of 5% and 16 for a concentration of 20%). Obtain the standard pointage from the chemical
manufacturer. For each one point lower strength, add 1.2 e of solvent per 100 I of bath.

surfaces. It is a highly corrosion-resistant bonding to Cold process: 40-45°C for 20-25 minutes will
protect the surfaces from corrosion and rust. The rust provide a phosphate coating of up to 3-3.5 g/m2.
may creep under the painted or scratched surfaces and 2 For the unaccelerated process:
crevices. This is a phosphate base chemical and can be For a heavy coating:
applied cold or hot. However, the cold process is not Hot process: 80-90°C for 5-7 minutes will provide a
recommended, as it may give a coat of about 3-5 g/m2 phosphate coating of more than 5 g/m2.
whereas for the equipment being discussed the coat must
be above 5 g/m2. Material
Concentration: An MS tank with a wall thickness of 3-4 mm, having a
heating arrangement and a thermostat temperature control
Hot process - 3-5% by volume of bath will be required. No protective lining is necessary as the
Cold process - 10-15% by volume of bath phosphate coating itself is protective.

Tonel; accelerator or oxidizing agent Checking the concentration of the bath
This is alkaline in nature, say, of sodium nitrite (NaNO,) Carry this out as for tank no. 1 (see above):
base and may be added to accelerate the process. At a
very high temperature, however, above 70"C, it becomes Pipette out 10 ml of bath solution.
ineffective. The bath temperature must therefore be kept Add a few drops of methyl orange indicator and shake.
below this. Titrate it against an N/10 NaOH solution until the
Concentration: colour changes to yellow.
Note the addition of NaOH, which will indicate free
250-3OOg/lOOO 1 of bath volume. acidity.
To the same solution add a few drops of phenol
Where the phosphate coating is required to be more than phthalein, and titrate it against N/10 NaOH until a
5 g/m2 an extra hot process is used, as noted later, when pink colour appears, which will indicate the total acidity
the use of toner (accelerator) becomes redundant, as it is of the bath. This is approximately 35 to 37 for a
ineffective above 70°C. concentration of 5% for a hot process and 60 to 64 for
a concentration of 10% for a cold process. Obtain the
Bath temperature and approximate time of dipping standard total acidity of the hot or cold process
1 For the accelerated process: chemicals from the manufacturer.
For a normal coating: For each one-point lower strength, add 125 to 150 ml
Hot process: 60-70°C for 2-5 minutes will provide chemical per 100 e of bath. Consult the manufacturer
a phosphate coating of up to 5 g/m2. for the exact details.