Page 430 - Industrial Power Engineering and Applications Handbook

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Checking the toner Checking the bath concentration
0 By starch iodide paper (original colour white) This can be done by checking the pH value of the bath
Dip a piece of this paper into the bath solution for a solution.
few seconds.
The change of colour of the paper will indicate the Use a universal pH paper. The pH value should be
condition of the toner, Le. between 2 and 3.
White - no toner If the value exceeds 3, add more chemical.
Pale mauve (light blue) - insufficient toner If it is less than 2, drain a part of the bath and replenish
Blue or dark blue - correct quantity of toner it with fresh water.
Black - excess toner.
Dqing
Precautions It is essential to dry passivated surfaces promptly to protect
1 The phosphated surface must be transferred to the them from moisture and atmospheric contamination. The
water rinsing tank without delay. drying may be carried out by blowing compressed air,
2 To protect the surface from contamination by foreign which is easier and more economical, or by placing in
matter, it should not be touched, wetted or subject to the same oven as for the paint. Special care need be
condensation. taken with hidden surfaces, such as in corners, bends
3 The sludge of the phosphate bath that settles at the and crevices, to ensure that there is no trapped moisture.
bottom, must be cleaned as frequently as possible.
The clear solution from the surface can be siphoned Sealing
into an empty rinsing tank. After cleaning the tank, The phosphate coating itself is not protective unless sealed
the clear solution can be poured back into the tank. with a protective coating of primer. Sealing is therefore
4 The phosphate solution is acidic. Continuous human carried out by applying a coat of primer within 12 hours
contact or splashing of bath solution must be avoided, of phosphating, if the atmosphere is dry, or immediately
and hands or skin washed clean with a dilute solution if it is humid. Otherwise the atmospheric humidity may
of 1-2% of ammonium bicarbonate. react with the surface and form a film of rust (Le. ferric
oxide (Fe,O,)).
Water rinsing
Nores
To rinse, wash the surface in clear, continuous running 1 The chemical concentrations, bath temperature and process times
water to remove all traces of soluble salts which may noted above are only indicative and for general reference. They
cause blistering on the surface. The tank can be similar may vary with the type of chemicals, the manufacturer and the
to the phosphating tank. It may, however, be coated with condition of the surface to be treated. Details may be obtained
an anti-corrosive paint to extend its life. from the chemical manufacturer to formulate the internal sheet
treatment process guidelines.
2 It is strongly recommended to check the concentration of all
Passivation the baths every day before commencing work. A passivation
solution particularly, must be changed frequently, rather than
This is the final neutralizing rinse after the pre-treatment adding more chemical to the same bath, depending upon the
to obtain a better corrosion resistance. The phosphated amount of work every day.
surfaces are treated with chromic acid-based or acidified
sodium dichromate solutions which are not affected by A13.3 Pre-treatment of non-ferrous components
moisture and thus protect the phosphate coating.
1 Degreasing - with neutral or non-caustic
based chemical, otherwise
Concentration
same as for ferrous metals.
2 Pickling and de-rusting - Not necessary, as there is
Hot process - 125-150 g/1000 1 of bath volume no scale formation or rust
Cold process ~ 250-500 g/lOOO 1 of bath volume on the non-ferrous sur-
faces.
A very high content of this acidic solution may dissolve 3 The rest of the process is almost the same as for ferrous
the phosphate coating. components.

Bath temperature and approximate dipping time A13.4 Size of tanks
Hot process - 60-70°C for 30-45 seconds These should be suitable to accommodate the size and
The hot process is generally not recommended as it volume of a switchgear or a controlgear assembly being
may dissolve the phosphate coating manufactured by the unit. The size noted below should
Cold process - 4045°C for 60 seconds or so. be adequate to meet most needs:

Tank for passivation Length - 3 m
This is similar to that for phosphating. It may, however, Width - 1 m
be coated with an anti-corrosive paint to extend its life. Depth - 1.2 m