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13/400 Industrial Power Engineering and Applications Handbook
Appendix: Painting procedure of 4 Curing the paint
switchgear and controlgear 5 Testing the painted surfaces
6 Providing a peelable coating compound, if necessary
assemblies and treatment of and
effluent 7 Effluent treatment and discharge of waste water.

A13.1 Introduction A13.2 Sheet pre-treatment (phosphate coating)
Painting of all metallic surfaces of a switchgear or a In Table A13.1 we describe the most common practices
controlgear assembly is an essential requirement to provide being adopted to pre-treat and phosphate ferrous and
it with an aesthetic appearance, on the one hand, and to non-ferrous surfaces, before applying paint:
prevent it from rust and corrosion, on the other. Painting
serves these purposes by providing the machine with a Degreasing and cleaning
hard and longer-lasting metallic surface. We describe
briefly, the basic procedure to paint and test painted Degreasing is a process to remove oil, grease, dirt and
surfaces. In the discussion, we have laid more emphasis swarf (file dust) ctc. from a surface.
on MS sheet-metal surfaces as these are more typical.
The painting procedure for other metal surfaces, Types of cleaners (degreasing agents)
although similar, the process of pre-treatment for cast There are several types of cleaners available for this
iron components or non-ferrous metals, such as aluminium purpose, for example:
and copper, may need more care. The process of pre-
treatment in such cases may vary slightly than for MS, 1 Alkaline cleaners (caustic based) These are caustic
as noted below. Such surfaces may require a change in soda based and are suitable for ferrous metals only.
the type of chemicals, their concentration and duration They are more effective in removing greases of
of treatment. The final surface preparation and painting vegetable oils, rather than mineral (petroleum) oils,
procedure, however, will remain the same for all. as they do not saponify the mineral oils.
The total painting procedure, may be divided into the 2 Neutral cleaners (non-caustic based) These are
rollowing operations: ethylene oxide condensates, and easily emulsify the
mineral oils and greases. They are more useful for
1 Sheet pre-treatment (phosphate coating) sheet-metal components, which contain no lead
2 Preparing the surface compound lubricants (as used for deep-drawing
3 Applying the final paint operations), and are also suitable for non-ferrous
Table Ai3.1 Process of sheet pre-treatment (phosphate coating)

hor ferrous metals
Pre-treatment process Heavily scaled and Heavily scaled, but Mildly scaled and For non-ferrous
heavily rusted surfaces mildly rusted-surfaces mildly rusted sufaces metals
(hot-rolled sheets) (hot-rolled sheets) (cold-rolled sheets)
1 Degreasing and cleaning J J J
1 Water rinsing J J J
3 Descaling or acid pickling J J
4 Water rinsing J J
5 De-rusting" J -
Second water rinsing or
6 Water rinsing J neutralizing is J"
recommended, in
HCI pickling
.~
~ ~ ~~
7 Zinc phosphatingb J J J J
8 Water rinsing J J J J
9. Passivationh J J J J
No. of tanks Nine-tank method Eight-tank method Seven-tank method Five-tank method
ahstead of de-rusting, a pickling process may also he sufficient, depending upon the surface condition of the sheets.
bAftcr the pickling process, if the phosphating bath contains traces of sulphate (SO,) or chloride (CI) salts, the phosphated surface may
become highly hygroscopic, and may absorb atmospheric moisture through even a very well painted surface, and show rusting with passage
of time (depending upon the atmospheric conditions at the place of installation). To avoid this and to achieve a long life for all painted
surfaces, it is recommended that all these salts are first neutralized. This is possible with the use of de-mineralized (DM) water, at least for
the make-up baths of phosphating and passivation. Where water is heavy and contains mineral salts, a small DM unit can he installed. It
is an inexpensive procedure for the quality of phosphate coating that would be achieved. This will also enhance the working life of these
bath solutions, and economize on their consumption. De-mineralization means removal of all sulphate (SO,) and chloride (Cl) salts.
Note: The transfer of jobs from one tank to another may he done by an overhead travelling hoist to handle bulky and heavy objects.