Page 428 - Industrial Power Engineering and Applications Handbook

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Heavily scaled but mildly rusted surfaces
Parameters H2s04 HCl
The process is similar to the above, except that de-rusting
Concentration 25% 30-50% and water rinsing tanks can now be eliminated if H2S04
(by weight) is used for the pickling. But when HCl is used, the second
Bath temperature 60 ? 5OC 4W5"C water rinsing tank should be retained, to remove all traces
Approximate time 15-20 min. 15-20 min. of HCl thoroughly before it enters the phosphating tank.
of pickling Otherwise the trapped traces of HC1 (chloride contents)
will contaminate the phosphating bath and adversely affect
Inhibitor: since the acid 0.25-0.5% 0.25-0.5% the phosphate coating. This may be shown by rusting,
attacks the metal and by weight by weight
causes pitting, an not immediately but in the course of time. It may be
inhibitor must be added noted that traces of chloride are not removed so easily
and hence the need for a second rinsing.
Notes In place of simple water rinsing, it is more appropriate
1 Concentration of acid and time of pickling will depend upon to add a neutralizing agent such as hexa-amine or sodium
the condition of the surface and the bath temperature. nitrite (NaN02) to make a bath of 34% concentration.
2 One or two dips during the course of pickling will give better In the bath, the job may be dipped for two or three
results and enhance the efficiency of the process, as it will
quickly remove loose scales. minutes at room temperature to neutralize all the trapped
3 To reduce metal attack and fumes, use an acid inhibitor during traces of acid. This method also helps to accelerate the
acid pickling. A 0.01% concentration is recommended. process of phosphating when the job is transferred to the
phosphating tank. The concentration of bath solution can
winters and cold climates. The heaters and thermostat be checked along similar lines to those for the toner used
must have a stainless steel body. in the phosphating tank.
3 Checking bath concentration Acid content can be The tank will be similar to the first rinsing tank. It is
checked with the help of a pH paper. If this indicates recommended that the rinsing be done at an elevated
more than 3.5, add more acid to make it less than this temperature of, say, 60-70°C, even when HC1 pickling
reading. is carried out in cold conditions to agitate the air and
4 Precautions easily remove all traces of chloride.
At lower concentration or lower bath temperature, The bath water may be checked for any acid traces
the acid will attack slowly on scale and rust and with the help of a pH paper. This should give a reading
will take longer to pickle. Therefore, monitor above 6, preferably around 7.
concentration and bath temperature.
H2S04 reacts with water and generates heat. When Note
using this chemical, pour it slowly into the water. When de-rusting is adopted after HCl pickling, the second water
Acid fumes, being heavy, will vaporize over the rinsing, as recommended above, is not essential as the traces of
tanks. They must be vented quickly to the atmosphere chloride, if any, will be neutralized in the de-rusting tank, which is
through an exhaust on the pickling tank otherwise a phosphoric acid-based rust solvent.
the completed phosphate surfaces may be adversely
affected. Apparently, the surfaces may not show Mildly scaled and mildly rusted sui$aces
rusting immediately, but may develop it while the Now the process of acid pickling may be eliminated if
equipment is in operation. desired. Instead, only the de-rusting process can be used,
5 Water rinsing To rinse, wash the surface in clear, as indicated in column 2 of Table A13.3. Alternatively
continuously running water to remove all traces of acid pickling may be carried out as before, but at a lower
acid from the surface. Two or three dips at room concentration and temperature, as noted in column 1.
temperature will be sufficient. The details of tank Since one cannot always be certain of the quality of
will be similar to those for the acid tank but without sheet surfaces it is advisable to follow the process of
any heating arrangement. acid pickling.
6 De-rusting This is a process to remove rust from the
surface. The procedure of de-rusting is given in Table Sand blasting
A13.3, column 2. The de-rusting chemical is The scale can also be removed by shot blasting using
phosphoric acid based, and does not contaminate the abrasive grits such as dry sand, less than 1 mm $. This
phosphating tank.
7 Tank material A mild-steel (MS) tank with a wall method is more suited for components not suited to the
dip method and cast iron components, in which the acid
thickness of 4-6 mm having a heating arrangement may become trapped in the porous surfaces. For sheet-
and a thermostat temperature control will be required. metal components and complicated shapes and crevices,
Since the phosphoric acid-based, rust solvent is the dip method alone is recommended.
corrosion resistant, no tank lining is necessary. The
heaters and thermostat may be of stainless steel or
lead-covered for better durability. Phosphating
8 Water rinsing To rinse, wash the surface in clear
water an MS tank with a thickness of 3-4 mm is Process
adequate. It may, however, be provided with corrosion- This is a process to provide a fine coat of zinc phosphate
resistant paint to extend its life. or zinc calcium phosphate on ferrous and non-ferrous