Page 187 - CW E-Magazine (3-12-2024)

Special Report

in these process areas can be brought to evolution reaction and will lower its area materials such as steel wool should
a minimum by good housekeeping to potential to the region where TiO is- be avoided. Titanium ignites spontane-
2
restrict and confine the corrosive fluids. stable.Alternatively, the titanium sur- ously in dry chlorine. Steel reacts with
faces can be coated with RuO . chlorine at an accelerated rate at high
2
Another important issue in any cell temperatures, igniting at about 250°C.
room is the stray or leakage currents- The industry standard material of Ti is passive in the presence of 0.5
that flow through the electrolyte feed and construction for chlorine headers is wt% moisture as depicted in Figure-14,
discharge headers into grounded equip- FRP with are sin-rich inner barrier. which shows the degree of passivity
ment. The leakage currents flow from of titanium depending on the dryness
the cells through a header to a reactor or The chlorine headers popular in (moisture content) of chlorine gas and
tank. The current depends on the volt- Europe are dual laminate headers, which the operating temperature of the gas.
age difference between the electrolyte are preferred over FRP for environmen- The presence of organic substances or
in the cells and at the target and on the tal reasons. The dual laminate has an rust increases the risk of steel ignition.
resistance of the electrolyte. Depending inner ring of less brittle PVC-C of 5-mm For dry chlorine, carbon steel Schedule
on the polarity of the current that the thickness for corrosion protection and 80 pipe is used in sizes up to 15-cm dia-
metallic pipes are exposed to, one has an outer layer of 10-15 mm thickness meter and schedule 40 is satisfactory
to either anodically or cathodically pro- of FRP for structural strength, the inter- for large diameter pipes.
tect the pipes. Anodic protection of the mediate layer being 5 mm of resin-rich
flanges is achieved by using concen- FRP 5I0 N or 197-3. The outer layer Liquid chlorine is generally stored
tric titanium rings coated with mixed contains a flame retardant and is added in vessels made from non-alloyed car-
metal oxides (RuO , IrO , PtO , etc.) after the dual laminate is stress re- bon steel or cast steel. Fine grain steel,
2
2
2
also called as Dimensionally Stable lieved. The chlorinated PVC (CPVC)/ stress-relieved and subjected to Charpy
Anodic (DSA) coating, to ensure the FRP dual laminate header is fabricated impact tests for low temperature im-
titanium flange area from reaching the from CPVC tubing up to 60 cm in dia- pact resistance, is used. Erosion of the
breakdown voltage of TiO , which is meter. The dual laminate headers have protective layer on steel is prevented
2
about 11.5 V. Cathodic protections, on a life of 8-10 years vs the 6-12 years of by keeping flow velocities less than 2
the other hand, is imposed by titanium life of FRP, at a comparable cost. meters per second. Organic materials,
blades. These require periodic replace- zinc, tin, aluminium, and titanium are
ment because of hydride formation. Titanium chlorine headers are used not acceptable in dry chlorine service.
in Europe and are claimed to reduce Copper, silver, lead and tantalum are
Titanium is stable in chlorine-con- the amount of chlorinated organics and acceptable for some equipment.
taining brine solutions. However, it is other non-volatile residues in gaseous
prone to crevice corrosion in the seal- and liquid chlorine. However, it suffers Wet chlorine is extremely corrosive,
ing areas and gaps between welded from crevice corrosion near the welds attacking most common metals except
titanium structures, as titanium can be (12, 13). Hastelloy C, titanium, and tantalum.
anodically oxidized to Ti either from Surface oxide films protect these met-
+
2
Ti or TiO or to Ti , the correspond- Chlorineprocessing and systems als from attack by the acids formed
3+
2
ing cathodic reaction being the chlo- The choice of construction mate- in chlorine hydrolysis. Tantalum is an
rine reduction reaction or the hydrogen rials for chlorine service depends on ideal construction material for service
evolution reaction. Figure-12 illustrates equipment design and operating condi- with wet and dry chlorine. However,
crevice corrosion of a titanium heat tions. Figure-13 shows the corrosion- it is expensive and normally used only
exchanger plate covered with synthetic resistance of various materials to dry in instruments such as transmitters,
rubber gasket. It can be seen that dam- chlorine, wet chlorine gas, and chlorine diaphragms, transducers, and thermo
aged areas of most of the parts are in aqueous solution. wells. FRPis used for wet chlorine. Rub-
covered with white TiO . This tendency ber-lined steel is also suitable for wet
2
of titanium toward crevice corrosion, Dry chlorine, with less than 40 ppm chlorine gas upto about 100°C. At low
particularly in gasketed areas, often can by weight of water, can be handled pressures and low temperatures, PVC,
be avoided by the use of alloys contain- safely below 120°C in equipment made CPVC, and reinforced polyester resins
ing Ni and Mo; the highest level of from iron, steel, stainless steels, Monel, are also used. PTFE, polyvinylidene
resistance is offered by alloys with Pd nickel, copper, brass, bronze, or lead. fluoride(PVDF), and polytetrafluoro-
or Ru. These catalyse the hydrogen Silicones, titanium, and high surface ethylene hexafluoropropylene (FEP)

Chemical Weekly December 3, 2024 187

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