Sulfuric acid is produced more than any
other chemical in this world for use in fertilizer, chemical, pigment, paint,
petroleum, textile and film, iron and steel and other applications. Besides of
these major applications, sulfuric acid is specifically used in producing dyes,
drugs, rayons, cellulose, alkylation of petroleum products to increase octane
rating, pickling of ferrous and nonferrous alloys, production of uranium from
ore, develop hydrogen fluoride from fluorospar, application use in copper, zinc
and nickel refining and treatment of organics in the development of alcohols
and treatment of organics in the production of alcohols and detergents.
Sulfuric acid is also used to add sulfur content that offer special characteristics
to the finished product like surfactants.
Corrosion by sulfuric acid
Stainless steel series 300 specifically
316/316l are widely used in sulfuric acid solutions for particular
applications. The applications include dilute or concentrated solutions at medium
or low temperatures. Austenitic stainless steel 300 series show active passive behavior
in sulfuric acid solutions.
Corrosion in sulfuric acid conditions is
electrochemical in nature and the total magnitude of metal loss is associated with
total corrosion current. The corrosion current is in proportion to current density
of the corroded metal if the corrosion is symmetrical on the metal. With increase
in temperature or chloride ion concentration, the critical current density
increases, the area of passivity decreases and pitting occurs.
Corrosion of stainless steel 316 is
observed in 93% sulfuric acid from a metallurgical plant stack gas. The acid comprised
of contaminants that made it more severe. The corrosion potential was noticed
to be stable passive at 25oC, rarely active at 75oC and at 95oC, the steel
became active for a short duration.
Anodic Protection
Anodic protection is a significant way of
increasing the service life of stainless steels and other materials in various
corrosion media and it has been successfully used to reduce corrosion level of
carbon steel and stainless steel in sulfuric acid application.
Austenitic stainless steels like SS 304,
304l, 316 and 316l can be anodically secured at actually all concentrations of
sulfuric acid up to boiling temperature. The corrosion rate can be minimized by
moving from freely corrosive active zone to secured passive zone.
A misconception about the passive state is
that lowers the corrosion rates. It is feasible to find very corrosion rates in
the passive state and hence there are practical limitations with respect to
anodic protection. Stainless steel type 316 is observed in the acid content of
3 – 92% at temperatures 34 to 121oC and it is found that anodic protection is
useful for the whole acid concentrations at temperatures up to 75oC.
Another factor considered is cathode
current density that will be limited by content polarization. The cathode to
anode surface area should be sufficiently big to passivate the anodic surface
at genuine rate. Hastelloy C276 wire and platinum were used as cathodes
for anodic protection. The harmful effect of chlorides in sulfuric acid can be
overcome by anodic protection. For example stress corrosion cracking of
Stainless steel type 304 in 5 to 50%
sulfuric acid at 30oC was prevented by anodic protection. Hence it is a
success.
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