Hot rolling processes associated with various metal
industries often leave behind inorganic oxide residues on treated metal
surfaces. Exposure to moisture also results in rusting of Ferro-Nickel based
metal parts. These oxides interfere with further cold processing like polishing,
electroplating, painting or enameling. Rust and scale formation is a thin layer
of metal oxide on the surface that is mostly removed by chemical cleaning also
called “Pickling”.
Pickling is carried out on oxide scaled surfaces of metals
with alloy content less than 6%. The ideal pickling solution, known as Pickle
Liquor is a 3%-10% solution of Hydrochloric acid (HCl) or Sulfuric acid (H2SO4).
Hydrochloric acid is chosen over Sulfuric acid because of faster pickling
action all the while minimizing base metal loss. It restricts formation of smut
and greatly decreases the risk of excessive attacking. It is also economic as heating expenses
needed for sulfuric acid are spared due to possibility of lower temperature pickling.
Metal pickling is essential for thoroughly removing scales
and inorganic surface contaminants before applying finishing layers like paint,
galvanizing and polishing. The result of the process depends on the type,
strength and temperature of the pickling liquor. In order to be economical, it
is chosen as a continuous cleaning process by mode of manual/mechanized
spraying or dipping in large acid vats. For small scale industries like
jewellery making and research laboratories, extensive use of pickle pots is
observed. Nevertheless, the corrosive action of diluted pickle acids cannot be
ignored. In most cases, there is a significant loss of base metal during
cleaning operations. For this reason, Acid Inhibitors are used with the acid
solution to reduce base metal corrosion.
Inhibitors
are natural or synthetic organic compounds added to acid solutions to reduce
metal surface corrosion. They slow or stop the chemical action of acid on the
base metal surface.
Inhibitors
are essential to:
Reduce base metal loss
Protect the surface from pitting attack by
excessive pickling or from poor surface condition quality
Reduce fumes resulting from excessive acid-metal
reactions
Help economize acid consumption without reducing
descaling efficacy
Control the rate of pickling
Rodine Acid Inhibitor is a series of acid
corrosion inhibitors designed to serve worldwide standards for acid inhibitors.
They are specially synthesized to provide protection to base metals from acidic
corrosion effectively. Rodine Acid Inhibitor products inhibit the corrosion of
metals against acids like Hydrochloric, Hydrofluoric, Sulfuric, Phosphoric,
Sulfamic, and organic acids blends. The most common products underlined in this
range include Rodine 213, Rodine 213 Spl, Rodine 31A,
Rodine 103, Rodine 103 NF, Rodine 92B, Rodine 130, and many more.
The process is limited to oxides and inorganic
scales and contamination by soils, oil, grease, soaps and lubricants need
additional processes with the use of Degreasing chemicals.
Other limitations of acid pickling include difficulty to handle products
because of their corrosive nature, the issue of hydrogen embrittlement
popularly associated with some alloy and high-carbon steels, occupational
hazards as the reactions produce toxic fumes in the factory shed. To obtain
desired descaling, without endangering metal surfaces, acid concentrations and
solution temperature need to be monitored quite strictly. Sludge is the
ultimate waste product of acid pickling. It contains acid rinse water, metallic
dissolved and suspended salts, waste spent acid etc. EPA considers it Hazardous
waste and recommends proper treatment before disposal mostly in the form of
Lime neutralization.
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