Environmental Engineering Reference
In-Depth Information
gas purging, or by vacuum treatment. The older practice of deactivation was to
pass the water over hot steel plates or scraps, the oxygen dissolved in water
getting consumed by reacting with iron in the process. The modern practice is
to use oxygen scavengers or ion exchange resins.
Sodium sulfite and hydrazine are the common oxygen scavengers. They react
with oxygen according the following reactions and remove oxygen in the
process:
2Na
2
SO
3
O
2
→
2Na
2
SO
4
(4.4)
N
2
H
4
O
2
→
N
2
2H
2
O
(4.5)
One ppm of oxygen requires 7.3 ppm of sodium sulfite. The reaction rate of
sulfide with oxygen at low temperature is increased with the addition of catalysts
like cobalt, manganese, or copper salts. Similarly, hydrazine also requires cata-
lysts like activated charcoal, metal oxides, and alkaline solutions of Cu
2
and
Mn
2
for speeding up of the reaction at low temperatures. Hydrazine is preferred
in high-pressure boilers because it reacts faster than sodium sulfide at elevated
temperatures and because it does not increase the salt content of the boiler as its
reaction products are all volatile.
Ion exchange resins are available with metal sulfites, ferrous hydroxide, or
manganous hydroxide, which react rapidly with oxygen. Reduction of the oxygen
level to below 0.002 ppm is achieved through the use of such resins. The used
resins can be regenerated by chemical treatment.
4.2.3 Inhibitors
An inhibitor is a chemical substance that when added in small concentrations to
an environment decreases the corrosion rate. The amount of inhibitor required
for an effective decrease in corrosion rate may be as low as a few ppm; a few
hundreths are often satisfactory, and rarely exceeding 1%. Hundreds of inhibitors,
both inorganic and organic compounds, are available and are being employed
for corrosion prevention in various water systems, in acid pickling solutions, and
in oil production and refining. Some common inhibitors used for such purposes
are summarized in Table 4.5. A special class of inhibitors, called vapor phase
inhibitors, are used in closed boxes and packages for protection against atmo-
spheric corrosion during storage and transit.
Inhibitor for a particular metal in a particular environment may not be effective
for the same metal in another environment or for another metal in the same envi-
ronment, and at times it increases the corrsion rate of another metal. A combina-
tion of two or more inhibitors sometimes is more effective than a single inhibitor
(synergistic effect). The overall effectiveness of an inhibitor depends on corrosi-
vity, pH, and temperature of the solution. Use of inhibitors is preferred in closed
and recirculating systems. However, in once-through systems like municipal wa-
