Environmental Engineering Reference
In-Depth Information
higher temperatures, lowering of temperature reduces the corrosion rate. The se-
verity of erosion corrosion can be reduced by decreasing the velocity of fluid
flow. On the other hand, pitting can be reduced in passivating metals and alloys by
avoiding stagnant conditions and ensuring some fluid flow. Potential pH diagrams
indicate immunity or passivity of metals at certain pH ranges, and an effective
means of corrosion control is to alter the pH of the electrolyte to such pH values.
For example, iron attains passivity and does not corrode at pH above 8.5.
The addition of alkali is a widely used practice in the treatment of boiler
feedwater, industrial and municipal waters. Many natural and municipal waters
contain calcium carbonate, which reacting with carbon dioxide is converted to
soluble calcium bicarbonate:
CaCO
3
CO
2
H
2
O i Ca(HCO
3
)
2
(4.1)
Addition of lime causes insoluble calcium carbonate to precipitate according to
the reaction:
Ca(HCO
3
)
2
Ca(OH)
2
→
2CaCO
3
2H
2
O
(4.2)
The deposit of CaCO
3
forms a hard and smooth protective film at a pH where
the precipitation just begins to occur. At higher pH values, the deposit is slimy
and porous. The difference between the pH of the water and the pH required to
precipitate CaCO
3
is referred to as Langlier index, which often is used to express
the condition of a water with respect to its tendency to deposit CaCO
3
.
The boiler tubes in oxygen-free hot water forms a protective film of magnetite
according to the reaction:
3Fe
4H
2
O
→
Fe
3
O
4
4H
2
(4.3)
The formation of the film is ensured by maintaining the pH of the feedwater at
a level of 9.5-11 with the addition of NaOH. In some high-pressure boilers, NH
3
is added instead of NaOH and the pH is maintained at 8.5-9.0. However, excess
alkali is damaging as the corrosion rate increases rapidly at pH values above 13
(Fig. 4.5).
Since steel is resistant to concentrated sulfuric and phosphoric acids at moder-
ate temperatures, increasing the concentration of acid, wherever permissible, con-
stitutes an effective means to reduce corrosion.
4.2.2 Removal of Corrosive Constituents
The corrosive tendency of the environment sometimes increases greatly in the
presence of certain corrosive constituents or chance contaminants. Chloride ions
present in water coolant are responsible for corrosion of nuclear reactors and
their elimination reduces corrosion. When such oxidizers as FeCl
3
or CuCl
2
are
present in hydrochloric acid, the corrosion of steel increases because of the addi-
tional cathodic reaction provided by the reduction of these oxidizers. The contam-
