Environmental Engineering Reference
In-Depth Information
to 500 ppm. At higher temperatures and higher hydrogen ion activity,
i
critical
in-
creases and the critical concentration of passivators also rises. Chloride and sul-
fate ions compete with the passivating ions for adsorption, thus interfering with
the process of passivation. The local breakdown of passivity in their presence
leads to pitting and higher amounts of passivators are required to prevent this.
Passivators are used extensively for the protection of steel in all types of water.
However, the choice of passivators varies from system to system. Chromates are
almost the inevitable choice for recirculating cooling waters, but they are not to
be used in antifreeze cooling waters because they tend to react with alcohols or
ethylene glycol. On the other hand, nitrites are not used in cooling tower waters
because they are gradually decomposed by bacteria. Chromates are toxic, which
restricts their application. For potable waters, nontoxic silicates or polyphos-
phates are used.
Cathodic Inhibitors
Cathodic inhibitors interfere with the cathodic processes and the rate of corrosion
is thereby decreased. They fall into three categories: cathode precipitates, oxygen
scavengers, and hydrogen evolution poisons.
Cathode precipitates and oxygen scavengers have been discussed in Sections
4.2.1 and 4.2.2, respectively. Calcium and magnesium carbonates, which are of-
ten present in natural waters, can be precipitated to form protective cathodic
deposits with the adjustment of pH. The addition of zinc sulfate also inhibits
corrosion by precipitating insoluble Zn(OH)
2
at increased alkalinity on the ca-
thodic areas according to the reaction:
ZnSO
4
2NaOH
→
Zn(OH)
2
Na
2
SO
4
(4.6)
Hydrogen evolution poisons interfere with the formation of hydrogen gas
(2H
ads
→
H
2
) to retard the overall rate of the cathodic reaction of hydrogen evolu-
tion. The corrosion rate is consequently decreased. Sulfides, selenides, and com-
pounds (usually oxides) of arsenic, antimony, and bismuth act as hydrogen evolu-
tion poisons. They are effective inhibitors in strong acids where the hydrogen
evolution reaction is rate controlling in the corrosion process. One difficulty is
presented by these inhibitors: They cause blistering and hydrogen embrittlement
in certain grades of steel because of the entry of the atomic hydrogen into the
metal. Also arsenic being toxic is restricted in use.
Organic Inhibitors
A good number of organic compounds are used as inhibitors. Common among
them are amines, imines, thiourea, mercaptans, guanidine, and aldehydes. The
compounds are chemisorbed on the metal surface forming a monolayer that inter-
feres with both the anodic and cathodic processes, although in many cases the
effect is unequal. The chemisorption is effected through the presence of a polar
