Environmental Engineering Reference
In-Depth Information
group in the molecular structure by which the molecules can attach themselves
to the metal surface. These include sulfur, nitrogen, amine, phosphorus, and hy-
droxyl groups.
Cationic inhibitors, such as amines, or anionic inhibitors, such as sulfonates,
will be adsorbed preferentially depending on whether the metal is charged nega-
tively or positively, respectively, with respect to the solution. Amines show better
performance as inhibitors for steel in phosphoric acid in the presence of iodides.
The explanation for this synergism is that the adsorption of iodide ions shifts the
surface charge of steel to more negative values where the adsorption of positively
charged amines is favored. The fact that a certain organic compound acts as a
good inhibitor for some metal but not another is explained from this specific
electronic interaction of the polar groups with the metal surface.
Molecular structure and size of the organic compound influence their inhibi-
tive action. The structures with benzene rings are particularly effective inhibitors.
Primary amines become more efficient as the chain length is increased. This is
presumably because of the steric effect, i.e., diffusion barrier provided by long
chains. However, for mercaptans and aldehydes, the efficiency is decreased with
the increase in chain length. The sharp decrease in corrosion rate with organic
inhibitors is shown in Fig. 4.7.
Organic inhibitors find wide application as
pickling inhibitors
. Acid pickling
of hot-rolled steel is necessary to remove mill scale. Pickling inhibitors resist
corrosion of the substrate metal. These inhibitors are also used for the acid clean-
ing of the pipes clogged with rust or to remove limestone crust from inside the
boiler tubes. Typical examples of pickling inhibitors are quinolin ethiodide,
o
- and
p
-tolythiourea, hexamethylene tetramine, formaldehyde, and
p
-thiocresol.
They are added in the concentration of 0.01-0.1%. Organic inhibitors are also
added to oils, greases, and waxes used as
slushing compounds
to temporarily
protect steel surfaces from rusting during shipment or storage.
Vapor Phase Inhibitors
Vapor phase inhibitors (VPIs) are compounds with low vapor pressure (0.0002-
0.4 mm Hg). In a closed system, they volatilize and the vapor condenses on the
metal surface to provide protection. In boilers, volatile basic compounds such as
morpholine and ethylenediamine are transported with steam to the condenser
tubes that prevent corrosion of the tubes by neutralizing carbonic acid and making
the environment alkaline. In closed containers and packages, volatile solids such
as the nitrite, carbonate, and benzoate salts of dicyclohexylamine, cyclohexylam-
ine, and hexamethylene imine are used for temporary protection of critical ma-
chine parts, ball bearings, cold-rolled steel coils, etc., during storage or transporta-
tion.
Dicyclohexylamine nitrite is a widely used vapor phase inhibitor that is often
impregnated in the waxed paper or cardboard used for wrapping and packaging.
