Page 669 - Corrosion Engineering Principles and Practice

622 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 623

the positive direction [14]. This positive shift generally observed
with most of the VCIs on ferrous metals is indicative of a preferentially
anodic effect of the inhibitors. This anodic effect may be related
either to a simple blocking effect of the anodic sites by the amine
part of the inhibitors or to the contribution of the anionic component
(i.e., the weak acid component).
In the case of nitrobenzoates, for example, it has been claimed
that an acceleration of the cathodic partial process by reduction of the
nitro group may lead, in addition to the effect of oxygen in the thin
electrolyte layer, to a complete passivation of iron or ordinary steels.
Contributions from the two parts of the dissociated molecule to the
inhibitive effect are very likely.

14.7 Inorganic (Nonmetallic) Coatings
Inorganic coatings can be produced by chemical action, with or
without electrical assistance, and include numerous classes of
materials, among them the hydraulic cements that can set underwater,
ceramics and clays, glass, carbon, silicates, and others. Some
treatments to produce inorganic coatings can change the surface layer
of a metal into a protective film of metallic oxide or compound that
has better corrosion resistance than the natural oxide film and
provides an effective base or key for supplementary protection such
as paints. In some instances, these treatments can also be a preparatory
step prior to painting.

14.7.1 Hydraulic Cement
Hydraulic cements are used to coat pipe inside and outside, especially
pipes that are to be buried or submerged, for example, water or sewer
lines. In the case of underwater piping to transport gas or liquid
hydrocarbons, for example, it may be mixed with barites or other
heavy materials to confer negative buoyancy. It also may be used in
mixtures of organic materials where its ability to maintain a
noncorrosive pH at the surface of steel is exploited.
Cast iron, ductile iron, or steel pipe may be coated at the mill by a
process during which the pipe is spun on the center of its longitudinal
axis while a mortar mixture is sprayed onto the inside surface in a
uniform, dense layer. After proper curing, provided the pipe is
handled carefully, this coating can protect the pipe interior against
attack by water and many other liquid and gaseous corrosive
environments.
A concrete coating on steel of any configuration can be protective
to the steel as long as it does not crack or spall off, as in Fig. 14.10
because the alkaline reaction of hydraulic cement maintains a high
pH at the steel surface, which effectively prevents corrosion. It does
this when encasing reinforcing steel, for example, where it functions
both as strength and protective component.