© 2006 by Taylor & Francis Group, LLC

40 Corrosion Control Through Organic Coatings
reverse of what is usually seen in the coatings world — is necessary to ensure
electrical conductivity. If the PVC is less than the CPVC, the zinc particles are not
in direct electrical contact with each other, and the insulating binder between the
particles prevents the bulk of the zinc dust from offering cathodic protection to the
steel.
Inorganic binders are silica-based. They can be further divided into two groups:
solvent-based partly hydrolyzed alkyl silicate (mostly ethyl silicate) and water-based
highly alkaline silicates. Inorganic ZRPs are conductive and are therefore used as
weldable or shop primers. They also have high porosities. With time (and corrosion
of the zinc), the matrix fills with zinc salts, giving a very dense barrier coat. Inorganic
ethyl silicate in partly hydrolyzed form sometimes has a storage stability problem.
Inorganic ZRPs require higher film builds than do the organic ZRPs. Schmid
recommends approximately 50 µm with an organic one-component binder, approximately
75 µm with an organic two-component binder, and approximately 100 µm
with an inorganic binder [88]. Other workers in the field have proposed film builds
of up to 140 µm for inorganic binders.
2.3.6 CHROMATES
The chromate passivating ion is among the most efficient passivators known. However,
due to health and environmental concerns associated with hexavalent chromium,
this class of anticorrosion pigments is rapidly disappearing.
2.3.6.1 Protection Mechanism
Simply put, chromate pigments stimulate the formation of passive layers on metal
surfaces [89]. The actual mechanism is probably more complex. Svoboda has
described the protection mechanism of chromates as “a process which begins with
physical adsorption which is transformed to chemisorption and leads to the formation
of compounds which also contain trivalent chromium” [90].
In the mechanism described by Rosenfeld et al. [91], CrO 4 2− groups are adsorbed
onto the steel surface, where they are reduced to trivalent ions. These trivalent ions
participate in the formation of the complex compound FeCr 2O 14−n(OH − ) n, which in
turn forms a protective film. Largin and Rosenfeld have proposed that chromates do
not merely form a mixed oxide film at the metal surface; instead, they cause a change
in the structure of the existing oxide film, accompanied by a considerable increase
in the bond energy between the iron and oxygen atoms. This leads to an increase
in the protective properties of the film [92].
It should perhaps also be noted that several workers in the field describe the
protection mechanism more simply as the formation of a normal protective mixed
oxide film, with defects in the film plugged by Cr 2O 3 [23,57].
2.3.6.2 Types of Chromate Pigments
The principal chromate-based pigments are basic zinc potassium chromate (also
known as zinc yellow or zinc chrome), strontium chromate, and zinc tetroxychromate.
Other chromate pigments exist, such as barium chromate, barium potassium chromate,
© 2006 by Taylor & Francis Group, LLC