© 2006 by Taylor & Francis Group, LLC

Composition of the Anticorrosion Coating 33
reaches an equilibrium value of 20 nm. The film prevents the outward
diffusion of iron. Phosphate ions do not appear to directly contribute to
the oxide film formation but rather act to complete or maintain it by
plugging discontinuities with anion precipitates of Fe(III) ions. Romagnoli
has noted that Pryor used soluble phosphates rather than the generally
insoluble phosphates used in coatings, so care should be taken in
extrapolating these results [37]. Other studies have found both oxyhydroxides
and iron phosphates incorporated in the protective film [59].
• Inhibitive Aqueous Extracts Formed with Certain Oleoresinous Binders
Inhibitive aqueous extracts form with certain oleoresinous binders. Components
of the binder, such as carboxylic and hydroxyl groups, form
complexes with either the zinc phosphate or the intermediate compounds
formed when the zinc phosphate becomes hydrated and dissociates. These
complexes can then react with corrosion products to form a tightlyadhering,
inhibitive layer on the substrate [21,39,43–46,52].
• Polarization of the Substrate
Clay and Cox [60] have suggested that nearly insoluble basic salts are
formed and adhere well to the metal surface. These salts limit the access
of dissolved oxygen to the metal surface and polarize the cathodic areas.
This theory was confirmed by the work of Szklarska-Smialowska and
Mankowsky [61].
2.3.3.2 Types of Zinc Phosphates
Because so many variations of zinc phosphates are available, it is convenient to
divide them into groups for discussion. Although no formal classes of zinc phosphates
exist, they have been divided here into groups or generations, more or less
by chronological development.
2.3.3.2.1 First Generation
The simplest, or first generation, zinc phosphate is made by either mixing disodium
phosphate and zinc sulfate solutions at boiling temperature or saturating a 68%
phosphoric acid solution with zinc oxide, also at boiling temperature. Both methods
give a precipitate with an extremely coarse crystalline structure. Further treatment
yields Zn3(PO4) 2 •4H2O, first generation zinc phosphate [37].
The usefulness of first-generation zinc phosphate is limited by its low solubility
[62]; only a small concentration of phosphate ions is available to protect the metal.
This is a problem because corrosion inhibition by phosphates takes place only when
the anion concentration is higher than 0.001M in a salt solution at pH 5.5 to 7.0 [57].
2.3.3.2.2 Second Generation
Zinc phosphates can be modified to increase their solubility in water or to add other
functional groups that can also act as inhibitors. This is usually achieved by adding
an organic surface treatment to the pigment or blending other inorganic inhibitors
with the zinc phosphate [23]. Table 2.4 shows the amount of phosphate ions in
milligrams-per-liter water obtained from various first-generation and subsequent
generation zinc phosphates [63]. It can be clearly seen why modifying phosphate
© 2006 by Taylor & Francis Group, LLC