Critical review of corrosion science and engineering

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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING 394 C ORROSION—MAY 2003 hand, exhibit relatively narrow ranges in pH where the reduced oxides are stable (Figure 14). The vana- dium oxide is relatively more stable toward high pH and the Mo oxide is stable toward lower pH values. Solely as oxides, the elements of Mo and V will never give the same stability as seen in analogous Cr 3+ ox- ide. On the other hand, the oxo-compounds of these elements (Mo and V) can form very stable polyoxo- metallates (POM) with each other, phosphates and tungstates. These metals are also gel-formers under the proper conditions. Hence, they should not be ruled out completely as chromate replacements. It also should be noted that vanadates and molybdates provide significant inhibition for aluminum, 122 par- ticularly in concert with other compounds. 123 Other reports have shown the inhibiting properties of molybdates and tungstates, 124 and molybdates and permanganates 125 for aluminum. In recent years, a process using permanganate has been commercialized, 126 and a direct permangan- ate-based conversion coating approach has been described. 127 Unlike oxoanions of Cr and Tc, the oxoanion of hypervalent Mn, permanganate, is ther- modynamically unstable with respect to the oxidation of water, although in sufficiently alkaline solutions it remains kinetically inert. 128 The hypervalent Tc behaves like chromate and forms a stable oxide film over a much broader pH range. Indeed, the technetates have been shown to actually exceed the corrosion inhibiting behavior of chromate with regard to corrosion of ferrous materi- als. 129 Cotton and Wilkinson in their classic text on inorganic chemistry point out the corrosion inhibit- ing properties of technetates. 1 Unfortunately for the search for chromate alternatives, technetate hardly offers a practical alternative to chromate. Nuclei for all of its isotopes are unstable with respect to β decay, the Tc 99 being the most stable with a 2.12 ° 10 5 year half-life decaying via a 0.29-MeV β emission. Refractive Metal Oxide Precursors The elements such as Nb, Hf, Ti, Zr, and Ta form very stable oxides in their highest oxidation state. Unfortunately, soluble and mobile precursors of these oxides remain difficult to stabilize in aqueous solution. Peroxo complexes and acid fluorides of Nb, Hf, Ti, and Zr exist at low pH. They form the basis of a number of patents (Appendix 1). Thin Zr coatings have been shown to rapidly form a very thin self- limiting layer. 130 Alkoxides of these species also can be used to form stable dispersions (sols), leading to sol-gel routes for coating formation. How such active precursors could be retained in the resulting film in a manner analogous to the retention of Cr 6+ by Cr conversion coatings is difficult to imagine. Rare Earths In recent years, the use of rare-earth compounds as corrosion inhibitors has become quite fashionable, FIGURE 13. Periodic Table of the elements, compounds of which (shaded) have been considered as alternatives for oxo-Cr 6+ inhibitors.
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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING C ORROSION—Vol. 59, No. 5
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  • Fall '19
  • pH, Chromates, Chromate conversion coating

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