As a final comment on the application of con ducting

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As a final comment on the application of con- ducting polymers for chromate replacement, con- ducting polymer duplex films comprised of the reduced and oxidized form may lead to the same electron transfer inhibiting structures suggested by Clark and McCreery 48 for inhibiting electron transfer at cathodes. FUTURE TRENDS Smart Coatings CCCs are effectively smart coatings in that they release a corrosion-inhibiting compound, Cr 6+ , when called upon to do so by the presence of a corrosive environment. Given the recent advances in nano- technology, self-organizing chemistries, and biomemetic technologies, advances in the develop- ment of smart, self-diagnosing corrosion protective coatings represent a fruitful area research effort. Inhibitor/Coating Discovery As suggested in the preceding section, the range of inorganic oxoanions or organic oxygen reduction inhibitor chemistries is vast. Conventional corrosion testing of any one of these tens of thousands of pos- sibilities would consume an inordinate amount of effort. Thus, development of methods for forming and screening large “libraries” of potential corrosion in- hibitors could identify that unique combination of properties as provided by chromate. In the past, pharmaceutical companies have used combinatorial methods to synthesize and screen classes of chemical compounds for a particular drug application. Re- cently, those in materials science have begun to use such methods for materials discovery including iden- tification of electrocatalysts and even corrosion in- hibitors in microelectronic processes. 170 It may be appropriate to use this methodology to discover com- pounds that would inhibit the corrosion of key aero- space alloys. CONCLUSIONS The oxoanions of hexavalent chromium uniquely inhibit the corrosion of many metals and alloys and have been particularly useful for protecting the high- strength aluminum alloys against corrosion. Part of the usefulness of hexavalent chromium lies in its ability to react with metallic surfaces to form an inert Cr 3+ oxide barrier with retained, releasable Cr 6+ . The ability of absorbed and reduced Cr 6+ (forming Cr 3+ hydroxide) to inhibit the reduction of oxygen repre- sents its main role in inhibiting corrosion, though other components of corrosion protection do exist. For example, speciated as an oxoanion, Cr 6+ appears to have an ability to inhibit pit initiation. While cor- rosion inhibition by Cr 6+ is well characterized, the details of Cr 6+ deposition are not. CCCs offer an especially attractive mix of characteristics including ease of application on a wide range of substrates, good corrosion resistance, and good adhesion. Supplemental bath ingredients such as F and Fe(CN) 6 3– are essential in making robust conversion coatings. Coating formation can be appropriately un- derstood in the context of a sol-gel process. Freshly formed conversion coating gels are well hydrated, and coating properties change dramatically as the gel dries. This has a range of practical consequences
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  • Fall '19
  • pH, Chromates, Chromate conversion coating

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