Although there is no general agreement on the details

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Although there is no general agreement on the details for the Cr 6+ -induced damage to DNA, the fol- lowing points are clear: —Cr 6+ is highly water soluble and passes through cell membranes. —Small molecule antioxidants, such as ascor- bate, glutathione, and L-cystine, appear to form highly reactive intermediates such as Cr 5+ stabilized by alpha hydroxy carboxylates and Cr 4+ , which in turn react either directly or through free radical in- termediates to damage DNA. A relation between the mechanism of Cr 6+ toxic- ity to corrosion inhibition mechanisms is not obvious or expected. However, the existence of Cr 4+ and Cr 5+ complexes in the aqueous intercellular environment leads to the question of the significance of such spe- cies in corrosion inhibition. For example, are similar species responsible for the very rapid deactivation of cathodic sites on metallic surfaces during the reduc- tion of Cr 6+ to the passivating Cr 3+ oxide? Are traces of Cr 4+ and Cr 5+ present in conversion coatings or chromate-containing paint films? Do they have any role in corrosion protection? The authors know of no active research considering the role of these interme- diate-valent Cr species. Definition of the role of corro- sion inhibition by Cr 6+ will not be complete until the importance of these organic-stabilized intermediate- valence state species are considered. Such consider- ations could have important implications for the mechanism of inhibition by chromate pigments in paints. Stable Cr 5+ can be synthesized 19 and used for such investigations. MECHANISM OF INHIBITION Recent investigation of corrosion protection by chromates and CCCs has focused on a number of specific working hypotheses regarding the mecha- nisms of inhibition. 20-24 They can be condensed to the following statements. Cr 6+ oxoanions: —being highly soluble and very mobile in solu- tion, are transported to sites of localized corrosion where they are reduced to Cr 3+ and irreversibly adsorbed at metal surfaces where they inhibit oxygen reduction; —inhibit pit initiation of Al and dissolution of active intermetallic phases in Al alloys; —modify the chemical composition of the surface of passive oxides and passivated intermetallic phases by adsorption and buffering; —adsorb on aluminum oxides, lowering the zeta potential, thereby discouraging adsorption of anions such as chloride, which promote dissolution and de- stabilization of protective oxides. CCCs: —provide a good, hydrophobic Cr 3+ hydroxide barrier with adhesion-promoting chemical and me- chanical properties; —store Cr 6+ oxoanions that can be slowly re- leased into an attacking solution. Released ions can migrate to and interact at defects to stifle corrosion; —inhibit anodic and cathodic reactions, at least temporarily, leading to corrosion inhibition.
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  • Fall '19
  • pH, Chromates, Chromate conversion coating

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