Critical review of corrosion science and engineering

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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING 396 C ORROSION—MAY 2003 from gibbsite and brucite. 138-139 The archetypical hydrotalcites are the naturally occurring minerals hydromagnesite (Mg 6 Al 2 [OH] 16 ·CO 3 ·4H 2 O) and pyroaurite (Mg 6 Fe 2 [OH] 16 ·CO 3 ·4H 2 O). These com- pounds consist of positively charged layered sheets of brucite in which Al 3+ or Fe 3+ cations occupy octahe- dral sites. Layers of anions and water reside between, and electrostatically bind together, the positively charged brucite sheets. Many synthetic hydrotalcites conforming to the generic formula, M x II M y III (OH) 2(x+y) ·A y/n ·nH 2 O, have been synthesized. 140-141 Hydrolithisite (Li 2 [Al 2 (OH) 6 ] 2 ·A·nH 2 O) forms spon- taneously and rapidly as a coating on metallic alumi- num and aluminum alloy surfaces when contacted with alkaline lithium salt solutions. 142-143 Under the proper conditions, in a matter of minutes, a compact polycrystalline mass forms as a thin inorganic coat- ing on alloy surfaces. 144 The resulting coating pro- vides useful levels of corrosion protection and renders an otherwise incompatible surface receptive to organic coatings. 145 Under hot, alkaline, and oxidizing conditions, hydrotalcite coatings with corrosion resistance suffi- cient to pass 168 h of salt spray exposure testing without pitting can be formed. 146 To maximize corro- sion resistance, these coatings can be sealed with a variety of transition metal-based solutions or seal- ants used on anodized coatings. Sealed coatings offer enough corrosion resistance to protect Al 2024-T3 from pitting in salt spray exposure for times up to 336 h. 147 Corrosion Inhibiting Pigments The chromate-containing, corrosion-inhibiting pigments include the sparingly soluble chromates of the alkaline earths, Ca, Sr, Ba, and Zn. They provide a source for active inhibiting Cr 6+ compounds. A number of chromate alternatives have been consid- ered, including molybdates, phosphates, metabo- rates, zinc oxide (ZnO), vanadates, and silicates. 148 The patent literature suggests the use of sol-gel technology for generating an endless variety of nonchromate pigments. 149-150 In addition to their bo- rate salts, organic acid salts of Ba have been consid- ered as corrosion inhibitors. Although Ba metaborate as it exists at pH 10 provides very good inhibition for the Cu-rich intermetallics of Al 2024, its ability to inhibit decreases with decreasing pH. 148 Sinko notes that effective replacement of hexavalent Cr for the inhibition of high-strength alloys requires an inhibitor for both the ORR and an- odic dissolution of the active material of the interme- tallic. Accordingly, he proposes the concept of providing both of these functions, inhibition of ORR, and inhibition of anodic dissolution/pitting, through the use of “hybrid” formulations 151-153 in which an organic (typically) ORR inhibitor is included with en- vironmentally benign anodic inhibiting anions. Such anodic inhibitors might include ox-compounds of Mo, P, and B. The organic ORR inhibitor takes over the role otherwise provided by Cr 6+ .
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  • pH, Chromates, Chromate conversion coating

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