Electrochemistry anodic protection

See the NACE Papers Oliver W. Siebert, Correlation of Laboratory Electrochemical Investigations with Field Applications of Anodic Protection, Materials Performance, vol. 20, no. 2, pp. 38-43, February 1981 Anodic Protection, Materials Performance, vol. 28, no. 11, p. 28, November 1989, adapted by NACE from Corrosion Basics— An Introduction. (Houston, Tex. NACE, 1984, pp. 105-107) J. Ian Munro and Winston W. Shim, Anodic Protection— Its Operation and Appheations, vol. 41, no. 5, pp. 22-24, May 2001 and a two-part series, J. Ian Munro, Anodic Protection of White and Green Kraft Liquor Tankage, Part I, Electrochemistry of Kraft Liquors, and Part 11, Anodic Protection Design and System Operation, Materials Performance, vol. 42, no. 2, pp. 22-26, February 2002, and vol. 42, no. 3, pp. 24-28, March 2002. 

Corrosion Kinetics and Applications of Electrochemistry 135 5.6.4 Anodic Protection... 

Anodic protection is a powerful technique used to mitigate corrosion of liquor tankage. However, the electrochemistry of Kraft hquors is complex due to the multiple oxidation states of sulfur compounds, the number of possible Fe-S-H20 reactions, and the existence of active-passive behavior. The electrochemical behavior may be further complicated because some Fe-S compounds are semiconductors. The major sulfur species in Kraft liquors are listed in Table 12.10. 

Anodic protection of a Kraft liquor tank was first successfully realized at the end of 1984, and the success of this system resulted in many commercial installations. Unfortunately, unexpectedly high corrosion rates were reported at localized areas in several of the tanks even though the remainder of the surfaces corroded at rates less than 0.13 mmyi.i Most of the problems experienced have been attributed to incomplete understanding of the electrochemistry of carbon steel in these liquors and the coexistence of active and passive areas, which had not been addressed properly in earlier control strategies. 

The electrochemistry of amino acids has been studied in strong acid solutions. In general, the compounds are decomposed to carboxylic acids, aldehydes, ammonia, and carbon dioxide. The results are reviewed by Weinberg [35]. The anodic oxidation mechanism has been studied in pH 10 buffer solution. Decarboxylation accompanied by C-N bond cleavage is the main reaction process [182]. The synthetically interesting Hofer-Moest decarboxylations of A/ -protected amino acids and a-amino malonic half esters under the formation of A/ -acyliminium ions is treated in the following section. 

Finally, the electrochemistry of porous metal oxides prepared as films from anodic treatment of metal electrodes will also be discussed. Porous metal oxide films on electrodes have applications in a variety of fields, from corrosion protection to batteries and catalysis. 

See, for example, U.K. Mudali, H.S. Khatak, and B. Raj, Anodic and cathodic protection. Chapter 5, Section 5.1 in Encyclopedia of Electrochemistry Volume 4, Corrosion and Oxide Films (A. Bard, M. Stattman, and G. Frankel, eds.), Wiley-VCH, Weinheun, 2003. 

Passivity has been known for several hundred years. Uhlig mentioned in his review on passivity that Lomonosov was the first in 1738 to detect that iron does not dissolve in concentrated nitric acid [3]. Ostwald described in his history of electrochemistry [4] that Keir observed in 1790 the passivity of iron again in concentrated nitric acid [5]. Similar observations were made in 1782 by Wenzel according to citations of Gmelin [6. In 1807, Hiesinger and Berzelius found that one could achieve passivity by anodic polarization [6], which was confirmed by Schonbein for iron in diluted nitric, sulfuric, and phosphoric acids [7], In their correspondence, Schonbein and Faraday discussed the nature of these observations and Faraday came to the conclusion that a thin film provides protection to the metal. He postulated its electronic conductivity based on his own experiments [8]. 

Electrochemistry plays a key role in corrosion and its control. Oxidation half-cell reactions produce anodic regions and reduction half-cell reactions cathodic regions. Cathodic protection is achieved when a more active metal is attached to the metal being protected from corrosion. The more... 

---