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Advances in Materials Science and Engineering
Volume 2015, Article ID 408138, 16 pages
http://dx.doi.org/10.1155/2015/408138
Research Article

Corrosion Inhibiting Mechanism of Nitrite Ion on the Passivation of Carbon Steel and Ductile Cast Iron for Nuclear Power Plants

1Materials Research Centre for Energy and Clean Technology, School of Materials Science and Engineering, Andong National University, 1375 Gyeongdongro, Andong 760-749, Republic of Korea
2Power Engineering Research Institute, KEPCO Engineering & Construction Company, 8 Gumiro, Bundang, Seongnam, Gyeonggi 463-870, Republic of Korea

Received 1 July 2015; Accepted 27 September 2015

Academic Editor: Randhir Singh

Copyright © 2015 K. T. Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

While NaNO2 addition can greatly inhibit the corrosion of carbon steel and ductile cast iron, in order to improve the similar corrosion resistance, ca. 100 times more NaNO2 addition is needed for ductile cast iron compared to carbon steel. A corrosion and inhibition mechanism is proposed whereby ion is added to oxidize. The ion can be reduced to nitrogen compounds and these compounds may be absorbed on the surface of graphite. Therefore, since nitrite ion needs to oxidize the surface of matrix and needs to passivate the galvanic corroded area and since it is absorbed on the surface of graphite, a greater amount of corrosion inhibitor needs to be added to ductile cast iron compared to carbon steel. The passive film of carbon steel and ductile cast iron, formed by NaNO2 addition showed N-type semiconductive properties and its resistance, is increased; the passive current density is thus decreased and the corrosion rate is then lowered. In addition, the film is mainly composed of iron oxide due to the oxidation by ion; however, regardless of the alloys, nitrogen compounds (not nitrite) were detected at the outermost surface but were not incorporated in the inner oxide.