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

K T Kim,Y S Kim,B T Lim,H W Kim,H B Park,H Y Chang

doi:10.1155/2015/408138

Abstract

While NaNO2addition can greatly inhibit the corrosion of carbon steel and ductile cast iron, in order to improve the similar corrosion resistance,ca.100 times more NaNO2addition is needed for ductile cast iron compared to carbon steel. A corrosion and inhibition mechanism is proposed wherebyNO2-ion is added to oxidize. TheNO2-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 NaNO2addition 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 byNO2-ion; however, regardless of the alloys, nitrogen compounds (not nitrite) were detected at the outermost surface but were not incorporated in the inner oxide.

Highlights

Since the operation period of nuclear power plants around the world increases each year, the degradations in buried pipes have become an important issue in the nuclear power industry
In the case of ductile cast iron, the effect of nitrite addition was similar to that of carbon steel, but the similar corrosion inhibition of ductile cast iron needs a significant addition of NaNO2
The corrosion of carbon steel can be inhibited at near 100 ppm NaNO2 addition, but the corrosion of ductile cast iron can be inhibited by an addition of more than 10,000 ppm NaNO2

Summary

Introduction

Since the operation period of nuclear power plants around the world increases each year, the degradations in buried pipes have become an important issue in the nuclear power industry. 30∼40 km per unit of nuclear power plant have been buried and operated, and depending on the application system and water chemistry, they are separately maintained as the large diameter pipes and the other pipes. About 70% of pipes are Prestressed Concrete steel Cylinder Pipe (PCCP) and Prestressed Concrete Pipe (PCP) Another pipework has been installed for conveying water for the fire-fighting system, which was made of carbon steel or cast iron [6]. Many reports have been presented on the corrosion inhibition of carbon steel but there are few reports on cast iron. In this work, corrosion inhibition effects of nitrite on carbon steel and ductile cast iron for nuclear power plant pipework using chemical and electrochemical methods were evaluated. This work attempts to clarify the corrosion inhibition mechanism between steel and iron by NaNO2 addition

Experimental Procedure

Corrosion Tests

Results and Discussion

Conclusions