Corrosion of SA 508 in High Temperature and Pressure Water Containing Silicon Compounds

Corrosion of SA 508 in High Temperature and Pressure Water Containing Silicon Compounds

Abstract:

Steam generators are crucial components of pressurized water reactors. The failure of the steam generator as a result of components degradation by corrosion has been a major cause of Pressurized Water Reactor (PWR) plant unavailability. Steam generator problems have caused major economic losses in terms of lost electricity production through forced unit out ages and, in cases of extreme damage, as additional direct cost for large-scale repair or replacement of steam generators. Steam generator tubes and tubesheet are susceptible to failure by a variety of mechanisms, the most majority of which are related to corrosion. The feedwater that enters the steam generators under normal operating conditions is extremely pure, but nevertheless contains low levels (generally in the µg/L concentration range) of impurities such as iron, chloride, sulphate, silicate, etc. When water is converted to steam and exits the steam generator, the non-volatile impurities are left behind. As a result, the concentration of the bulk steam generator water is considerably higher than that in the feedwater. When the silicon compounds concentration is higher the localized corrosion appears and can have a catastrophic result consisting in the loss of components integrity and the development of a large quantity of soluble corrosion products and deposits. For low concentration the effect of the silicon compounds on the behaviour of steam generator materials has never been considered adequately. The information available is contradictory, and it is possible to encounter in the literature old results that indicate an inhibition or an aggressive effect of silicon compounds in caustic environment. The purpose of this paper consists in assessment of localized corrosion behaviour of the tubesheet material (carbon steel SA 508 cl.2) at the normal secondary circuit parameters (temperature = 2600C, pressure = 5.1MPa). The testing environment was the demineralized water containing different silicon compounds, such as silica (SiO2) and sodium metasilicate (Na2SiO3) used separately or together, at pH=9.5 regulated with morpholine and ciclohexilamine (all volatile treatment - AVT). The paper presents the results of metallographic and X - ray diffraction examinations as well as the results of electrochemical measurements. Keywords: steam generators, feedwater, silicon compounds, demineralized water, morpholine, ciclohexilamine

Issue: 2007, Volume 58, Issue 12

Pages:

Publication date: 2001/1/1

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This article is published under the Creative Commons Attribution 4.0 International License

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