Decomposition of Typical Organic Substance in Water Supply of Boiler and Corrosivity of Its Decomposition Products

doi:10.11902/1005.4537.2016.125

Journal of Chinese Society for Corrosion and protection

2017, Vol. 37

Issue (6): 597-604 DOI: 10.11902/1005.4537.2016.125

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Decomposition of Typical Organic Substance in Water Supply of Boiler and Corrosivity of Its Decomposition Products

Nana WANG¹, Fengtao WANG², Liang CHANG², Zhiping ZHU¹(

)

1 Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, School of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410014, China
2 Henan Electric Power Research Institute, Zhengzhou 450052, China

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Abstract

Since the retrofitting of a thermal power unit for heat delivery, correspondingly, the supplied water of boiler significantly increased from ordinary 3%~5% to above 50%, thereby the conductivity of the water generally exceeded, resulting in serious acid corrosion troubles in low pressure cylinder of the steam turbine. It is known that humic acid is a typical organic substance related with corrosion troubles in the water system, thus the decomposition behavior of humic acid at 350 ℃ was studied in concentration range 0~20 mg/L, while decomposition products specially from the test by 7.5 mg/L of humic acid at different time intervals were extracted for composition determination with ion chromatography and TOC analyzer. Thereafter the effect of low molecular organic acids with impurities on the corrosion behavior of steel 1Cr13 was characterized by means of electrochemical methods as well as SEM, EDS and XRD. The results showed that high temperature decomposition products of humic acid contained low molecular organic acids mainly of acetic acid and formic acid,and impurity ions of SO₄^2-, Cl^-, NO₃^- and F^-. Obviously, the ions of SO₄^2-, NO₃^- and Cl^- were aggressive to the steel 1Cr13, in the contrary, F^- exhibited inhibition effect with the increasing concentration.

Key words: make-up water anion humic acid decomposition product TOC

Received: 25 August 2016

ZTFLH:

TG172.42

Fund: Supported by National Natural Hunan Province Science & Technology Key Grant Project Foundation (2013GK2016)and Henan Electric Power Company Research Institute Self-Financing Research Project Foundation (19151209)

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Cite this article:

Nana WANG, Fengtao WANG, Liang CHANG, Zhiping ZHU. Decomposition of Typical Organic Substance in Water Supply of Boiler and Corrosivity of Its Decomposition Products. Journal of Chinese Society for Corrosion and protection, 2017, 37(6): 597-604.

URL:

https://www.jcscp.org/EN/10.11902/1005.4537.2016.125 OR https://www.jcscp.org/EN/Y2017/V37/I6/597

Fig.1 Basic unit structure of humic acid

Fig.2 pH values of gas and liquid phases generated after decomposition of humic acid at 350 ℃ for 12 h

Fig.3 Contents of various anions in the liquid phase generated after decomposition of humic acid at 350 ℃ for 12 h

Fig.4 Contents of various anions in the gas phase generated after decomposition of humic acid at 350 ℃ for 12 h

Fig.5 Concentrations of various anions in the liquid phase generated after decomposition of humic acid with the content of 7.5 mg/L at 350 ℃

Fig.6 Contents of TOC in the gas/liquid phase generated after decomposition of humic acid at 350 ℃ for 12 h

Fig.7 Concentrations of TOC in the gas/liquid phase generated after decomposition of humic acid with the content of 7.5 mg/L at 350 ℃

Fig.8 Polarization curves for 1Cr13 steel after immersed in different concentrations of NaCl solutions at 80 ℃

Fig.9 Nyquist plots of 1Cr13 steel after immersed in different concentrations of NaCl solutions at 80 ℃

Fig.10 Equivalent circuit used for quantitative evaluation of ElS spectra

Table 1 Polarization parameters for 1Cr13 steel after immersed in NaCl solutions with different concentrations at 80 ℃

Table 2 EIS parameters for 1Cr13 electrodes after immer-sed in different concentrations of NaCl solution at 80 ℃

Fig.11 SEM image (a) and EDS result (b) of Cl^- corroded specimens surfaces at 80 ℃

Fig.12 Surface images of sample before (a) and after (b) corrosion in 50 mg/L Cl^- solution at 80 ℃

Fig.13 SEM image (a) and EDS result (b) of F^- corroded specimens surfaces at 80 ℃

Fig.14 XRD pattern of F^- corroded specimens surfaces at 80 ℃

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