Corrosion products on brass condenser tube cooled by seawater.
Key words: corrosion products, condenser tube, seawater
Seawater is the commonly used cooling fluid to remove unwanted heat from heat transfer surfaces in steam condensers of the coastal power plants. Coastal power plants are usually designed to have one or two large steam condenser with open seawater circulation on the tube side and closed loop steam cooling system on the shell side. For example leakage from one hole on condenser tube with diameter of 0.15 mm can with NaCl contaminate all steam condensers for 3-4 hours (Heaton, 1977). Durability of relatively thin-walled tube, selected primarily for heat transfer efficiency, must endure over long periods of time because its reliability and efficiency affect the overall plants work. Deposits and films, which accumulate and grow from seawater on the tube inner wall surface has influence on heat transfer capacity and its ability to condense steam. The efficiency of heat transfer in the condensers of steam turbines has effect on the power production. Besides that seawater can cause corrosion of the tube wall that can cause leakage from the tube side and contaminate steam that is on shell side. Contaminated steam causes corrosion damage in turbine flow section that reduces the reliability of condensing steam turbines. Corrosion of steam condenser tubes causes great problems in coastal power plants. Hydrotalcite was detected as a corrosion product on inner wall of aluminium brass condenser tube cooled with unpolluted seawater (Yamamoto, et al., 1981). In a little polluted seawater was detected paraatacamite as a corrosion product in aluminium brass condenser tubes (Castle, et al., 1976). This paper presents results of investigation of corrosion products formed on surface of the inner wall of steam condenser tube that was cooled by seawater. Results were obtained by light microscope, AES-profile analysis and X-ray powder diffraction analysis.
2. MATERIALS AND METHODS
Material that was investigated in this study was cut out from the original condenser tube from Thermal Power Plant Plomin 1 (TPPP 1), 125 MW-s that was cooled by Plomin bay seawater about 370 hours.
Chemical analysis showed (Table 1) that the condenser tube material corresponds to the CuZn20Al2.00 alloy. Steam condenser from this TPPP 1 operated with about 18000 [m.sup.3] x [h.sup.-1] of steam whose reheat temperature was 535 [degrees]C. The average temperature of cooling seawater was about 19 [degrees]C. Seawater velocity was 1.5 m x [s.sup.-1]. The tubes size was 22.4 x 25 x 7000 mm with plane ends and with diameter of 22.4 mm. Tubes material was heat-treated before use. For the analysis performed in this work each specimen was cut through to obtain a cross-section of condenser tube wall with corrosion products formed on inner tube wall surface during operation of steam condenser. Then the specimens were polished, washed, dried and prepared to obtain a microstructure. Surface analyses of corrosion products were performed with light microscope, AES-profile analysis and by X-ray powder diffraction analysis.
3. RESULTS AND DISCUSSION
Figure 1 shows the cross sectional microstructure of inner wall of steam condenser tube obtained by light microscope after 370 hours of condenser operations. Microstructure of this Al-brass contains a few crystal twins. Course microstructure shows that recrystallization was done at the temperature of grains growth. From the Figure 1 it is obvious that pitting corrosion was damaged inner wall of steam condenser tube (Pomenic, 2002).
[FIGURE 1 OMITTED]
Figure 2 shows schematic picture of crack in steam condenser tube. Area A and area near the crack were analyzed by AES-profile analysis.
[FIGURE 2 OMITTED]
AES-profile analysis was used to analyze corrosion products formed on inner wall of aluminium brass, CuZn20Al2.00, steam condenser tube (from Thermal Power Plant Plomin 1, 125 MW-s) cooled by Plomin bay seawater (Figures 3 and 4). . Results obtained by AES-profile analysis were performed by PHI model SAM 545A, Physical Electronics, Eden-Prairie, Minnesota, USA. AES-profile analysis was obtained under these conditions: [Ar.sup.+], 3 keV, 15 mA; depth -10 nm/min.
[FIGURE 3 OMITTED]
On the layer surface was detected: Cl, Cu, S, Mg, Fe, Ca, Al, O, C and Zn. Concentration of Cu increase from 4 at% to the 65 at % after 60 min. Concentration of Zn increase from 4 at% to the 11 at%. Concentration of Al decreases from 19 at% to the 9 at%. Concentrations of elements Ca, O and Mg at the start were higher and after 60 min decreases to about 10 at%.
[FIGURE 4 OMITTED]
At the start there were detected: Cu, Zn, Al, O, C, Fe and S. Analyzed depth of corrosion products was 1.2 im. Concentration of Cu increase from 12 at% to the 30 at % after 120 min. Concentration of Zn increase from 14 at% to the 24 at%. At the start Al is not detected, but after half time it was detected at 14 at% and after 120 min was 18 at%. Oxygen from 17 at% increases to the 27 at%. Iron and sulphur were detected only on layer surface (Pomenic, 2003).
X-ray powder diffraction analysis was used to detect which crystalline compounds were present in corrosion products. Results obtained by X-ray powder diffraction analysis were performed on Philips PW 1710 X-ray powder diffractometer with use of standards. These analyses are powder X-ray diffractograms, which were computer interpreted. In Table 2 are shown D-s and intensities from which the present crystalline compounds were detected in analyzed corrosion products from inner wall of steam condenser aluminium brass tube, CuZn20Al2.00, cooled by seawater. X-ray powder diffraction analyses were showed that the main crystalline compound detected was [Ca.sub.0.7] [(Mn Mg).sub.0.26] x C[O.sub.3] with 80%. It is not sure that Mn is present in this compound because it can be changed with another element. CaC[O.sub.3] is present with 10%. Si[O.sub.2] and [Cu.sub.2]S were present only at detection limit with about 3%. [Cu.sub.2]O is not sure because it is covered by maximums of CaC[O.sub.3] and [Ca.sub.0.7] [(Mn Mg).sub.0.26] x C[O.sub.3].
Pitting corrosion was occurred in steam condenser aluminium brass tube, CuZn20Al2.00 cooled by seawater. Sulphur detected in corrosion products by AES-profile analysis indicates seawater pollution. Also, results obtained by X-ray powder diffraction analysis showed seawater pollution because in corrosion products paraatacamite, [Cu.sub.2][(OH).sub.3]Cl (in little polluted seawater) and hydrotalcite, [Mg.sub.6][Al.sub.2]C[O.sub.3][(OH).sub.16] x 4[H.sub.2]O (in unpolluted seawater) were not detected.
Castle, J., E., Epler, D., C., Peplow, D., B., (1976). ESCA Investigation of Iron Rich Protective Films on Aluminum Brass Condenser Tubes, Corrosion Science, Vol.16, 1976, pp. 145-157
Heaton, W., E., (1977). Impingement Corrosion of Condenser Tubes, British Corrosion Journal, Vol. 12, No.1,
Pomenic, L., (2002). A WDX, EDX Analysis of Corrosion Products on the Inner Wall of Steam Condenser Tubes Cooled by Seawater, 8th international Scientific Conference on Production Engineering, pp. IV-061, ISBN 953-97181-3- 9, Brijuni, June 13-14, 2002, Croatian Association of Production Engineering, Zagreb
Pomenic, L., (2003). Surface Characterization of Corrosion Layers on Brass Condenser Tube Cooled by Seawater, RIM 2003 4nd International Scientific Conference on Production Engineering, pp. 147-150, ISBN 9958-624-16-8, Bihac, September 25-27, Bosnia and Herzegovina, Karabegovic, I., Jurkovic, M., Dolecek, V.
Yamamoto, H., Kunieda, H., (1981). Failure Analysis of Aluminum Brass Condenser Tubes in Power Plants, DECHEMA, pp. 1476-1481, Frankfurt am Main
Table 1. Chemical analysis of steam condenser tube material Chemical composition / wt % Cu Zn Al Sn Ni Fe Mn 77.47 20.58 1.61 0.21 0.00 0.59 0.30 Table 2. Results of X-ray powder diffraction analysis D Intensity Crystalline compounds 9.3283 88 3.0257 481 2.4932 118 2.7779 153 CaC[O.sub.3] 2.0912 123 1.8928 258 1.8743 567 1.8060 143 3.2737 467 [Cu.sub.2]O 2.2432 681 3.3417 379 Si[O.sub.2] 3.7846 259 2.9768 3149 2.7827 128 2.4537 467 [Ca.sub.0.7] [(Mn Mg).sub.0.26] x C[O.sub.3] 2.2432 681 2.0588 589 1.8928 258 1.8734 548
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|Publication:||Annals of DAAAM & Proceedings|
|Date:||Jan 1, 2005|
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