^ Fig 5. Photograph of the complete set-up for internal corrosion.
Article by Nicolas Larché, Josefi n Eidhagen, Sandra Le Manchet, Hervé Marchebois, Ulf Kivisäkk, and Dominique Thierry, Institut de la Corrosion, Brest France
In the project, super austenitic UNS S31266 was used as tube plate and showed very good galvanic compatibility with UNS S32707, and similar high Pitting Resistant Equivalent Number (PREN) close to 50 (cf. PREN of more commonly used super duplex and super austenitic stainless steels is closer to 40). UNS S32707 and S31266 have similar open circuit potential (OCP) in the range of +600mV/SCE (Saturated Calomel Electrode) in chlorinated seawater at 30°C . From this background it was proposed to further investigate higher temperatures in chlorinated seawater to assess the internal pitting resistance of UNS S32707 tubes expanded in UNS S31266 tube plates, using controlled full scale seawater flow loops. In parallel, laboratory exposures have been performed to assess the limits of use of the selected stainless steel grades regarding temperature and the residual chlorine content.
3. Experimental procedure
The microstructure of all the tested alloys was checked with optical microscopic examination according standard NFA 05- 150 12/85. A normal duplex (austenite/ferrite) microstructures with 50%-ferrite content was found for S32750 and S32707. A normal austenite structure was found for UNS S31254 and S31266. None of the tested materials shown microstructure defects. The typical aspect of microstructure of the etched UNS S32707 and S31266 is shown in Fig. 1.
3.2. Heat exchanger testing
Calculations were done to reach internal skin temperatures of 85°C and 95°C . With bulk seawater at 35°C, it corresponds to regulation of the external heating blocks at 135°C and 155°C, respectively. To increase the heat flux even more, an additional test was performed for six month with 20°C circulating seawater. To get an internal skin temperature of 95°C the external heating blocks were regulated to 170°C. All the test conditions are gathered in Table 3.
To monitor possible corrosion activity in the seawater, calibrated reference electrodes (AgAgCl/KCl/Gel) were fixed in each seawater loop and high-impedance (>1011Ω) corrosion potential data loggers were used to continuously measure and record the open-circuit potentials (OCP). A photograph of the whole test set-up with the internal seawater loop is presented in Fig. 5.
3.3. Laboratory Crevice Corrosion Testing
4.1. Heat Exchanger testing
4.1.1. Control of monitoring system
4.1.2. Potential measurement
4.1.3. Visual inspection
4.2. Laboratory crevice corrosion testing
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Pascal Moullec is acknowledged for the design and support of the full scale seawater fl ow loop.