Fig 5. Photograph of the complete set-up for internal corrosion. — Corrosion resistance of heat exchangers in seawater desalination plants

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Featured Story – Corrosion resistance of heat exchangers in seawater desalination plants

December 23, 2020

This paper examines the corrosion of high-alloy stainless steels used in heat exchangers which are working in service in seawater desalination plants where chlorination of the seawater is used to limit microbial activity and can therefore provide a quite aggressive environment.

^ 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

1. Summary

High-alloy stainless steels are often considered for the severely corrosive environment of seawater desalination plants. This is particularly true in heat exchangers units, where chlorination of the seawater can be used to limit microbial activity, making the environment quite aggressive. Chlorination oxidizes and increases the corrosion potential to approximately +600 mV/SCE for stainless steels and leads to higher susceptibility to localized corrosion. The so-called super duplex or super austenitic (6Mo) stainless steels can be used in seawater-cooled heat exchangers at limited temperatures, otherwise titanium alloys are generally recommended for equipment integrity over the service life.

The corrosion resistance and limits of use of hyper duplex UNS S32707 tubes expended and welded on UNS S31266 tube plates have been investigated. Seawatercooled heat exchangers made of these high-grade alloys have been tested during 18 months in different simulated service conditions using natural 0.5 ppm-chlorinated seawater. The fl ow loops have been designed to control the heat exchange between external and internal part of the tubes. In this study the corrosive electrolyte (i. e. chlorinated seawater) was circulating inside the exchangers at a fl ow rate of 2 m/s. The results showed a pitting corrosion resistance of the UNS S32707 tubes at a heat fl ux representative to a tube skin temperature up to 95°C inside in the seawater, exceeding the limits of use of the more commonly used super duplex UNS S32750 or super austenitic UNS S31254. In parallel, laboratory experiments using standard crevice assembly confi rmed the higher crevice corrosion resistance of UNS S31266 in chlorinated seawater, compared to UNS S31254 or S32750. These new results also show a good correlation with the fi eld service experience and help to challenge the use of Titanium in seawater-cooled heat exchangers.

2. Introduction

Seawater desalination process is a demanding application in terms of material corrosion resistance. Natural and chlorinated seawaters are known to be corrosive for most metallic alloys, and titanium alloys are commonly used for heat exchanger applications, especially at temperatures exceeding 40°C ^[1]. However, titanium can be expensive and difficult to supply. Then looking into alternative alloys such as high-alloyed stainless steels becomes of interest for seawater heat exchangers. Due to the susceptibility of stainless steels to localized corrosion including for some commonly used high grade alloys ^[2-5], the actual corrosion risk of the candidate materials must be carefully addressed. This risk will strongly depend on the exact service conditions (e. g. temperature, chlorination, oxygen content, etc.), on the metallurgy (e. g. cast or wrought alloys) and on the geometrical configuration of the areas in contact with seawater, as well as surface contamination ^[5-7]. From this background a fi t-for-purpose study was conducted to evaluate the corrosion resistance of selected high-alloyed stainless steels for tube heat exchanger applications. The corrosion resistance was evaluated using full scale tube heat exchangers, involving chlorinated seawater fl ow loops. The full scale set-up was developed and used in a previous program to evaluate the internal pitting corrosion resistance of duplex UNS S32205, superduplex UNS S32750 and hyperduplex UNS S32707 ^[8]. The results from this previous study showed limitation of UNS S32750 at a tube wall temperature of 50°C without pitting corrosion in chlorinated seawater (see summarized results in Table 1). The results also confirmed that standard duplex stainless steel is not resistant at these temperatures. Hyper duplex tubes UNS S32707, on the other hand show a full pitting corrosion resistance under the same conditions of exposure ^[8]. From these results, it was proposed to better define the limits of use of UNS S32707 tubes with increasing heat fluxes.

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 ^[9]. 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

3.1. Materials

UNS S32707 is a hyper duplex stainless steel, used for tubular heat exchanger applications and included in ASTM⁽¹⁾A789 ^{[10, 11]}. UNS S31266 is a super austenitic stainless steel, which is used for plate applications ^[12]. In this study the materials have been tested in a heat exchanger prototype (full scale fl ow loop) and at laboratory scale using plate coupons and standard crevice assembly. For laboratory testing, the UNS S31254 and UNS S32750 have also been tested for comparison purpose. The chemical composition and the PRENW for each tested material are presented in Table 2.

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

UNS S32707 tubes in dimension 19.05×1.65 mm were expanded manually with plastic deformation of the tubes in the S31266 tube plates, driven with electronically controlled torque, as illustrated in Fig. 2. The tube is considered as expanded when the reduction of the tube thickness (i. e. plastic deformation) is measured to be about 10%. The control is done manually with an electronic gauge. After the expansion the tube ends were welded on the tube plate according to the parameters the schematic drawing given in Figure 3. After welding, the tube and tube sheets were pickled in hydrofluoric-nitric acid (85% water, 10% nitric acid and 5% hydrofluoric acid) for 2 hours and followed by a carefully water rinsing of the global system.

The principle set-up used to evaluate internal corrosion resistance of the built exchangers is given in Figure 4, showing the approximated theoretical calculation of the tube skin temperature. It is constructed with regulated heating blocks on the outside of the tubes to create a certain heat flux between the outside tube surface and the inside tube surface. Two heating blocks on the outside surface of the tube were used to heat a length of about 20 cm along each tube. Residual chlorinated seawater (0.5 ppm) regulated at 35°C was circulating inside the tubes. The chlorination was obtained with electrolysis of seawater between two coated titanium electrodes connected to a regulated power supply. Natural seawater directly and continuously pumped from the bay of Brest (France) was used for the corrosion tests. Residual chorine was continuously controlled with a platinum electrode for a chlorine content control at 0.5 ±0.1 ppm. The chlorination control was regularly calibrated with a manual chlorine content measurement performed with a spectrometer unit. The temperature was controlled at 35°C ±1°C with the use of regulated titanium heater. The flow rate of the chlorinated seawater inside the tubes was controlled at 2 m/s.

Calculations were done to reach internal skin temperatures of 85°C and 95°C [8]. 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.

During exposures, non-destructive evaluation of the corrosion was performed using an endoscopic camera after 6 and 12 months of exposure. After exposure, a destructive evaluation was performed using visual, binocular and microscopic evaluation tools, to evaluate the corrosion.

3.3. Laboratory Crevice Corrosion Testing

Crevice corrosion tests were performed on UNS S31266, S31254 and S32750, which are all candidate material for tube plate/water box applications (where crevice corrosion can be critical). Tests were performed using standard CREVCORR-type crevice assemblies ^{[13, 14]}.

This standard assembly was developed within European funded project (CREVCORR) which included the development and qualification of a crevice corrosion test for stainless steels to be used in marine environments ^[13]. It has the following characteristics: crevice formers are made of polyvinylidene fl uoride (PVDF), all fasteners are made of titanium grade 2 and are electrically isolated from the tested specimen, and disc springs are used to keep a measurable and constant pressure between the crevice formers and the specimen. According to the standard “CREVCORR” testing method, the crevice former should be tightened to the test specimens with a force of about 900N (i. e. pressure of about 3 N/mm²), which corresponds to a torque of 3 N·m with the crevice assembly that is used. A schematic representation and a photograph of the crevice assembly are shown in Fig. 6. The standard parameters of the CREVCORRmethod were modified in order to increase the severity of the crevice geometry. The aim was to reach a higher pressure of about 20 N/mm² at the gasket location which is more representative of severe crevice configurations such as flanges or bolting systems^{[5, 15]}.

The PVDF gaskets were all polished with 600 grit paper to ensure a similar surface finish on crevice formers. The crevice formers were all mounted on specimens in immersion in seawater in order to get seawater under the crevice formers from the start of the exposure. The anode (surface under crevice formers) to cathode (surface in contact with bulk environment) ratio was 1:30, using plate specimens of 150 mm x 100 mm. Base material plates were all tested with as-received surface state, corresponding to a roughness Ra = 2 to 3 µm. Exposure test were performed using natural seawater, heated and chlorinated by electrolysis. The tested residual chlorine content ranged from 0.5 to 15 ppm, and the tested temperatures ranged from 20°C to 50°C. All the corrosion tests have been performed during 3 months, using five replicates for each tested condition of exposure.

4. Results

4.1. Heat Exchanger testing

4.1.1. Control of monitoring system

Control of the test environment was done during the 18 months of exposure. The residual chlorine content, pH, dissolved oxygen, conductivity and temperature of the seawater remained stable during the 18 months of exposure, as shown in Table 4. The control of the heat fluxes for the 3 tested conditions is given in Table 5. It shows a stable control of the desired temperatures.

Fig. 7. Open-circuit potential vs time curve of UNS S32707/S31266 exchanger for the two heat fluxes of 135°C_{OD temp} → 85°C_{ID temp} → 35°C_seawater and 155°COD temp → 95°C_{ID temp} → 35°C_seawaterfor 18 months in 0.5 ppm chlorinated seawater

Fig. 8. Open-circuit potential vs time curve of UNS
S32707/S31266 exchanger for internal corrosion
testing at heat flux of 170°C_{OD temp} → 95°C_ID

temp → 20°C_seawater, for 6 months in 0.5 ppm
chlorinated seawater (2 replicates).

Fig. 9. Photo of the internal surface of UNS S32707
tube in contact with flowing chlorinated (0.5 ppm)
seawater tested with the heat flux 135°C_{OD temp} →
85°C_{ID temp} → 35°C_seawater, for 18 months
(no corrosion)

Fig. 10. Photo of the internal surface of UNS S32707
tube in contact with fl owing chlorinated (0.5 ppm)
seawater tested with the heat flux 155°C_{OD temp} →
95°C_{ID temp} → 35°C_seawater, for 18 months
(no corrosion).

Fig. 11. Photo of the internal surface of UNS S32707 tube in contact with fl owing chlorinated (0.5 ppm) seawater tested with the heat flux 170° C_{OD temp} → 95° C_{ID temp} → 20° C_seawater, for 6 months (no corrosion).

Fig. 12. Visual inspection of one of the S31266/S32707 welds and S31266 tube plate in contact with flowing chlorinated (0.5 ppm) seawater at 35°C for 18 months (no corrosion).

Fig. 13. Summary of the crevice corrosion results using CREVCORR assembly with the severe 20 N/mm² gasket pressure

4.1.2. Potential measurement

The open-circuit potential vs. time curves of the exchangers tested for internal corrosion at 135°COD temp. → 85°CID temp → 35°C_seawater and 155°C_{OD temp} → 95°C_{ID temp} → 35°C_seawater, are given in Figure 7. The heating blocks were started after about 2 week of circulation of 0.5 ppm chlorinated seawater at 35°C. The open-circuit potential of both exchangers stabilized after about 120 days of exposure at potential around +700 mV/SCE. This is in good line with the potential of non-corroded stainless steels in 0.5 ppm chlorinated seawater at temperatures above 30°C ^{[9, 16]}. The open-circuit potential curves for the heat flux of 170°C_{OD temp} → 95°C_{ID temp} → 20°C_seawater is given in Figure 8. The heating was started after 2 weeks of exposure. In this condition, the open-circuit potential stabilized to lower values, at about +550mV/SCE, this is probably due to the lower temperature of the bulk seawater.

From the open-circuit potential monitoring curves in Figure 7 and Figure 8, no obvious sign of localized corrosion was detected. The corrosion resistance was confirmed during the exposure with the use of endoscopic inspections after day 150 and 360. After these non-destructive observations (2 hours of flow stop per inspection), the loop was restarted.

4.1.3. Visual inspection

After the test period the samples were cut and inspected visually. The visual aspects of the tested tubes after exposure are given in Figure 9, Figure 10 and Figure 11 for the 3 tested heat fluxes. For all the tested tubes no pitting corrosion was detected. Independently of the tested durations, the internal tube surfaces showed stable coloured (orange/red) oxide layer. The tinted oxide corresponds to a stable thickening of the passive layers in oxidizing media which never turned into corrosion initiation.

In the water boxes (0.5 ppm chlorinated seawater at 35°C), The UNS S32707/S31266 welds resisted pitting corrosion during the 18 months of exposure. Also no crevice corrosion initiated on S31266 tube plates at aramid-gaskets. A photograph of the weld and tube plate after 18 months of exposure is given in Figure 12.

4.2. Laboratory crevice corrosion testing

The results from crevice corrosion testing using CREVCORR-type assemblies are summarized in Figure 13.

It allows the comparison of the limits of application between the high alloys S31266 and the more commonly used superaustenitic and superduplex S31254 / S32750.

It should be underlined that the limits of application defined in Figure 13 are related to the use of a crevice geometry involving PVDF-CREVCORR gasket applied at a pressure of 20 N/mm², which can be considered as severe crevice configuration, generally used for ranking purpose. Other crevice geometry can lead to other results and crevice corrosion resistance can be further optimized with the use of other gasket material such as aramid gasket ^{[15, 16]}. A synergic effect of residual chlorine and temperature was observed, with critical conditions for UNS S31266 for 2 ppm chlorine content at 40°C and 0.5 ppm chlorine content at 50°C. When the temperature is decreased to 35°C, no crevice corrosion initiated from 1 to 15 ppm of chlorine content. When compared to crevice results of S31254 and S32750 using similar crevice assemblies (PVDF-CREVCORR-20N/mm²), the highest resistance of S31266 is clearly confirmed, since crevice corrosion initiates for 0.5 ppm chlorine content at 30°C on S31254 and S32750.

5. Conclusions

The localized corrosion behaviour of full scale tube heat exchangers made of high alloy stainless steels was evaluated in chlorinated seawater at different heat fluxes. The main conclusions are listed below:

UNS S32707 hyperduplex tubes resisted internal pitting corrosion in the most demanding tested heat flux of 170°C_{Outer skin temp}. → 95°C_{inner skin temp}. → 20°C_{bulk seawater temp}. For comparison purpose UNS S32750 superduplex tubes showed severe pitting at 105°C_{Outer skin temp}. → 70°C_{inner skin temp}. → 35°C_{bulk seawater temp}.

In the full scale loop, the UNS S32707/UNS S31266 welds resisted pitting corrosion in the water box operating in 0.5 ppm chlorinated seawater at 35°C. In this condition, the tube plates UNS S31266 resisted crevice corrosion at the aramid-gaskets used to fix the water boxes

From laboratory test results, synergic effect was observed between temperature and residual chlorine content in terms of crevice corrosion risk of high grade stainless steels

With severe crevice geometries (i. e. PVDF CREVCORR with gasket pressure of 20 N/mm²) crevice corrosion resistance was significantly improved with the use of UNS S31266 compared to UNS S31254 and S32750.

The results from this study showed that the use of high grade hyperduplex S32707 and superaustenitic S31266 increases the limits of applications of stainless steels in seawater applications. It should be underlined that the limits of application in terms of crevice corrosion could probably be further extended with the use of less severe crevice configurations (e. g. use of aramid-type gasket). In demanding seawater applications such as heat exchangers, S32707 and S31266 can thus be considered as possible alternative to titanium, with better corrosion resistance than UNS S32750 and S31254.

6. References

[1] J. Houben, Cooling Water Reliability, an End-Users View, MTI EUROTAC, 2006

[2] Strandmyr O. and Hagerup O., Field Experience with Stainless Steel Materials in Seawater Systems, Corrosion’98, Houston, NACE, paper N°. 707, 1998.

[3] Havn T., Material Engineering and Fabrication Experiences, Corrosion, NACE paper N°56, 1995.

[4] Johnsen R., North Sea Experience with the Use of Stainless Steel in Seawater Applications, EFC Publication 10, The Institute of Materials, 1993

[5] N. Larché, D. Thierry, V. Debout, T. Cassagne, J. Peultier, E. Johansson and C. Tavel-Condat, Crevice Corrosion of Duplex Stainless Steels in Natural and Chlorinated Seawater, Duplex World 2010, October 11-13 2010.

[6] H. Yakuwa, M. Miyasaka and K. Sugiyama, Evaluation of Crevice Corrosion Resistance of Duplex and Super duplex Stainless Steels for Seawater Pumps, Corrosion, NACE paper N°09194, 2009.

[7] A. M. Grolleau, H. Le Guyader and V. Debout, Prediction of Service Life of Nickel Based Alloys N06625 and N06059 and Super Austenitic Stainless Steel S31266 in Seawater using InHouse Crevice Corrosion Tests, Nace corrosion, paper N°09192, 2009.

[8] N. Larché, D. Thierry, “Corrosion performance of 25%Cr super duplex stainless steel for different seawater applications,” Stainless Steel World Conference, (Maastricht, Netherlands, 2011)

[9] D. Thierry, C. Leballeur, N. Larché, “Galvanic series in seawater as a function of temperature, oxygen content and chlorination,” CORROSION 2016, paper no. 7058 (Houston, TX: NACE, 2016), p. 7-8.

[10] ASTM A789/A789M-13 (latest revision), “Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Tubing for General Service” (West Conshohocken, PA: ASTM)

[11] “Sandvik SAF 2707 HD-Tube and pipe, seamless,” Datasheet updated 2016-03-14 14:09, http://smt.sandvik.com/en/materials-center/material-datasheets/tube-and-pipe-seamless/sandvik-saf-2707-hd/ (July 11, 2016)

[12] “UR™66: A high strength super austenitic stainless steel with PRENW = 55,” http://www.industeel.info/products/stainlesssteels/super-austenitic/ ur-66/ (July 11, 2016)

[13] B. Espelid, Development of a New Crevice Corrosion Qualification Test for Stainless Steels, Stainless steel world , 2003.

[14] Standard ISO 18070:2015, Corrosion of metals and alloys — Crevice corrosion formers with disc springs for flat specimens or tubes of stainless steels in corrosive solutions, 2015

[15] R. Francis, Factors affecting gasket selection for stainless steels in seawater. Corrosion, NACE paper N°. 07262, 2007.

[16] N. Larché, P. Boillot, P. Dezerville, E. Johansson, J.M. Lardon, D. Thierry,Crevice corrosion performance of high alloy stainless steel and Ni based alloy in desalination industry Desalination and Water Treatment , Volume 55, Issue 9, 2015

7. Acknowledgement

Pascal Moullec is acknowledged for the design and support of the full scale seawater fl ow loop.