Super duplex stainless steels in Seawater Reverse Osmosis applications

By Dr Glenn Byrne and Mr Nicholas Hicks, Rolled Alloys, Temperance, Michigan

Seawater Reverse Osmosis (SWRO) projects (circa 1980 to 1990) utilised various lower grades of stainless steel without success (Table 1(1)). Experience in Saudi Arabia (highlighted in yellow) shows operators selected alloys based on the lowest cost and then gravitated to the next lowest cost option when the alloy originally selected leaked and needed to be replaced. The Malta experience (highlighted in green) appears to be particularly painful. It took about 10 years of trial and error before the industry accepted that 6% Molybdenum alloys were, at that time, the only commercially available stainless grades suitable for use in high-pressure seawater feed (HPSF) and brine reject (BR) pipework systems. Having said this, some operators still insist that alloys such as 316L, 904L and 2205 have worked well.
By the early 1990s super duplex stainless steels (SDSSs) had entered the market. Good experience in chlorinated seawater cooling, fire protection systems, and seawater pumping in offshore oil and gas applications gave SWRO contractors the confidence to use these alloys. Also, the cost differential between SDSS and 6% Mo grades of about 30% was attractive to the SWRO industry.
By the early 2000, SDSSs had become the material of choice for new build HPSF and BR pipe work. Contractors who continued to use 6%Mo alloys did so because they had good previous experience with using them and didn’t want to change. Also, membrane and energy recovery technology improved significantly at this time, and their costs fell. This made SWRO plants readily scalable and affordable for large-scale municipal use. Consequently, there was a sudden increase in the number of new build projects, many of which used SDSS grades (Figure 1).
The capacity of SWRO plants being built increased by an order of magnitude, typically from 20,000m3/day to 200,000m3/day. Today, we have plants making 600,000m3/day of drinking water. The trend of using SDSS continues, with almost all new build and expansion projects specifying the use of SDSS for HPSF and BR pipework systems.

Table 1. Project experience and material performance

Testing corrosion resistance

Our company developed a test method to allow us to assess the corrosion resistance of various grades of steel in seawater service. This recognises that critical pitting (CPT) and the relative critical crevice temperatures (RCCT) in seawater are not material constants, they vary with potential. Stainless steels in natural seawater adopt a potential of about +250 to +300mV (SCE) after a biofilm has formed. If the seawater is chlorinated, the potential rises to about +600mV (SCE). In the case of SWRO, the sea water is chlorinated to disinfect it, and then it is dechlorinated by adding chemicals like Sodium Metabisulphite. This must be done because the polyamide membranes are chlorine intolerant and would oxidise in the presence of chlorine and let too much salt pass through. Because these chemicals are also oxygen scavengers, they cause the potential to fall to between +100 and +200mV (SCE). This potential is controlled by the RO contractors as the membranes work best in this range. Figure 2 shows the equipment used to determine the CPT and RCCT at a given potential in an artificial seawater solution. It also shows the types of specimens used. The CPT of the parent material is measured by testing the bullet sample on the end of the brass studding. To measure RCCT, a crevice is formed by placing an O-ring seal around the bullet sample. The term “relative” is used because the measurement made is relative to the O-ring seal crevice geometry.

The samples are placed in the artificial seawater solution and polarised to the potential of interest. A heated water bath then slowly increases the temperature of the solution, and the current density of the sample is measured continuously. The temperature when the current density consistently exceeds 10µA/cm2 is taken as the CPT or RCCT (2).
Figure 3 shows the results of testing the CPT of welds and the RCCT of parent material of several stainless steels at different potentials. From this several things become clear.

1. 316L sufferer’s crevice corrosion at temperatures below the normal ambient range for SWRO plants worldwide and is hence unsuitable for this service.
2. The RCCC of both 904L and 2205 is sensitive to potential within the ambient temperature range of plants worldwide. Small changes in potential cause large changes in RCCT. This means that performance is dependent on the water treatment used by the plant. This may explain why some contractors claim good experience with these grades in certain applications. If the water treatment is such that the potential achieved is towards +100mV (SCE) the RCCT will be higher than most plant ambient temperature conditions and the alloy will perform well (but this comes with a high cost of water treatment chemicals). However, if the potential is close to +200mV (SCE) the RCCT is below the ambient temperature range for most RO plants around the world and performance will be poor.
3. The limiting case for SDSS is the CPT of welds in the “as welded” condition. Welds made using a consumable with a pitting resistance equivalent number (PREN) of about 40.5, have a CPT of about 50ºC at +200mV (SCE), which is about 10ºC higher than the normal maximum design temperatures for SWRO plants in the Middle East.
4. If these welds are pickled in acid, their CPT increases to about 80ºC at +200mV (SCE). Hence, we recommend that all welded spools be post weld acid pickled.
5. The CPT of welds in the “as welded” condition can be increased to about 68ºC by the use welding consumables with a minimum PEREN of 42. This means that contractors may choose to omit the acid pickle and make cost savings. It is also notable that in this case the CPT of the welds is close to the RCCT of the parent material.

We have also considered the case for the BR, where the chloride content is increased by a factor of about 2. Figure 4 shows the effect of chloride content on the RCCT at different potentials. These results show that RCCT is much more sensitive to potential than chloride content.
Variation of chloride content between 20,000ppm and 60,000 ppm, at a given potential, makes little difference in RCCT. At +200mV (SCE) the measured RCCT is significantly higher than the ambient temperature of plants around the world. This shows that SDSS are very robust under both HPSF and BR conditions. Both SDSS and 6%Mo grades perform well in SWRO service, but they are not immune to corrosion. For example, the crevice formed between the rubber seal and the pipes where mechanical connections are used can be a potent location for the initiation of crevice corrosion. This form of joint is highly utilised by the desalination industry. During our own early experiences in the Middle East, and contrary to experience in Mediterranean conditions, we experienced leaks in low single figure percentage numbers at mechanical joints. This reflects the stochastic nature of crevice corrosion (3).
By testing pipe samples, with machined and un-machined surfaces, creviced using polyacetal washers, specially machined to suit the curvature of the pipes being used, we have found that skim machining of the sealing face significantly increases the RCCT of the connection (Figure 5). These results don’t relate directly to the performance limits of the mechanical joint, but they do show the benefit of the machining operation.
Our company now machines the sealing face connections as standard (Figure 6). Subsequent, long-term, experience in Middle East conditions (4) supports the testing results.
To work well, these alloys must be properly manufactured, fabricated, and installed. Sophisticated users, like the oil and gas industry, are very aware of this (5). We recommend that similar disciplines be adopted by the desalination industry (6).

Summary and conclusions

SDSS are now the alloys of choice for HPSF and BR pipework systems in SWRO plants. This is because these alloys are the most commercially attractive solution that provides the necessary corrosion resistance in the most severe SWRO conditions. However, the continued successful application of these grades requires the engineering contractors to apply a disciplined approach to manufacturer selection, product specification and quality, fabrication and installation processes and procedures.

Acknowledgements

I would like to thank the directors and shareholders of Rolled Alloys for permission to publish. I would also like to acknowledge the contributions of Dr Roger Francis and Mr Geoff Warburton.

About this Tech Article

This tech article appeared in Stainless Steel World, April 2024 magazine. To read many more articles like these on an (almost) monthly basis, subscribe to our magazine (available in print and digital format) – SUBSCRIPTIONS TO OUR DIGITAL VERSION ARE NOW FREE.

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