[ Super Duplex ]
[ Super Duplex ]
is about 10 º C higher than the normal maximum design temperatures for SWRO plants in the Middle East . d ) If these welds are pickled in acid , their CPT increases to about 80 º C at + 200mV ( SCE ). Hence , we recommend that all welded spools be post weld acid pickled . e ) The CPT of welds in the “ as welded ” condition can be increased to about 68 º C 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 .
Chloride ( mg / L )
100,000 22 % Cr Duplex
ZERON 100
200mV SCE
10,000
600mV SCE 600mV SCE 400mV SCE
1,000 0 20 40 60 80 100
Critical Crevice Temperature (° C ) Figure 4 . Shows how the RCCT varies with Chloride content at different potentials .
As Received
Machined 22 % Cr Duplex
As Received
Machined
ZERON 100
20 40 60 80 100 Relative Critical Crevice Temperature (° C )
Figure 5 . Crevice corrosion below the sealing face at a mechanical joint and the effect of skim machining the sealing face .
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 ) . Figure 6 . Skim machined sealing faces of mechanical joints in pipe spools .
24 Stainless Steel World April 2024 www . stainless-steel-world . net