Stainless Steel World Magazine April 2024 | Page 23

thick walled glass tube

ZERON 100X WELDING WIRE ptfe seal

DURAMASTIC LAQUER specimen

65 mm

30 mm

10 mm

Figure 2 . Test configuration and sample types used when measuring CPT and RCCT

13 mm 4 mm

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 10 µ A / cm 2 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 . a ) 316L sufferer ’ s crevice corrosion at temperatures below the normal ambient range for SWRO plants worldwide and is hence unsuitable for this service . b ) 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

Potential ( mV ( SCE ))

700

600

500

400

300

200

100 c ) 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 . 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 50 º C at + 200mV ( SCE ), which

The Effect of Potential on the Relative Critical Crevice Temperature and the Critical Pitting Temperature of Stainless Steels in Artificial Sea Water . chlorinated

natural

HP Feed range of potentials

904L

2205 ( W )

ZERON 100 ( W , PREN 40 )

ZERON 100 W , PREN

ZERON 100 & 6 %

ZERON 100 W , PREN 40 + ACID

CPT " As Welded " ( PREN 40.7 ) RCCT Parent Material CPT " As Welded " ( PREN 42 ) CPT Pickled Welds ( PREN 40.7 ) 316L 904 L 2205 (" As Welded ")

Figure 3 . The effect of potential on RCCT and CPT of stainless steels in artificial seawater .

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