Stainless Steel World Magazine April 2023 | Page 21

[ Duplex ] demonstrated that under oxidising conditions , the alloy was susceptible to SCC at chloride concentrations as low as 15 mg / L , at 130 ° C . The potentials of stainless steels are not reported for many fluids , but they are known for seawater at ambient temperature ⁹ . If a biofilm forms , the potential is around 300 mV SCE , and the conditions are moderately oxidising . If chlorine or hypochlorite is injected to prevent the formation of a biofilm (~ 0.5 mg / L ), the potential rises to around 600 mV SCE , and the conditions are very oxidising . If the oxygen is removed from the seawater , the potential is ~ -100 mV SCE with 200 ppb oxygen , which is roughly neutral . More negative potentials can indicate reducing conditions . Reducing the pH can also increase the susceptibility to chloride SCC . Figure 5 shows the time to failure for some stainless steels in 40 % calcium chloride at 100 ° C ( near neutral redox potential ) at two pH values . It can be seen that the threshold stress for 2205 decreases at the lower pH . This is because the potential is moving to the active-passive transition region . The factors affecting redox potential are interactive , and this must be borne in mind when selecting materials . As an example , some corrosion tests were carried out on S32760 super duplex in a simulated mineral processing fluid . The fluid composition was : Chloride - 1 to 90 g / L MgSO ₄ - 75 g / L NiSO ₄ - 9.2 g / L CoSO ₄ - 1.0 g / L MnSO ₄ - 2.7 g / L FeCl ₃ - 1.5 g / L H ₂ SO ₄ - 38 g / L With the acid the pH was low , and the ferric chloride made the solution oxidising . Autoclave tests were conducted at different temperatures and the results are shown in Table 3 . The results show that SCC only occurred at the highest temperature and chloride concentration , while pitting only occurred at the lowest temperature and a relatively high chloride concentration . This is because the type of corrosion depends on exactly where you are on the polarisation curve ( Figure 3 ) under any particular set of conditions .

[ Duplex ] demonstrated that under oxidising conditions , the alloy was susceptible to SCC at chloride concentrations as low as 15 mg / L , at 130 ° C . The potentials of stainless steels are not reported for many fluids , but they are known for seawater at ambient temperature ⁹ . If a biofilm forms , the potential is around 300 mV SCE , and the conditions are moderately oxidising . If chlorine or hypochlorite is injected to prevent the formation of a biofilm (~ 0.5 mg / L ), the potential rises to around 600 mV SCE , and the conditions are very oxidising . If the oxygen is removed from the seawater , the potential is ~ -100 mV SCE with 200 ppb oxygen , which is roughly neutral . More negative potentials can indicate reducing conditions . Reducing the pH can also increase the susceptibility to chloride SCC . Figure 5 shows the time to failure for some stainless steels in 40 % calcium chloride at 100 ° C ( near neutral redox potential ) at two pH values . It can be seen that the threshold stress for 2205 decreases at the lower pH . This is because the potential is moving to the active-passive transition region . The factors affecting redox potential are interactive , and this must be borne in mind when selecting materials . As an example , some corrosion tests were carried out on S32760 super duplex in a simulated mineral processing fluid . The fluid composition was : Chloride - 1 to 90 g / L MgSO ₄ - 75 g / L NiSO ₄ - 9.2 g / L CoSO ₄ - 1.0 g / L MnSO ₄ - 2.7 g / L FeCl ₃ - 1.5 g / L H ₂ SO ₄ - 38 g / L With the acid the pH was low , and the ferric chloride made the solution oxidising . Autoclave tests were conducted at different temperatures and the results are shown in Table 3 . The results show that SCC only occurred at the highest temperature and chloride concentration , while pitting only occurred at the lowest temperature and a relatively high chloride concentration . This is because the type of corrosion depends on exactly where you are on the polarisation curve ( Figure 3 ) under any particular set of conditions .