[ Duplex ]
Transpassive
90
80
70
Transition Zone
Potential |
Passive |
Reduction of Area (%)
60
50
40
30
|
Ductile Failure No Cracking |
Brittle Failure Cracking |
Current Density
Active
Figure 3 . Schematic diagram of the polarisation curve for a stainless steel .
20
10
0 0 200 400 600 800 1000 1200 1400 Potential ( mV Ag / AgCl 2
)
15mg / L Chloride
1,000mg / L Chloride
Figure 4 . The three different ductility regions for S32760 super duplex stainless steel in aerated solution at 130 ° C 8 .
600 600
500 500
Time to Failure ( h )
400
300
200
Time to Failure ( h )
400
300
200
100 100
0 0
0 0.2 0.4 0.6 0.8 1
0 0.2 0.4 0.6 0.8 1 1.2
Stress / 0.2 % Proof Stress Stress / 0.2 % Proof Stress Stress
304L 316L 2205
( A ) ( B )
316L 2205 2507
Figure 5 . Threshold stress versus time to failure in 40 % calcium chloride at 100 ° C and different pH values [ A = pH 6.5 ; B = pH 1.5 ]
when reducing chemicals are present . One of the most common is hydrogen sulphide , as found in sour oil and gas wells . Stainless steels can suffer SCC in sour fluids , but only if chloride is also present 10 . The use of metals in sour oil and gas fluids is specified in ISO 15156 / NACE MR0175 11 . For duplex stainless steels the limits of use are 10 kPa partial pressure of H 2 S for S32205 , and 20 kPa for super duplex . However , it is well known that the chloride content and insitu pH also affect the resistance to SCC . Francis combined the results of SCC tests from a number of laboratories to derive the safe use limits for duplex and super duplex stainless steel , as shown in Figure 6 7 . The results show that the duplex alloys can be used at higher H 2 S partial pressures than the ISO 15156 / NACE MR 0175 limit at lower chloride concentrations , and at higher pH values .
Service experience There have been numerous successful applications of duplex stainless steels in industry because of their good resistance to SCC . Many of these have been recorded in the proceedings of the numerous duplex stainless steel conferences held since 1991 . Table 4 shows some examples from the refinery and petrochemical industries , where duplex alloys have been chosen for their resistance to corrosion and SCC 7 . There have been a few failures of duplex alloys by SCC under oxidising conditions . Linton et al reported the failure of a super duplex separator vessel in an ammonium nitrate plant 12 . The environment was 82 % ammonium nitrate , 16 % water , 0.8 % ammonia and 0.8 % nitric acid at ~ 180 ° C . Only a few mg / L chloride were reported in the fluid , but the nitric acid would make the fluid very oxidising , and the results in Figure 4 suggest that at 180 ° C even a few mg / L chloride would cause SCC under strongly oxidising conditions . Super duplex S32550 was used for agitator shafts in the autoclave at a mineral processing plant . The autoclave was pressurised with oxygen at 31 bar at 220 ° C , and the fluid contained
~ 30g / L sulphuric acid and less than 50 mg / L chloride . Severe cracking occurred in several shafts as shown in Figure 7 . As above , the oxygen made the environment highly oxidising , and it would not take much chloride for SCC to initiate . There have been many examples of the use of duplex stainless steels in sour brines outside the limits in ISO 15156 / NACE MR0175 , as shown in Figures 6A and 6B , without any SCC failures . Woollin and Maligas report the SCC failure of a super duplex stainless steel in a brine containing 20,000 mg / L chloride and 200 kPa H 2 S at pH 3.5 12 . This point lies above the boundary curve in Figure 6B at pH 3.5 , so the failure is hardly surprising . In reducing alkaline environments , duplex stainless steels are very resistant to both SCC and corrosion . The Kraft pulp and paper process is a good example , where sodium sulphide is used as a catalyst with the caustic soda . Duplex stainless steels have found a wide variety of uses in Kraft mills 7 , and Figure 8 shows a S32304
22 Stainless Steel World April 2023 www . stainless-steel-world . net