Duplex stainless steels and chloride SCC – how resistant are they ?
This paper discusses the conditions under which duplex can suffer chloride SCC and describes some service successes and failures .
By Roger Francis , Nickel Institute
It is well known that the 300 series austenitic stainless steels , such as 316L ( UNS S31603 ), are susceptible to chloride stress corrosion cracking ( SCC ) at low concentrations of chloride and at temperatures exceeding 55 ° C 1 . This has caused numerous service failures , as described , for example , in reference 1 . Duplex stainless steels are well known to have good resistance to chloride SCC , and there is data that suggests the chloride and temperature limits for different grades of duplex stainless steels 1 . What is less well known is that failures can occur within these limits under certain environmental conditions .
Standard tests Note : the nominal compositions of the alloys discussed in this paper and their UNS numbers are shown in Table 1 .
There are a number of standard tests for SCC , and some modifications of them , such as the ASTM G36 boiling magnesium chloride test 2 and the G123 test in boiling acidified sodium chloride 3 . Table 2 presents the results of tests on some austenitic and duplex stainless steels 4-6 . It can be seen that everything fails in the G36 test , and it is just the time to failure that varies . In the other tests , the austenitic alloys fail , and the duplex alloys pass , which doesn ’ t help in assessing alloys for SCC resistance in a specific fluid unless it is the one in the test . Another test used to compare the SCC resistance of stainless steels is immersion in aerated 5 % sodium chloride at around the 0.2 % proof stress and determining the temperature at which SCC initiates . The results for some alloys are shown in Figure 1 , and while this is a little more useful than the data in Table 2 , it doesn ’ t help very much with alloy selection 7 . By doing a lot of tests in solutions with different chloride contents and at different temperatures , it is possible to produce curves such as those in Figure 2 . The data were generated in pure aerated solution , making the conditions mildly oxidising , and this has proved useful for many applications . However , there are service environments where the conditions are strongly oxidising or strongly reducing .
Redox potential Figure 3 shows the typical polarisation curve for stainless steel in a corrosive solution . When it resists corrosion it is in the passive zone , but if the conditions become more reducing , the potential can decrease , and the alloy moves into the active zone , where general dissolution occurs . Under oxidising conditions the alloy can move into the transpassive zone , where increased dissolution may occur , or pitting if chlorides are present . The redox potential indicates whether the conditions are reducing , oxidising or neutral . SCC is possible at the transition zones between active and passive , and passive and transpassive . Atkinson et al carried out slow strain rate tests on S32760 super duplex stainless steel in aerated solutions at 130 ° C , and chloride concentrations from 15 to 1,000 mg / L 8 . The results in Figure 4 show that at lower potentials the alloy had good resistance to cracking , while at potentials ≥ 600 mV SCE the alloy was very susceptible to cracking . From 400 mV to 600 mV SCE there was a transition region where the ductility decreased as the potential increased . These results clearly
Table 1 . Nominal compositions of the stainless steels discussed in the paper . NAME UNS No . NOMINAL COMPOSITION ( wt %) PREN *
Fe C Cr Ni Mo N Cu W
304L Austenitic
316L Austenitic
Alloy 33 Austenitic
S30403 Bal 0.02 18 10 – – – – 18
S31603 Bal 0.02 17 10 2 – – – 24
R20033 Bal 0.01 33 32 1 0.4 0.7 – 43
Lean Duplex
S32101 S32304 S32003
Bal Bal Bal
0.02 0.02 0.02
21.5 23 20.5
22 % Cr Duplex S32205 Bal 0.02 22.5 5 3.3 0.17 – – 35
Super duplex
S32760 S32750 S32550
Bal Bal Bal
Bal = Balance * PREN = % Cr + 3.3 (% Mo + 0.5x % W ) + 16x % N .
0.02 0.02 0.02
25 25 25
1 4.8 3
7 7 6.5
0.3 0.3 2
3.5 3.5 3.5
0.15 0.10 0.15
0.25 0.25 0.25
– – –
0.7 – 1.5
– –
0.7 – –
25 24 30
41 41 40
20 Stainless Steel World April 2023 www . stainless-steel-world . net