Ferritic steels have a body centred cubic crystal structure (BCC), austenitic steels have a face centred crystal structure (FCC). The change in structure comes with the addition of Nickel to the steel. The minimum nickel content in 304 is 8% which is about as bad as it gets for SCC, refer to a Copson curve.

The lower alloyed Ferritic steels may be more resistant to SCC but may be more susceptible to other forms of corrosion such as pitting and crevice corrosion.

Cheers

Graham Gedge

Felix

Although austenitic 300 series SS are prone to chloride stress cracking in the harsh chloride environment above 60C and your operation is at 100C which is a problem. However, any micro examination has been carried out in the new and chloride pitted and cracked parts to see any sensitization depleting chromium content of the part. Also SS 304L has any Mo about 2 to 3% , if not then try SS 304L with ~3% Mo and see how long it last.

Dr. K

Dear Felix,

Basic reason is minimize heat input in the case of ferritic avoid the formation of high CLSCC and they avoid loss of ductility in compared to little high heat input of austenitic. Susceptibility mechanism may be defined in support of temperature Arrhenius equation.

AISI 304 stainless steel around 8 %-10% nickel content; molybdenum improves resistance to localized corrosion in chloride-containing environments have good corrosion resistance but are susceptible to chloride stress corrosion cracking. They resist oxidation and retain their higher strength at high/elevated temperatures. 400-series, ferritic grades have 11 - 20% chromium and no nickel and these grades are typically magnetic. They generally cost less than 300-series grades and even though 400-series grades resist chloride stress corrosion cracking they are generally less corrosion resistant than common 300-series grades under similar service conditions. Chloride attacks plays more with higher heat input. Balance heat is higher in austenitic.

But except CLSCC, ferritic is more prone to corrode in comparison to austenitic of high nickel grade.

Hope it is helpful for you.

Ashish

In addition to above comments, the ferritic stainless steels however pose difficulties with regarding welding. If this is an issue and need corrosion resistance the duplex stainless steels provide a composite material with FCC and BCC structures providing good SCC, pitting resistance with fabrication options.

Duplex steels would provide good corrosion resistance as well as fabrication, but make sure to select the right duplex or super duplex steel.

Dr. K