Duplex stainless steels are resistant to chloride stress corrosion cracking (SCC) thanks to the ferrite in their mixed grain structure. In normal circumstances, super duplex stainless steels should not suffer chloride SCC in seawater, even at elevated temperatures approaching their sensible long-term operating limit of 250°C.
Why chloride SCC matters
One of the main failure risks for stainless steels is chloride stress corrosion cracking. The combination of an applied stress and simultaneous corrosion can cause sudden, catastrophic failure at operating loads well below the nominal strength of the alloy.
SCC develops when small corrosion pits grow into large cracks under the combined influence of stress and corrosion. Pitting corrosion can advance more rapidly than general corrosion, but the growth of deep pits tends to slow over time. With added stress, a pit turns into a crack whose geometry acts as a stress concentrator, and it then advances far more rapidly.
How the families compare
Ferritic and martensitic stainless steels are highly resistant to SCC, while austenitic stainless steels are highly susceptible. Duplex and super duplex grades lie in between. Austenitic and super austenitic grades were once more commonly used in aggressive environments for their superior corrosion resistance, but the incidence of SCC in seawater and sour oil and gas service encouraged the uptake of duplex grades.
At lower temperatures, super duplex stainless steels resist SCC at chloride concentrations several times higher than seawater, as found in water treatment, desalination and chemical process industries. Because pH and the presence of other species also influence behaviour, it is difficult to set a chloride limit in isolation.
Duplex stainless steels based on 22% chromium are still considered resistant to SCC, but at slightly lower temperatures or concentrations than 25% chromium super duplex grades. Duplex grades are therefore more widely specified in water treatment facilities, where the salinity of natural, brackish or contaminated water is lower than in seawater.