One of the main risks of failure for stainless steels is from chloride stress corrosion cracking. Here, the combination of an applied stress and simultaneous corrosion can result in sudden and catastrophic failure, at operating loads well below the nominal strength of the alloy. Duplex stainless steels are resistant to stress corrosion cracking, due to the presence of ferrite in their mixed grain structure.
Stress corrosion cracking results from small corrosion pits developing into large cracks under the combined influence of an applied stress and corrosive conditions. Whilst pitting corrosion can advance more rapidly than general corrosion, the growth of deep pits will tend to slow over time. However, in the case of SCC the addition of stress will change the pit into a crack, the geometry of which acts as a stress concentrator and will advance far more rapidly.
Ferritic and martensitic stainless steels are highly resistant to SCC, whilst austenitic stainless steels are highly susceptible. Therefore, duplex and super duplex stainless steels lie somewhere in between. Austenitic and super austenitic stainless steels were more commonly used in aggressive environments due to their superior corrosion resistance. However, the incidence of SCC in seawater and ‘sour’ Oil and Gas applications encouraged the uptake of duplex grades.
In normal circumstances, super duplex stainless steels should not suffer from chloride stress corrosion cracking in seawater. This is true even at elevated temperatures, approaching the sensible long-term operating limits for these grades of 250degC. At lower temperatures, super duplex stainless steels would be resistant to SCC at increased chloride concentrations – several times that found in seawater – as may be seen in water treatment, desalination and chemical process industries. Due to the influence of other elements, such as the pH level and presence of other species, it is difficult to set a chloride limit in isolation.
Duplex stainless steels, based upon 22%Cr, are still considered resistant to SCC, but at slightly lower temperatures or concentrations than 25%Cr super duplex grades. Therefore, duplex grades are more widely specified in water treatment facilities, where the salinity in natural, brackish or contaminated water is less than in seawater.
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