Super duplex stainless steels are frequently used in seawater environment. The steel has good general corrosion resistance, but is susceptible to localized corrosion in seawater. In natural seawater biofilm formation has an important influence on the crevice corrosion resistance and the propagation rate at temperatures up to 30-40°C. In chlorinated seawater the tendency is even stronger, but the propagation rate is generally lower than in natural seawater.

The critical crevice corrosion temperatures of super duplex stainless steel (UNS S32760) have been studied by use of potentiostatic tests. The tests have been performed under potentiostatic control where well-defined crevices were polarized 25 mV/h to a given potential (from +250-600 mV Ag/AgCl). The temperature was then increased 4°C/24h until crevice corrosion initiated.

The investigation reveals that the general tendency is that the critical crevice corrosion temperature (CCT) is decreasing with increasing potential up to +400 mV Ag/AgCl before the CCT starts increasing again. The test results indicate that large metastable pitting activity around +400 mV Ag/AgCl may lead to early initiation, i.e. lower initiation temperature.

The test method is suitable for ranking material/alloy performance, but the critical crevice corrosion temperatures found are probably higher than they would be in service conditions.

Subject
Pits, Materials, Corrosion rate, Repassivation potentials, Corrosion initiation, Natural seawater, Biofilms, Crevices, Crevice corrosion, Electric potential, Pitting, Corrosion potential, Test specimen
Keywords:
Crevice corrosion, critical crevice corrosion temperature, CCT, super duplex stainless steel, biofilm, chlorination, metastable pitting, potentiostatic testing
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2013
Association for Materials Protection and Performance (AMPP)
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