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51313-02763-Crevice Corrosion on Super Duplex—Effect of Potential on Critical Crevice Corrosion Temperature

Product Number: 51313-02763-SG
ISBN: 02763 2013 CP
Author: Anne Britt Høydal
Publication Date: 2013
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Super duplex stainless steels are frequently used in seawater environments. The steel show good general corrosion resistance but are susceptible to localised corrosion in natural seawater. The effect of potential on critical crevice corrosion temperature has been studied since high temperatures and alteration of potential due to biofilm formation or chlorination deteriorate the corrosion resistance.

In natural seawater biofilm formation has an important influence on the crevice corrosion resistance and propagation rates at temperatures up to 30-40°C. A biofilm will give a potential rise to around +300 to +350 mV Ag/AgCl/seawater. In chlorinated seawater the tendency is even stronger (+ 600 mV Ag/AgCl/seawater) but the propagation rate is generally lower than in natural seawater.

The critical crevice corrosion temperatures of UNS S32670 have been studied by use of potential tests. The tests have been performed under potentiostatic control where well-defined crevices were polarised 25 mV/hour to given potentials (+250 to +600 mV Ag/AgCl/seawater). The temperature was then increased 4°C/24 hours until crevice corrosion initiated.

According to the investigations the critical crevice corrosion temperature tends to decrease with increasing potential. The test results indicate that large metastable pitting activity around + 400 mV Ag/AgCl/seawater may lead to early initiation i.e. lower initiation temperature.

The test method is suitable for ranking materials/alloy performance but the critical crevice corrosion temperatures under service conditions however may be significantly lower than the values determined by laboratory tests.

Super duplex stainless steels are frequently used in seawater environments. The steel show good general corrosion resistance but are susceptible to localised corrosion in natural seawater. The effect of potential on critical crevice corrosion temperature has been studied since high temperatures and alteration of potential due to biofilm formation or chlorination deteriorate the corrosion resistance.

In natural seawater biofilm formation has an important influence on the crevice corrosion resistance and propagation rates at temperatures up to 30-40°C. A biofilm will give a potential rise to around +300 to +350 mV Ag/AgCl/seawater. In chlorinated seawater the tendency is even stronger (+ 600 mV Ag/AgCl/seawater) but the propagation rate is generally lower than in natural seawater.

The critical crevice corrosion temperatures of UNS S32670 have been studied by use of potential tests. The tests have been performed under potentiostatic control where well-defined crevices were polarised 25 mV/hour to given potentials (+250 to +600 mV Ag/AgCl/seawater). The temperature was then increased 4°C/24 hours until crevice corrosion initiated.

According to the investigations the critical crevice corrosion temperature tends to decrease with increasing potential. The test results indicate that large metastable pitting activity around + 400 mV Ag/AgCl/seawater may lead to early initiation i.e. lower initiation temperature.

The test method is suitable for ranking materials/alloy performance but the critical crevice corrosion temperatures under service conditions however may be significantly lower than the values determined by laboratory tests.

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