Evaluation of the corrosion risk of duplex stainless steel UNS S82551 in treated seawater injection service

• In seawater handling systems, the available well tubing materials are Glass Reinforced Epoxy lined low alloy steel or Corrosion Resistant Alloy's (CRA) such as super duplex stainless steel. However, in treated seawater the corrosion risk can be controlled so that lower grade alloys can be considered. Recent efforts have focused attention on better dissolved oxygen controls which permits the investigation and possible use of more cost effective materials such as the duplex stainless steel UNS S82551. Full scale corrosion testing of tubes joined together with a proprietary premium threaded connection was performed in controlled seawater loops simulating service conditions at 30° C. The flow rate and dissolved oxygen were controlled at 5 m/s and <20ppb, respectively. Weekly dissolved oxygen excursions corresponding to 24h at 100ppb followed by 1 hour at 300ppb were included during the 5 months exposure. Corrosion results of UNS S82551 tubing were compared to UNS S31803 and UNS S39274. In parallel, laboratory exposures of coupons were performed in dissolved oxygen controlled cells with and without CREVCORR crevice formers, allowing the measurement of electrochemical potentials as function of dissolved oxygen content (e. g. biofilm ennoblement monitoring) and the related corrosion resistance. The results showed that dissolved oxygen content should be properly controlled below critical values to avoid crevice corrosion of the lesser alloyed duplex stainless steels. In the full scale loop test, UNS S82551 tubes did not exhibit crevice corrosion at threaded connection interfaces under the defined test conditions. 

Nyckelord

Crevice corrosion

Dissolved oxygen

Duplex stainless steels

Treated seawater

Alloy steel

Corrosion

Corrosion resistance

Corrosion resistant alloys

Cost effectiveness

Dissolution

Electrochemical corrosion

High strength steel

Materials handling

Oxygen

Seawater

Stainless steel

Tubing

Dissolved oxygen contents

Dissolved oxygen control

Duplex stainless steel

Electrochemical potential

Service conditions

Super duplex stainless steel

Threaded connection

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