Corrosion Behavior of Duplex Stainless Steels in Arabian Seawater

A study has been conducted to investigate the corrosion behavior of duplex stainless steels in seawaters of Kingdom of Saudi Arabia.  The main goal of the study has been to find out suitable alloys for seawater handling system particularly pumps in seawater desalination plants as the conventional materials like austenitic stainless steel 316, Ni-resist iron and copper base alloys failed unexpectedly during early years of operation.

The corrosion behavior of stainless steels (mainly duplex steels) was investigated at 2 locations in Saudi Arabia (Arabian Gulf at east coast and Red Sea at west coast).  Ten duplex stainless steels comprising of 7 simple duplex and 3 superduplex steels were employed for the study. Additionally, 3 steels namely: superaustenitic, austenitic cast-iron and austenitic type SS 316 were also used for comparison purposes. Both cast and forged duplex steels were used during the studies.  The immersion tests were also carried out in chlorinated seawater.  The immersion time varied from 365 to 470 days.  The corrosion rates of the alloys were determined on the basis of mass loss data.  The localized attack was studied by the macroscopic examination and pit depth determination.  The average pit depth, maximum pit depth and average pit number were determined.

The results of the investigations indicate that superduplex steel are the most resistant to general and localized corrosion attack in two seawaters locations.  The superaustenitic SS has nearly the same resistant as that of superduplex steels.  In general, forged alloys have much higher corrosion rates than cast duplex steels. Alloys having PRE_N greater than 38 have extremely low corrosion rates.  Presence of copper and tungsten appears to increase the corrosion resistance of duplex steels appreciably.

The crevice corrosion behavior of alloys was evaluated by determining rate of crevice corrosion occurrence, "R" in different seawaters.  The value of "R" reflects the tendency of crevice corrosion in the alloys.  Super duplex steels which are invariably forged or rolled have little or no tendency to undergo crevice corrosion whereas the cast duplex steels have variable tendencies (R value varies from 30 to 65%) of crevice corrosion occurrence.

On the basis of corrosion study, it has been found that amongst all the alloys studied super duplex DP3W and SF 2507 have outstanding corrosion resistance against general and pitting and crevice corrosion (localized corrosion) in seawater.