Duplex stainless steels have been widely used as a material for chemical plants in the chloride environments because of their excellent corrosion resistance compared with other commercial types of austenitic stainless steels and superior ductility and weldability to the ferritic alloys. However, they don't have the required corrosion resistance to the hot sea water particularly when it's in the welded condition. The main concern has arisen over welds of duplex stainless steels which contain small amount of sigma phase (s ), and the effects may have on their corrosion resistance.

Several works were carried out to investigate the effect of sigma phase on the pitting corrosion of the duplex stainless steels. However, there were no published work have utilized the scratch technique to the evaluate this effect. Practically, the complete fabricated products such as tubes, pipes etc. made of duplex stainless steel or any other type of alloy steels are subjected to the mechanical damage during the operation, installation, handling which in turn may reduce its corrosion resistance owing to the localized distortion of the passive film particularly in the chloride containing environment.

This paper reports the effect of different sigma phase content on the pitting potential of the 22/5 type of duplex stainless steel in aqueous solution of 0.6 M NaCl at room temperature and 80+2°C using the scratch test technique. The same number of specimens were tested in nitrogen purged aqueous solution of 0.6 M NaCl at room temperature and 80°C. Sigma phase (s ) in the test specimens was precipitated thermally by heating at 850°C for different duration.

The test results showed that sigma phase markedly decreases the pitting potential of the tested duplex stainless steel specimens. In addition, the reduction in the pitting potential found to be strongly temperature dependent. The change in the test temperature found to have more effect on the pitting potential than that caused by the sigma phase content. The most interesting results of this work is that test in the nitrogen purged solution render the test specimens more susceptible to the pitting corrosion compare when tests were carried out in solution without nitrogen purging.

Microscopically examined specimens showed most of pits to occur on scratches passed through the ferritic phase of the tested duplex stainless steel specimens.