CREVICE CORROSION OF HIGH ALLOY STAINLESS STEELS IN SWRO PILOT PLANT 1

SUMMARY The steels containing about 12% Cr and other alloying elements are referred to as stainless steels (SS) since they have good tarnish resistance. Although, FeCr ferritic stainless steels are used commercially in cutlery and household applications, the greatest tonnage of SS are the austenitic Fe-Cr-Ni alloys containing about 18% Cr. These alloys have better ductility and weldability and good corrosion resistance. In seawater, general corrosion on these alloys is negligible but they are prone to localized corrosion due to high chloride content. The attack is particularly severe in crevices such as occurring in overlapping surfaces, under seal and gaskets, elbows and similar areas. Although, crevice corrosion and pitting can cause only a small metal loss yielding little amount of corrosion products but still sufficient to produce membrane blockage. In such a scenario, the contamination of effluent brine or the product water from the alloy is not ruled out. Resistance to crevice attack and pitting can be improved by adding Mo to the alloys for example, most commonly used stainless steels 316 has 17% Cr, 12% Ni and 2.5% Mo. However, this type of alloys is subjected to corrosion attack in more severe environment and stagnant condition. Under such conditions, high alloy stainless steels containing 6% Mo are used which have good resistance to localized attack. The research work carried out in the present project was aimed to study the corrosion performance of some austenitic and duplex stainless steels in crevice forming environment created in RO plant specially in feed and brine lines. Some electrochemical studies were also carried out to investigate the behavior