Stainless steels such as AISI Type 304 SS are being used for seawater piping applications due to their desirable mechanical properties and good weldability. However, they are susceptible to pitting and crevice corrosion in chloride bearing environments. Thus a new generation of highly alloyed stainless steels such as Avesta 254 SMO with high molybdenum contents has been developed for improved localised corrosion resistance in seawater. These steels are also susceptible to the formation of undesirable secondary phases such as sigma and chi which degrades both mechanical and corrosion properties. Alternatively, the main alloying element can be surface alloyed onto the surface of a suitable substrate by laser surface alloying. In this paper, austenitic stainless steel surface alloys of varying molybdenum contents have been formed on 304 SS by laser surface alloying and characterised by optical and scanning electron microscopy. The corrosion behaviour was ascertained by electrochemical and immersion tests.

Stainless Steels are required for many applications for ship building as well as for offshore structures such as oil exploration. AISI type 304 stainless steel is not very suitable for such applications as it has a strong tendency for pitting and crevice corrosion. Even type 316 and 317 stainless steels which have respectively 2.5 and 3.5% Mo are not very effective in these environments. Commercially available stainless steels, viz., Avesta 254 SMO is being employed for such applications because of its strong resistance to pitting and crevice corrosion. This is mainly because of high Mo concentration (6.5%). Such steels are not only costly but are prone to form deleterious phases such as delta ferrite and sigma during welding or other heat treatment operations. Hence, an alternative technique to restrict Mo at the surface is needed. In the present work, surface alloys consisting of an austenitic stainless steel with Mo content as high as 10-12% have been formed on stainless steel type 304 substrates. These steels show enhanced passivity and strong resistance to pitting corrosion.