Material Grade

ALLOY 254 SMO

Introduction 254 SMO* is a high-alloy austenitic stainless steel developed for use in seawater and other aggressive chloride-bearing media. The steel is characterized by the following properties: * Excellent resistance to pitting and crevice corrosion * High resistance to general corrosion * High resistance to stress corrosion cracking * Higher strength than conventional austenitic stainless steels * Good weldability In solutions containing halides such as chloride and bromide ions, conventional stainless steels can be readily attacked by local corrosion in the form of pitting corrosion, crevice corrosion or stress corrosion cracking. In acid environments, the presence of halides also accelerates general corrosion. In pure sulphuric acid, 254 SMO is much more resistant than AISI 316, and in naturally aerated sulphuric acid containing chloride ions 254 SMO exhibits higher resistance than 904L,

Intergranular corrosion 254 SMO has a very low carbon content. This means that there is very little risk of carbide precipitation during heating, for example when welding. The steel passes the Strauss test (ASTM A262, practice E) even after sensitizing for one hour at 600-1000°C (1110-1830°F). However, due to the high alloying content of the steel, inter-metallic phases can precipitate at the grain boundaries in the temperature range 600-1000°C (1110-1830°F). These precipitations do not involve any risk of intergranular corrosion in the environments in which the steel is intended to be used. Thus, welding can be carried out without any risk of intergranular corrosion.

Pitting corrosion Its high chromium content and particularly the molybdenum content give 254 SMO excellent resistance to pitting and crevice corrosion. The high nitrogen content also improves pitting resistance. The results of laboratory determination of the critical pitting temperature (CPT) in 3 % NaCl are shown in Figure 3, where it can be seen that 254 SMO possesses very good resistance in water containing chlorides 254 SMO is, therefore, a suitable material for use in seawater.

Crevice corrosion The weak point of conventional stainless steels is their limited resistance to crevice corrosion. In seawater, for example, there is a considerably greater risk of crevice corrosion under gaskets, deposits or fouling. Tests in natural seawater at 60°C (140°F) have shown that 254 SMO can be exposed for prolonged periods without suffering crevice corrosion. Figure 4 shows the results of accelerated crevice corrosion tests.