what is austenitic stainless steel type

Stainless steels are immune to CO2 corrosion due to the formation of protective oxide film to prevent corrosion of the underlying metals. Two types of austenitic stainless steel are discussed here, i.e. austenitic stainless steel and highly alloyed austenitic stainless steel. Austenitic stainless steel includes type SS316, SS304. SS304 is not commonly used for marine environment due to less chromium, nickel and molybdenum content which shows lower corrosion resistant than SS316. SS904 and SS6Mo are grouped under highly alloyed austenitic stainless steel which exhibit higher corrosion resistant.
SS316 is immune to CO2 corrosion. However, it is more susceptible to attack from the offshore marine environment. It is noted that SS316 material should have minimum 2.5% Mo to improve CISCC and pitting resistance of the material. However, in oxygen free environments and temperature below 60˚C, there has no problem of chloride pitting and cracking
SS316 is widely used on offshore installations when CRAs are required. The “L” type of SS316 has lower carbon content than the standard version of the alloy, typically maximum 0.03wt% compared to maximum 0.08 wt% for standard version. The lower carbon content gives better resistance towards sensitization during welding and heat treatment.
SS904 is a material with chemistry between SS316 and SS6Mo. The corrosion resistance is comparable to DSS, except that resistance to H2S failure is equivalent to SS316. The austenitic microstructure enables the material to be used to low temperatures. Typical applications will be cladding material for vessels, where a more robust material than SS316 is required, and vessels, where resistance to both high and low temperature is required. SS904 will offer a cost effective material selection to SS6Mo.
Highly alloyed austenitic stainless steel, SS6Mo has a corrosion resistance in seawater comparable to SDSS; both may be used in piping systems up to 20˚C without problems. SS6Mo is however significantly more costly due to relatively higher alloy elements such as nickel and molybdenum. One advantage of SS6Mo compared to SDSS in seawater is however better resistance towards hydrogen induced stress corrosion cracking (HISC) when exposed to cathodic protection. SS6Mo has, like all other austenitic materials, good low temperature properties. It is commonly used for systems where both low and high temperature may occur, and where SS316L cannot be used due to sour service restriction.

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