Duplex Steels

The chemical composition of the duplex steels is balanced to yield relatively equal corrosion resistance in the austenite and ferrite phases, hence the term “duplex” stainless steel refers to the dual austenite/ferrite microstructure. 

Unlike Type 316L stainless steel which has a microstructure that consists primarily of austenite which is achieved primarily by adding sufficient nickel to the alloy to stabilize it,  duplex stainless steels have a chemical composition that is adjusted to produce a microstructure that consists of approximately equal parts ferrite and austenite. For the most common grades such as 2205 duplex stainless steel, used as comparison here out, this is accomplished by reducing the nickel content to about 5% and adjusting the magnesium and nitrogen additions to yield approximately 40 – 50 % ferrite.

The increased nitrogen content and fine grained microstructure of duplex stainless steel results in higher strength levels than the common austenitic grades such as types 304L and 316L.  In the solution annealed condition the duplex steel will have about double the yield strength of Type 316L. 

The most common mode of corrosive attack with stainless steel in pharmaceutical applications is pitting in the presence of chloride-bearing environments. The increased levels of Cr, Mo, and N in duplex stainless steel will provide substantially better pitting and crevice corrosion resistance than the 316L grade.  In standardized critical pitting temperature tests, utilizing a standardized test solution of 6% ferric chloride, the 2205 grade has a critical pitting temperature (CPT) that is intermediate between that of Type 316L and 6% Mo super austenitic stainless steels as shown in the chart at the right. It should be noted that the CPTs measured in ferric chloride provide a reliable ranking of the relative pitting resistance but should not be used to predict the critical pitting temperature in other chloride-bearing environments.