Corrosion Resistance of Duplex Stainless Steel

Duplex stainless steel exhibit a high level of corrosion resistance in most environments where the standard austenitic grades are used. However, there are some notable exceptions where they are decidedly superior. This results from the alloying that is undertaken.

Acids:

Both S32205 and S32507 duplex stainless steel outperform many high nickel austenitic stainless steel in solutions containing up to 15% acid. They are better than type 316L or 317L through at least 40% acid. The duplex grades can also be very useful in oxidising acids of this kind containing chlorides. The duplex stainless steel do not have sufficient nickel to resist the strong reducing conditions of mid-concentration sulphuric acid solutions, or hydrochloric acid. Their resistance to oxidising conditions makes duplex stainless steel good candidates for nitric acid service and the strong organic acids. Although types 304 and 316 will handle these strong organic acids at ambient temperatures, 2205 and other duplex grades are superior in many processes involving organic acids at high temperature. The duplex stainless steel are also used in processes involving halogenated hydrocarbons because of their resistance to pitting and stress corrosion.

Caustics:

The high chromium content and presence of ferrite provides for good performance of duplex stainless steel in caustic environments. At moderate temperatures, corrosion rates are lower than those of the standard austenitic grades.

Pitting and crevice corrosion and resistance:

Pitting and crevice corrosion and resistance are generally measured with reference to CPT* and CCT**.These are critical temperatures for crevice and pitting on stainless steel. The high Chromium and Molybdenum and Nitrogen contents in duplex grades provide very good resistance to chloride-induced localised corrosion in aqueous environments. Depending on the alloy content, some duplex grades are among the best performing stainless steel. Because they contain relatively high chromium content, duplex stainless steel provide a high level of corrosion resistance very economically.

Reference material: Practical Guidelines for the Fabrication of Duplex Stainless Steel – ISBN 978 -1 -907470 -00 -4 – 2nd Edition 2009, published by International Molybdenum Assoc. (IMOA) London UK

The range of duplex stainless steel allows them to be matched for corrosion resistance with the austenitic and ferritic steel grades. There is no single measure of corrosion resistance. However, it is convenient to use the Pitting Resistance Equivalent Number (PREN) as a means of ranking the grades. 

PREN = %Cr + 3.3 x %Mo + 16 x %N 

The following table shows how the duplex stainless steel compare with some austenitic and ferritic grades. 

It must be emphasised that this table is only a guide to material selection. It is always important to assess the suitability of a particular with a full knowledge of the corrosive environment.

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