Duplex stainless steels containing typically chromium, nickel, molybdenum and nitrogen, and having approximately equal volume fractions of austenitic and ferritic phases in their microstructure, are used in aggressive chemical and marine environments where high corrosion resistance is required, such as in heat exchangers, desalination plants, food pickling and mine waters. However, there is always a demand for improved corrosion resistance in these steels to cope with increasingly severe environmental conditions, and much work has been done to meet this. In particular, it is known that small additions of the platinum group metals improve corrosion resistance in stainless steel (1). Similar improvements have been observed after adding platinum group metals to duplex stainless steels with low-chromium contents, but no research had been done on the effects of adding platinum group metals to high-chromium duplex alloys.

Now, research from South Africa, involving immersion in sulphuric acid and electrochemical measurements, has shown that the addition of just 0.28 per cent of ruthenium to duplex stainless steel: iron-29 per cent chromium-14 per cent nickel-3 per cent molybdenum can increase the corrosion resistance of the base alloy by improving the hydrogen evolution efficiency and by inhibiting anodic dissolution (2). Adding ruthenium moved the corrosion potential towards more noble values. As the nitrogen content in these alloys was extremely low, in order to obtain the desired microstructure the nickel content was higher than usual; this additionally benefited the observed corrosion resistance.