Super duplex stainless steel has now been in service since 1989, with well over 1 million welds, and generally few problems have occurred. However, during welding, particularly of small diameter thin walled pipe, it is possible for small amounts of sigma phase to be precipitated in the low temperature heat affected zone. The depletion of chromium and molybdenum around the precipitates reduces the local corrosion resistance, and the question that is frequently asked is, how much sigma phase can be tolerated before corrosion resistance is significantly impaired?

In the paper a simple model is presented for the depleted zone around sigma precipitates produced during the welding of Zeron 100 super duplex stainless steel. The requirements for corrosion to propagate are evaluated, and the precipitate size is shown to be the most critical factor. The tolerable level of sigma phase is shown to vary hugely with particle diameter.

Using typical precipitate sizes found in Zeron 100, the model’s predictions are shown to correlate well with the results of corrosion tests of welds in chlorinated sea water. Similar results are found for sour oil and gas process brines and flue gas desulphurisation slurries.

The results of the modelling exercise have important implications for engineering fabrication of super duplex stainless steels. Because of the dramatic influence of precipitate size there is no single figure for the tolerable level of sigma phase which is acceptable. It is concluded that fitness for purpose tests for welds are more relevant than metallography and counting of sigma particles.