Abstract
The effect of the microstructure (ferrite/austenite ratio, grain size and inclusions) on the Pitting Resistance of a UNS S32550 Duplex Stainless Steels (DSS) was studied in chloride containing solutions. Increasing the ferrite content in the matrix of the UNS S32550 (by annealing the alloy at a higher temperature) resulted in a lower Critical Pitting Temperature (CPT) of the alloy. Re-annealing the material to increase the grain size of the austenite resulted in an increased CPT. Microscopy on the inclusions before electrochemical measurements showed that the precursor sites for pitting on UNS S32550 were found to be inclusions that are complex in composition where pits nucleate and propagate at temperatures above the CPT in artificial sweater. These inclusions were analyzed and found to be inhomogeneous in nature and consisting of a mixture of various elements (Si, Al, Mg, Ca, Ti, Mn and S). After analysis of the particles, in-situ observation of the particles in the artificial seawater showed that the pits started at those inclusions with further dissolution of the ferrite matrix.
