Abstract
Duplex Stainless Steels (DSS) are highly important engineering materials due to their high corrosion resistance combined with high strength and moderate alloy cost (lower nickel and molybdenum content) compared to standard austenitic grades. They are widely used in various industrial sectors, such as oil & gas (pipes and storage tanks), desalination (evaporators and pumps) and pulp and paper (digesters and bleaching reactors) industries. In this paper, the influence of alloying elements on the passive film properties is first investigated after surface preparation and after ageing. The influence of the microstructure (Grain Orientation Spread (GOS) and Grain Surface Area (GSA)) on the corrosion resistance is then studied in NaCl solution by means of Critical Pitting Temperature (CPT) tests.
Surface analysis was carried out in each phase of three DSS grades. No significant differences were observed in the properties of the passive films formed after etching. After ageing, there is a slight thickening of the passive films. The ratio Cr / Fe increases significantly for all grades whereas the ratio O2- / (OH- +H2O) increases only for grades with a pitting resistance equivalent number (PREN) greater than 25.5. N-H bonds were found at the outer interface whereas M-N bonds were located at the inner interface. Only N-H bonds were found for grade #3. This was attributed to the large amount of this element in grade #3.
CPT measurements were carried out in ferrite grains of grade #1. It was shown that all the grains with a GOS value < 1.30 remain passive (no pitting). For GOS > 1.30, metastable / stable pitting was systematically detected. It was found that the higher the GSA, the greater the CPT. Therefore, the GOS and the GSA seem to be two very important metallurgical parameters to predict pit initiation in DSS. The corrosion resistance will be increased by decreasing GOS or increasing GSA.
