THE INFLUENCE OF SENSITIVE TEMPERATURE ON THE LOCALIZED CORROSION RESISTANCE OF DUPLEX STAINLESS STEEL SAF2304

Duplex stainless steels (DSS), characterized by a two-phase microstructure of ferrite (alpha) and austenite (gamma), have an attractive combination of mechanical strength and corrosion resistance in various aggressive environment. DSS SAF2304 shows wide application potential due to its lower cost compared with conventional DSS and better corrosion performance than austenite steel. However, precipitations of detrimental phases inevitably occur when DSS is heated to temperatures ranging from 300 degrees C to 1000 degrees C during manufacturing and welding procedures. These precipitations will lead to the reduction of corrosion resistance of DSS due to the presence of chromium-depleted zones around them. This work investigates the influence of sensitive temperature on the localized corrosion resistance of DSS SAF2304. The resistances to intergranular corrosion and pitting corrosion of DSS SAF2304 annealed at various temperatures ranging from 600 degrees C to 950 degrees C for 2 h were investigated by means of double loop electrochemical potentiodynamic reactivation (DL-EPR) technique in a solution of 1 mol/L H2SO4+1 mol/L HCl+0.2 mol/L NaCl at 30 degrees C with a scanning rate of 1.667 mV/s and critical pitting corrosion temperature (CPT) technique in a solution of 1 mol/L NaCl with a rising rate of 1 degrees C/min; respectively. The morphologies and microstructures of the specimens after electrolytic etching in 30%KOH, oxalic acid and potassium metabisulfite were characterized by OM and SEM techniques. A same trend was observed by the different evaluating techniques, which suggested that both of the resistances of intergranular corrosion and pitting corrosion of DSS SAF2304 decreased with the annealing temperature increased from 600 degrees C to 700 degrees C, while a contrary trend was found from 750 degrees C to 950 degrees C. In particular, the samples annealed at 700 and 750 degrees C suffered the severest corrosion. The relationship between microstructure and localized corrosion resistance was revealed by the evolution of the microstructure; and it was found that the deterioration of the resistance to localized corrosion was due to the formation of chromium depleted zones around the precipitation of Cr2N.