New High-Resolution Solution for Measuring Degree of Sensitization of Duplex Stainless Steel 2205 Using Double-Loop Electrochemical Potentiodynamic Reactivation Technique

Abstract

In the present work, a new high-resolution solution for measuring degree of sensitization (DOS) of duplex stainless steel (DSS) 2205 (UNS S31803) by double loop-electrochemical potentiodynamic reactivation (DL-EPR) technique was obtained and the solution parameters were optimizing using the Taguchi method. Sensitized samples in three different conditions were chosen and five solution main factors including acid and two types of depassivator concentrations, scan rate, and surface roughness were considered. To reduce the number of experiments, Taguchi orthogonal array (OA) was used. The resulting optimal condition was to be 0.5 M sulfuric acid (H 2 SO 4), 0.002 M sodium thiosulfate (Na 2 S 2 O 3), 0.02 M sodium chloride (NaCl), and 120 grid in surface roughness and 30 mV/min scan rate. By introducing the new solution, a DOS value of 78% was obtained in the DSS2205 sample sensitized at 850°C for 40 min. Moreover, topography variations of non-sensitized and sensitized samples measured by atomic force microscopy elucidated the mechanism of austenite/ ferrite boundary attack and carbide precipitates formation in sensitized samples extended to the ferrite phase.

Characterization of the Native Passive Film on Ferrite and Austenite Phases of Sensitized 2205 Duplex Stainless Steel

“…Electrochemical measurements.-The degree of sensitization was evaluated by double-loop electrochemical potentiodynamic reactivation (DL-EPR) test, which was conducted in a solution containing 0.5 M H 2 SO 4 , 0.005 M Na 2 S 2 O 3 , 0.02 M NaCl at 25 ± 1°C, and a scan rate of 100 mV/min, according to procedures described previously. 5 The potential was scanned in the anodic direction from 50 mV below open circuit potential (OCP) toward a point of 100 mV vs. SCE, in the initial region of the passive region. Then, scanning direction was reversed and the potential was reduced from 100 mV vs. SCE back to the cathodic region, resulting in an anodic loop and a reactivation loop for sensitized sample, 26,38 while it shows cathodic behavior for solution annealed sample.…”

“…Duplex stainless steels (DSSs) containing both austenite and ferrite phases in their microstructure are of interest for various applications in chemical and petrochemical industries, gas refineries, desalinations, marine environments, and in the paper and pulp industry, due to their advantageous corrosion resistance and superior mechanical properties. [1][2][3][4][5] In DSSs, chromium (Cr) and molybdenum (Mo) are concentrated slightly in the ferrite phase, whereas nitrogen (N) and nickel (Ni) are partitioned toward the austenite phase. 6,7 It has been shown previously 2,6,[8][9][10] that the selective dissolution of ferrite or austenite phases greatly depend on the electrochemical potential and composition of each phase.…”

Magnetic and Electrochemical Techniques as Tools to Detect Deleterious Phases in Duplex Stainless Steels

“…[27][28][29][30][31][32][33][34][35][36][37] Improving the choice of experimental parameters by using optimizing methods has been proposed. 38 In our laboratory, we are not following this approach of finding the best experimental condition for each class of DSS. Our main purpose is to establish a quality control protocol to be used in the field, as it would be very hard to work in the field using a new protocol for each DSS needing to be analyzed.…”