Examination of the Effect on Tungsten on the Corrosion Properties of 25% Cr Super Duplex Stainless Steel by Advanced Electrochemical Techniques

Abstract

In this thesis, the effect of Tungsten (W) on localized corrosion resistance of 25Cr super duplex stainless steel (SDSS) was investigated. Firstly, the effect of W on precipitation kinetics of intermetallic phases in SDSS was studied. Three SDSSs differing in their W content (0, 0.6 and 2.1 wt.%) were isothermally heat treated (IHT) at different combinations of temperature and time followed by microstructure analyses with different microscopy techniques. A critical pitting corrosion test was performed to correlate the actual microstructure with the corrosion resistance of the materials. Secondly, the three SDSSs were tested in the solution annealed (SA) condition and their critical crevice corrosion repassivation potential (ER,Crev) was measured using a potentiodynamicgalvanostatic-potentiodynamic (PD-GS-PD) technique at different temperatures. Critical crevice corrosion repassivation temperature (TR,Crev) was determined from the ER,Crev. Additionally, the two SDSSs with 0 and 2.1 wt.% W were further tested in natural seawater polarized to different fixed potentials. The temperature was increased stepwise to obtain the critical crevice temperature (CCT) and, after initiation of crevice corrosion, the temperature was decreased stepwise to measure TR,Crev, which was compared with the output from the PDGS- PD technique. In addition, some samples with selected IHT temperature-time combinations from the microstructure characterization were tested with the same procedure in natural seawater. Lastly, the W-free and the 2.1 wt.% W SDSSs were tested in the SA condition in solutions simulating crevice corrosion environments at different temperatures. Two different solutions were used: 1M HCl as solution reference for 0 pH, and 7M LiCl adjusted to 0 pH to represent the solution in a stable propagating crevice corrosion attack. Galvele’s acidification model was employed to estimate the critical potential (Ecrit) values of these materials and compare them with the results from the other used techniques. Results obtained with all the techniques demonstrated a beneficial effect of W on the localized corrosion resistance of SDSS. In SA specimens, the higher the W content, the larger the enhancement. On the other hand, results showed that there exists a W content range within which W retarded and decreased the amount of deleterious phase precipitation on IHT specimens. Outside this range, W became detrimental as the deleterious phases precipitated earlier and in larger amount than the reference W-free material.