Effect of Ferritic Phase Content on Pitting Corrosion Behavior of Cast Duplex Stainless Steel
Authors: Yamamoto Ryotaro , Yakuwa Hiroshi , Miyasaka Matsuho , Hara Nobuyoshi 
Published: February 2020

1. Introduction
Duplex stainless steel (DSS) has been increasingly used in seawater desalination plants, oil & gas-related facilities, and chemical plants. This is because it has high corrosion resistance and high strength compared to austenitic stainless steels. It has been known that the phase ratio between the ferritic (α) and the austenitic (γ) phases can greatly affect the corrosion resistance of DSS. However, the data for the corrosion behavior of the cast DSS with different α/γ phase ratio is limited. The objective of this study is to elucidate the effect of α phase content on pitting corrosion behavior of the cast DSS.
2. Experimental procedures
2-1. Material
The chemical composition of the cast DSS is shown in Table 1. The DSS was annealed isothermally to control α phase content in the DSS. The specimen whose α phase content was 32 vol% (1100 °C for 48 hours, WQ), 43 vol% (1250 °C for 48 hours, WQ) and 50 vol% (1300°C for 48 hours, WQ) was prepared from 25%Cr-8.4%Ni-2%Mo DSS (TP No. "1B+Ni" in Table 1), and the the specimen whose α phase content was 50 vol% (1100 °C for 48 hours, WQ), 58 vol% (1150 °C for 48 hours, WQ), 70 vol% (1275 °C for 48 hours, WQ) and 77 vol% (1300°C for 48 hours, WQ) was prepared from 25%Cr-5.6%Ni-2%Mo DSS (TP No. 1B in Table 1). The α phase content of the prepared specimens were measured by an image analyser from the optical micrographs (magnification: x200).
2-2. Pitting potential measurement
The surface of the specimens was finished with 1 μm diamond paste. The pitting potential measurement was carried out in a concentrated artificial seawater (TDS: ca. 4.7%) at 80±2 °C under deaerated atmosphere by purging argon. Potentiostatic polarization treatment was performed at -0.65 V vs. an Ag/AgCl (Saturated KCl) electrode (SSE) prior to the test, then the corrosion potential of the specimen was measured for 10 minutes. Potentiodynamic anodic polarization was performed at a sweep rate of 20 mV/min. The potential when the current density reached 1 A/m2 was defined to be the pitting potential (V'c,100). Throughout this paper, the potential values are expressed in V vs. SSE.
2-3. Potentiostatic polarization measurement
The surface of the specimens was finished with 1 μm diamond paste. 25 wt% NaCl aqueous solution at pH= 0 was used as the test solution to simulate a severe environment in a pit. The solution temperature was 40±2 °C. The test was conducted under deaerated atmosphere by purging argon. Potentiostatic polarization treatment was performed at -0.65 V prior to the test, then the corrosion potential of the specimen was measured for 10 minutes. The potentiostatic polarization was conducted. The potentiostatic conditions were -0.30 V, -0.25 V, -0.20 V, and -0.15 V. After the corrosion test, surface observation by a scanning electron microscope (SEM) and the depth profile measurement in the corroded area by a contact probe profilometer were conducted.
3. Results
The pitting potential with different α phase content are listed in Table 2. The pitting potentials of the specimen prepared from 25%Cr-5.6%Ni-2%Mo DSS were approximately 0.17 ~ 0.18V and the pitting potentials of the specimen prepared from 25%Cr-8.4%Ni-2%Mo DSS were approximately 0.22 ~ 0.25V. The pitting potential of the cast DSS in the concentrated artificial seawater (TDS: ca. 4.7%) does not depend on the α phase content. The dependence of the preferentially dissolved phase on the holding potential, which determined by the SEM observation after the potentiostatic test, are also listed in Table 2. γ phase was preferentially dissolved at the higher holding potential and α phase was preferentially dissolved at the lower holding potential. With decreasing α phase content, γ phase was tendency to preferential dissolution at the lower holding potential. After the potentiostatic polarization test, SEM observation and the depth profile measurement in the corroded area were conducted to estimate the corrosion rate of each phase. The corrosion rate of the α phase tended to increase with increasing α phase content. In particular, as for the specimen in the α phase content of 58 vol% or more, the corrosion rate of α phase at higher potential range was significantly increased. The results of the SEM/EDS analysis in corroded area suggest that increase in corrosion rate of the α phase with increasing α phase content was caused by the Cr nitride formation in the α phase.

Figure 1