Evaluation of selective corrosion in UNS S31803 duplex stainless steel with electrochemical impedance spectroscopy

Abstract

The precipitation of certain phases, in particular of sigma phase, in duplex stainless steels makes them susceptible to selective corrosion. In the present work, the effect of heat treatments upon selective corrosion of UNS S31803 duplex stainless steel was investigated in 2 M H₂SO₄ + 0.5 M NaCl + 0.01 M KSCN solution at 30 °C using electrochemical impedance spectroscopy (EIS). For this purpose, samples were solution annealed at 1050 °C for 30 min and, subsequently, heat-treated at 750 °C or 850 °C for times varying from 1 h to 30 h in order to precipitate the sigma phase and other phases. After these heat-treatments the specimens were immersed in the test solution and submitted to EIS tests. These tests, performed at the open circuit potential after 84 h of immersion in the test solution, showed higher impedances for samples tested at 850 °C than at 750 °C, implying lower selective corrosion rates for the former samples. Moreover, for samples tested for longer times at 850 °C, there are evidences of an increase of the high frequency phase angle, which seems to be associated with the recovery of Cr and Mo content in the depleted zone near the precipitated sigma phase. These results are in good agreement with those obtained in tests performed with the electrochemical potentiodynamic reactivation double loop method.

Introduction

Due to their microstructure, composed of approximately equal volumetric fractions of ferrite (α) and austenite (γ), duplex stainless steels display adequate mechanical properties and resistance to corrosion in an ample range of applications, thus justifying their increasing use in several fields such as chemical processing, electrical energy generation and others. Notwithstanding these advantages, in some applications involving welding and high temperature, these steels are susceptible to the precipitation of additional phases that affect both the corrosion and the mechanical properties.

Depending on temperature, different phases can precipitate, such as sigma (σ), chi (χ), nitrides (Cr₂N) and carbides (M₂₃C₆ and M₇C₃) [1], [2], [3]. In a duplex stainless steel such as UNS S31803, heat-treated at 850 °C for 5 h, the main components of sigma phase are Fe, Cr and Mo in approximate wt% amounts of 57, 31 and 7, respectively. The remaining 5 wt% is Ni, Mn and Si [4]. The chi phase, which is formed in lesser amounts, has also the same main components in approximate amounts of 56 wt% Fe, 17 wt% Cr and 27 wt% Mo [5]. During their precipitation these phases remove from their vicinity elements responsible for the corrosion resistance of the steel. In this way, regions depleted in these elements are formed, making the material susceptible to selective corrosion in a manner similar to sensitization of austenitic stainless steels, which renders them susceptible to intergranular corrosion. The latter is a peculiar type of selective corrosion, in which the chromium depleted regions form along the grain boundaries. In duplex stainless steels the regions susceptible to selective corrosion are chromium and molybdenum depleted and they usually form along the interfaces between the precipitated sigma phase and the austenite grains. There is no corrosion observed along the austenite grain boundaries [6].

The harmful effect of the sigma phase on the corrosion resistance of duplex stainless steels was reported in several works [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. This effect has been evaluated through different tests, such as general corrosion tests [9], pitting potential tests [9], [11], [12], [14], [16], crevice corrosion tests [14], intergranular corrosion tests [12], [15], [16], [17] and stress corrosion tests [12]. Moreover, this effect has been of great concern in welding of duplex stainless steels, because during this process the sigma phase is formed in the heat affected zones (HAZ) [10], [18], [19]. Leaks in pipelines fabricated with this type of steel were reported to occur in these zones [18].

According to Truman and Pirt [8] this effect is a consequence of chromium and molybdenum content decrease in the matrix phases, because the sigma phase is considerably richer in these elements than either the ferrite or austenite. Adhe et al. [16] proposed that this decrease is confined to regions adjacent to sigma phase. Wilms et al. [14], on the other hand, observed that the localized corrosion starts next to the sigma phase in the newly formed secondary austenite. Using mass balance Kobayashi and Wolynec [4] determined theoretically the composition of this new austenitic phase showing that it is poor in Cr and is Mo free, which explains its severe selective corrosion.

More recently the susceptibility to selective corrosion of duplex stainless steels has been evaluated with the double loop electrochemical potentiokinetic reactivation (DL-EPR) technique [17], [20], [21], [22], [23]. In this technique, first developed for austenitic stainless steels, the testing solution consists of 0.5 M H₂SO₄ + 0.01 M KSCN at 30 °C. The role of KSCN is to help to break the passive film during the reactivation cycle of the test. For duplex stainless steels, this solution showed itself inadequate. Attempts were made to run the test at 70 °C [20] or to increase the KSCN content to 0.5 M [22]. Lopez et al. [17] proposed to add NaCl to the solution and to increase the amount of sulfuric acid. In this way, they increased the ability of the solution to break the passive film during the reactivation cycle of the test, since it is well known that the chlorides are highly efficient in doing this. Chaves and Wolynec [23] conducted comparative DL-EPR tests in solutions proposed by Garz et al. [20], Lopez et al. [17] and a third solution similar to the latter, in which the KSCN was replaced by 0.01 M thiocetamide (C₂H₅NS). They found that the testing solution proposed by Lopez et al. [17] is the most efficient for detecting this susceptibility. Using their solution, which consists of 2 M H₂SO₄ + 0.5 M NaCl + 0.01 M KSCN at 30 °C, they [24] investigated the selective corrosion kinetics of UNS S31803 duplex stainless steel.

To obtain further knowledge about this selective corrosion, it was decided to perform also electrochemical impedance spectroscopy (EIS) tests in the above solution and to check the possibility of these EIS tests being used to assess the susceptibility of duplex stainless steels to selective corrosion. The present work reports the results obtained by this technique and compares them with those obtained with the DL-EPR tests.