A New methodology of Nondestructive Testing for Quantitative Evaluation of Sigma Phase in Duplex Stainless Steels

Abstract

Duplex Stainless Steels (DSSs) are widely used in the oil and gas industry. When the steel is subjected to a thermal gradient, as in welding may occur precipitation of intermetallic phases. Among these phases, there is the sigma phase, which even in small quantities reduces drastically the mechanical and corrosion resistance properties. According to some studies in the literature, the limit amount of sigma phase present in steel in welded joints must be in a maximum volume fraction of 2,5%. In search to improve the detection sensitivity of the sigma phase, this paper presents results obtained by Linear Sweep Voltammetry (LSV) in Duplex Stainless Steel UNS S31803 as a Nondestructive Testing (NDT) model. The innovation in the application of this test is to use a microcell-based system that allows a reduced area of analysis obtaining density of currents in the order of microamperes and in the use of selective electrolytes to the elements of the oxides present in the intermetallic phases. With the use of these systems and through the optimization of control parameters it was possible to detect very low amounts of deleterious phases such as the sigma and chi phases.

Evaluation of voltametric profiles for quantitative estimation of sigma phase (σ) content in duplex stainless steel UNS S31803: an ohmic and layers porous resistance models (LPRM) analysis

“…The KOH concentration used for this analysis confirm the suitable condition (pH 13.97), as already described by (HASKEL et al, 2015;HASKEL et al, 2019;DE SAMPAIO et al, 2020), for a selective interaction with the intermetallic phase.…”

“…Electrolytic solutions like KOH, NaOH presents high sensibility for reaction with Cr in the σ and χ phases that is oxidized to higher states (Cr +3 to Cr +6 ). This selective behaviour is an advantage to be used for metallographic and potentiodynamic tests (HASKEL et al, 2019;DE SAMPAIO et al, 2020). From the results obtained by (HASKEL et al, 2019), electrochemical cells with micro electrode concepts with seems to be better for the analysis, providing current densities in the order of microamperes and an outstand sensitivity result for small levels of σ and χ deleterious phases quantification, this has also been proved by (DE SAMPAIO et al, 2020).…”

Sigma and Chi Phases Analysis in UNS S32750 Superduplex Stainless Steel by Optimized Linear Sweep Voltammetry in Alkaline Medium

“…Thus, the occurrence of these peaks were correlated with deleterious phases precipitated identified by LOM. Recently, Haskel et al, 61 tested several concentrations of KOH and scanning rates in UNS S31803 duplex stainless aged specimens. In this work, a sigma-free specimen was distinguished from another one aged at 870 °C, which containing 0.2% sigma, using a 3.0 mol l −1 KOH solution and a scan rate of 1.0 mV s −1 .…”

Sigma Phase Detection in Duplex Stainless Steel Based on Correlation to Current and Potential in Micro Electrode Electrochemical of Surface

“…Linear sweep voltammetry (LSV) is an effective technique used to detect the σ phase in DSS 18 . Haskel et al 19 used this technique to detect the χ and σ phases in DSS UNS S31803 presenting up to 0.9% intermetallic phases. Results in the aforementioned study which showed variations in charge density and current density, advocated for the feasibility of developing an on-site non-destructive technique assay.…”