Development of an electrochemical reactivation test procedure for detecting microstructural heterogeneity in Ni-Cr-Mo-W alloy welds

Abstract

Alloy-22 (UNS N06022), a Ni-Cr-Mo-W based alloy shows excellent corrosion resistance in both oxidizing and reducing environments and is a candidate material for fabrication of outer wall of nuclear waste containers [1–3]. This alloy is considered, as readily weldable [3–6]. In solution-annealed condition, wrought Alloy22 reveals a single-phase austenite structure without any grain boundary secondary phase precipitates [1, 2]. Topologically close packed (TCP) phases such as μ, σ , and P, which are stable at higher temperatures, were observed to be retained at room temperature as terminal solidification constituents during solidification of weld metal [6]. As the chemical composition of these TCP phases is rich in Mo and W, presence of these phases could result in localized depletion of alloying elements in the matrix and make the alloy more susceptible to corrosive attack. Moreover, the weld microstructure revealed segregation of Mo and W at the interdendritic regions [3, 4]. Multipass welding and post weld heat treatment could increase the quantity of secondary phase precipitation. As the corrosion and toughness properties are affected by the presence of TCP phases and segregation of alloying elements [3], it is important to quantify these microstructural changes in comparison with that of solution annealed condition. A rapid nondestructive testing technique such as electrochemical potentiokinetic reactivation (EPR) test could be an appropriate tool for such microstructural evaluation, especially in-situ service conditions. EPR test has been successfully applied for quantifying the degree of sensitization in austenitic and duplex stainless steels [7, 8]. There is no standard EPR test procedure available for detecting intergranular corrosion susceptibility of NiCr-Mo alloys. Recently the present authors reported a test procedure to detect Cr depletion in thermally aged Alloy-22 wrought materials using an electrolyte of 1 M H2SO4 + 0.5 M NaCl + 0.01 M KSCN [9]. However, this test solution could not reactivate the weld metal, as the Cr profile was not affected by the solidification structure and the TCP phases [4]. In this short communication, development of a testing procedure, which could detect the presence of Mo and W depleted regions in Alloy-22 welds, is reported. 38 mm thick Alloy-22 (UNS N06022) plates (chemical composition 0.002% C, 0.14% Mn, 0.009% P, 0.001% S, 0.04% Si, 59.2% Ni, 20.62% Cr, 13.91% Mo, 0.01% Co, 0.01% Cu, 2.68% W, 2.8% Fe and