Intergranular corrosion behaviour of AISI 316 stainless steel electron-beam clad on plain carbon steel

A previous study [1] carried out to characterize the electron-beam cladding and alloying of plain carbon steel containing about 0.4wt% C with AISI 316 stainless steel showed that a surface duplex structure can be obtained from an initially fully austenitic clad material. The interface between the overlayer material and the substrate was characterized by a steep concentration gradient of the stainless steel alloy elements when the electron-beam parameters had been adjusted to clad the surface. In this case the chemical composition of the surface layer as well as the electrochemical corrosion behaviour were found to be practically identical to that of the original AISI 316. It seemed of interest to study the microstructural changes after thermal treatments in the carbide precipitation range and to investigate their effect on the intergranular corrosion resistance of the clad layer. It is well known that thermal ageing of FeCrNi alloys in the temperature range 400-850 °C produces carbide precipitation at grain boundaries [2]. The chromium carbides (mainly Cr23C6) act as a sink for Cr and, as a consequence of the low diffusion coefficient of Cr in austenite, a zone with Cr content lower than the matrix content is created adjacent to the carbide. This Cr-impoverished zone is retained and is responsible for the intergranular corrosion of FeCrNi alloy by the theory of Cr depletion [3]. The generally superior resistance of duplex steels to intergranular corrosion has been known for over 50 years [4]. A series of investigations has demonstrated that the resistance of two-phase austenitic ferritic steels is a strong function of the alloy microstructure [5]. With these premises, the purpose of this letter is to describe the results of a preliminary study on the susceptibility to intergranular corrosion of the AISI 316 clad layer ageing treatments in the caribide precipitation range. The chemical composition of the AISI 316 sheets cladded on C40 carbon steel (0.4 wt % carbon) is shown in Table I. The processing conditions have been described elsewhere [6]. Two heating treatments were performed at 650 °C for 100 and 500 h in a vacuum furnace, followed by air-cooling. Microstructural observations were carried out after the thermal treatments and after the electrochemical tests by means of scanning electron microscopy. Energy-dispersive X-ray (EDX) microanalysis was performed on some precipitates. The susceptibility to intergranular corrosion was evaluated by means of the electrochemical potentiokinetic reactivation (EPR) test following the procedure reported in [7]. The microstructural observations on the clad layer thermally treated at 650 °C for 100 h revealed the presence of Cr-, Tiand Mo-rich carbides, precipitated intragranularly. Fig. 1, obtained after diamond polishing, shows the alpha-phase finely distributed and the discrete intragranular precipitates. Fig. 2a, obtained after nitric acid etching, shows the preferential etching around the intragranular precipitates, most of which have been detached as well as some slight attack on the ferritic phase. Fig. 2b shows some of the precipitates whose composition has been determined by EDX and is reported in Table II.