Characterization of 475°C Embrittlement of Duplex Stainless Steel Microstructure via Scanning Kelvin Probe Force Microscopy and Magnetic Force Microscopy

Abstract

Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure of 22Cr-5Ni duplex stainless steel have been correlated to microstructure development with aging treatments at 475 • C. Magnetic force microscopy (MFM) was employed to differentiate crystallographic phases to provide complementary information. The absolute Volta potentials of both ferrite and austenite increased after 5 hours of aging, indicating electrochemical ennoblement of the entire microstructure. Longer aging resulted in a gradual decrease of measured Volta potentials in both phases. The microstructure showed after 255 hours aging up to 2.5-times larger potential differences than in the as-received condition, indicating impaired electrochemical nobility. In the as-received microstructure, the ferrite phase was less noble than the austenite, whereas after 5 hours aging both phases had similar, balanced Volta potentials which indicated a balanced nobility of ferrite and austenite. Longer aging treatment caused severe loss of nobility for the entire microstructure, with ferrite showing larger changes in Volta potential than the austenite. Spinodal microstructure decomposition and associated phase reactions of the ferrite, with elemental redistribution in the austenite, are the reason for the observed changes in microstructure nobility. Duplex Stainless Steels (DSSs) have been used for structural and functional applications in a broad range of industry sectors.1 Their two-phase microstructure typically consists of a balanced fraction of ferrite (δ) and austenite (γ) resulting in beneficial properties, such as high strength and toughness, with outstanding corrosion and stress corrosion cracking resistance. However, when exposed to higher temperature regimes DSSs suffer from microstructure changes due to phase transformations arising from strong interactions of alloying elements.2-4 These microstructure changes can be accompanied by the occurrence of embrittlement and loss in corrosion resistance, which restricts their use in high-temperature applications.When the material is exposed to temperatures between 600 • C and 1000• C, the microstructure can decompose into several secondary phases.5-7 Ferrite can undergo eutectoid transformations into several phases, including σ-phase with secondary austenite (γ 2 ), χ-phase, carbides (mainly M 23 C 6 ), nitrides (CrN and Cr 2 N), or carbonitrides. 1,5,[7][8][9] Chromium nitrides can cause significant changes in microstructure performance 10,11 and hardness, 12 despite their small volume fractions and sizes (usually below 2 μm).12,13 Precipitation is typically accompanied by significant reductions in localized corrosion resistance, and is mainly attributed to the formation of element-depleted regions adjacent to secondary phases. [5][6][7] Exposure to lower temperatures between 250• C to 550• C, often referred to "475• C embrittlement", also results in variations of mechanical and electrochemical properties.14-28 This has been attributed to phase separation mechanisms, prim...

Time-temperature-precipitation and property diagrams for super duplex stainless steel weld metals

“…Nilsson et al [34] heat-treated 22Cr-3Mo-8Ni weld metal and reported that spinodal decomposition of ferrite caused embrittlement accompanied by a hardness increase at the same temperature. Örnek et al [35] used Volta potential testing and verified that ferrite decomposition reduced the Volta potential difference between ferrite and austenite in type 2205 DSS resulting in a similar etching behavior of the two phases. Therefore, the etching response and hardness increase together are strong indications of ferrite decomposition, so-called 475°C embrittlement, occurring in the weld metal in the 10 min sample.…”

Volta Potential Evolution of Intermetallics in Aluminum Alloy Microstructure Under Thin Aqueous Adlayers: A combined DFT and Experimental Study

“…The magnitude of the Volta potential difference between dissimilar microstructural constituents is a quantitative measure for microgalvanic corrosion [35,36,47,49,50,52,[57][58][59][60][61][62]. Therefore, respectively.…”

“…In a galvanic couple, the phase with higher electrochemical nobility has typically higher Volta potentials and usually acts as the cathode, whereas that with lower Volta potentials acts as the anode [47,48]. For a detailed interpretation of the polarity the reader is referred to previous works [47,49,52].…”

“…Potential line profiles were sectioned from selected sites and plotted in potential vs. distance charts, from which the Volta potential difference between the IMP and the alloy matrix was obtained. Interpretation of SKPFM data was done in accordance to previous works [49][50][51][52].…”

Time-dependent in situ measurement of atmospheric corrosion rates of duplex stainless steel wires

“…38 Selective dissolution of the ferrite phase may be more likely in DSS due to the lower corrosion potential of the ferrite than that of the austenite in chloride-containing environments. 36,[39][40][41][42][43] However, selective attack of the austenite may also be possible if the oxidising power of the electrolyte is sufficiently high, where the ferrite remains passive. [39][40][41][42] The extent of surface corrosion and the dissolved volume slightly increased with exposure time, as shown in Fig.…”