The carbon migration between a ferritic steel and an austenitic steel was studied in submerged arcwelded 5Cr-0.5Mo/21Cr-12Ni dissimilar metal welds (DMWs) after aging at 500 ЊC for various times and after long-term service in technical practice. The distribution of carbon, chromium, nickel, and iron in the areas around the weld interface was determined by electron probe microanalysis, and the microstructural aspect in the carbon-depleted/enriched zone was characterized by optical microscopy and transmission electron microscopy (TEM). Furthermore, the precipitation sequences and composition characteristics of the carbides were identified by diffraction pattern microanalysis and energy-dispersive X-ray (EDX) microanalysis. It was found (1) that there exists a coherent relationship between intracrystalline M 23 C 6 and the austenitic matrix; (2) that the composition of M 23 C 6 in the carbon-enriched zone is independent of the duration of aging and service; (3) that the maximum carbon concentration is determined by the carbide type, the composition characteristic of precipitated carbides, and the concentration of carbide-forming Cr adjacent to the weld interface in the carbonenriched zone; and (4) that the carbon migration in the 5Cr-0.5Mo/21Cr-12Ni DMWs can be described by a diffusion model.

Co-Sn alloy coatings are electrodeposited from a choline chloride/ethylene glycol eutectic-based electrolyte, named Ethaline, containing CoCl2·6H2O and SnCl2·2H2O without any complexing agent or additive. Cyclic voltammetry shows that the co-electrodeposition of Sn and Co is feasible due to the adjacent reduction potentials of Sn(II) and Co(II) in the solution. X-ray diffraction reveals that the films mainly contain Co3Sn2 with expanded lattice parameters and pure Sn phases. Interestingly, a self-organization of layered structure with different Co content for each layer and mixed amorphous-nanocrystalline phases are formed in the electrodeposited film, which is revealed by cross-section transmission electron microscopy observations. The formation mechanism of the layered film is discussed based on the particular fluctuation of the current-time curve. The layered Co-Sn alloy coating with a total thickness of about 400 nm exhibits enhanced corrosion resistance showing a corrosion potential of −326 mV (vs. Ag/AgCl) and a corrosion current density of about 0.09 μA cm−2 in a 3.5 wt.% NaCl aqueous solution.

The complexation behavior and co-electrodeposition mechanism of Au-Sn alloy in a highly stable non-cyanide bath were studied by electrochemical analysis and quantum chemical calculation based on density functional theory (DFT). The interactions between metal ions and multiple complexing agents were revealed, and the mechanism on the high stability of the Au-Sn bath was clarified. The complexing agents potassium pyrophosphate (K 4 P 2 O 7 ) and ethylene diamine tetraacetic acid (EDTA) have a synergetic complexation to Sn ions, which exist in one valence state of Sn 2+ to form three complex ions, i.e., [Sn(P 2 O 7 )] 2− , [Sn(P 2 O 7 ) 2 ] 6− and Sn-EDTA; while the complexing agents of 5,5-dimethylhydantoin (DMH) and sodium sulfite (Na 2 SO 3 ) have a complexation to Au ions, which exist in two different valence states (i.e., Au 3+ and Au + ) to form two complex ions of [Au(DMH) 4 ] − and Au[(SO 3 ) 2 ] 3− . Moreover, the existence of [Au(DMH) 4 ] − inhibits the decomposition of [Au(SO 3 ) 2 ] 3− . Meanwhile, EDTA participates in some form of coordination with Au + , inhibites the decomposition of [Au(SO 3 ) 2 ] 3− and facilitates the reduction of [Au(SO 3 ) 2 ] 3− . The co-electrodeposition of Au-Sn alloy is a diffusion-controlled process, and the nucleation of Au-Sn alloy is a progressive nucleation process. The antioxidant catechol significantly increases the overpotential of Au-Sn co-electrodeposition and improves the brightness of electrodeposited films of Au-Sn alloys.

Au−30 at-Sn eutectic alloy was fabricated by sequentially pulse electroplating Au and Sn films on Si chips. Three kinds of Au/Sn/Au triple layer films were prepared in the present work: Au/Sn/Au (6/6/1 μm) films, Au/Sn/Au (6/6/6 μm) films and Au/Sn/Au (8/6/1 μm) films. The microstructure and phase transformation in Au/Sn/Au films during aging and reflow soldering were investigated. For Au/Sn/Au (6/6/1 μm) films during aging at 100 and 150°C, the layered AuSn/AuSn2/AuSn4 structure formed in the reaction region. Furthermore, the Sn film was completely consumed, and AuSn4 finally transformed into AuSn and AuSn2 after aging at 150°C for 15 h. For Au/Sn/Au (6/6/6 μm) films during aging at 150°C, the electroplating sequence had an important effect on the formation of Au−Sn phases. An Au5Sn layer was present at the Au II/Sn interface but not at the Au I/Sn interface. For Au/Sn/Au (8/6/1 μm) films, the micropores that formed preferentially along the Au5Sn/AuSn interface remarkably decreased with increasing reflow temperature from 280 to 310°C. After reflowing for 10 s, the microstructure was not an Au−Sn eutectic; however, after reflowing for 60 s, coarsened primary Au5Sn phase and typical Au−30 at-Sn eutectic microstructure of fine eutectic phases (AuSn+Au5Sn) formed.

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