Microstructure and Electrochemical Corrosion Behavior of Fe-Cr System Alloys as Substitutes for Ni-Based Brazing Filler Metal

doi:10.1007/s40195-016-0457-3

Abstract

Abstract:

：Two new Fe-Cr system alloys, Fe-20Cr-43Ni-10P (mass%) and Fe-20Cr-20Ni-8P-5Si-2Mo (mass%), have been developed as substitutes for the expensive Ni-based brazing filler metal used in brazing exhaust gas recirculation coolers. The microstructures and melting properties of the alloys were analyzed by electron probe X-ray microanalyzer and differential scanning calorimetry. The electrochemical characteristics of the alloys were investigated by potentiodynamic polarization testing in an electrolyte solution made in accordance with the standards of the Automobile Manufacturers Association of Germany. Furthermore, the corrosion behaviors of the alloys were investigated by constant-potential polarization testing and surface characterization. It is found that both alloys are composed of solid-solution phases and phosphide phases. The solid-solution phases serve as the anode, and the phosphide phases serve as the cathode in the corrosion reaction for both alloys. Fe-20Cr-43Ni-10P exhibits galvanic corrosion on the entire surface. In contrast, Fe-20Cr-20Ni-8P-5Si-2Mo is attacked at a few localized areas so that the cavities form on the surface. The corrosion potential (E_corr) is lower than that of Ni-29Cr-6P-4Si (mass%) for both alloys. This means that the Fe-Cr system alloys are more easily corroded than Ni-29Cr-6P-4Si. The corrosion rate and corrosion resistance cannot be investigated by the corrosion current density (i_corr) and polarization resistance (R_p), respectively, because of localized corrosion of the two alloys.

Key words: Iron-chromium system alloy, Microstructure, Electrochemical corrosion, Brazing

Kangdao Shi,Takahiro Tsunoda,Ikuo Shohji,Kotaro Matsu,Yasuhiro Taguchi. Microstructure and Electrochemical Corrosion Behavior of Fe-Cr System Alloys as Substitutes for Ni-Based Brazing Filler Metal[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(8): 697-706.

Figures/Tables 15

Fig. 1 Schematic diagram of the specimen

Fig. 2 DSC curves of brazing filler metals: a Fe-20Cr-43Ni-10P, b Fe-20Cr-20Ni-8P-5Si-2Mo

Fig. 3 Microstructures of cross sections of the as-cast brazing filler metals (back-scattered electron images): a Fe-20Cr-43Ni-10P, b Fe-20Cr-20Ni-8P-5Si-2Mo

Fig. 4 Potentiodynamic polarization curves of the specimens

Fig. 5 Surface microstructures of Fe-20Cr-43Ni-10P brazing filler metal after constant-potential polarization in potential of -0.11 V/SSE (back-scattered electron images): a polarized for 3 h, b polarized for 50 h, c polarized for 100 h

Fig. 6 Surface microstructures of Fe-20Cr-20Ni-8P-5Si-2Mo brazing filler metal after constant-potential polarization in potential of -0.17 V/SSE (back-scattered electron images), a polarized for 3 h, b polarized for 50 h, c polarized for 100 h

Fig. 7 Mapping analysis results of the surface for Fe-20Cr-43Ni-10P after constant-potential polarization in potential of -0.11 V/SSE for 100 h

Fig. 8 Mapping analysis results of surface for Fe-20Cr-20Ni-8P-5Si-2Mo after constant-potential polarization in potential of -0.17 V/SSE for 100 h

Fig. 9 Current density dependence on time during constant-potential tests (potential: -0.11 V/SSE for Fe-20Cr-43Ni-10P; potential: -0.17 V/SSE for Fe-20Cr-20Ni-8P-5Si-2Mo)

Fig. 10 Surface morphologies of Fe-20Cr-43Ni-10P after constant-potential polarization test in potential of -0.11 V/SSE for a 3 h, b 10 h, c 50 h, d 100 h (secondary electron images)

Fig. 11 Cross-sectional morphologies of Fe-20Cr-43Ni-10P after constant-potential polarization test in potential of -0.11 V/SSE for a 50 h, b 100 h (back-scattered electron images)

Fig. 12 Surface morphologies of Fe-20Cr-20Ni-8P-5Si-2Mo after constant-potential polarization test in potential of -0.17 V/SSE for a 3 h, b 10 h, c 50 h, d 100 h (secondary electron images)

Fig. 13 Cross-sectional morphologies of Fe-20Cr-20Ni-8P-5Si-2Mo after constant-potential polarization test in potential of -0.17 V/SSE for a 50 h, b 100 h (back-scattered electron images)

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