Formation of Super-Corrosion Resistant Bulk Amorphous Alloy Coatings by Thermal Spraying and Properties of Coatings Having Nano-Structure after Crystallization

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Alloy powders of Fe-10Cr-10Mo-2C-8P (alloy A), Fe-16Cr-30Mo-4C-2B (alloy B) and Fe-16Cr-30Mo-2C-1B-7P (alloy C) were made for thermal spraying. The coatings of these alloys were prepared on the mild steel substrate by atmospheric plasma spraying(APS) and high velocity flame spraying(HVOF) processes. Perfect amorphous coatings were obtained by the HVOF process from all alloys. Whereas, the perfect amorphous coatings were obtained by the APS process from only alloy B and C. The current density of the coating of alloy Awas close to that of the coatings of alloys B and C from the anodic polarization curves of the HVOF coatings in 1N HCl solution. Therefore, it was thought that the corrosion resistance of the coatings of all alloys was almost the same. The corrosion resistance of the APS coating of alloy A was the same as SUS316L plate from the immersion test in HCl solution. On the other hand, the corrosion resistance of the coatings of alloys B and C was excellent, because the corrosion rate of the APS coatings of alloys B and C was 1/100 of that of the alloy A. The tempered coatings of alloys A,B,and C were composed of ferrite, M_<23>C_6,M_7C_3 carbides and M_3P phosphide, ferrite, M_<23>(C,B)_6, M_7(C,B)_3 borocarbides and ferrite, M_<23>(C,B)_6,M_7(C,B)_3 borocabides and M_3P phosphide, respectively. It was recognized that the fine-grained structure of 10nm in diameter was formed in the coating of alloy B after crystallization from TEM observation. Also, the HVOF coating of alloy C held the nano-structure after tempered at 973K for 100h. The corrosion resistance of the tempered coatings was inferior to that of the as-sprayed coatings in resistant to hydrochloric acid solution. However, it was showed that the tempered coatings were superior to nickel-based self-fluxing alloy coating in resistant to hydrochloric acid solution.