Abstract
The composition of a commercial duplex stainless steel was modified with boron additions (3.5, 4.5, and 5.5 wt.%) and processed by rapid-quenching techniques: Melt-spinning, copper-mold casting, and high-velocity oxygen fuel (HVOF). Spray deposition was also used to produce alloys as the process may induce rapid-solidified-like microstructures. These processing routes led to microstructures with distinguished corrosion resistance. Among the alloys with different boron contents, the 63.5Fe25Cr7Ni4.5B composition enabled the production of fully amorphous ribbons by melt-spinning. The cooling rate experienced during copper-mold casting, high-velocity oxygen fuel, and spray deposition did not ensure complete amorphization. The crystalline phases thereby formed were (Fe,Cr)2B and (Fe,Mo)3B2 borides in an austenitic-matrix with morphology and refinement dependent of the cooling rates. Fully amorphous 63.5Fe25Cr7Ni4.5B ribbons exhibited outstanding corrosion resistance in chloride-rich alkaline and acid media with negligible corrosion current densities of about 10−8 A/cm² and a broad passivation plateau. Although the specimens of the same composition produced by HVOF process and spray deposition exhibited lower corrosion resistance because of intrinsic porosity and crystalline phases, their corrosion behaviors were superior to those of AISI 1045 steel used as substrate with the advantage to be reinforced with hard borides known to be resistant against wear.
Highlights
Rapid solidification processed alloys have been attracting scientific and industrial attention over the last decades, and they have opened new possibilities in metallurgy fields regarding the design of microstructures with improved physical and mechanical properties [1]
The ultimate corrosion resistance of Cr-containing bulk amorphous steels (BAS) is superior to their crystalline counterparts
The nominal compositions were adjusted by adding pure chromium and nickel, Table 1, to ensure high corrosion resistance
Summary
Rapid solidification processed alloys have been attracting scientific and industrial attention over the last decades, and they have opened new possibilities in metallurgy fields regarding the design of microstructures with improved physical and mechanical properties [1]. Even though spray forming is not considered a rapid solidification technique, the microstructure evolution of materials processed by this route exhibits interesting features resulting from two completely different stages of solidification [18]: At the atomization (high cooling rates, ranging from 102 to 104 K/s) and after deposition (low cooling rates, ranging from 0.1 to 10 K/s). The presence of Cr as low as 4–6 at.% in amorphous steels enables the formation of a highly stable and resistant passivating film because of the fast and selective dissolution of Fe and consequent efficient enrichment of corrosion resistant elements on the surface [23] In this case, the ultimate corrosion resistance of Cr-containing BAS is superior to their crystalline counterparts. Results point out a strong corrosion resistance dependency regarding the resulted microstructure induced by processes with different cooling rates
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