Corrosion of high-velocity-oxygen-fuel (HVOF) sprayed non-crystalline alloy coating in marine environment

Amorphous alloys owing to their high hardness, strength, corrosion and wear resistance are regarded as one of the potential coating materials for applications in severely corrosive environment. A high-velocity-oxygen-fuel (HVOF) spraying process was employed in this study to prepare an amorphous Fe alloy coating on SUS316 substrate. The prepared Fe-40-Cr-19-Mo-18-C-15-B-8 alloys had high hardness but the micro-structural examination showed micro-voids and micro-cracks permeated in the coating. Both x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements confirmed the combined amorphous and nano-crystalline phases in the coating. Thermal stress after cooling from the spray process caused the emergence of intensive cracking. The potentio-dynamic polarization curves of the prepared coating were measured in four different solutions, respectively: 0.5 M H2SO4, 1 M NaOH, 3.5 wt% NaCl, and seawater. Among them, the prepared coating had better corrosion resistance in neutral solutions. The corrosion tests under different applied over-potentials in seawater further revealed nearly ten-fold increase in corrosion current when the over-potential was raised from -100 mV(SCE), +200 mV(SCE), +600 mV(SCE), to +800 mV(SCE), respectively. Although the coating surface roughened considerably at higher over-potentials, oxidation of Mo and Cr elements in this Fe alloy during corrosion test, confirmed from the measured six-valenced ions of Mo6+ and Cr6+ in the x-ray photoelectron spectroscopy (XPS) analysis, was beneficial in lowering the corrosion current.

Automated summary

Amorphous Fe alloy coating was prepared on SUS316 substrate using HVOF spraying process, showing high hardness but micro-voids and micro-cracks. XRD and TEM confirmed the presence of amorphous and nano-crystalline phases in the coating. Corrosion tests revealed better resistance in neutral solutions and significant increase in corrosion current in seawater with higher over-potentials, attributed to oxidation of Mo and Cr elements in the Fe alloy.