The corrosion behavior and microstructure of high-velocity oxy-fuel sprayed nickel-base amorphous/nanocrystalline coatings

The corrosion characteristics of two Ni-Cr-Mo-B alloy powders sprayed by th e high-velocity oxy-fuel (HVOF) process have been studied using potentiodyn amic and potentiostatic corrosion analysis in 0.5 M H2SO4. The deposits wer e also microstructurally characterized using x-ray diffraction (XRD), scann ing electron microscopy (SEM) (utilizing both secondary electron and backsc attered electron modes), and transmission electron microscopy (TEM), Result s from the microstructural examination of the two alloys have revealed a pr edominantly amorphous/nanocrystalline face centered cubic (fcc) matrix cont aining submicron boride precipitates as well as regions of martensitically transformed laths. Apparent recrystallization of the amorphous matrix has also been observed i n the form of cellular crystals with a fee structure. The oxide stringers o bserved at splat boundaries were found to be columnar grained alpha-Cr2O3, though regions of the spinel oxide NiCr2O4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistan ce to corrosion in 0.5 M H2SO4, as revealed by potentiodynamic tests. They both had rest potentials approximately equal to -300 mV saturated calomel e lectrode (SCE) and passive region current densities of similar to 1 mA/cm(2 ). Microstructural examination of samples tested potentiostatically reveale d the prevalence of degradation at splat boundaries, especially those where significant oxidation of the deposit occurred.