Cyclic Oxidation and Hot Corrosion Behaviors of Gradient CoNiCrAlYSi Coatings Produced by HVOF and Diffusional Processes

Conventional and gradient CoNiCrAlYSi coatings were produced using high-velocity oxy-fuel (HVOF) and an additional step of diffusional over aluminizing (pack cementation) techniques on the Inconel-738 substrate. Hot corrosion and cyclic oxidation performance of the conventional and the gradient coatings were investigated by exposing samples to a molten salt of Na2SO4-20 % wt. NaVO3 at 880 A degrees C and 1-h exposure at 1100 A degrees C in air. Corrosion and cyclic oxidation rates were determined by measuring the weight gain at regular time intervals. X-ray diffraction, field emission scanning electron microscopy, and electron probe microanalysis techniques were used to characterize the coatings and the thermally grown oxide. Increase in the amount of beta aluminum-rich phase and the formation of high-chromium layer beneath the outer aluminum-rich layer and the low surface roughness of gradient coating increased the hot corrosion and oxidation resistance by a factor of 1.7. In addition, the gradient coating showed better rehealing of the thermally formed alumina scale due to its possession of more beta phase as a Al reservoir.