Corrosion resistance of investment casting samples of CF8 stainless steel in different passivation conditions

Abstract

This study evaluated the corrosion performance of specimens of investment casting CF8 stainless steel, in different passivation conditions. The samples of a metallurgical industry were heat treated, blasted with steel grit, and surface treated with an aqueous solution used in the passivation process, containing 22% nitric acid and 4% hydrofluoric acid in two different situations: immediately after reset (new solution) and after use (depleted solution). Also used was a solution with a proposed passivation concentration of 30% nitric acid. The performance of the samples was evaluated through accelerated corrosion testing with salt spray, potentiostatic polarization curves, and the morphology of the passive layer was analyzed by scanning electron microscopy (SEM). In the images we have observed the lamellar morphology of the passive oxide film obtained on the samples that have undergone treatment. The specimen treated by the proposed process, obtained a more compact passive layer with a more uniform appearance and showed the best result on exposure to salt spray, where the first change from the initial condition occurred after 192 h of exposure, while all other test specimens showed changes after 48 h of exposure. Also the electrochemical tests showed a passivation zone with lower current density and higher passivation potential. Thus, the best performance was presented by the samples that passed through the pickling and passivation treatment with the proposed solution.