Characterization of black coating on Fe360 steel obtained with immersion in aqueous solutions

Abstract

Blackening process of steel is widely used in many industrial applications to create a protective layer against corrosion, environmental attack and to improve the aesthetic features. This study is focused on obtaining a black coating on a low carbon steel surface employing three different solution. In particular, two solution based on sodium nitrate and sodium hydroxide were used at boiling point, while the third based on selenous acid at ambient temperature. The samples were immersed for 5, 15 and 30 min in the boiling solutions, while for cold solution it was chosen 1 min of immersion time. The morphology of the protective layer were studied with stereo-microscope in order to evaluate the color, and Scanning Electron Microscope (SEM) to identify any defects in the surface, X-ray diffraction (XRD) was used to evaluate the composition of the protective layer. The corrosion resistance was measured performing potentiodynamic polarization tests and electrochemical impedance spectroscopy tests. The layer obtained with 15 min immersion time for the two boiling solutions is dense and adherent as shown by SEM analysis. For the other immersion times the layer was not uniform with porosities. Cold solution lead to a not homogenous layer reducing corrosion resistance.