Fabrication and corrosion resistance of super-thick compound layer by austenitic gas oxynitrocarburizing

In order to improve the corrosion resistance of the Q235 low carbon steel, the austenitic gas oxynitrocarburizing was applied to produce a super-thick compound layer. The microstructure and phase composition of oxynitrocarburized layers was characterized by using optical microscope, X-ray diffraction, scanning electron microscopy equipped with energy dispersive X-ray analyzer and transmission electron microscopy, respectively. The corrosion resistance was measured systematically by electrochemical impedance spectroscopy, potentiodynamic polarization and neutral salt spray tests. The results show that a thick compound layer more than 30 μm could be produced on the surface of the Q235 steel by austenitic gas oxynitrocarburizing in a short time. The phase composition of the compound layer mainly consisted of the ε-Fe3N1-x or γ′-Fe4N phases, which depends on the cooling method after oxynitrocarburizing. The austenitic gas oxynitrocarburizing could dramatically improve the corrosion resistance of the Q235 steel which was even superior to that of stainless steels in breakdown potential. The corrosion resistance of the oxynitrocarburizied layer depended on its compactness, thickness, nitrogen content and the oxide film covered on the surface. Among these influence factors, the nitrogen content and compactness were the dominated factors. The fast growth of compound layer could be attributed to the fine microstructure and phase transitions. The austenitic gas oxynitrocarburizing offered a potential way to expand the application of low carbon steels in the aggressive corrosion environment.