Layer characteristics and corrosion performance of nitrocarburized and post-oxidized API 5L Grade B steel

Hydrocarbon transmission pipelines are critical infrastructure systems in which corrosion-related degradation can lead to severe environmental, economic, and safety consequences. Improving the corrosion resistance of pipeline steels through surface engineering can reduce failures. In this study, API 5L Grade B steels were nitrocarburized using a salt bath followed by a post-oxidation process. Treated steels were characterized by optical and scanning electron microscopes (SEM–EDS), X-ray diffraction (XRD), microhardness, statistical layer-thickness analysis, and electrochemical techniques (Tafel and EIS). These treatments produced a layer (diffusion region) composed of iron nitrides and sealed by magnetite, increasing surface hardness and thickness as a function of temperature and time. Statistical analysis revealed that layer formation shows a stochastic behavior. Meanwhile, electrochemical results demonstrated a significant improvement in corrosion performance under optimally treated conditions; conversely, surface degradation and reduced protective effectiveness were observed in overtreated samples. These results highlight the importance of optimizing processing parameters to enhance the pipeline's reliability.