Plasma-assisted surface engineering of API 5L X70 steel via cathodic cylinder deposition of Al2O3-ZrO2 coatings and plasma nitriding

Hydrogen transport through metallic pipelines challenges material structural integrity, as exposure to hydrogen-containing environments can promote corrosion and embrittlement. In this context, this work investigated the effects of cathode cylinder plasma deposition (CCyPD) and plasma nitriding (PN) on the mechanical and electrochemical properties of API 5L X70 steel. We deposited Al 2 O 3 –ZrO 2 coatings by CCyPD using a compacted target of 95 wt% Al 2 O 3 and 5 wt% ZrO 2 , with samples held at cathodic and anodic potentials; we nitrided samples at 673 and 773 K. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FEG-SEM), Vickers microhardness, and potentiodynamic polarization. XRD confirmed the formation of Al 2 O 3 and ZrO 2 phases in the CCyPD coatings, as well as Fe 3 N and Fe 4 N nitrides in the nitrided samples. FEG-SEM analyses revealed continuous layers with good apparent adhesion in both treatments. Microhardness increased from 173 HV to 643 HV after nitriding at 773 K, while the coatings obtained presented values close to 280 HV. In the electrochemical tests, the XN5 condition exhibited the most positive corrosion potential (Ecorr = −0.155 ± 0.040 V), while the Al 2 O 3 -ZrO 2 coatings presented the lowest corrosion current density, suggesting a barrier effect provided by the ceramic coating. The results indicate that plasma nitriding promotes greater gains in surface hardness. At the same time, the coatings obtained by CCyPD exhibit better electrochemical performance under the investigated conditions, supporting further investigation of their behavior in hydrogen-containing environments.