Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities

Black coatings were successfully formed on Grade 2 (G2) and Grade 5 (G5) titanium alloy by means of a direct-current Plasma Electrolytic Oxidation (PEO) process at a very low current density of 0.05 A/cm2. The impact of two different treatment times (30 min and 60 min) was examined. The electrolyte for the PEO process was a phosphate base solution Na5P3O10 containing FeSO4 and (NH4)6Mo7O24 as coloring additives. PEO-coated samples were subjected to optical, morphological, structural, chemical, and electrochemical characterization. XRD, EDS, and XPS data analyses revealed that anion MoO42- and metal cation Fe3+ were successfully incorporated into the coatings. The results demonstrated that PEO-coated samples prepared after 60 min exhibit a stronger black color than those created after 30 min, with an absorptance maximum of 0.86. Furthermore, all prepared PEO coatings improve the corrosion resistance of bare titanium. Among them, the 60-minute PEO coatings on both alloys were the ones with the best corrosion properties.