Titanium and aluminum alloys are widely used metals in the aerespace industry due to their high strength-to-weight ratio. Surface treatments of titanium and aluminum alloys play a key role in overcoming the limitation in their application, related to the moderate corrosion properies in aggressive environments, and can be also employed to proper functionalize the surface for particualr applications. Regarding this, the development of black coatings can be very useful. In fact the coating can not only increase wear and corrosion properties but can reduce refelctions and consequently increase absorption, making the substrates attractive for a lot of otpic applications in the aerospace field. Morehover the obtainment of black coatings can be also useful for aestethic reasons. Among the different surfcace treatments that can be perfomed on light alloys Plasma Electroltyic Oxidation (PEO) seems one of the most promising and more widely studied in the recent years. PEO is a process developed from tradtional anodic oxidation but that works at higher potentials and current densities. It is an environmentally friendly alternative to conventional anodizing since its electrolyte is generally less toxic and the obtained coatings are charactacteirzed by excellent corrosion resistance and good thermal and tribological properties. Different compounds, mainly oxides, can be found in the obtained coatings and the composition of the coatings is influenced not only by the composition of the substrate but also by the composition of the employed electrolyte, due to the fact that compounds from the electrolyte can be incorporated into the coatings.  

In the present study, a black PEO coating was produced on Grade 2 and Grade 5 titanium alloys and on 6061 and 7075 aluminum alloys using different types of compunds into the eletrolyte and working with different current modes (direct current or pulsed current). The surface morphology and the cross section of the samples were analyzed using scanning electron microscopy (SEM), and the phase composition was evaluated using X-ray diffraction (XRD). Black intensities were determined using ultraviolet–visible (UV-VIS) spectrometry, and electrochemical corrosion properties of the coatings were assessed by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests.

The UV observation evidenced very high absorbance values for all the samples whereas the corrosion properties of the samples resulted strongly linked to the current mode and to the composition of the eletrolytes. The differences in the corrosion properties where mainly linked to the microstrctural and morphological changes in the coating, with the different additives that create different porosity grades in the coatings surface.