The electrochemical behavior of commercially pure titanium (CP Ti) and Ti-6Al-4V (Grade 5) alloy in phosphate buffered saline solution (PBS, pH = 7.4) at 37 °C (i.e., in simulated physiological solution in the human body) was examined using open circuit potential measurements, linear and potentiodynamic polarization and electrochemical impedance spectroscopy methods. After the impedance measurements and after potentiodynamic polarization measurements, the surface of the samples was investigated by scanning electron microscopy, while the elemental composition of oxide film on the surface of each sample was determined by EDS analysis. The electrochemical and corrosion behavior of CP Ti and Ti-6Al-4V alloys is due to forming a two-layer model of surface oxide film, consisting of a thin barrier-type inner layer and a porous outer layer. The inner barrier layer mainly prevents corrosion of CP Ti and Ti-6Al-4V alloy, whose thickness and resistance increase sharply in the first few days of exposure to PBS solution. With longer exposure times to the PBS solution, the structure of the barrier layer subsequently settles, and its resistance increases further. Compared to Ti-6Al-4V alloy, CP Ti shows greater corrosion stability.

Anodic and spontaneous corrosion of different types of stainless steel (AISI 304L, AISI 316L and 2205 DSS) in phosphate-buffered saline solution (PBS, pH = 7.4) at 37 °C (i.e., in simulated physiological solution in the human body) were examined using open circuit potential measurements, linear and cyclic polarization, and electrochemical impedance spectroscopy methods. After the anodic and spontaneous corrosion, the surface of the tested samples was investigated by light and scanning electron microscopy (SEM) with EDS analysis. It has been established that the tendency of the examined steel materials towards local corrosion decreases in the order: AISI 304L < AISI 316L < 2205 DSS. Namely, the possibility of repassivation and the resistance to local corrosion increases in the same order. The corrosion resistance of steel samples at open circuit potential is a consequence of forming a natural oxide film with a bi-layer structure and consists of an inner barrier and an outer porous film. The inner barrier film has a small thickness and extremely high resistance, while the outer porous film is much thicker but also has significantly lower resistance. The inner barrier layer mainly prevents corrosion of examined steel samples in order: AISI 304L < AISI 316L < 2205 DSS. Light microscopy and SEM/EDS analysis after pitting and spontaneous corrosion showed damage on the AISI 304L and AISI 316L surface, while the surface of 2205 DSS was almost undamaged by corrosion.

The effect of temperature (from 288 to 308 K) and concentration of sulfide ions (up to 40 ppm) on the corrosion behavior of AISI 304L and AISI 316L stainless steels in seawater was studied with measurements of open-circuit potential, linear and potentiodynamic polarization, and electrochemical impedance spectroscopy. An increase in temperature and pollutant concentration negatively affects the corrosion stability of stainless steels at the open circuit (the resistance, compactness, and thickness of the surface layer decrease and the corrosion current increases), in the passive region (the passivation current increases, the depassivation potential decreases, and the passive potential region narrows), and in the transpassive potential region (the rate of metal dissolution increases). The occurrence of pitting corrosion on the surface of the samples was confirmed with optical microscopy and a non-contact 3D profilometer. A few large pits (depth 80–100 μm and width 100 μm) were formed on the surface of AISI 304L steel, while several smaller pits (depth 40–50 μm and width 50 μm) were formed on the surface of AISI 316L steel. With increasing temperature and sulfide ion concentration, the width, depth, and density of the pits increased on both steel samples. In the studied temperature and concentration range of sulfide ions, the AISI 316L steels exhibited higher corrosion resistance. Overall, the influence of sulfide ions on steel corrosion was more pronounced than the influence of temperature.

Ti and Ti6Al4V alloy are extensively utilized in structural parts in engineering applications and the production of medical implants due to their excellent mechanical properties, lightweight, and high corrosion resistance. This study comprehensively evaluates their corrosion behavior in three challenging aquatic environments: brackish water, seawater, and seawater bittern. Utilizing open circuit potential (E_OC) measurements with polarization techniques (linear and potentiodynamic) and electrochemical impedance spectroscopy (EIS) measurements, the research highlights distinct environmental influences on corrosion performance. Notably, Ti and Ti6Al4V alloy demonstrated exceptional stability with the highest polarization resistance and lowest corrosion current in brackish water, while seawater bittern presented the most demanding condition for Ti6Al4V. Additionally, the analysis of the electrode surfaces after polarization measurements using optical microscopy, optical profilometry, and SEM/EDS tests revealed minor damage, indicating the high corrosion resistance of these materials. This study advances the understanding of Ti and Ti6Al4V alloy performance in diverse environments and offers valuable insights for optimizing their use in harsh aquatic conditions, particularly for applications requiring durability and longevity.