Titanium and its alloys have superior electrochemical properties compared to other alloy systems due to the formation of a protective TiO₂ film on metal surfaces. The ability to generate the protective oxide layer will depend upon the type of alloy to be used. The aim of this work was to characterize the electrochemical corrosion behavior of titanium Ti-CP2 and alloys Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-4V, and Ti Beta-C. Samples were anodized in 1 M H₂SO₄ and H₃PO₄ solutions with a current density of 0.025 A/cm². Electrochemical tests on anodized alloys were carried out using a three-electrode cell and exposed in two electrolytes, i.e., 3.5 wt % NaCl and 3.5 wt % H₂SO₄ solutions at room temperature. Scanning electron microscopy (SEM) was used to observe the morphology of anodized surfaces. The electrochemical techniques used were cyclic potentiodynamic polarization (CPP) and electrochemical noise (EN), based on the ASTM-G61 and G199 standards. Regarding EN, two methods of data analysis were used: the frequency domain (power spectral density, PSD) and time-frequency domain (discrete wavelet transform). For non-anodized alloys, the results by CCP and EN indicate icorr values of ×10⁻⁶ A/cm². However, under anodizing conditions, the icorr values vary from ×10⁻⁷ to ×10⁻⁹ A/cm². The PSD Ψ⁰ values are higher for non-anodized alloys, while in anodized conditions, the values range from −138/−122 dBi (A²·Hz⁻¹)^1/2 to −131/−180 dBi (A²·Hz⁻¹)^1/2. Furthermore, the results indicated that the alloys anodized in the H₃PO₄ bath showed an electrochemical behavior that can be associated with a more homogeneous passive layer when exposed to the 3.5 wt % NaCl electrolyte. Alloys containing more beta-phase stabilizers formed a less homogeneous anodized layer. These alloys are widely used in aeronautical applications; thus, it is essential that these alloys have excellent corrosion performance in chloride and acid rain environments.

AlCrN/TiSi, AlCrN/TiCrSiN and AlCrN/AlCrN + CrN coatings were deposited on Inconel 718 alloy by physical vapour deposition (PVD). The corrosion behaviour of uncoated and coated specimens was evaluated using electrochemical impedance spectroscopy (EIS) at open circuit potential in a 3.5 wt.% NaCl and 2 wt.% H₂SO₄ solutions. The EIS data acquired were curve fitted and analysed by equivalent circuit models to calculate the pore resistance, the charge transfer resistance and the capacitance. The Nyquist diagrams of all systems showed one part of the semicircle which could relate that reaction is a one step process, except for the AlCrN/TiCrSiN and AlCrN/AlCrN + CrN coatings in H₂SO₄ solution, for which two semicircles related to active corrosion in substrate alloy were found. However, from the Bode plots, it was possible to identify two the time constants for all systems exposed to NaCl and H₂SO₄ solutions. According to electrochemical results, the corrosion resistance of the AlCrN/TiSiN coating was better in the NaCl solution, whereas the AlCrN/AlCrN + CrN coating show better performance in the Sulphuric Acid solutions.

Increasingly stringent environmental regulations in different sectors of industry, especially the aeronautical sector, suggest the need for more investigations regarding the effect of environmentally friendly corrosion protective processes. Passivation is a finishing process that makes stainless steels more rust resistant, removing free iron from the steel surface resulting from machining operations. This results in the formation of a protective oxide layer that is less likely to react with the environment and cause corrosion. The most commonly used passivating agent is nitric acid. However, it is know that high levels of toxicity can be generated by using this agent. In this work, a study has been carried out into the electrochemical behavior of 15-5PH (precipitation hardening) and 17-4PH stainless steels passivated with (a) citric and (b) nitric acid solutions for 60 and 90 min at 49 °C, and subsequently exposed to an environment with chlorides. Two electrochemical techniques were used: electrochemical noise (EN) and potentiodynamic polarization curves (PPC) according to ASTM G199-09 and ASTM G5-13, respectively. The results obtained indicated that, for both types of steel, the passive layer formed in citric acid as passivating solution had very similar characteristics to that formed with nitric acid. Furthermore, after exposure to the chloride-containing solution and according with the localization index (LI) values obtained, the stainless steels passivated in citric acid showed a mixed type of corrosion, whereas the steels passivated in nitric acid showed localized corrosion. Overall, the results of the $R_n$ values derived show very low and similar corrosion rates for the stainless steels passivated with both citric and nitric acid solutions.

Advanced high-strength steels (AHSS), are commonly used in the manufacture of car bodies, as well as in front and rear rails, and safety posts. These components can be exposed to corrosive environments for instance, in countries where de-icing salts are used. In this work, the corrosion behavior of four AHSS steels with dual-phase [ferrite-martensite (DP) and ferrite-bainite (FB)] steels were studied by means of electrochemical noise (EN) measurements according to the ASTM G199-09 standard in NaCl, CaCl₂ and MgCl₂ aqueous solutions at room temperature. The direct current (DC) trend data from EN were removed by a polynomial method of statistical and spectral analysis. According to the noise resistance (R_n) values obtained for the DP and FB dual-phase steels, both the martensite/bainite content and morphology of the phase constituents have an important effect on the corrosion behavior of these steels. The L.I. (localization index) (0.00054 to 0.15431), skewness (−6.18 to 7.35) and kurtosis (high values 37.15, 74.84 and 106.52) were calculated. In general, the results indicated that the main corrosion process is related to uniform corrosion. Corrosion behavior of AHSS steels exposed in NaCl solution could be related to the morphology of the phase constituents exposed in NaCl, CaCl₂ and MgCl₂ solutions.

Aluminum is a material widely used in aeronautical and transport industries due to its excellent mechanical and corrosion resistance properties. Unfortunately, aluminum alloys are susceptible to corrosion, which limits their use in some corrosive environments. The aim of this work is to characterize hard coat film fabricated by anodizing in a citric–sulfuric acid system using electrochemical techniques. The anodization process was carried out using an aluminum alloy AA 6061 anodization bath: a mix of citric and sulfuric acid solutions were used. For the anodizing process, two current densities were used, 1 and 7.2 A·cm⁻². Anodized specimens obtained under different conditions were exposed to a 3.5 wt.% NaCl solution, and their electrochemical behavior was studied by electrochemical impedance spectroscopy (EIS) and cyclic potentiodynamic polarization (CPP) according to ASTM G106-15 and ASTM G5-13, respectively. Scanning electron microscopy (SEM) was employed to determinate the morphology and thickness of coatings. The results showed improved corrosion resistance in 6061 aluminum anodized in citric–sulfuric acid electrolyte compared to those anodized in sulfuric acid solution.

Custom 450 stainless steel and AM 350 stainless steel are both precipitation hardening stainless steels, which are widely used in a variety of aerospace applications. The former steel exhibits very good corrosion resistance with moderate strength, whereas the latter is used for applications requiring high strength along with corrosion resistance. In this study, the corrosion behavior of CUSTOM 450 and AM 350 stainless steels passivated in (a) citric acid and (b) nitric acid solutions for 50 and 75 min at 49 and 70 °C, and subsequently exposed in 5 wt. % NaCl and 1 wt. % H₂SO₄ solutions are investigated. Two electrochemical techniques were used: electrochemical noise (EN) and electrochemical impedance spectroscopy (EIS) according to ASTM G199-09 and ASTM G106-13, respectively. The results indicated that passivation in nitric acid made the surface prone to localized corrosion. Statistical and PSD values showed a tendency toward pitting corrosion. On the whole, passivated CUSTOM 450 stainless steel showed the best corrosion behavior in both, NaCl and H₂SO₄ test solutions.