Precipitation-hardening (PH) stainless steels (SS) are widely used in various aerospace applications. These steels exhibit good mechanical and corrosion resistance. The electrochemical behavior of 15-5PH, 17-4PH, Custom450 and AM 350 stainless steels passivated with citric and nitric acid baths for 60 and 90 min at 25 and 49 °C were evaluated in 5 wt.% sodium chloride (NaCl) and 1 wt.% sulfuric acid (H₂SO₄) solutions. The electrochemical behavior was studied with potentiodynamic polarization curves (PPC) according to the ASTM G5-13 standard. The results indicated that there are two characteristic mechanisms that are present in the potentiodynamic polarization curves. When the PHSS is immersed in an H₂SO₄ solution, there is a secondary passivation, and in the NaCl solution, there is a pseudo-passivation (not stable passivation film). The current densities in the NaCl solution were between 10⁻⁴ and 10⁻⁵ mA/cm², while those of H₂SO₄ were recorded around 10⁻² and 10⁻³ mA/cm². Citric acid does work as a passivating solution, and in some cases, the corrosion resistance of the stainless steel was comparable to that of nitric acid.

Precipitation-hardening stainless steels, like AM 350 and Custom 450, are extensively utilized in various aerospace applications. The latter steel is utilized for applications needing great strength and corrosion resistance. In contrast, the former steel has a good corrosion resistance and moderate strength. The purpose of this study was to analyze transient frequencies in the electrochemical noise of Custom 450 and AM 350 stainless steels that had been passivated for 60 and 90 min at 25 and 49 °C using baths of citric and nitric acid and then immersed in solutions containing 1% sulfuric acid (H₂SO₄) and 5% sodium chloride (NaCl). The potentiodynamic polychromatic curves employed electrochemical techniques and noise (EN) based on the ASTM-G5 and G199 standards. Two methods of data analysis were applied concerning EN: the domain of frequencies (power spectral density, PSD) and the time–frequency domain (Hilbert-Huang Transform). The PHSS passivated in citric acid indicated current densities in the H₂SO₄ solution between 10⁻² and 10⁻³ mA/cm², while those in the NaCl solution were recorded around 10⁻⁴ and 10⁻⁵ mA/cm². The citric acid functions as a passivating agent. The results of the electrochemical noise analysis show that the PHSS passivated in nitric acid displayed a greater corrosion resistance. Moreover, there is a tendency for PHSS to be passivated in nitric acid to corrode locally.

Precipitation-hardenable stainless steels (PHSSs) are widely used in various applications in the aerospace industry. PHSSs are used in various parts that need to combine mechanical properties with corrosion resistance when aircrafts are in corrosive environments. This study aimed to analyze the corrosion kinetics of CUSTOM 450 and AM 350 stainless steels that were passivated in acid baths for a period of 120 min at temperatures of 25 and 50 °C and then immersed in solutions containing 1 wt. % sulfuric acid (H₂SO₄) and 5 wt. % sodium chloride (NaCl). The electrochemical technique used was cyclic potentiodynamic polarization (CPP) based on ASTM G61-86. Microstructural characterization was performed via optical microscopy (OM) and scanning electron microscopy (SEM). The results revealed that the potentiodynamic polarization curves have two distinct corrosion mechanisms. The immersion of PHSSs in the H₂SO₄ solution produces an activation–passivation reaction. The NaCl solution has pseudopassivation (unstable passivation film). The values of the icorr current densities in the solutions of sodium chloride and sulfuric acids are between 10⁻³ and 10⁻⁵ mA/cm²; the stainless steels are susceptible to localized pitting corrosion in both test solutions, with positive hysteresis in the CPP.

Precipitation-hardenable stainless steels (PHSS) are widely used in various applications in the aeronautical industry such in as landing gear supports, actuators, and fasteners, among others. This research aims to study the pitting corrosion behavior of passivated martensitic precipitation-hardening stainless steel, which underwent passivation for 120 min at 25 °C and 50 °C in citric and nitric acid baths before being immersed in solutions containing 1 wt.% sulfuric acid (H₂SO₄) and 5 wt.% sodium chloride (NaCl). Electrochemical characterization was realized employing electrochemical noise (EN), while microstructural analysis employed scanning electron microscopy (SEM). The result indicates that EN reflects localized pitting corrosion mechanisms. Samples exposed to H₂SO₄ revealed activation–passivation behavior, whereas those immersed in NaCl exhibited pseudo-passivation, indicative of an unstable oxide film. Current densities in both solutions ranged from 10⁻³ to 10⁻⁵ mA/cm², confirming susceptibility to localized pitting corrosion in all test conditions. The susceptibility to localized attack is associated with the generation of secondary oxides on the surface.

Stainless steels are used in the aeronautical industry for their corrosion resistance and good mechanical performance. The chemical treatment used to improve corrosion resistance is passivation, forming a compact, continuous, adherent chromium oxide film. This research aimed to investigate the effect of citric acid at different concentrations (citric acid; citric acid + oxalic acid, citric acid + hydrogen peroxide, and citric acid + hydrogen peroxide + ethanol) on AM 350 stainless steel passivated for 90 and 120 min at 25 and 50 °C and immersed in 5% by weight sodium chloride (NaCl) solutions. The electrochemical technique used was electrochemical impedance spectroscopy (EIS) based on ASTM-G106. The EIS (equivalent circuit) results indicate that there are one and two constant phase elements (CPE), which indicate the presence of various factors on the stainless steel surface, such as roughness and the formation of porous and passive layers, respectively. A double-layer system was employed for some samples. However, when the ethanol was added to the passivation bath, the behavior changed to a one-time constant system. The AM 350 passivated in citric and oxalic acid presented the higher corrosion resistance with values of 6 × 10⁵ Ω·cm².

This study presents research on optimizing the parameters of the passivation process for precipitation-hardening stainless steels (PHSS) to improve the corrosion resistance of AM350 and CUSTOM 450 alloys, which are extensively utilized in the aerospace and aviation sectors, since, as this is a complex process, it requires the implementation of a robust methodological approach that allows for multi-response optimization. Experiments were designed using the Taguchi method, which offered a strong framework for examining the impact of material, type of passivation solution, concentration, temperature, and passivation process time on the corrosion resistance of both PHSS alloys. To confirm the ideal PHSS passivation process parameters and measure the significance of each component, gray relational analysis (GRA) and analysis of variance (ANOVA) were also employed. The combined use of the Taguchi/GRA represents a robust and efficient methodological approach to the multi-response optimization of complex processes, overcoming the limitations inherent in the individual application of each technique. It was determined that the optimized parameters were a PHSS AM 350, a solution composed of a combination of citric acid and oxalic acid, acid concentration of 25% v/v, temperature of 50 °C, and time of 120 min. This combination of parameters resulted in significant improvements of up to 55% in corrosion resistance in the H₂SO₄ and NaCl evaluation solutions, demonstrating the effectiveness of the optimized conditions. This work emphasizes the efficacy of integrating Taguchi, GRA, and ANOVA techniques to significantly reduce the corrosion rate of PHSS undergoing the passivation process using alternatives to nitric acid. The integration of the Taguchi methodology with GRA enables the normalization and combination of responses with different scales and performance criteria into a single gray relational index, facilitating the overall evaluation of the system.