Investigation into the Effects of Citric Acid on the Corrosion Behavior of AM 350 Stainless Steel Using Electrochemical Impedance Spectroscopy

Abstract

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.

1. Introduction

Due to its unique properties, stainless steel (SS) is essential in various industries. From construction to food, medical, automotive, and aerospace applications, it is indispensable for its durability, corrosion resistance, versatility, and esthetics [1,2]. These steels are preferred for demanding environments and critical applications due to their ease of cleaning and maintenance and their ability to maintain their mechanical properties under extreme conditions [3,4].

Stainless steel is a ferrous alloy characterized by low-carbon steel. It contains 10.5% chromium by weight, making it a corrosion-resistant alloy. In its chemical composition, stainless steel may contain other alloying elements such as nickel (Ni), molybdenum (Mo), nitrogen (N), or titanium (Ti), among others. The addition of these alloying elements improves the performance of these steels. Their appearance, formability, and mechanical or thermal resistance allow improvements to their behavior under high temperatures. Stainless steels are classified into five different families or types known as austenitic, ferritic, martensitic, duplex, and precipitation-hardening, and the latter is based on the type of heat treatment used and on the crystalline structure [4,5,6,7].

Precipitation-hardening stainless steels (PHSSs) are characterized by the mechanical resistance obtained from hardening by aging heat treatment. They are iron–chromium–nickel alloys. In the aeronautical sector, the most commonly used PHSSs are 15-5PH, 17-7PH, 17-4PH, Custom 450, and AM350, and they are manufactured for components such as blades, rotors, and turbine shafts, as well as structural components of aircraft [8,9,10,11].

Stainless steels stand out for their corrosion resistance, unlike conventional steel, which oxidizes and forms a reddish rust layer under normal conditions. This is because chromium adheres to oxygen to form a thin, transparent film of chromium oxide on the surface of the steel. This oxidation film is known as the passive layer, which prevents the further oxidation of the stainless steel. In the case of mechanical or chemical damage, the passive layer can be repaired autonomously in the presence of oxygen [12,13,14,15,16].

A literature review shows that nitric acid (HNO₃) is a powerful oxidant that facilitates the creation of passive films on stainless steels during the passivation process [11]. Nitric acid must be highly concentrated to be effective. However, many questions have been raised regarding the production of harmful-to-health toxic vapors by nitric acid in passivation baths. Moreover, the traditional disposal of used nitric acid presents environmental problems due to the generation of nitrates in wastewater and toxic sludge containing heavy metals. However, in recent years, alternative solutions have emerged that are non-toxic and environmentally sustainable, such as citric acid (C₆H₈O₇), which is inexpensive and can be extracted from fruits and vegetables. C₆H₈O₇ is frequently used in solutions around 20%, while HNO₃ can reach solutions of 40% based on international regulations. Since 2008, NASA and Boeing have reported research into passivating stainless steels in the presence of citric acid to improve corrosion resistance [17,18,19,20,21,22].

Direct and alternating current electrochemical techniques have been used to characterize the electrochemical behavior of various passivated stainless steels. Electrochemical impedance spectroscopy (EIS) is a technique that allows us to understand the corrosion mechanism, analyze the oxide layer, and establish the system’s kinetics [23].

In 2004, the corrosion behavior of stainless steels with various molybdenum contents in solutions with and without oxygen was studied, where it was concluded that the presence of certain alloying elements, such as molybdenum, caused a decrease in the pH of the solutions [24]. The same authors, in 2007, investigated the growth of the chromium oxide film in passivated SS where the alloying elements determined the stability of the oxidation film [25]. Bragaglia et al. presented results on aeronautical components of stainless steels passivated with citric acid, indicating that the corrosion kinetics decreased compared to nitric acid [26]. According to the research of Marcelin et al. [27], the properties of the passive film control the electrochemical process of corrosion in martensitic stainless steels. Lara et al., in 2020, used electrochemical techniques such as potentiodynamic polarization curves and electrochemical noise to compare austenitic stainless steel 304 against 15-5PH and 17-4PH in citric and nitric acid baths and immersed in NaCl and H₂SO₄. The corrosion kinetics were lower when citric acid was used [28]. In recent studies, Gaona et al. [1,6,8,9,23] have investigated the localized corrosion of PHSSs in acid baths using electrochemical techniques, such as cyclic potentiodynamic polarization, electrochemical noise analysis in time, frequency, and time–frequency, and electrochemical impedance spectroscopy.

The present work aims 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 wt. % sodium chloride solution, using electrochemical impedance spectroscopy (EIS) based on ASTM-G106. Stainless steels such as AM350 are used in the aeronautical industry and are exposed to different atmospheres such as marine and industrial environments.

2. Materials and Methods

2.1. Materials

The stainless steel used was AM 350 (AMS 5548; Aerospace Material Specifications) in the shape of 2-inch cylindrical bars. The nominal chemical composition is reported in

Table 1. The chemical composition of the AM 350 (wt.%) [29].

Element	AM 350
Fe	Balance
C	0.07–0.11
Cr	16.0–17.0
Ni	4.0–5.0
Mo	2.50–3.25
Mn	0.50–1.25
N	0.07–0.13
Si	≤0.50
S	0.030

. AM 350 is a chromium–nickel–molybdenum stainless steel that can be hardened by martensitic transformation or precipitation hardening. AM 350 may have an austenitic phase for best formability or a martensitic microstructure with high strength depending on the heat treatment. In the annealed condition, alloy AM 350 is essentially austenitic, forming characteristics similar to those of the AISI 300 series stainless steels. It has a higher rate of work hardening, and cold forming will cause martensite formation proportionate to the amount of deformation [23].

The samples were cut from a cylindrical bar with a diameter of 2 inches [30]. Subsequently, all samples were prepared by metallography technique (for grinding and polishing, we employed 400-, 500-, 600-, and 800-grade SiC sandpaper) as indicated in the ASTM E3 [31]; the cleaning of the samples was performed by ultrasound [32] using an ethyl alcohol solution and a rinse with distilled water and air, respectively.

2.2. Passivation Treatment

In the passivation treatment, we controlled parameters such as bath concentration, temperature, and immersion times based on the SAE/ASM2700 and ASTM G967 standards [33,34,35]; see Figure 1.

i.

Passivated AM 350 steel samples were cleaned by ultrasound (10 min in ethanol) and immersed in deionized water.

ii.

The passivation baths used were as follows:

a.

citric acid (C₆H₈O₇) 25% p/v;

b.

citric acid (C₆H₈O₇) 25% p/v + oxalic acid (C₂H₂O₄) 10% v/v;

c.

citric acid (C₆H₈O₇) 25% p/v + hydrogen peroxide (H₂O₂) 10% v/v;

d.

citric acid (C₆H₈O₇) 25% p/v + hydrogen peroxide (H₂O₂) 10% v/v + ethane (C₂H₆O) 5% v/v.

iii.

The bath temperatures were 25 and 50 °C, and the immersion times were set at 90 and 120 min.

iv.

The samples were rinsed with deionized water and air-dried to finish the treatment.

The nomenclature used to identify each of the passivated samples is given in the following

Table 2. Nomenclature of the AM 350 samples.

Passivation Baths	Temperature (°C)	Concentration	Time (min)
citric acid (C6H8O7)	25 and 50	(C6H8O7) 25% p/v	90 and 120
citric acid (C6H8O7) + oxalic acid (C2H2O4)	25 and 50	(C6H8O7) 25% p/v + (C2H2O4) 10% v/v	90 and 120
citric acid (C6H8O7) + hydrogen peroxide (H2O2)	25 and 50	(C6H8O7) 25% p/v + (H2O2) 10% v/v	90 and 120
citric acid (C6H8O7) + hydrogen peroxide (H2O2) + ethane (C2H6O)	25 and 50	(C6H8O7) 25% p/v + (H2O2) 10% v/v + (C2H6O) 5% v/v	90 and 120

.

2.3. Corrosion Measurements

A three-electrode electrochemical cell (Figure 2) configuration was used for the corrosion measurements, with the working electrode (material to analyze), a saturated calomel electrode (SCE) as a reference electrode, and a platinum wire as a counter electrode. The tests were carried out in duplicate at room temperature using a Potentiostat/Galvanostat/ZRA (produced by Solartron 1287A, Bognor Regis, UK). The tests were evaluated in solutions containing 5 wt. % NaCl solution. The parameters used in the electrochemical impedance spectroscopy (EIS) technique included a frequency range of 0.01 to 100,000 Hz, the application of a 10 mV RMS amplitude, and 35 points per decade [36]. The results analysis was performed utilizing “Zview-4” software (https://www.scribner.com/software/68-general-electrochemistr376-zview-for-windows/ ScribnerAssociates, Inc. by Berek Johnson, Southern Pines, NC, USA), and the EIS diagrams were examined in terms of electrical equivalent circuits.

2.4. Microstructural Characterization

Scanning electron microscopy (SEM, JEOL-JSM-5610LV, Tokyo, Japan) was used to determine the microstructure of AM 350 PHSS, using the secondary electron (SE) detector at 500×, operating at 20 kV, WD = 14 mm.

3. Results

3.1. SEM Microstructural Analysis

The SEM technique was used to study the microstructures of the AM 350 PHSS in initial conditions. Figure 3 shows an SEM-SE micrograph of AM 350 SS (a) containing austenite (γ) and delta (δ) ferrite phase, respectively, and an EDS Spectrum (b) in which the presence of alloying elements such as chromium, nickel, iron, silicon and carbon can be observed. The presence of austenite (γ) in semi-austenitic steels is a thermodynamically stable phase that can be transformed by an aging treatment. Likewise, alloying elements were found to indicate the presence of the delta ferrite (δ) phase [16,23].

3.2. Electrochemical Impedance Spectroscopy (EIS)

Our investigation into AM350 stainless steel passivated in different concentrations of citric acid allowed us to observe the effect on localized corrosion. The interpretation of EIS data, based on Nyquist and Bode diagrams, is the subject of various theories and models for simulating experimental data. Their physical interpretation was enhanced by using equivalent circuits, which are models that seek to interpret and relate each element of the electrical circuit with the electrochemical phenomena that occur at the metal/film and film/solution interfaces, as well as within the film. These models provide kinetic information on corrosion and a mechanistic study on the growth and degradation of the passive film [37,38,39].

According to the structure of the oxide layer, two equivalent circuits were proposed that model the system’s behavior, which can be seen in Figure 4. The equivalent circuit models are the ones that best fit the Nyquist diagrams of the passivated stainless steel samples exposed to a 5 wt. % NaCl solution, where R_s represents the electrolyte resistance, CPE₁ and CPE₂ represent the constant phase element of the porous layer and the protective layer for the passivated film, and R₁ corresponds to the resistance of the porous layer and R₂ to the resistance of the protective layer. The CPEs, constant phase elements, represent the impedance response for electrochemical systems in corrosion and often reflect a reactivity distribution [40,41,42,43]. The simple Faradaic reaction without diffusion can be defined in terms of a CPE, as indicated in the following Equation (1):

$$Z(\omega )=R_s+{\displaystyle \frac{R_{ct}}{1+{\left(j\omega \right)}^{\alpha }Q{R}_{ct}}}$$

(1)

The ohmic resistance is R_s, and the charge transfer resistance is identified as R_ct. The parameters α and Q (CPE) are frequency-independent. When α = 1, Q has capacitance units (μF·cm⁻²), representing the interface capacitance. When α < 1, the electrochemical system exhibits behavior attributed to its heterogeneity or continuously distributed time constants for charge transfer reactions. Regardless of the cause of the CPE behavior, the phase angle associated with CPE is frequency-independent and is expressed in Equation (2) [44].

$$Z(\omega )=R_e+{\displaystyle \frac{1}{{\left(j\omega \right)}^{\alpha }Q}}$$

(2)

Figure 5 shows the Nyquist and Bode diagrams of AM350 passivated in citric acid exposed to NaCl electrolyte. The figure shows how C/25/25/120 presented a higher resistance to corrosion in Nyquist and Bode |Z| diagrams, obtaining values of 3 × 10⁵ Ω·cm², and the sample C/50/25/120 presented the lower corrosion resistance, with values of 1.5 × 10⁵ Ω·cm². The equivalent circuit in Figure 3 shows how two time constants are present for all samples except the sample C/50/25/90, which only presents one time constant. The first layer is related to the passive layer, where the highest resistance is for C/25/25/120; that resistance is associated with R_ct (14,283 Ω·cm²) (see

Table 3. Electrochemical characteristics from Nyquist diagrams of AM 350 samples in a citric acid (C₆H₈O₇) bath and immersed in 5 wt. % NaCl solution.

Sample	Rsol (Ω·cm)	CPE1 (μF/cm2)	n1	R1 (Ω·cm)	CPE2 (μF/cm2)	n2	R2 (Ω·cm)	X2
C/25/25/90	17 ± 1.5	1.45 × 10−5 ± 1.5 × 10−7	0.84 ± 0.1	11,360 ± 40	8.16 × 10−6 ± 2 × 10−8	0.59 ± 0.05	1,994,500 ± 58	5.4 × 10−4
C/25/25/120	16 ± 2.0	1.01 × 10−5 ± 2.1 × 10−7	0.86 ± 0.1	14,283 ± 11	8.49 × 10−6 ± 1 × 10−8	0.62 ± 0.05	3,654,800 ± 63	5.0 × 10−4
C/50/25/90	17 ± 1.3	2.06 × 10−5 ± 1.6 × 10−7	0.81 ± 0.1	13,204 ± 20	5.62 × 10−6 ± 1.8 × 10−8	0.65 ± 0.05	1,383,400 ± 87	6.4 × 10−4
C/50/25/120	16 ± 0.9	4.29 × 10−5 ± 2.2 × 10−7	0.88 ± 0.1	962,470 ± 23	-	-	-	1.9 × 10−3

). The CPE in the first layer is 8.49 × 10⁻⁶ μF/cm². The values obtained for n are of the order of 0.8, indicating that the behavior is associated with the capacitive system of a double layer. Also, this value indicates that the passive layer created is homogenous. The behavior of n in the second time constant is near 0.5, which indicates that impeded diffusion is occurring, so the generation of oxide by a diffusion model is not occurring due to non-homogenous ion transfer. All the samples presented CPE₁ values of the ×10⁻⁵ F/cm² order; however, the lowest value was C/25/25/120, indicating that the passive layer has acceptable conditions against corrosion.

Figure 6 shows the Nyquist and Bode diagrams for AM350 passivated in citric and oxalic acid exposed to a NaCl solution. The results show (see

Table 4. Electrochemical characteristics from Nyquist diagrams of AM 350 samples in citric acid (C₆H₈O₇) + oxalic acid (C₂H₂O₄) bath immersed in 5 wt. % NaCl solution.

Sample	Rsol (Ω·cm)	CPE1 (μF/cm2)	n1	R1 (Ω·cm)	CPE2 (μF/cm2)	n2	R2 (Ω·cm)	X2
C.O/25/25.10/90	15 ± 1	1.65 × 10−5 ± 0.9 × 10−7	0.86 ± 0.02	27,519 ± 21	5.96 × 10−6 ± 1 × 10−8	0.76 ± 0.02	2,063,800 ± 120	1.6 × 10−3
C.O/25/25.10/120	15.81 ± 0.5	1.16 × 10−5 ± 0.7 × 10−7	0.87 ± 0.06	27,929 ± 36	3.97 × 10−6 ± 0.6 × 10−8	0.65 ± 0.02	5,091,900 ± 98	5.2 × 10−4
C.O/50/25.10/90	17.91 ± 0.8	3.00 × 10−5 ± 1.8 × 10−7	0.87 ± 0.02	1,624,200 ± 240	-	-	-	1.1 × 10−3
C.O/50/25.10/120	18.2 ± 0.4	3.86 × 10−5 ± 1.3 × 10−7	0.89 ± 0.03	1,022,800 ± 603	-	-	-	1.3 × 10−3

) how C.O/25/25.10/120 presents values of 7 × 10⁵ Ω·cm² according to the Bode |Z| diagram. On the other hand, C.O/50/2/.10/120 presents a resistance of 2 × 10⁵ Ω·cm². That behavior can be observed in the Nyquist diagram, with a high semicircle value for C.O/25/25.10/120. The samples C.O/25/25.10/90 and C.O/25/25.10/120 present a double-layer behavior; both samples present similar resistance with values of 2.7 × 10⁴ Ω·cm², the capacitive values are also of the order of ×10⁻⁵ μF/cm². The value of the first n is 0.86 and 0.87, indicating that a homogenous passive layer is formed; on the other hand, the second value of R is 20 × 10⁵ Ω·cm² for C.O/25/25.10/90 and 50 × 10⁵ Ω·cm² for C.O/25/25.10/120, with an n value of 0.76 and 0.65. These values are associated with the possibility of a secondary passive layer at the interface of metal; for sample C.O/25/25.10/120, the value of 0.65 indicates that the surface is heterogeneous and that an impeded diffusion occurs. For sample C.O/25/25.10/90, the value of 0.75 indicates that the surface is heterogeneous. Therefore, the resistance of C.O/25/25.10/120 is higher than that of C.O/25/25.10/90. The CPE values are of the ×10⁻⁵ F/cm² order in the first layer, and in the second, they decrease to ×10⁻⁶; the lower value indicates that the resistance to ionic transference is higher.

Figure 7 shows the Nyquist and Bode diagrams of AM350 passivate in citric acid with hydrogen peroxide immersed in NaCl solution. Sample C.P/50/25.10/90 presents the highest resistance to corrosion: 4 × 10⁵ Ω·cm². On the other hand, C.P/50/25.10/120 presents the lower corrosion resistance with values of 0.9 × 10⁵ Ω·cm²; however, it is the only sample that presents a double-layer behavior, with 14.52 × 10⁴ Ω·cm², and an n of 0.94. This means that the passive layer created is more homogenous than the passive layer of the sample mentioned in Figure 3 and Figure 4. The other samples present the behavior of one time constant, indicating that the passive layer created only presents one interface (see

Table 5. Electrochemical characteristics from Nyquist diagrams of AM 350 samples in citric acid (C₆H₈O₇) + hydrogen peroxide (H₂O₂) bath and immersed in 5 wt. % NaCl solution.

Sample	Rsol (Ω·cm)	CPE1 (μF/cm2)	n1	R1 (Ω·cm)	CPE2 (μF/cm2)	n2	R2 (Ω·cm)	X2
C.P/25/25.10/90	16 ± 0.5	2.50 × 10−5 ± 1 × 10−7	0.91 ± 0.02	584,230 ± 86	-	-	-	1.0 × 10−3
C.P/25/25.10/120	17 ± 0.8	1.65 × 10−5± 2 × 10−7	0.93 ± 0.02	4,155,700 ± 70	-	-	-	1.7 × 10−3
C.P/50/25.10/90	16 ± 0.8	2.24 × 10−5± 1 × 10−7	0.91 ± 0.02	1,201,800 ± 63	-	-	-	1.2 × 10−3
C.P/50/25.10/120	16 ± 0.9	4.07 × 10−5± 1 × 10−7	0.94 ± 0.02	145,210 ± 23	7.67 × 10−5 ± 1.6 × 10−7	0.92 ± 0.02	130,830 ± 17	5.5 × 10−3

). The CPE values are of the ×10⁻⁵ F/cm² order, but C.P/50/25.10/120 presents the highest value, with 4.07 × 10⁻⁵ F/cm², indicating that ionic transference is higher, so the corrosion resistance decreases.

Figure 8 shows the Nyquist and Bode diagrams of AM350 passivated in citric acid with hydrogen peroxide and ethane exposed to NaCl solution. In these samples, CPE/25/25.10.5/90 presents a higher impedance with values of 15.9 × 10⁵ Ωcm²; in contrast, CPE/25/25.10/120 presents a lower resistance of 41.95 × 10⁴ Ω·cm². The behavior of these samples is related to a one-time-constant system, and the values of n (

Table 6. Electrochemical characteristics from Nyquist diagrams of AM 350 samples in citric acid (C₆H₈O₇) + hydrogen peroxide (H₂O₂) + ethane (C₂H₆O) bath and immersed in 5 wt. % NaCl solution.

Sample	Rsol (Ω·cm)	CPE1 (μF/cm2)	n1	R1 (Ω·cm)	X2
C.P.E/25/25.10.5/90	17 ± 0.5	2.42 × 10−5 ± 6 × 10−7	0.91 ± 0.15	1,530,900 ± 130	1.7 × 10−3
C.P.E/25/25.10.5/120	18 ± 0.3	3.99 × 10−5 ± 7 × 10−7	0.90 ± 0.17	419,510 ± 99	1.6 × 10−3
C.P.E/50/25.10.5/90	17 ± 0.5	3.02 × 10−5 ± 3 × 10−7	0.90 ± 0.15	1,197,500 ± 178	1.1 × 10−3
C.P.E/50/25.10.5/120	17 ± 0.2	2.83 × 10−5 ± 4 × 10−7	0.90 ± 0.1	464,940 ± 169	1.1 × 10−3

) indicate that the passive layer is homogenous due to the value of 0.9, very near to 1, which would indicate perfect capacitance; for this reason, charge is distributed on the surface in a more homogenous way. Sample CPE/25/25.10.5/120 presents the higher CPE, with 3.99 × 10⁻⁵ F/cm², indicating that the ionic transference is higher than the other samples. This result is related to the lower corrosion resistance of the sample.

4. Discussion

The AM350 steel presented a passive behavior and two corrosion systems. The first system showed a one-time-constant behavior, while the other presented a double-layer behavior, which is the most common in a system with a protective layer. The double-layer behavior occurs at low and medium frequencies due to a redox reaction on the material surface [43,44,45,46]. For some authors [47,48,49], a passive layer with only one time constant is associated with an unstable system due to the decomposition of the oxide layer. In a double-barrier system, after the dissolution of the first interface, a process of retained diffusion occurs for some samples, which indicates that an oxide layer is continuously being created; meanwhile, one time system indicates that the passive layer is degraded and the oxide generated by the corrosion process is unstable.

The one-time-constant system is associated with a metal dissolution up to the passivation process [50]; however, when one time constant presents n values of 0.9, this indicates that the surface is heterogenous and is not susceptible to pitting corrosion. For this reason, samples that present a layer system are more susceptible to localized corrosion [51], because the first layer is related to the passive layer and the second is related to the charge transfer and capacitance inside pits and cracks in the surface. Therefore, the n values of the second barrier are between 0.5 and 0.6, indicating a heterogenous surface and a retained diffusion process.

Double-layer systems are more connected to active pits because the surface area is inside. This behavior increases in the presence of Cl⁻, which attacks preference zones; however, the formation of Cr oxides protects the material against corrosion. When samples are submerged in NaCl solution, an unstable passivation film causes an increase in current density, affecting the corrosion kinetics of stainless steel [52,53,54,55,56].

It is important to consider that NaCl is not a passivation electrolyte; however, the behavior shown for samples with values for n near to 0.5 indicates that a diffusion process is retained, meaning that the material can be a pseudopassivate [57,58,59,60,61]. This occurs due to the generation of an oxide layer of Cr, as the following equation shows:

$${\mathrm{C}\mathrm{r}}^{3+}+3{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{C}\mathrm{r}\left(\mathrm{O}\mathrm{H}\right)}_3+3{\mathrm{e}}^{-}$$

(3)

$$\mathrm{C}\mathrm{r}{\left(\mathrm{O}\mathrm{H}\right)}_3+\mathrm{C}\mathrm{r}+{3\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O}+3{\mathrm{e}}^{-}$$

(4)

One time constant indicates that the material has a passive layer in constant active behavior. However, the values of n that are near 0.9 indicate that the passive layer generated is heterogeneous.

Figure 9 shows the results for the R_ct of each sample; this value is associated with the transference resistance and directly with the resistance of the passive layer. The sample anodized in citric acid + hydrogen peroxide shows higher corrosion resistance. This behavior is associated with revisiting hydrogen peroxide, which generates a redox process; however, the oxide layer created is more stable. The corrosion resistance increases when time increases for this passivation bath, which means that the passive layer is more stable when the time is longer and the temperature is 25 °C.

The effect of temperature can be observed for samples passivated in citric + oxalic acid, where the temperature increases R_ct resistance. This behavior is related to the catalytic effect of temperature. However, that effect is contradictory for citric acid + hydrogen peroxide and citric acid + hydrogen peroxide + ethane, where temperature leads to a decrease in the corrosion resistance of the passive layer. This occurs when the high temperature generates an increase in passive layer vacancies [62,63,64].

For this reason, it is difficult to determine a temperature and time homogenous for all baths because each one leads to different properties in the passive layer.

5. Conclusions

After studying the corrosion behavior of AM350 stainless steels passivated in citric acid at different concentrations (citric acid; citric acid + oxalic acid; citric acid + hydrogen peroxide and citric acid + hydrogen peroxide + ethanol) for 90 and 120 min at 25 and 50 °C and exposed to 5 wt. % sodium chloride solution and using electrochemical impedance spectroscopy, the following conclusions can be drawn:

• The impedance parameters provided relevant information concerning the role of chlorides and corrosion resistance.
• The equivalent circuit presented a one-time-constant and two-time-constant corrosion system.
• The results indicate that a two-time-system or double-layer equivalent circuit is associated with a system that is susceptible to pitting due to the behavior of the second barrier, where the n values are 0.6, indicating that the second layer is heterogeneous due to cracks and pittings.
• The AM 350 stainless steels passivated in citric and oxalic acid presented the higher corrosion resistance with values of 6 × 10⁵ Ω·cm² for sample C.O/25/25.10/120, indicating that with an increase in time, the passivate will be generated in a better way.
• After evaluation using EIS, it was observed that adding peroxide and ethanol helped to make the passivation process more stable at 50 °C because when passivation was performed in that media, the corrosion resistance increased when the passivation temperature increased. However, when the time increased, the corrosion resistance decreased.
• The citric acid passivation process in stainless steels could represent an environmentally friendly alternative to the frequently used nitric acid passivation process.