Caracterización electroquímica de la pasivación de acero inoxidable empleado en la fabricación de fuselajes de helicópteros.

El presente trabajo de investigación se desarrolló a partir de la necesidad que se tiene por parte de una empresa Mexicana del sector aeronáutico por reducir los costos de producción generados por la falta de procesos internos como es el proceso de pasivado de aceros inoxidables, empleado en la fabricación de fuselajes para helicópteros militares. En general cuando se habla de este tipo de helicópteros se realizan investigaciones teniendo como primer punto a considerar la seguridad de quienes lo operan, en conjunto con los factores a los que estará expuesto, de entre los cuales se puede mencionar: condiciones ambientales, peso y resistencia a la corrosión, entre otras. Por lo anterior a pesar de los avances tecnológicos los aceros inoxidables continúan siendo los materiales que por excelencia persevera su uso. Los aceros inoxidables debido a su contenido de cromo superior al 11% permiten tener un material con excelentes propiedades mecánicas y resistencia a la corrosión. Los aceros inoxidables endurecibles por precipitación, conocidos como PH (por sus siglas en inglés, Precipitation Handening), presentan en su composición cobre, aluminio, titanio, niobio y molibdeno que después de ser sometidos a un tratamiento térmico de envejecimiento ofrecen una alternativa de elevadas características mecánicas y de maquinabilidad, mismos que se pueden clasificar en función de su estructura en estado recocido y del comportamiento resultante tras el tratamiento de envejecido como: austeníticos, semi-austeníticos o martensíticos . Estos aceros inoxidables PH, tienen la capacidad de formar una capa de óxido superficial, generalmente de óxido de cromo que ayuda a mejorar su resistencia a la corrosión, una forma de generar la formación de esta capa es mediante un proceso de pasivado. Algunas compañías como Boeing han realizado investigaciones que involucran diferentes agentes pasivantes, dado que actualmente se emplea ácido nítrico y es altamente tóxico a la salud, la NASA ha investigado el uso de ácido cítrico para los aceros inoxidables PH empleados en las plataformas de despegue de los transbordadores espaciales, en todos los casos la capa pasiva a obtener debe ser continua, adherente y homogénea. Este trabajo de investigación consistió en la caracterización de aceros inoxidables 15-5PH y 17-4PH, sometidos a un proceso de pasivación en: ácido cítrico, ácido nítrico y dicromato de sodio con temperaturas de pasivación de 25 y 49°C, con tiempos de inmersión en el agente pasivante de 30 y 90 minutos. La formación y mecanismo de corrosión se evaluó mediante técnicas electroquímicas como curvas de polarización potenciodinámicas y ruido electroquímico. Una forma de obtener el espesor de la capa pasiva obtenida se llevó a cabo mediante la técnica de espectroscopia de fotoelectrones emitidos por rayos X. Los resultados señalan que el acero inoxidable 15-5PH pasivado en ácido cítrico en presencia de NaCl a una temperatura de 25 °C con un tiempo de inmersión de 30 minutos en el medio pasivante presentan la mejor velocidad de corrosión. Las técnica de XPS muestra espesor de capa pasiva de hasta 12.45 nm. Esta capa obtenida está constituida por óxidos de fierro y de cromo en donde el níquel no presenta variación en su concentración. ABSTRACT This research work was developed based on the need of a Mexican company in the aeronautical sector to reduce the production costs generated by the lack of internal processes such as the passivation process to stainless steels, used in the manufacture of fuselages for military helicopters. In general when talking about this type of helicopter, investigations are carried out with the first point to consider the safety of those who operate it, together with the factors to which it will be exposed, among which we can mention: environmental conditions, weight and resistance to corrosion, among others. Therefore despite technological advances stainless steels are still the materials that par excellence continues to be use. Stainless steels due to their chromium content higher than 11% allow having a material with excellent mechanical properties and resistance to corrosion. Stainless steels hardened by precipitation, known as PH (Precipitation Handening), present in their composition copper, aluminum, titanium, niobium and molybdenum that after being submitted to a thermal treatment of aging offer an alternative of high mechanical and machinability characteristics, which can be classified according to their structure in state annealing and the resulting behavior after aging treatment as: austenitic, semi-austenitic or martensitic. These stainless steels PH, have the ability to form a surface oxide layer, usually chromium oxide that helps improve its resistance to corrosion, one way to generate the formation of this layer is through a passivation process. Some companies, such as Boeing, have carried out research involving different passivating agents, since nitric acid is currently used and is highly toxic to health. NASA has investigated the use of citric acid for PH stainless steels used on the take-off platforms of space shuttles; in all cases the passive layer to be obtained must be continuous, adherent and homogeneous. This research work consisted in the characterization of stainless steels 15-5PH and 17-4PH, submitted to a passivation process in: citric acid, nitric acid and sodium dichromate with passivation temperatures of 25 and 49 ° C, with immersion times in the passivating agent of 30 and 90 minutes. The formation and corrosion mechanism was evaluated by electrochemical techniques such as potentiodynamic polarization curves and electrochemical noise. One way of obtaining the thickness of the passive layer obtained was carried out by means of the photoelectron spectroscopy technique emitted by X-rays. The results indicate that the 15-5PH stainless steel passivated in citric acid in the presence of NaCl at a temperature of 25 ° C with an immersion time of 30 minutes in the passivating medium present the best corrosion rate. The XPS technique shows passive layer thickness up to 12.45 nm. This layer obtained consists of iron and chromium oxides in which the nickel does not vary in its concentration

Estudio del comportamiento de la corrosión de los aceros inoxidables am 350 y custom 450 pasivados en medio ácidos

En este trabajo se estudia el comportamiento de los aceros inoxidables AM 350 y Custom 450 en medios corrosivos de NaCl al 5% y H2SO4 al 1%. Los cuales fueron pasivados en ácido cítrico y ácido nítrico, a temperaturas de 25 y 49oC, con tiempos de inmersión de 60 y 90 minutos. La técnica Electroquímica empleada fue Polarización potenciodinámica, utilizando un potenciostato/galvanostato/ZRA "Gill AC Instruments". Las demandas de corriente en solución de NaCl son de 10-4 mA/cm2, mientras que las de H2SO4 están alrededor de 10-2 y 10-3 mA/cm2. También se realizó la determinación de la microestructura de los materiales establecidos, su morfología y composición química, a través de la microscopía óptica y microscopía electrónica de barrido. Se comprobó que el que ácido cítrico funciona como solución pasivante y en algunos casos la resistencia a la corrosión es equiparable con la del ácido nítrico

Cyclic potentiodynamic polarization on titanium alloys anodizing in alkaline solutions

Titanium alloys are used in different industries such as biomedical, aerospace, aeronautic, chemical, and naval. Those industries have high requirements with few damage tolerances. The aim of this work was to study the corrosion behavior of titanium alloys anodizing and non-anodizing in alkaline (KOH and NaOH) solutions, exposed in 3.5%wt. NaCl and 3.5% wt. H2SO4 solutions at room temperature using cyclic potentiodynamic polarization (CPP) according to standards in order to obtain electrochemical parameters as the passivation range (PR), corrosion type, passive layer persistence, corrosion potential (Ecorr), and corrosion rate. The alloy Ti Beta-C anodized presented better corrosion resistance than Ti-6Al-4V in both media. The smallest corrosion rate is presented in Beta-C samples (4.72 E-8 A/cm2) and the highest corrosion rate is CP2 (1.61 E-5 A/cm2

Electrochemical corrosion behavior of passivated precipitation hardening stainless steels for aerospace applications

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

Corrosion behavior of aluminum-carbon fiber/epoxy sandwich composite exposed on NaCl solution

For years, the aeronautical industry has employed different types of materials to satisfy its high-performance requirements. Fiber-metal laminates are used due to their combination of lighter weight and the high mechanical properties of reinforced metal and carbon. We therefore made two different composites of laminate-metal and laminate-metal-laminate of carbon fiber-reinforced polymer and aluminum with an ALCLAD layer. The samples were characterized by salt fog (0, 48, and 96 h) at 5 wt% NaCl and electrochemical impedance spectroscopy (EIS) with an electrolyte of 3.5 wt% NaCl. All samples were studied by electron scanning microscopy (SEM). The results demonstrated that the samples of laminate-metal-laminate presented an adsorption process after 0 and 48 h of salt fog exposition; meanwhile, the samples of laminate-metal showed a capacitive behavior for all the samples; however, corrosion resistance decreased when the salt fog exposition time increased

Electrochemical Noise Analysis Using Experimental Chaos Theory, Power Spectral Density and Hilbert–Huang Transform in Anodized Aluminum Alloys in Tartaric–Phosphoric–Sulfuric Acid Solutions

Aluminum and its alloys find widespread applications across diverse industries such as the automotive, construction, and aeronautics industries. When these alloys come into contact with ambient air, an Al2O3 thin oxide layer is naturally formed, typically measuring 2 to 4 nm and exhibiting remarkable hardness and protective qualities, rendering the alloys corrosion-resistant in specific atmospheric and chemical environments. This study aimed to characterize the electrochemical behaviors of anodized AA2024 and AA7075 alloys within a complex three-component electrolyte composed of tartaric–phosphoric–sulfuric acid (TPSA) solutions. The anodized specimens were subsequently exposed to 3.5 wt.% NaCl solution at room temperature, and their electrochemical performances were meticulously evaluated using an electrochemical noise (EN) analysis in accordance with ASTM G-199, respectively. In the EN, three methods of data analysis were used: the time domain analysis (chaos analysis: application of Lyapunov exponent and dimension correlation), the frequency domain analysis (power spectral density, PSD), and the time–frequency domains analysis (Hilbert–Huang transform, HHT). Scanning electron microscopy (SEM) was used to observe the morphologies of the anodized surfaces. The results indicated that the AA2024-0, AA2024-1, and AA2024-2 alloys and the AA7075-2 and AA7075-3 samples exhibited mixed corrosion according to the Lyapunov constant, with a notable inclination towards localized corrosion when analyzed using the PSD and HHT methods. The surface was not homogenous, and the corrosion process was predominately localized in specific zones

Surface, Chemical, and Tribological Characterization of an ASTM F-1537 Cobalt Alloy Modified through an Ns-Pulse Laser

Metallic biomaterials are considered safe materials for the fabrication of orthopedic prostheses due to their mechanical stability. Among this group, cobalt-chromium-molybdenum alloys are commonly used. Nevertheless, adverse reactions on tissues caused by the liberation of metallic ions are a limitation. Therefore, the modification of biometallic material surfaces has become a topic of interest, especially the improvement of the wear resistance to retard the degradation of the surface. In this work, dimples obtained at different processing parameters by an ns-pulse laser were texturized on an ASTM F-1537 cobalt alloy. Surfaces were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy, and Raman spectroscopy. The mechanical integrity of the surface was evaluated using a 3D surface analyzer and Vickers indentation tests. The tribological response was studied employing a ball-on-disc tribometer under lubricated conditions tracking the coefficient of friction, volume loss, wear rate, and surface damage by SEM. The variation of the laser power, repetition rate, and process repetitions slightly modified the chemistry of the surface (oxides formation). In addition, the rugosity of the zone treated by the laser increased. The texturized samples decreased the wear rate of the surface in comparison with the untreated samples, which was related to the variation of the dimple diameter and dimple depth

Use of Electrochemical Noise for the Study of Corrosion by Passivated CUSTOM 450 and AM 350 Stainless Steels

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

Corrosion Resistance of Hard Coat Anodized AA 6061 in Citric–Sulfuric 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

Alternative to Nitric Acid Passivation of 15-5 and 17-4PH Stainless Steel Using Electrochemical Techniques

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 Rn values derived show very low and similar corrosion rates for the stainless steels passivated with both citric and nitric acid solutions