Corrosion Performance of Steel Coated with Different Duplex Systems and Exposed to Salt Spray and Continuous Immersion Tests

55%Al-Zn alloy coated steel sheets pre-treated with 5% Fe (NO3)3 + 15% H3PO4 solution, coated with acrylic (AC), alkyd (AQ), vinyl (VL), solvent-based epoxy (ES) or waterborne epoxy (EA) corrosion-inhibiting primers + a barrier topcoat alkyd paint, and exposed to different media were studied. Exposure conditions included salt spray or continuous immersion in solutions containing chloride or sulphate ions. The state of the exposed samples was evaluated by visual inspections, standardized physicochemical tests and electrochemical impedance (EIS) measurements. Their results allowed inferring that: 1) under immersion conditions, the best performance was offered by the ES samples; 2) most of the painting systems performed satisfactorily during their exposure to the salt spray chamber; and 3) the correlation between them permitted to explain some problems arising in service and opened the door to extend the useful life by adjusting the painting system formulation.Centro de Investigación y Desarrollo en Tecnología de Pinturas (CIDEPINT

Synthetic and natural iron oxide characterization through microparticle voltammetry

Se presentan los resultados de un estudio sobre el uso de la Voltametría de Micropartículas (VMP), aplicada a la identificación de óxidos y oxihidróxidos de hierro. Mediante esta técnica, fue posible diferenciar hematita, goethita, magnetita y maghemita, en muestras sintéticas y naturales. La medición de pigmentos sintéticos tratados a diferentes temperaturas, evidenció la existencia de un patrón de comportamiento que permite diferenciarlos. Se constató que la ubicación del pico de corriente (valor de potencial), varía en función de la especie mineral, el tamaño de grano y el grado de cristalinidad, el área, en relación con la concentración de especie electroactiva y el ancho, con la distribución de tamaños de partícula. En muestras con elevadas concentraciones de óxidos y oxihidróxidos de hierro, los picos se definieron a corrientes del orden de los mA (miliamperes) y en muestras de paleosuelos con un contenido de hierro total inferior al 6%, los picos se registraron a corrientes del orden de los µA (microamperes). De esta manera, se constata la posibilidad de aplicación de la técnica en estudios ambientales y paleoambientales que tengan en cuenta a estos minerales. Las principales ventajas de la VMP, respecto a los métodos convencionales, se refieren a la rapidez y simplicidad de aplicación y a la posibilidad de procesar pocos microgramos de muestra, sin limitación de su grado de cristalinidad. A diferencia de los métodos magnéticos, fuertemente condicionados por la presencia de magnetita, la VMP resulta altamente sensible a la detección de especies débilmente magnéticas.We are hereby presenting the results obtained from a study on using Microparticle Voltammetry (MPV) for identifying Iron oxides and oxy–hydroxides. This technique allowed us to distinguish different mineral species, such as hematite, goethite, magnetite and maghemite, in both synthetic and natural samples. By measuring synthetic pigments at different temperatures, evidence was found of an electrochemical behavioral pattern which allowed differenciating them. The current peak location (potential value) proved to vary according to the mineral species, grain size and chrystallinity degree. The area varies in terms of electroactive concentration of the species given. Width varies according to particle size distribution. In high iron oxide and oxy–hydroxide concentration samples, peaks were defined at current values of mA (milliampers) and in paleosol samples having an overall iron content lower than 6%, peaks were recorded at currents of µA (microampers). Therefore, a possibility arises of applying this technique to environmental and palaeo–environmental studies of these minerals. MPVs main advantages compared to conventional methods are speed and simplicity as well as the fact that it allows processing a few sample grains, in spite of its chrystallinity degree. Unlike magnetic methods – strongly influenced by the presence of magnetite– MPV is highly sensitive for detecting weakly magnetic species.Centro de Investigación y Desarrollo en Tecnología de Pintura

Aleaciones metálicas usadas en implantes quirúrgicos

Los implantes quirúrgicos se utilizan para sustituir y restaurar tejidos óseos. Los materiales metálicos habitualmente usados para confeccionar dispositivos implantables son los aceros inoxidables ASTM F138 y ASTM F745, la aleación de cobalto ASTM F75, la aleación Ti-Al-V y el Ti Grado 4. En el presente trabajo se analizan las propiedades mecánicas, electroquímicas y magnéticas de estos materiales, a fin de establecer una comparación entre los mismos. Los resultados indicaron que el Ti-Al-V y el Ti Grado 4 presentan las mejores propiedades como biomateriales, con resistencias mecánicas entre 550 y 895 MPa y módulos de elasticidad entre 100 y 120 GPa. Los estudios electroquímicos mostraron que estos materiales poseen una mayor estabilidad en su condición pasiva, seguido por el ASTM F75. Al mismo tiempo, las aleaciones de Ti experimentan una menor magnetización ante campos magnéticos externos.Surgical implants are used to replace and restore bone tissue. The metallic materials commonly used to elaborate implantable devices are stainless steels ASTM F138 and ASTM F745, the ASTM F75 cobalt alloy, the alloy Ti-Al-V and Ti Grade 4. In this work it was analyzed the mechanical, electrochemical and magnetic properties of these materials, in order to establish a comparison between them. The results indicated that the Ti-Al-V and Ti Grade 4 have the best properties as biomaterials, with mechanical strength between 550 and 895 MPa and elastic moduli between 100 and 120 GPa. Electrochemical studies have shown that these materials have a greater stability in its passive condition, followed by the ASTM F75. At the same time, Ti alloys have a lower magnetization to external magnetic fields

Effect of zinc crystals size on galvanized steel deformation and electrochemical behavior

Hot-dip galvanized steel sheets with different spangle sizes were deformed by means of rolling and tension. The change of preferential crystallographic orientation and of superficial characteristics due to the deformation was analyzed by means of both X-rays diffraction and optical and scanning electronic microscopy. A correlation between such changes and the involving deformation modes was intended to be done and the spangle size influence on these modes was studied. Coating reactivity change due to the deformation was investigated by means of quasi-steady DC electrochemical tests. The results allow to infer that, in great spangle samples, the main deformation mechanism is twinning whereas in small spangle ones, pyramidal slip systems happen as well. The increase of the reactivity with the deformation is greater in tension than in rolling and it is more important in small than in great spangle samples

Resistencia a la corrosión por picado del acero inoxidable AISI 316L nitrurado iónicamente

El acero inoxidable AISI 316L es uno de los materiales habitualmente usados para confeccionar prótesis e implantes quirúrgicos, debido a su elevada resistencia a la corrosión. Sin embargo, este acero es susceptible de sufrir corrosión por picado en medios que contengan iones Cl-, tal el caso de algunos fluidos del cuerpo humano. Con el fin de mejorar las características anticorrosivas de este acero, se realizaron tratamientos térmicos superficiales de nitruración iónica a 400ºC. La resistencia a la corrosión por picado fue evaluada mediante ensayos potenciostáticos de corriente continua, observándose que el tratamiento de nitruración iónica mejora la resistencia a la corrosión de este acero debido a la formación de una capa superficial de austenita sobresaturada en nitrógeno, denominada austenita expandida.Stainless steel AISI 316L is one of the metallic alloys used as biomaterial due to their satisfactory mechanical and corrosion resistance. Nevertheless, this steel is susceptible to suffer pitting corrosion by the action of the Cl- ions present in the fluids of the human body. In order to enhance its corrosion resistance, ion nitriding treatment was performed at 400ºC. The susceptibility to localized corrosion was evaluated through potentiostatic tests and the results indicated that nitrided AISI 316L exhibited a higher pitting corrosion resistance than the same steel without nitriding treatment. This behaviour can be attributed to the superficial layer of nitrogen supersaturated austenite, also called expanded austenite

Anti-Corrosion Performance of Cr+6-Free Passivating Layers Applied on Electrogalvanized

Hexavalent chromium-based passivation treatments have been successfully used as promoters of conversion coatings for many years. Their effectiveness is without question although there are many problems with regard to their environmental suitability. Hexavalent chromium compounds are carcinogenic and toxic. These problems have lead researchers to evaluate other potential systems, with lower toxicity, to ascertain if they can replace chromates as effective passivators. Researchers have proposed several alternative passivation treatments; these are processes based on molybdates, permanganates, titanates, rare earth metal and Cr3+ (considered to be non-carcinogenic) compounds. In this work, zinc coatings obtained from free-cyanide alkaline bath and submitted to a Cr3+ based passivation treatment with different colors were studied. The corrosion behavior was studied by polarization measurements and mainly by electrochemical impedance spectroscopy in 0.6 N NaCl solution. Morphological observations on the coatings surface were also performed. The results indicate that the green-colored Cr3+ passivated coatings have a good corrosion resistance followed by yellow and blue-colored passivation respectively. They could be a less polluting alternative to the traditional chromated coatings.Centro de Investigación y Desarrollo en Tecnología de Pintura

Electrochemical Characterization of Chromate Free Conversion Coatings on Electrogalvanized Steel

The chromate conversion treatment is widely used, but it requires highly toxic chromic acid solutions with the consequent effluent disposal and ecological problems. The removal of these toxic chemicals is considered a priority within European Union. The corrosion resistance of three alternative treatments applied on electrogalvanised steel, and immersed in aerated 0.3 M Na2 SO4 solution, pH 10, at 25 °C, was investigated using electrochemical techniques. Their performance was compared with the obtained using the traditional Cr6+-based treatment in the same conditions. The achieved results show that the alternative coatings exhibited discrete protective properties in the sulphate solution. The nitro-cobalt chemical conversion treatment showed similar protective properties than the traditional Cr6+-based treatment, while with the Cr3+-based treatment those were very poor. The phosphate treatment initially performed acceptably but as the time elapsed, its protective properties decreased.Facultad de Ingenierí

Behaviour of two eco-compatible duplex systems used in the construction industry against corrosion

Two types of coated steels are currently competing in the construction industry: the hot-dip galvanized steel (HDGS) and the 55%Al–Zn alloy coated steel (HDZAS). Both systems are typically coated by a chromate-based conversion film, which is dangerous for human health and environment. Furthermore, for aesthetics purposes and/or to prolong their expected or planned service life, they are painted designing each paint system as a function of the service conditions to which they will be subject to. The present work reports on a comparative study of two duplex systems: steel/metal coating/organic coating. In both cases, the chromate-based pretreatment was replaced by another one that features γ-mercaptopropyltrimethoxysilane (MTMO) as an active element. The organic coating used was a water-based polyurethane. Behavior of this paint against corrosion was assessed by exposing the samples in salt spray or humidity chambers, and monitoring its evolution by electrochemical impedance spectroscopy, visual observation, scanning electron microscopy and energy dispersive X-ray analysis. The results revealed that: MTMO was suitable for promoting adhesion both in HDGS and HDZAS; in humidity chamber, both duplex systems provided good protection; the presence of corrosion products of the base steel and paint blistering was detected in the HDGS/paint system seventy-two days after exposure; however, 150 days after that, none of these effects were observed in the HDZAS/paint system; differences in behavior were significant in Salt spray chamber: speed of paint delamination from the mark done and the bulk of corrosion products generated was much higher in HDGS/paint system than in HDZAS/paint.Centro de Investigación y Desarrollo en Tecnología de Pinturas (CIDEPINT

Temporary protection of electrogalvanized steel by nanofilms of mercaptopropyltrimethoxysilane (MTMO)

The ɣ-mercaptopropiltrimetoxisilane (MTMO) has proven to be a good temporary corrosion protection on steel, but there are few studies on galvanized steel. In this paper the effectiveness of some specific silane films was studied. They were synthesized from MTMO hydrolyzed with ethanol or methanol, applied by different methods on electrogalvanized steel and cured at different temperatures. The corrosion behavior was evaluated by polarization curves, cyclic voltammetry and humidity chamber. From the results it was concluded that the alcohol used in the hydrolysis and the curing temperature have a marked influence on the protection level afforded by the conversion film.Centro de Investigación y Desarrollo en Tecnología de Pinturas (CIDEPINT

Characterization and Corrosion Resistence of Galvanized Steel//Passivation Compositive/Polyuethane Paint Systems

The corrosion performance of electrogalvanised steel sheets pre-treated with a Cr3+ or Cr+6-based conversion layer and then covered with polyurethane-waterborne topcoat paint has been studied. The pre-treated metallic panels were coated with one of the three tested polyurethane (PU) topcoat paints, in which the dispersion type was the formulation variable. The pigment was TiO2 (rutile) with a PVC value of 10. Before and after the immersion in 0.05 M NaCl (pH 5.70) or 0.1 M Na2SO4 (pH 6.36) solutions, replicates of the different samples were subjected to standardized tests (porosity, gloss and color, hardness, flexibility). During the immersion, blistering and rusting degrees were evaluated through periodical visual inspections, while the coated steel performance was monitored by Electrochemical Impedance Spectroscopy (EIS) measurements. Initial (dry) and final (wet) paint adhesion was also determined. EIS data were interpreted and discussed in terms of the time dependence of the electrical (paint coating) and electrochemical (steel substrate) parameters associated with interfacial processes describing the metal/paint system deterioration. According to the electrochemical properties, visual inspection a standardized tests results, it was concluded that the studied polyurethane-based polymeric films applied on pretreated electrogalvanised steel provided a very effective protection against corrosion as a result of their excellent barrier properties.Centro de Investigación y Desarrollo en Tecnología de Pinturas (CIDEPINT