Effect of Laminar Flow on the Corrosion Activity of AA6061-T6 in Seawater

The electrochemical behaviour and surface changes on AA6061-T6 alloy exposed to Caribbean seawater from the Cozumel Channel for 30 days under laminar flow (0.1 ms−1) were studied, these contrasting then with stationary conditions. Open circuit potential monitoring and electrochemical current fluctuations, considered as electrochemical noise (EN), were employed as two nondestructive methods. The calculated corrosion current, based on Rn, was one order higher in laminar flow. The fluctuations of current were transformed in the frequency domain. Their power spectral density (PSD) plots were obtained in order to gain information concerning the dynamic of the spontaneous release of energy during the corrosion process. The value of the exponent β in PSD graphs suggested that the localised corrosion on AA6061-T6 surface occurs as a persistent stationary process, which dynamic is controlled by oxygen diffusion. The changes in the morphology and elemental composition of the formed layers revealed that the localised attacks occurred in the vicinity of intermetallic particles rich in Fe and Cu, which act as cathodes

Comparative cyclic voltammetry and surface analysis of passive films grown on stainless steel 316 in concrete pore model solutions

The study of passive layers grown on AISI 316 stainless steel in two model solutions, saturated Ca(OH)2 and cement extract (CE) solution, show that each solution simulates the concrete pore environment in a different way. A more resistant passive layer is formed in CE solution due to its distinctive composition, homogeneity, thickness and porosity. The CE-exposed sample withstood several hours of cathodic polarization before the characteristic cyclic voltammetry peaks of the non-passivated AISI 316 reappeared. This is in contrast to the saturated Ca(OH)2-exposed sample, which showed the characteristic CV peaks after several minutes of cathodic polarization. XPS spectra of the passive layer formed in saturated Ca(OH)2 indicate that the amount of Ca is as much as 10 times larger than that in the passive-layer formed in the CE solution. Films formed in CE solution contained Si and S, which are part of the cement mixture composition. Topographical dissimilarities in the passive films are also found in the AFM and optical microscopy images. The passive layer formed in CE solution is homogeneous and covers the metal surface completely. The film formed in saturated Ca(OH)2 is rough, crystalline, and nonhomogeneous. The differences in the composition and electrochemical behavior of passive layers formed on AISI 316 in saturated Ca(OH)2 solution or CE model solution suggest that CE solutions are superior for simulating concrete pore environments and that CE solutions create a more resistive passive film. In addition, passive films formed without polarization were superior to films formed on AISI 316 under anodic polarization

Species composition and assemblage structure of microfouling diatoms growing on fiberglass plates off the coast of Yucatán, Mexico

Background. It is generally accepted that exopolymer films secreted by diatoms promote the onset of macrofouling on surfaces of materials used in marine environments. However, few studies provide precise information at species level regarding the microfouling process. The use of anti-fouling paints on different surfaces to create a toxic environment may aid in precluding development of the initial diatom microfilm leading to macrofouling. Goals. To describe the species composition and the structure of fouling diatoms. Methods. We analyzed assemblages growing on fiberglass plates, coated and uncoated with antifouling paint, fixed on PVC stands submerged at a 10-m depth. Because diatoms are opportunistic, fast growing microalgae that proliferate on many substrates, our hypothesis was that diatom assemblages growing on fiberglass surfaces, coated or uncoated with antifouling paint, would not differ in their structure. Results. Floristic analysis yielded 170 diatom taxa and similar assemblages with high values of diversity occurring on both surfaces. Conclusions. In keeping with our hypothesis, both colonized fiberglass surfaces compare to living substrata that favor the growth of diatom associations that exhibit high species diversity. This information will be useful in environmental studies, such as pollution abatement, and for the design and maintenance of fishing boats and industrial equipment prone to biofouling.Background. It is generally accepted that exopolymer films secreted by diatoms promote the onset of macrofouling on surfaces of materials used in marine environments. However, few studies provide precise information at species level regarding the microfouling process. The use of anti-fouling paints on different surfaces to create a toxic environment may aid in precluding development of the initial diatom microfilm leading to macrofouling. Goals. To describe the species composition and the structure of fouling diatoms. Methods. We analyzed assemblages growing on fiberglass plates, coated and uncoated with antifouling paint, fixed on PVC stands submerged at a 10-m depth. Because diatoms are opportunistic, fast growing microalgae that proliferate on many substrates, our hypothesis was that diatom assemblages growing on fiberglass surfaces, coated or uncoated with antifouling paint, would not differ in their structure. Results. Floristic analysis yielded 170 diatom taxa and similar assemblages with high values of diversity occurring on both surfaces. Conclusions. In keeping with our hypothesis, both colonized fiberglass surfaces compare to living substrata that favor the growth of diatom associations that exhibit high species diversity. This information will be useful in environmental studies, such as pollution abatement, and for the design and maintenance of fishing boats and industrial equipment prone to biofouling

Primeras etapas de corrosión de metales en agua de mar artificial: II. Acero inoxidable AISI 304

RESUMENEn esta investigación se ha evaluado las primeras etapas del proceso de corrosión del acero inoxidable AISI 304 sumergido en agua de mar artificial en ausencia de incrustaciones de “bio-fouling” marino. Muestras planas del acero inoxidable AISI304 preparadas según la norma internacional ASTM G1 (2003), fueron sumergidas por cuatros meses en agua de mar artificial preparadas según la norma ASTM D1141 (2008). La velocidad promedio mensual de corrosión fue obtenida determinando la pérdida de masa. La mayor velocidad de corrosión de las muestras fue observada en el tercer mes de inmersión (0.40 g m-2), alcanzando un valor final al cuarto mes de 0.35 gm-2 (4.4 x 10-5 mm de penetración de la corrosión en el metal). El proceso de corrosión fue acompañado por una liberación (runoff) de iones de hierro de 6.00x10-7 g m-2 en el agua de mar en el cuarto mes. El potencial de corrosión tuvo un valor inicial positivo (+0.045 V vs SCE) y después mostró una desviación hacia valores más negativos, alcanzando un valor de -0.35 V en el cuarto mes, debido a la ruptura de la capa pasiva del acero y la formación de múltiples picaduras, como consecuencia de los ataques de los iones agresivos de cloruros. Las imágenes SEM de la superficie del acero revelaron que las picaduras fueron formadas en los granos del acero y así mismo, en los límites entre estos, cuyos tamaños y área creció con el avance del proceso de corrosión. Los patrones de rayos-X mostraron los picos específicos correspondientes a la matriz del acero (Fe), así como los de los óxidos mixtos Cr0.19Fe0.7Ni0.11, que forman parte de la capa pasiva en la superficie del AISI 304. Con el desarrollo de las picaduras cambió la intensidad de estos picos.Palabras clave: corrosión en agua de mar; corrosión acuosa del acero AISI 304, agua de mar artificial, runoff de hierro

Primeras etapas de corrosión de metales en agua de mar artificial: III. Aluminio

RESUMENEn el presente trabajo se ha estudiado las primeras etapas del proceso de corrosión del aluminio sumergido en agua de mar artificial, y por lo tanto, en ausencia de incrustaciones de “bio-fouling marino” en la superficie del metal. Muestras planas de aluminio electrolítico preparadas según la norma internacional ASTM G1 (2003) fueron sumergidas por cuatros meses en agua de mar artificial preparado según la norma ASTM D1141 (2008). La velocidad de corrosión de Al calculada mediante fórmulas establecidas (Raichev et al., 2009:28) fue aproximadamente doble (4.18 g m-2) en el segundo mes que del primer mes (2.85 g m-2), alcanzando un valor final de 4.35 g m-2. El proceso de corrosión fue acompañado por liberación (runoff) de iones de Al+3 (3x10-3 a 3.0x10-4 g m-2). El potencial de corrosión durante los primeros dos meses mostró un descenso hacia valor más negativo (-0.76 V vs SCE) con el desarrollo del proceso de corrosión y posteriormente se observó una tendencia a potencial menos negativo (≈ -0.73 V) debido a los productos de corrosión de Al y/o reparación de la capa pasiva en la superficie del metal. La morfología de la superficie del Al (examinada con SEM) reveló la formación de múltiples picaduras de aproximadamente 20 μm de diámetro/longitud debido al ataque de los cloruros del agua de mar artificial. Como producto de corrosión apareció el hidróxido de aluminio, nordstrandita, Al (OH)3. El estudio presenta una comparación del comportamiento corrosivo de Al con la del Cu y acero inoxidable AISI 304 previamente reportados en esta misma revista.Palabras clave: corrosión acuosa del aluminio; agua de mar artificial; runoff de aluminio

Primeras etapas de corrosión de metales en agua de mar artificial: I. cobre

El cobre y sus aleaciones son utilizados en estructuras expuestas al agua del mar, donde están sometidos al ataque corrosivo, principalmente debido al alto contenido de iones de cloruro. El objetivo de este trabajo fue estudiar las primeras etapas de corrosión de cobre en agua de mar artificial (conocida como agua del acuario) con la finalidad de descartar la bio-corrosión debida a la incrustación de bio-organismos presentes en el mar (bio-fouling), enfocándose al desarrollo del proceso de corrosión en su ausencia. Muestras planas de Cu electrolítico fueron sumergidas por 1, 2, 3 y 4 meses en el agua del mar. A estos periodos de tiempo fue registrado su potencial de corrosión Ecorr y la pérdida de masa (Vcorr); así mismo, se analizó la morfología (mediante SEM) del ataque corrosivo y la composición (mediante XRD) de los productos de corrosión. Al inicio la fase mayoritaria cristalina fue la cuprita (Cu2O), conocida como patina, una capa protectora del metal, la cual posteriormente se transformó en paratacamita, Cu2(OH)3Cl. La velocidad de corrosión mostró un crecimiento no lineal, alcanzando un valor de 141 g m-2 (0.015 mm) en el cuarto mes siendo 0.21 g m-2 la liberación (runoff) de iones de cobre. Con el tiempo Ecorr tuvo una tendencia hacia valores más negativos, relacionado con las transformaciones en la fase cuprita. Las imágenes de SEM revelaron que con el avance del proceso de corrosión la superficie de cobre mostró una degradación más profunda y no uniforme

Mg-Ca0.3 Electrochemical Activity Exposed to Hank’s Physiological Solution and Properties of Ag-Nano-Particles Deposits

This work compares the degradation of Mg and Mg-Ca0.3 alloy when they are exposed for 14 days to Hank’s solution at 37 °C. A combination of immersion test, electrochemical techniques (PDP, EIS, EN), and surface characterization methods (SEM-EDS, XRD, and XPS) were carried out. The pH change over time, the lower mass loss (≈20%), and the lower concentration of the released Mg2+ ions (≈3.6 times), as well as the lower level of the surface degradation, allowed to consider the positive effect of Ca, presenting Mg-Ca0.3 alloy with lower electrochemical activity than that of Mg. The positive effect of Ca may be due to the formed layer characteristics on the alloy surface, which impedes the cathodic hydrogen evolution and Mg-ions release. The electroless deposited Ag-nano-particles (Ag-NPs) on Mg-Ca0.3 surface were characterized by SEM-EDS, XRD, UV-Vis, and contact angle. The agar-diffusion test was used to compare the growth of Staphylococcus aureus and Escherichia coli bacteria on Mg-Ca0.3 in the presence of Ag-NPs deposits in different size. Zeta-potential of the bacteria was negative, with respect to pH of the Mueller-Hinton culture broth. The greater antibacterial effect of S. aureus was attributed to its more negative zeta-potential, attracting more released Ag+ ions

Characterization by XRD and FTIR of Zeolite A and Zeolite X Obtained from Fly Ash

Fly ashes from the fluidized bed boiler were used to obtain zeolites by the alkaline fusion process, followed by a hydrothermal procedure. The melting was carried out with 98 % granulated NaOH, mixed with ashes in mass ratios 1.2/1, 1.4/1 and 1.6/1 and using three melting temperatures, 450 °C, 550 °C and 650 °C. For the crystallization of the zeolites, the hydrothermal procedure at 90 °C was used. The crystalline phases of the zeolitic materials were quantified by XRD analysis. The morphology of the crystals was identified by SEM. The FTIR spectra revealed that the transformation of sodalite mineral to zeolite A and zeolite X increased with the NaOH/CVT ratio and the temperature

Procurement and characterization of cellulose nanocrystals from cassava bagasse (Manihot esculenta Crantz)

ABSTRACT Objective: Obtaining and characterizing cellulose nanocrystals from cassava bagasse. Design/methodology/approach: Cellulose nanocrystals were obtained from cassava bagasse by acid hydrolysis (HCI), ultrasonication, centrifugation, dialysis, deep freezing and lyophilization. The cassava bagasse and the cellulose nanocrystals obtained were physiochemically characterized by Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD) and Scanning Electron Microscopy with Coupled Elemental Analysis (SEM-EDS). As an additional technique, Atomic Force Microscopy (AFM) was used. Results: The analyzes performed show that the cellulose obtained was type Iβ. This study reports a percentage of crystallinity of cassava bagasse cellulose of 37.1%, increasing the percentage to 48% of crystallinity in cellulose nanocrystals. The diameters of the cassava bagasse fibers were reported to be 2 μm and their elemental composition (SEM-EDS) mainly constituted by carbon (C), oxygen (O) and traces of (N). The morphology observed through AFM of the nanocrystals of cassava bagasse (Manihot esculenta) was rod-shaped, with a helical appearance without residual charge, with diameters between 8.7 and 9.3 nm. Limitations on study/implications: The acid hydrolysis process showed a low percentage of crystallinity, although higher than other works reported for cassava bagasse. Findings/conclusions: The results obtained show the possibility of obtaining cellulose nanocrystals from cassava bagasse ().    Objective: To procure and characterize cellulose nanocrystals from cassava bagasse. Design/methodology/approach: Cellulose nanocrystals were obtained from cassava bagasse by acid hydrolysis (HCI), ultrasonication, centrifugation, dialysis, deep freezing and lyophilization. The cassava bagasse and the cellulose nanocrystals obtained were physicochemically characterized by Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD) and Scanning Electron Microscopy with Coupled Elemental Analysis (SEM-EDS). As an additional technique, Atomic Force Microscopy (AFM) was used. Results: The analyses performed show that the cellulose obtained was type Iβ. This study reports a percentage of crystallinity of the cassava bagasse cellulose of 37.1%, increasing the percentage to 48% crystallinity in cellulose nanocrystals. The diameters of the cassava bagasse fibers were reported to be 2 μm and their elemental composition (SEM-EDS) mainly constituted by carbon (C), oxygen (O) and traces of nitrogen (N). The morphology observed through AFM of the nanocrystals of cassava bagasse (Manihot esculenta) was rod-shaped, with helicoidal appearance without residual charge, with diameters between 8.7 and 9.3 nm. Limitations on study/implications: The acid hydrolysis process showed a low percentage of crystallinity, although higher than other works reported for cassava bagasse. Findings/conclusions: The results obtained confirm the possibility of obtaining cellulose nanocrystals from cassava bagasse (Manihot esculenta)