9 research outputs found
Electrochemistry and surface chemistry of stainless steels in alkaline media simulating concrete pore solutions
No full textDespite the increased use of stainless steel for concrete reinforcement in harsh chloride environments, comparatively little is known about the surface chemistry of these materials in alkaline media simulating concrete pore solutions. This work is concerned with a combined electrochemical and XPS surface analytical investigation on austenitic, ferritic and duplex stainless steels in simple NaOH and more complex alkaline concrete pore solutions. The results show that the passive films on these materials change with immersion time, the ferritic and duplex stainless steels becoming enriched in chromium oxy-hydroxide, the austenitic steel strongly enriched in nickel hydroxide. The composition of the metal layer beneath the surface film is strongly enriched in nickel and depleted in chromium and iron. The results are discussed with respect to the relation between the Fe(II) content in the films and the open circuit potential (OCP) during exposure, the film growth mechanism and localized corrosion resistance
Electrochemical and XPS surface analytical study on the reactivity of Ni-free stainless steel in artificial saliva
No full textNowadays there is a strong demand to replace CrNi stainless steels used for biomedical applications such as brackets in orthodontics with a new generation of more biocompatible austenitic, nickel-free alloys. The aim of this work is the investigation of the growth and stability of the surface films formed on DIN 1.4456 Ni-free stainless steel (18% Cr, 18% Mn, 2% Mo) in artificial saliva (pH 7.9) at 37°C by electrochemistry and XPS surface analyses and the assessment of the ions that leach from the steel. These results show that the open circuit potential asymptotically increases and the corrosion current density decreases from 1 to 24 hours exposure time to the artificial saliva. XPS provides evidence that the surface film present on the surface after mechanical polishing transforms into a passive film that becomes progressively enriched in Cr(III) oxy-hydroxides whereas iron and manga¬nese oxides are depleted. It is found that the instantaneous corrosion rate decreases exponentially with the amount of Cr(III) in the passive film. The results are discussed regarding the biocompatibility, thus the release of ions into solution, of the new nickel-free stainless steel. Based on the results presented in this paper and literature evidence a comprehensive model is proposed that substantiates the biocompatibility of these alloy
An XPS investigation on glucose oxidase and Ni/Al hydrotalcite interaction
No full textIn this work, a Ni/Al hydrotalcite (HT) was used as glucose oxidase (GOx) immobilizer. Small-area and angle-resolved X-ray photoelectron spectra were recorded on HTs electrosynthesized on Pt in the absence and in the presence of GOx, and compared with those obtained for a Pt surface, modified with the electrosynthesized HT, on which a drop of GOx solution was deposited. The simultaneous electrodeposition of HT + GOx resulted in a compact deposit, thicker than the XPS sampling depth (>10 nm), that is not homogeneous in the lateral and in-depth composition. The presence of GOx can be deduced comparing the N1s spectra of HT and HT + GOx: in the latter, the N1s component at 400 eV binding energy (BE) is predominant whilst, depending on the analyzed point, a small or no contribution from the component at 407.2 eV, due to nitrate, is revealed. Angle-resolved XPS provides evidence on the in-depth composition of anions, cations and GOx. The results highlight the crucial role played by nickel in GOx immobilization. On the basis of the results, it can be suggested that enzyme activity is unevenly distributed and is localized in small areas, where Ni concentration is higher
Fe10Cr and Fe15Cr alloys as standards for stainless steel surface characterization by XPS
No full textInfluence of major anions on As(V) adsorption by synthetic 2-line ferrihydrite. Kinetic investigation and XPS study of the competitive effect of bicarbonate
No full textThe potential competitive effect of background electrolytes (Na(2)HPO(4) center dot 2H(2)O, NaHCO(3), Na(2)SO(4) and NaCl solutions) on arsenate adsorption onto synthetic 2-line ferrihydrite has been studied by means of kinetic batch experiments conducted at pH values from 4.0 to 10.0 and at anionic concentrations of 0.01 and 0.1 M. The results indicate that the adsorptive capacity of ferrihydrite for arsenate decreases strongly in the presence of phosphate species at pH in the range of 4-10 and in the presence of bicarbonate at pH 8.3 as a consequence of their competitive effect. Analogously to phosphate, a surface interaction of inner-sphere type between ferrihydrite and bicarbonate is suggested. Chloride has negligible effects on arsenate adsorption processes, confirming it as an outer-sphere ion that does not compete with the inner-sphere binding peculiar to arsenate onto ferrihydrite. Sulphate exhibits an intermediate behaviour; at 0.01 M concentration, the competitive effect of sulphate is similar to that of chloride, whereas at 0.1 M concentration sulphate shows a moderate influence on arsenate adsorption. The results of the kinetic studies can be summarised by the following order of competitive capacity: phosphate > carbonate > sulphate > chloride. The process of arsenate adsorption follows pseudo-second order kinetics and the reaction half-time notably increases in the presence of strong competitor anions such as phosphate and carbonate with respect to an ineffective competitor anion such as chloride. Modelling of arsenate adsorption with PHREEQC, according to the Generalized Two-Layer Model, confirms that the pH effect is notably less important than the competitive effect of carbonate species in determining the amount of arsenate adsorbed onto ferrihydrite at pH 8.3 in 0.1 M NaHCO(3) solution, whereas the model greatly underestimates the competitive effect of carbonate species at pH 8.3 in 0.01 M NaHCO(3) solution. The results of the batch experiments in 0.1 M NaHCO(3) solution are substantiated by XPS analyses of ferrihydrite after immersion in the same solution, both with and without dissolved arsenate. XPS confirms the interaction between ferrihydrite surface and arsenate; the binding energy of As3d shifts towards higher binding energies after adsorption with respect to the pure compound Na(2)HAsO(4)center dot 7H(2)O taken as reference standard. In presence of carbonate species, the As3d binding energy is found at intermediate values. XPS quantitative analysis shows a depletion of arsenate on ferrihydrite surface, providing further evidence of the competition of the two species (i.e. arsenate and bicarbonate) for the ferrihydrite adsorption sites. Important environmental implications concerning arsenic mobility, as well as possible application in various fields (e.g. irrigation agriculture, soil decontamination, water treatment), might derive from these findings
An XPS investigation on glucose oxidase and Ni/Al Hydrotalcite interaction”
No full textIn this work, a Ni/Al hydrotalcite (HT) was used as glucose oxidase (GOx) immobilizer. Small-area and angle-resolved X-ray
photoelectron spectra were recorded on HTs electrosynthesised on Pt in the absence and in the presence of GOx, and compared
with those obtained for a Pt surface,modified with the electrosynthesized HT, on which a drop of GOx solution was deposited.
The simultaneous electrodeposition of HT+GOx resulted in a compact deposit, thicker than the XPS sampling depth (>10 nm),
that is not homogeneous in the lateral and indepth composition. The presence of GOx can be deduced comparing the N1s
spectra of HT and HT + GOx: in the latter, the N1s component at 400 eV binding energy (BE) is predominant whilst, depending
on the analyzed point, a small or no contribution from the component at 407.2 eV, due to nitrate, is revealed. Angle-resolved
XPS provides evidence on the indepth composition of anions, cations and GOx. The results highlight the crucial role played by
nickel in GOx immobilization. On the basis of the results, it can be suggested that enzyme activity is unevenly distributed and
is localized in small areas, where Ni concentration is higher
Influence of HCOO<sup>–</sup> on Calcite Growth from First-Principles
No full textOrganic molecules in aqueous solutions
are good candidates in the
inhibition of some biogenic crystal growth. The formic acid HCOOH
is considered to investigate at an atomic level the interaction between
the carboxyl functional group −COO<sup>–</sup> and the
(101̅4) hydrated surface of calcium carbonate, CaCO<sub>3</sub>, in the form of calcite. Ab initio simulations based on the density
functional theory are performed to study the adsorption of undissociated
and dehydrogenated HCOOH in the presence of water. Relevant adsorption
energies obtained for HCOO<sup>–</sup> + H<sub>2</sub>O on
calcite predict that water is essential in the stabilization of the
carboxyl group in its deprotonated form. The interfacial properties
and the trend of adsorption energies for different coverages are given
in detail. The dissociation barriers of HCOOH on hydrated calcite
are evaluated with the climbing-image nudged elastic band (CI-NEB)
method
