Iduvirges Lourdes Muller

Pitting potential of high purity binary aluminium alloys—II. AlMg and AlZn alloys

The pitting potential of binary AlCu, AlZn, and AlMg alloys, with a alloying content from 0 to 5%, were studied in 1.0 M NaCl solutions. The pitting potential of the alloy was found to increase as much as 200 mV by alloying aluminium with copper. The alloying of aluminium with magnesium had small effect on the pitting potential of the alloy. The alloying of aluminium with zinc, on the other hand, produced a decrease of as much as 200 mV in the pitting potential. As predicted by the transport mechanism for pitting, the alloying elements were found to modify the pitting potential by changing the anodic behaviour of the alloy in the pit-like solution. The maintenance of a localized acidification on the metal-solution interface was found to be the main cause for pitting.

Passivation of pure iron in alkaline solution containing silicate and sulphate—Galvanostatic and Potentiostatic studies

Electrochemical studies were carried out with an iron disk electrode using galvanostatic and potentiostatic techniques. A different electrochemical behavior of pure iron was observed in a solution containing silicate when compared to a solution without silicate, specially if it contains sulphate. It appears that silicate inhibits the film thickening while sulphate accelerates the film thickening, at pH 12.

A RRDE study of the electrochemical behavior of iron in solutions containing silicate and sulphate at pH 10–13

Abstract Electrochemical studies were carried out with an iron disk/platinum ring electrode using cyclic voltammetry technique in solutions of x mol/l NaOH +y ppm SiO2 +z mol/l Na2SO4 (x=hydroxyl concentration necessary to promote pHs=10; 10.5; 11; 11.5; 12; 12.5 and 13, respectively; y=0 or 500; z=0 or 0.05). The results show that there is no detectable dissolution in a pH range of 11–13, even if sulphate is present in the solution, although this specie promotes passive film thickening. Fe (II) and Fe (III) soluble species were detected at pH=10.5 only in the absence of silicate and at pH=10, even if silicate is present in the solution.

Electrochemical behaviour of iron in neutral solutions of acetate and benzoate anions

The electrochemical behaviour of iron in pH 6 aqueous solutions of benzoate and acetate anions mixed in varying concentrations and constant ionic strength was investigated. For that purpose cyclic voltammetric experiments were carried out at a disc electrode either static and/or rotating. A high surface coverage is observed in the presence of benzoate anions, thus favouring the establishment of a passive state. The overall process is discussed in terms of a three-dimensional film, the growth mechanism of which is under diffusion and migration control. It was also observed that the addition of chloride ions to the mixed electrolyte solutions containing increasing benzoate concentration shifts the breakdown potential and the repassivation potential toward more positive values.

Stress corrosion cracking and hydrogen embrittlement susceptibility of an eutectoid steel employed in prestressed concrete

The stress corrosion and hydrogen embrittlement behavior of AISI 1080 steel employed in concrete prestressing tendons was studied with different experimental techniques. A simulated concrete pore solution, with and without contaminants such as chloride, sulfate and thiocyanate ions was used. For comparison purposes the standard 20% ammonium thiocyanate solution was also employed. Polarization curves, slow strain rate tests and fracture mechanics tests were used to evaluate the influence of parameters such as potential, temperature (between 0 and 100 °C), and tempering temperature of the steel. The results have shown that the fracture mechanism of the stress corrosion cracking process is associated with hydrogen action.

Characterization of Siloxane-poly(methyl methacrylate) Hybrid Films Obtained on a Tinplate Substrate Modified by the Addition of Organic and Inorganic Acids

Tinplate is used to food packaging and other types of packages. The corrosion resistance of the tinplate has been study due the necessity of an alternative to high environmental impact of chromatization process. Therefore protective coatings as hybrid films base elaborations with different acids are studied to improve the barrier effect against corrosion. The objective of this work is characterize hybrid films deposited on a tinplate from a sol made up of the alkoxide precursors 3-(trimethoxysilylpropyl)methacrylate (TMSM), tetraethoxysilane (TEOS) and poly(methyl methacrylate) (PMMA) together with one of three acids (acetic, hydrochloric or nitric acid) and to verify their action against the corrosion of the substrate. The films were obtained by a dip-coating process and cured for 3 hours at 160 °C. The film hydrophobicity was determined by contact angle measurements, and the morphology was evaluated by SEM. FTIR measurements were performed to characterize the chemical structures of the films. The electrochemical behavior of the coatings was evaluated by techniques open circuit potential monitoring (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results demonstrate that the siloxane-PMMA films improve the protective properties of the tinplate, with the films obtained by acetic acid addition exhibiting the greatest improvement.

Corrosion behaviour of iron and AISI 304 stainless steel in tungstate aqueous solutions containing chloride

O comportamento do ferro e do aco inoxidavel AISI 304 frente a corrosao em solucao aquosa de tungstato de sodio em presenca de cloreto foi investigado usando as tecnicas de voltametria ciclica, medidas de potencial de circuito aberto e impedância eletroquimica. Os resultados mostraram que a acao inibidora do ânion tungstato esta relacionada com sua adsorcao sobre a superficie eletrododica e depende marcadamente da presenca de oxigenio dissolvido no meio. Filmes formados por polarizacao anodica a baixas velocidades de varredura de potencial e rotacao do eletrodo sao mais estaveis permitindo um decrescimo na taxa de corrosao. Medidas de impedância eletroquimica em potencial de circuito aberto mostraram que a acao inibidora do tungstato e consideravelmente favorecida em tempos de imersao prolongados.

The influence of acetate concentration on the dissolution of iron in aqueous solutions

Abstract The influence of sodium acetate concentration in the range of 0.1–1.0 M, on the dissolution process of iron in aqueous solutions with pH values between 5 and 7, has been investigated. Cyclic voltammetry using rotating disc and ring-disc electrodes was employed. The voltammograms present two anodic peaks and in the first one (more negative potential) the reaction order with respect to acetate was 0.1 and at the second one a value of 0.3 was found at pH = 5. With respect to the influence of pH, an order of reaction of 0.2 was found in relation to OH− ions, both for the first and the second peak. Kinetic parameters were determined under non-stationary conditions and compared with those found in the literature. It was observed that the passivation is easier to achieve with the increase of acetate concentration or pH.

The corrosion behaviour of mild steel and type 304 stainless steel in media from an anaerobic biodigestor

Abstract Mild steel and AISI 304 stainless steel samples were exposed to the aqueous solutions from an anaerobic biodigestor of wastewater, showing quite different behaviours. Carbon steel presented generalized corrosion whereas stainless steel, as-received or sensitized, tended to show some localized corrosion. Both materials presented bacteria attachment, particularly sulphate reducing bacteria (SRB), forming biofilms which were abundant and mixed with corrosion products on the mild steel surface and thin and patchy on stainless steel surfaces. Different types of anaerobic and aerobic bacteria were detected in the medium. Experiments were carried out both at 1 and at 7 atmospheres pressure, in presence of a gaseous phase containing N 2 , CO 2 and CH 4 . Potentiodynamic experiments were conducted in order to have a better insight on the electrochemical behaviour of the material in this medium.

The electrochemical behaviour of Fe in acetate solution containing chloride

The effect of chloride ions on the dissolution and passivation of iron in 1 M sodium acetate solution, pH 5, has been studied using voltammetric and rotating ring disc electrode (RRDE) techniques. Soluble Fe(II) and Fe(III) species have been detected during the active-to-passive transition and pitting potential regions. The addition of chloride ions at higher concentrations than 700 ppm in this medium causes the rupture of the anodic film accompanied by pitting corrosion of Fe.