Corrosion and protection characteristics of phosphate coatings: effect of yttrium oxide as additive

Abstract

The corrosion and protection characteristics of phosphate coatings formed in a phosphating solution containing mainly ZnO, H₃PO₄ and NaF, using Y₂O₃ as an additive, were investigated through SEM, polarisation curves and EIS diagrams. The results show that the corrosion protection of phosphate coatings has been improved when Y₂O₃ is added to the phosphating solution, making the free corrosion potential shift to the positive direction and causing the corrosion current to decrease. The protection ability of phosphate coatings depends mainly on their barrier performance. The phosphate coatings formed in the phosphating solution with 10 and 20 mg L^–1 Y₂O₃ have finer crystal structures and smaller porosity; therefore, they exhibit better corrosion resistance and adhesion properties than those without Y₂O₃ and with 40 mg L^–1 Y₂O₃.     

Corrosion resistance of Ni/Al2O3 coatings in NaCl solution

Abstract

This paper presents results of a research on the corrosion resistance of composite Ni/Al₂O₃ coatings electrochemically deposited from Watts baths containing different amounts (0, 20, 100 g dm^?3) of Al₂O₃ particles. Potentiodynamic tests and electrochemical impedance spectroscopy (EIS) measurements were carried out in a 3% solution of NaCl. The coatings with about 6 wt-% of corundum, deposited from a bath with 100 g dm^?3 of a powder, showed the best protective properties. The rate of corrosion of such coatings after 7 days of exposure in the NaCl solution was over two times slower than that of coatings containing 2 wt-% of Al₂O₃ and six times slower than that of a standard nickel coating. Two equivalent circuits: one consisting of two RC circuits and the other one made up of three RC circuits were used for the analysis of the impedance spectra. Regardless of the presence and amount of the Al₂O₃ particles in the nickel coating, during first day of exposure in the NaCl solution a layer of nickel oxides and hydroxides forms on the surface of the coatings increasing their corrosion resistance. In the case of coatings with 6 wt-% of Al₂O₃, the passive layer is least vulnerable to the aggressive action of Cl^? ions.     

Electrodeposition of high performance multilayer coatings of Zn–Co using triangular current pulses

Abstract

Compositionally modulated alloy (CMA) coatings of Zn–Co were electrodeposited on to mild steel from an acid chloride bath containing thiamine hydrochloride, as an additive. Electroplating was carried out galvanostatically from a single bath containing Zn²⁺ and Co²⁺ ions. Gradual change in composition in each layer was effected by triangular current pulses, cycling between two cathode current densities. Compositionally modulated alloy coatings were developed under different conditions of cyclic cathode current density and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. The formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy. The corrosion resistances of CMA and monolithic alloy coatings were compared with that of the base metal. Compositionally modulated alloy coating at optimal configuration, represented as (Zn–Co)_{2·0/4·0/300}, was found to exhibit ～80 times better corrosion resistance compared with monolithic (Zn–Co)_3·0 alloy, deposited for the same length of time from the same bath. Improved corrosion resistance was attributed to the formation of n-type semiconductor film at the interface, supported by Mott–Schottky plots. Decrease in corrosion resistance at high degree of layering was found, and is due to lower relaxation time for redistribution of solutes in the diffusion double layer, during plating.     

Citric acid as natural corrosion inhibitor for aluminium protection

Abstract

The inhibition effect of citric acid on the corrosion behaviour of aluminium in 2M NaCl solution (pH 2) was studied with the help of potentiodynamic polarisation, electrochemical impedance spectroscopy (EIS) and linear polarisation resistance (LPR) techniques. The experimental results showed that citric acid inhibits the corrosion of aluminium in NaCl solution and the inhibition efficiency depends on its concentration. The protection efficiency increases with citric acid concentration up to a critical value of 1·0 × 10^?5M. At higher concentrations, the inhibition efficiency reduced again with increasing concentration. The mechanism of inhibition was attributed to the adsorption of citric acid onto the metal surface.     

Chemical and structural properties of ancient metallic artefacts: multitechnique approach to study of early bronzes

Abstract

The results are presented of a characterisation study of the microstructure and microchemistry of archaeological bronze (Cu–Sn) artefacts from the eighth to the sixth century bc. Metallographic examination, with optical and electron optical microscopy, has been performed on polished sections of early Iron Age studs and bracelets found in incineration tombs of the Necropolis of Chiavari in Italy. A heterogeneous microstructure of the bronze was observed, exhibiting a wide range of grain sizes, and a predominant α-phase solid solution containing α/δ and α/? eutectoid phases decorated with a high density of inclusions. The composition of grain boundary surfaces was determined, using scanning Auger microscopy (SAM), on specimens fractured in vacuo. The extent of tin segregation at the grain boundaries was 3–5 times greater than that in the grain interiors. Copper rich sulphides occasionally containing the oligoelements (iron and lead) were identified as the predominant type of inclusions formed at the grain interfaces and within the grains. Analysis of the corrosion patina at the surface of the bronze artefacts was conducted by combining X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and SAM. The patina was found to exhibit a multilayered structure and a complex chemical composition forming various crystallographic phases including malachite, cuprite, and copper–tin oxide. Corrosion of the underlying bronze matrix proceeded along the grain boundaries, where the sacrificial corrosion of tin reacting with diffused oxygen and chlorine was identified. The results of this study have been used to clarify the metallurgy and manufacturing processes of the examined finds, and to evaluate the state of their degradation.     

Inhibition of pure iron in sulphuric acid by N,N-dipropynoxymethylamine acetate and its synergism with chloride

Abstract

The inhibition mechanism of N,N-dipropynoxymethylamine acetate on pure iron in H₂ SO₄ solution and its synergism with chloride was studied by potentiodynamic polarisation and electrochemical impedance techniques. Results showed that the inhibition mechanism was the mixed deactivation mechanism. Molecules of the inhibitor blocked the active sites on the iron surface and had a distinct inhibition synergism with chloride on the surface of the pure iron. Inhibition efficiencies and synergism were related to the concentration and molecular structure of the inhibitor.     

Characterisation of titanium oxide films by potentiodynamic polarisation and electrochemical impedance spectroscopy

Abstract

Titanium is used for many applications, ranging from chemical, biomedical, and decorative. Its versatility is due to its excellent mechanical properties, high corrosion resistance and biocompatibility. Moreover, its oxides, grown by anodising techniques, can have different colourations, which make them useful for applications related to art and architecture; in their crystalline form, they show excellent photochemical properties, which are widely used in chemical synthesis and in the cleansing of pollutants. The aim of this work is to characterise the titanium dioxide films, grown anodically in sulphuric acid 0·5 M at three different anodising potentials, by potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). When comparing the obtained oxide films, a progressive increment in their crystallinity and electrical resistance was observed with the increase of the anodising potential.     

Alternative methodology for on site monitoring of corrosion and remediation of reinforced concrete

Abstract

Difficulties associated with the interpretation of site data collected over long periods of time from commonly used corrosion monitoring techniques for steel reinforcement in concrete, such as corrosion potential measurements and linear polarisation resistance, often make it difficult to assess accurately the extent of corrosion. An alternative methodology for the interpretation of data is proposed, based upon a model of the quality of the passive film on the steel surface. This model leads to a representation of the corrosion state by means of the relationship, over a long period of time, between the corrosion potential and the logarithm of the linear polarisation resistance. For the reinforced concrete panels tested in this study, data points representing this relationship closely fitted a family of results. This led to the development of a so called 'monitoring control diagram', MCD, in which for a fixed geometry and fixed experimental conditions, a relationship between the corrosion potential and polarisation resistance of steel exists. The establishment of an MCD may enable the development of a useful monitoring tool.