J. de Damborenea

Inhibition of 5083 aluminium alloy and galvanised steel by lanthanide salts

Lanthanide salts are being considered as an environmentally friendly alternative to the classic systems based on chromates. The inhibitor behaviour of CeCl3 has been studied for both AA5083 alloy and galvanised steel in aerated NaCl solutions. For AA5083, cerium appears as dispersed islands whilst in the galvanised steel samples, it is stochastically dispersed onto the metallic surface, forming a film. These differences can be interpreted in terms of the distribution of cathodic areas for each material. In the case of AA5083, Al6–(Mn,Fe,Cr) acts as permanent cathodic site whilst permanent cathodic sites do not exist in the galvanised steel.

Cerium: a suitable green corrosion inhibitor for tinplate

Abstract In chloride solutions, tinplate exhibits localised corrosion processes due to the defects or imperfections of the outer Sn layer, which leaves the Sn–Fe alloyed layer exposed to aggressive solutions. The anodic character of the external Sn with respect to the internal alloyed layer, will lead to its dissolution acting as a sacrificial anode. The addition of cerium salts to the aggressive media decreases the pitting susceptibility of tinplate by means of cerium precipitation on the cathodic sites (bare Sn–Fe areas). This induces to a change of the controlling mechanism suggesting that cerium is a cathodic inhibitor for tinplate, which could be corroborated by removal selectively the outer Sn layer and leaving the Sn–Fe layer directly exposed to the solution. In these conditions, the corrosion process of the steel base will take place through the defects of the Sn–Fe layer, which presents a cathodic character in comparison to the base metal, inducing the precipitation of cerium on the alloyed layer. Scanning electron microscopy and energy dispersive spectroscopy analysis revealed that the location of cerium precipitates depend on the distribution of the cathodic areas that could lead either to a continuous layer covering the surface or to precipitates inside the mechanical failures.

Growth mechanisms of cerium layers on galvanised steel

In recent times, a good deal of research efforts have been devoted to develop new environmentally friendly corrosion inhibitors as an alternative to the classic systems based on chromates. In this way, rare earth salts have demonstrated a high efficiency in different metal/corrosive systems. Lanthanide salts are classified as cathodic inhibitors, although, it is not clarified the species which contribute to the formation, in the cathodic areas, of the oxide/hydroxide of the lanthanide ion. In this work, the inhibitor behaviour of CeCl3 for galvanised steel in aerated NaCl solutions has been investigated. SEM and EDS studies showed that Ce is initially stochastically dispersed onto the metallic surface, forming a continuous film on its surface after prolonged immersion time. Electrochemical calculations seem to indicate that, under experimental conditions only Ce3+ is expected on the metallic surface and this is consistent with experimental findings.

Corrosion properties of tin prepared by laser gas alloying of ti and ti6al4v

Abstract TiN coatings were prepared by laser surface alloying of commercially pure (c.p.) Ti and Ti6Al4V alloy in a nitrogen atmosphere. Surface gas alloying was carried out using a 5 kW Spectra–Physics transverse flow CO2 laser operating in TEM 20 at a scan rate of 0.3 m\min and an input flow of 30 l\min. After laser gas alloying, surface tracks of a light gold colour could be seen by the naked eye. The microstructure of the Ti alloy revealed a surface layer of TiN dendrites in a nitrogen-enriched alpha matrix covering the melted surface. The corrosion properties of the material are discussed through the results of anodic polarization test and electrochemical impedance spectroscopy in HCl solution. An improvement in corrosion behaviour, associated with the presence of a good TiN coating, was observed.

Adsorption and inhibitive properties of four primary aliphatic amines on mild steel in 2 M hydrochloric acid

Abstract Four n-alkyl amines (6, 8, 10 and 12 carbon atoms) were investigated as corrosion inhibitors for mild steel in 2 M hydrochloric acid solution using gravimetric and polarization techniques. The influences of temperature (278–308 K) and inhibitor concentration (10−1-10−6 M) were studied. Protection efficiency improved when the inhibitor concentration and the length of the alkyl chain were increased. The four amines tested were adsorbed on the mild steel surface according to a Frumkin isotherm.

Corrosion behaviour of steels after laser surface melting

Abstract Laser surface melting (LSM) were carried out in three types of stainless steels — austenitic, martensitic and ferritic — in order to improve pitting corrosion resistance. After the LSM, most of the steels present a shift in the pitting potential to more noble values, a longer passivity stage and lower passive current density than the substrate. These improvements are achieved by means of a modified surface layer, more homogeneous with a fine cellular dendritic structure free of large precipitates. Nevertheless, the corrosion resistance depends critically on the laser processing parameters, particularly for the cases of ferritic and martensitic stainless steels. Therefore, care must be taken in the choice of the laser-processing parameters that leads to optimal properties in each material.

Electrochemical impedance spectroscopy for studying the degradation of enamel coatings

Vitreous coatings can be used as a means for the corrosion protection of metals and alloys due to their excellent dielectric properties. Their use is of particular interest in the case of highly aggressive media (different pH levels, temperatures up to 600 °C). However, ceramic coatings exposed to aggressive agents and thermal cycles can experience devitrification and spalling problems, with the consequent loss of their protective properties. The present study uses the electrochemical impedance spectroscopy technique to characterize, in a non-destructive way, the appearance of surface defects induced by the effect of temperature and chemical attack on a vitreous enamel deposited on steel. By means of analysis of the impedance diagrams obtained, a relationship is established between the loss of the coatings protective properties and the mechanism by which the corrosion process takes place.

Electrochemical and in vitro behaviour of sol–gel coated 316L stainless steel

The corrosion resistance of AISI 316L stainless steel for biomedical applications, was significantly enhanced by means of inorganic and hybrid SiO2 sol–gel coatings deposited by dip-coating. Coatings of 0.5–1.1 μm with different hybrid character were obtained by varying the temperature of thermal treatment from 400 to 550 °C. The pitting potential is increased up to 1 V and the current intensities were reduced up to 10−8 A/cm2. After one month of immersion in simulated body fluids, all the coatings showed a high degree of corrosion resistance and integrity, being slightly better the behaviour of the hybrid coatings. Not any bioactivity signal was observed.

Electrochemical behaviour of SiO2 sol-gel coatings on stainless steel

SiO2 coatings onto stainless steel substrates have been prepared by sol-gel in order to study the performance and mechanism of attack in different corrosive solutions. The electrochemical behaviour of the samples has been evaluated by Electrochemical Impedance Spectroscopy using NaCl and HCl as electrolytes. Comparative tests have been performed on samples with one and two silica layers as well as on uncoated ones. SiO2 coatings produce no important protection of stainless steels subjected to electrochemical corrosion. This behaviour may be explained by micropores and microcracks produced during the coating sintering.

Electrochemical modelling of exfoliation corrosion behaviour of 8090 alloy

The susceptibility of alloy 8090 T8171 to exfoliation corrosion has been analysed using the electrochemical impedance spectroscopy technique (EIS). Corrosion susceptibility was determined in a standard medium (EXCO tests). This acid solution creates a very severe attack in a short period of time and makes it possible to distinguish the different stages of the corrosion process. The EIS technique enables collection impedance spectra which show a variation with increasing exposure time and so is appropriate for recording these stages. The Nyquist diagram is initially described by a well defined capacitive semicircle, which suggests a corrosion process under activation control. This response is linked to the existence of small pits on the surface of the working electrode. However, when the immersion time increases, the attack causes the appearance of large pits on the electrode, leading to an impedance response comprised by two overlapping semicircles; the high-frequency semicircle related with the flat part of the working electrode and the low frequency arc associated with the part covered by pitting.