M. Marcos

Lanthanide compounds as environmentally-friendly corrosion inhibitors of aluminium alloys: a review

Abstract Currently, chromates are among the most common substances used as inhibitors or incorporated in anticorrosive pretreatments of aluminium alloys. However, these compounds are highly toxic and their use produces serious environmental hazards. Consequently, an intense research effort is being undertaken to replace chromates by more ecological compounds. In recent years, several authors have begun studies of the behaviour of lanthanide compounds in the corrosion protection of metallic alloys. After a brief overview of the use of chromates and their ecological alternatives in corrosion protection systems, the paper reviews progress from the literature published to date concerning the application of lanthanide compounds as corrosion inhibitors of aluminium alloys in aqueous solution. Aspects of the mechanism involved in the cathodic inhibition process by lanthanides are also dealt with. In addition, these compounds have been used to prepare chromate-free conversion coatings by full-immersion and electrochemical activation methods. Treatments proposed for these conversion coatings are discussed in detail. From the comprehensive review presented in this article, it is concluded that lanthanide compounds fulfil the basic requirements for consideration as components of more environmentally-friendly formulations: low toxicity and acceptable protective capacity. However, methods proposed up to now present various limitations that restrict their application in practice. Further research efforts are necessary to develop feasible treatments for industry.

Using wavelets transform in the analysis of electrochemical noise data

Various concepts developed in the framework of wavelet transforms have been adapted to the analysis of electrochemical noise measurements (ENM). Wavelet transforms are proposed as an alternative tool to overcome the limitations of FFT in the analysis of ENM data. In this context, the theoretical basis of wavelet analysis is briefly reviewed. Then, both methods are applied to the study of various different corrosion systems covering a wide range of ENM signals. The results reported here demonstrate, firstly, that wavelet analysis is applicable to those systems in which FFT techniques also work. Moreover, in cases where FFT fails, wavelet analysis provides valuable knowledge about the behaviour of the system. Using wavelets, the different ENM components contributing to the original signal can be characterised. Each component is defined by a set of wavelet coefficients, which contain information about the time scale characteristic of the associated corrosion event.

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.

Influence of chemical composition on the pitting corrosion resistance of non-standard low-Ni high-Mn–N duplex stainless steels

Abstract The pitting corrosion resistance of a new family of duplex stainless steels has been evaluated. These non-standard duplex stainless steels are characterised by low Ni content and high N and Mn levels. Potentiodynamic polarisation scans in NaCl solution have been carried out to determine pitting potentials. A crevice-free cell has been used to perform the electrochemical tests. An exponential equation is obtained in the regression analysis between the pitting potential and chemical composition which allows an estimate of the pitting resistance of these new duplex stainless steels.

CeCl3 and LaCl3 binary solutions as environment-friendly corrosion inhibitors of AA5083 Al–Mg alloy in NaCl solutions

Abstract Lanthanum and cerium chloride and their binary mixtures have been investigated as corrosion inhibitors of AA5083 Al–Mg alloy in aerated 3.5% NaCl aqueous solution. Their inhibitor power was evaluated by using electrochemical techniques together with full immersion tests. The highest protection degree was found for the binary solution doped with 250 ppm CeCl3 and 250 ppm LaCl3. Scanning electron microscopy and energy dispersive spectroscopy enabled us to confirm the cathodic nature of the inhibition process and to establish some features of the inhibition process.

Localized alkaline corrosion of alloy AA5083 in neutral 3.5% NaCl solution

The corrosion process of the alloy AA5083 in an aerated solution of NaCl at 3.5% has been studied. The results obtained indicate that the main process that this alloy undergoes, under the conditions of exposure studied, is related to localized corrosion that takes place as a consequence of the process of alkalinization around the cathodic precipitates existing in the alloy. The pitting formed presents a hemispherical morphology that is clearly different from crystallographic pitting. The formation of crystallographic pitting has not been observed, even in samples submitted to tests of very long duration. In order to obtain the formation of crystallographic pitting, it is necessary to polarize the alloy at the nucleation potential of pitting and, in addition, the density of the current must be above a critical value. Only when the layer of oxide is eliminated does the formation of crystallographic pitting take place by simple exposure in an aerated solution of NaCl at 3.5%.

Inhibitor properties of “green” pigments for paints

Abstract Zinc chromate is one of the anticorrosive pigments most frequently used in the formulation of primers. However, its environmental aggressiveness and toxicity severely restrict its use, and different green alternatives have been proposed in order to replace zinc chromate. In the last decade, the behaviour of zinc phosphate as anticorrosive pigment has been intensively researched. During this time, various modifications have been made to this family of pigments to improve its properties, and a “second generation” of phosphate pigments, incorporating elements such as molybdenum, aluminium, or iron, has been produced. In this paper, the inhibitive properties of zinc phosphate and three second-generation phosphates have been investigated, using zinc chromate pigment as a reference. Pigment extract solutions, at different values of pH, have been used as corrosive media. Carbon steel samples were immersed in such solutions and their corrosion rates were measured using electrochemical techniques. The data obtained suggest that zinc chromate provides the highest percentage of inhibition in neutral and basic solutions, but phosphate-based pigments showed better results in acid solutions. Given this performance advantage, together with their less harmful environmental impact, these phosphate-based pigments can be proposed as realistic alternatives to chromates in the formulation of protective paints for use in acidic conditions.

WAVELET TRANSFORM-BASED ANALYSIS FOR ELECTROCHEMICAL NOISE

Abstract Wavelet transforms are presented as a useful tool to analyse electrochemical noise data. Various concepts developed in the framework of wavelet transforms have been adapted to study electrochemical noise measurements. The most relevant feature of this method of analysis is its capability of decomposing electrochemical noise records into different sets of wavelet coefficients, which contain information about corrosion events occurring at a determined time-scale. Thus, this mathematical approach could become an alternative tool which solves the limitations of other more established procedures for the analysis of electrochemical noise data, such as statistical or Fourier transform-based methods.

Protection against corrosion in marine environments of AA5083 Al–Mg alloy by lanthanide chlorides

Abstract The addition of small concentrations of lanthanide chlorides to 3.5% NaCl aerated aqueous solutions inhibits uniform and pitting corrosion processes of AA5083 Al–Mg alloy. Full immersion tests combined with different electrochemical techniques have been used in order to determine the protection degree and the inhibition character supplied by the rare earth salts. Additional Scanning Electron Microscopy and Energy Dispersive Spectroscopy data have allowed us to establish some features of the inhibition mechanism.

Use of wavelets to study electrochemical noise transients

Two kinds of time records are found when measuring electrochemical noise: those presenting characteristic transients or spikes, and others in which neither transients nor spikes can be distinguished. Although the former are likely to be more informative than the latter, most of the methods developed for analyzing electrochemical noise are focused on studying the latter. This paper suggests a methodology for automatically studying the appearance of the transient, which could represent a significant advance in the field of corrosion monitoring and laboratory investigation. The proposed methodology is based on a mathematical tool called the wavelet transform, whose principal characteristic is that it enables a simultaneous analysis of signals in both time and scale. Thus a subsequent interscalar analysis of the signals can reveal the existence of transients and even provide a comparative characterization of their size and scale.