Influence of aeration on the localized trenching on aluminium alloys

This paper aims to make the difference between alcalinisation and micro-galvanic corrosion in the mechanism of the corrosion of Al alloys. Using a pH microprobe, the pH changes inside a pure water droplet deposited on a thermal aged 2011 aluminium alloy were scanned. These pH variations, depending on the position inside the droplet, were related to the heterogeneous corrosion attacks of the aluminium surface quantified by AFM measurements. A modelling investigation is proposed to evaluate the distinct roles of alcalinisation, due to the oxygen reduction reaction (ORR), and the galvanic coupling between intermetallic particles and the surrounding matrix. It can be concluded that the former controls corrosion initiation and lateral extension whereas the latter governs the propagation kinetics.     

Modelling of anodic dissolution of pure aluminium in sodium chloride

A mathematical model for simulating a passive aluminium (Al) surface with a pit in which active electrochemical metal dissolution occurs has been developed. The model includes hydrolysis products of Al and the species obtained as a result of homogeneous reactions between chloride and Al³⁺ ions and Al hydrolysis products. The model does not assume the equilibrium state in solution: all terms in homogeneous reactions are treated explicitly using kinetic constants taken from the literature. The validity of assuming reaction equilibrium has been addressed. Solution potential values and species concentrations are predicted for different dissolution current densities. The acidity in the pit is explained by the hydrolysis of Al³⁺; an analytical expression for the pH values at the pit bottom for a given dissolution current density is presented.The model is applied to a real capillary geometry used in electrochemical microcell experiments. It was found that for r_cap/r_pit < 100, where r_cap and r_pit are the capillary end and pit radii, respectively, the insulating capillary wall affects the species concentrations and the solution potential. Moreover, for r_cap/r_pit < 20, the shape of the capillary, which might not be cylindrical, should be taken into account.     

Integrated AFM and SECM for in situ studies of localized corrosion of Al alloys

Rolled 3xxx series Al alloys, e.g., EN AW-3003, are generally used as fin or tube material in heat exchangers for automobiles. With reducing fin thickness, maintaining fin material integrity is of increasing importance. This study aimed at exploring the differences in intrinsic corrosion properties between EN AW-3003 and a newly developed Al-Mn-Si-Zr fin alloy using state-of-the-art local probing techniques. Volta potential mapping of both alloys by scanning Kelvin probe force microscopy (SKPFM) indicates a cathodic behaviour of constituent intermetallic particles (>0.5 μm) relative to the alloy matrix. Compared to EN AW-3003, the Al-Mn-Si-Zr alloy has a smaller number of particles with large Volta potential difference relative to the matrix. In situ atomic force microscopy (AFM) measurements in slightly corrosive solutions showed extensive localized dissolution and deposition of corrosion products on EN AW-3003, and only a small number of corroding sites and “tunnel-like” pits on Al-Mn-Si-Zr. Probing the ongoing localized corrosion process by integrated AFM and scanning electrochemical microscopy (SECM) revealed more extensive local electrochemical activity on EN AW-3003 than on Al-Mn-Si-Zr. In all, the lower corrosion activity and smaller tunnel-like pits resulted in lower material loss of the Al-Mn-Si-Zr alloy, a beneficial property when striving towards thinner fin material.     

Effects of Mn on the localized corrosion behavior of Fe-18Cr alloys

Effects of Mn on the localized corrosion, anodic dissolution behavior, and repassivation kinetics of Fe-18Cr-xMn (x = 0, 6, 12) alloys were examined using potentiodynamic tests with or without micro-electrochemical cell, electrochemical impedance spectroscopy (EIS), and rapid scratch electrode tests. The addition of Mn to Fe-18Cr alloy significantly degrades passivity by decreasing the resistance to localized corrosion, and also by expanding the active region in the noble direction. The decrease in the resistance to localized corrosion of the alloys is due primarily to an increase in the number and size of Mn-containing oxides, acting as initiation sites for pitting corrosion. It was demonstrated using a micro-electrochemical test that the inherent protectiveness of passive film is also considerably reduced by the Mn addition, even though the deleterious influences of Mn-containing oxides are completely excluded. Mn facilitates the metal dissolution reaction by enhancing the activity of iron adsorbed intermediate or by generating second intermediate species (possibly manganese adsorbed intermediate) acting as another dissolution path. Further, the Mn addition appears to suppress the passivation process by reducing the activity of Cr-adsorbed species in an acidic solution, and hence the repassivation rate is significantly decreased with Mn content of the alloys.     

Predictive modeling of localized corrosion: An application to aluminum alloys

Corrosion prevention in light-weight alloys is currently an area of major research for civilian, aerospace, and defense applications. In order to understand thoroughly the complex corrosion system and hence develop effective and “green” corrosion prevention strategies, predictive modeling is believed to be an essential tool. This work presents a new finite element method (FEM)-based corrosion model, specifically tailored for localized pitting corrosion of aluminum alloys. The model distinguishes itself from existing ones by its strong predictive power and high generality. By resorting to this methodology, not only corrosion rate but also pit stability can be quantitatively evaluated for a wide range of systems involving heterogeneous alloy microstructure, complex pit morphology, and versatile solution chemistry. Moreover, the knowledge discovered can shed light on the control of pit repassivation, which will eventually lead to effective corrosion inhibition approaches. A thorough investigation of pitting corrosion of aircraft aluminum alloys is presented in order to demonstrate the efficacy and attractiveness of our method.     

Studies of the anodic dissolution of aluminium alloys containing tin and gallium using imaging with a high-speed camera

Imaging with a high-speed camera at a resolution of 10–20 μm has been used for the direct observation of the anodic dissolution of aluminium alloys containing Sn and Ga. The imaging allows confirmation that hydrogen bubble evolution occurs from the Sn inclusions within rounded pits during both open circuit corrosion and anodic dissolution. Using microelectrodes with only a few Sn inclusions in their surface, it is shown that the evolution of H₂ is not continuous and may be correlated with a potential oscillations between −1.50 V (where H₂ evolution occurs) and significantly less negative potentials (where no H₂ is evolved). It is proposed that this potential shift is associated with pH changes resulting from H₂ evolution itself.     

Noise resistance and shot noise parameters on the study of IGC of aluminium alloys with different heat treatments

In the present paper, the effect of heat treatment on the susceptibility to intergranular corrosion (IGC) of aluminium alloys is analysed. Samples of aluminium alloys AA2024 and AA7075 were first subjected to different heat treatments. Then the susceptibility of these samples to IGC was determined by means of normalized tests, based on the immersion of the samples in an aggressive medium and the subsequent evaluation of the attack, using metallographic analysis. In order to quantity the IGC suffered by the samples, both the degree and the depth of the attacks were measured. In addition, electrochemical noise (EN) signals were recorded during the normalized tests. This technique is especially interesting for the study of corrosion processes of systems with low impedance, such as those faced in this paper, since it does not modify the corrosion potential of the system. Three parameters were used to analyse the EN signals: noise resistance (R_n) and two shot noise parameters, the characteristic charge (q) and the characteristic frequency (f_n). Finally, the relationship between the results of the metallographic analysis and those obtained from the analysis of EN signals was established. Unfortunately, a poor correlation between the shot noise parameters and the degree of IGC was found, due to both the high localization and high activities of all systems.     

Scanning Kelvin probe force microscopy for the in situ observation of the direct interaction between active head and intermetallic particles in filiform corrosion on aluminium alloy

This article presents for the first time an in situ high-resolution study of the interaction between the active head in filiform corrosion (FFC) and intermetallic particles within an aluminium alloy. For the first time direct evidence will be provided that the intermetallic particles directly determine the so far seemingly random course of the filaments. Both the segments of active filaments and the intermetallic particles (IMPs) were successfully imaged in a humid air (ca. 85% RH) environment by scanning Kelvin probe force microscopy (SKPFM) through a plasma polymer coating of about 340 nm thickness. In order to be able to do that, the experimental parameters need to be adjusted in such a way, that the width of the filaments is small enough to be well within the scan window of SKPFM (100 μm × 100 μm). Also it is important that the small IMPs can still be mapped by SKPFM through the coating. This was successfully achieved by use of a HDMSO plasma polymer film. Surface potential values in the head region of the propagating filaments were found to be 200 mV lower than the interface between intact plasma polymer and the aluminium alloy, indicating the active region. On the other hand, the surface potential values in the trailing filament tail are found to be about 250 mV higher than background, pointing out the cathodic site and superpassivation due to the accumulated corrosion products in this region. It was found that the direction of the filament is determined by the location of the IMPs nearest to the active head.     

Simulation and analysis of industrial crystallization processes through multidimensional population balance equations. Part 1: a resolution algorithm based on the method of classes

In order to obtain constant solid properties with particles exhibiting a low order of symmetry, it is necessary to monitor and to control several distributed parameters characterising the crystal shape and size. A bi-dimensional population balance model was developed to simulate the time variations of two characteristic sizes of crystals. The nonlinear population balance equations were solved numerically over the bi-dimensional size domain using the so-called method of classes. An effort was made to improve usual simulation studies through the introduction of physical knowledge in the kinetic laws involved during nucleation and growth phenomena of complex organic products. The performances of the simulation algorithm were successfully assessed through the reproduction of two well-known theoretical and experimental features of ideal continuous crystallization processes: the computation of size-independent growth rates from the plot of the steady-state crystal size distribution and the possibility for MSMPR crystallizers to exhibit low-frequency oscillatory behaviours in the case of insufficient secondary nucleation.     

Potentiodynamic and potentiostatic deposition of polyaniline on stainless steel: Electrochemical and structural studies for a potential application to corrosion control

Polyaniline (PAn) films were electrodeposited on stainless steel 304 (SS) from 0.5 M H₂SO₄ solution containing 0.1 M aniline (An) by using potentiodynamic and potentiostatic techniques. In particular, PAn films were prepared as follows: (i) by cyclic potential sweep (CPS) deposition upon varying the upper potential limit (E_l) of the polymerization potential region between 0.8 and 1.1 V, while the lower potential limit was equal to −0.2 V; (ii) by potentiostatic deposition upon varying the applied potential (E_appl) between 0.8 and 1.1 V. The potential sweep rate (dE/dt) was also varied for the An polymerization during the CPS deposition. Variation of the E_l, dE/dt and E_appl affects the PAn growth leading to films of different electrochemical and structural properties. The electrochemical properties of the PAn were examined by using cyclic voltammetry. Scanning electron microscopy was used to reveal the structure and morphology of the PAn films. Monitoring the open circuit potential (E_OC) of the PAn-coated SS electrode in 0.5 M H₂SO₄ shows that the SS remains in the passive state. PAn films can also provide protection to SS in chloride-containing 0.5 M H₂SO₄ for the studied period of time, although pits were detected during prolonged immersion. The protection efficiency seems to be related with the parameters E_l, dE/dt and E_appl varied during polymerization. The mechanism of the SS protection provided by the PAn coatings is discussed in terms of the active role of PAn in corrosive environments.     

Electrochemical sensor based on a poly(para-aminobenzoic acid) film modified glassy carbon electrode for the determination of melamine in milk

A novel and simple sensor is developed in this paper for melamine detection, which is based on an electropolymerized molecularly imprinted polymer (MIP) of para-aminobenzoic acid (pABA). The poly(para-aminobenzoic acid) (P-pABA) film was deposited in a pABA solution by potentiodynamic cycling of potential with and without the template (melamine) on a glassy carbon electrode. The surface feature of the modified electrode was characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The molecular imprinted sensor was tested by differential pulse voltammetry (DPV) to verify the changes in redox peak currents of hexacyanoferrate. Several important parameters controlling the performance of the P-pABA were investigated and optimized. In the optimal conditions, the relative redox peak currents of hexacyanoferrate were linear. The concentration of melamine ranged from 4.0 μM to 0.45 mM, with a linear correlation coefficient of 0.9992. The detection limit was 0.36 μM (S/N = 3). The MIP sensor was successfully applied to the determination of melamine in milk products and showed high selectivity, sensitivity, and reproducibility. The results of this research demonstrate that it is feasible to use the molecular imprinting methodology when preparing sensing devices for analytes that are electrochemically inactive.