Effect of plasma treatment on corrosion-electrochemical behavior of titanium in molten mixture of cesium and sodium chlorides

The effect of high-temperature pulsed plasma (HTPP) on the corrosion-electrochemical behavior of titanium in a molten eutectic mixture of cesium and sodium chlorides in an argon atmosphere at temperatures of 790–810 K is studied. Upon the HTPP treatment in an oxygen or helium environment, a modified corrosion-resistant layer with a thickness up to 20 μm is formed on the titanium surface and decreases the corrosion rate of titanium in a chloride melt by a factor of 10–16.     

A two step anodizing process of aluminium as a means for improving the chemical and physical properties of oxide films

An anodizing process was developed to form corrosion resistant and hard oxide films on aluminium. The process consists of two steps: first the formation of chromate/phosphate treated layer on the surface of aluminium and secondly anodizing in a sulphuric acid solution. The anodic oxide films formed by the present process contain Cr(III) and phosphate species mostly in the outer part of the porous layer. The films formed by the present process provided a better corrosion resistance to the substrate aluminium from pitting in a chloride medium than the films formed by conventional anodizing and sealed in a boiling chromate solution. Further, Vickers hardness on the cross section of film increased compared with the films formed by conventional anodizing. This two step process can be developed to form other composite oxide films by using different treatments for the first step.     

Passivation of Al–Cr–Fe and Al–Cu–Fe–Cr complex metallic alloys in 1 M H2SO4 and 1 M NaOH solutions

The corrosion mechanisms of Al–Cr–Fe and Al–Cu–Fe–Cr complex metallic alloys have been investigated by potentiodynamic and potentiostatic polarization. Very good passivation of the Al–Cr–Fe surface is exhibited from 1 M H₂SO₄ to 1 M NaOH solutions, which was confirmed by ICP-OES analysis over a period of 273 days. Potentiostatically formed passive films analysed by XPS revealed chromium enrichment in the outermost layer of the aluminium oxy-hydroxide film. Although Al–Cu–Fe–Cr showed passivation during potentiodynamic polarization, heavy active corrosion at OCP was revealed by ICP-OES. For the Al–Cu–Fe–Cr alloy, the 10% content of Cr is insufficient to maintain a protective “chemically stable” Cr oxide/hydroxide.     

High temperature oxidation of FeCrAl‐alloys – influence of Al‐concentration on oxide layer characteristics

The superior high temperature oxidation resistance of FeCrAl alloys relies on the formation of a dense and continuous protective aluminium oxide layer on the alloy surface when exposed to high temperatures. Consequently, the aluminium content, i.e. the aluminium concentration at the alloy–oxide layer interface, must exceed a critical level in order to form a protective alumina layer. In the present study the oxidation behaviour of six different FeCrAl alloys with Al concentrations in the range of 1.2–5.0 wt% have been characterised after oxidation at 900 °C for 72 h with respect to oxide layer surface morphology, thickness and composition using scanning electron microscopy, energy dispersive X‐ray spectroscopy and Auger electron spectroscopy. The results show that a minimum of 3.2 wt% Al in the FeCrAl alloy is necessary for the formation of a continuous alumina layer. For Al concentrations in the range of 2.0–3.0 wt% a three‐layered oxide layer is formed, i.e. an oxide layer consisting of an inner alumina‐based layer, an intermediate chromia‐based layer and an outer iron oxide‐based layer. In contrast, the 1.2 wt% Al FeCrAl alloy is not able to form a protective oxide layer inhibiting extensive oxidation.     

Using EIS to analyse samples of Al–Mg alloy AA5083 treated by thermal activation in cerium salt baths

This paper describes a study undertaken of the morphological and anticorrosive characteristics of surface layers formed on the Al–Mg alloy AA5083 from solutions of Ce(III), by means of various heat treatments while immersed in baths of cerium salts. SEM/EDS studies have demonstrated the existence of a heterogeneous layer formed by a film of aluminium oxide/hydroxide on the matrix and a series of dispersed islands of cerium deposited on the cathodic intermetallics. With the object of evaluating the degree of protection provided by the layers formed and of characterising the particular contribution of the electrochemical response of the system in NaCl, the results obtained by means of EIS are presented and discussed.     

Protective action towards aluminium corrosion by silanes with a long aliphatic chain

A noticeable and persistent protective action towards aluminium corrosion can be obtained by treatments with silane molecules containing a long aliphatic chain (e.g., n-octadecyl-trimethoxy-silane, C18), which can markedly increase the barrier properties of the silanic coating. At variance with other silanic compounds, the layer of C18, although not uniform, tends to cover the whole aluminium surface completely and, in fact, its EIS spectra initially present one time constant only. It is likely that v.d.Waals interactions between the alkyl chains of C18 can lead to the formation of a polymolecular layer. The aqueous salt solutions may permeate through this layer, but with greater difficulties than in the case of an efficient di-silyl derivative (e.g., 1,2-bis-trioxymethyl-silyl-ethane, BTSE). C18 coatings noticeably retard not only the cathodic oxygen reduction reaction, but also the anodic metal oxidation process. A very prolonged action is also found towards the pitting process in the particularly aggressive chloride solution.     

Preparation and characterisation of aluminium pigments coated with silica for corrosion protection

Aluminium pigments with a layer of silica were prepared by a sol–gel method using tetraethoxysilane as precursor and ethylenediamine as catalyst. Under the optimum conditions, the corrosion protection factor can reach 99.3% and average grain almost remains the same size after coating, indicating that the coated aluminium pigments have excellent chemical stability and good dispersibility. FTIR and EDS analyses demonstrate that a layer of silica coating has been formed on the flaky aluminium particle. SEM, AFM and BET analyses show that a smooth and dense silica coating layer has been formed.     

Corrosion resistance of lamellar aluminium pigments coated by SiO2 by sol–gel method

The aluminium pigments were coated with SiO₂ by sol–gel method to improve their stability. The effects of formulation factors, such as medium of reaction, adding sequence of catalyst and number of coating, were investigated. The stability of the coated aluminium pigments in acid was examined by measuring the hydrogen generation amount. It was found that the coating layer formation is due to the condensation of tetraethyl orthosilicate (TEOS) to form a dense 3D cross-linked layer on the surface of aluminium. The optimized sequence of adding catalysts would be hydrochloride first, then ammonia. Stability tests confirmed that the aluminium pigments have better corrosion resistance after coating with SiO₂.     

Modification of piston surfaces by compressed plasma flow

Surface of the casted pieces made of aluminium piston alloys was treated by a plasma jet produced by the magnetoplasma compressor, with an aim to improve their exploitation properties. Fast piston surface heating and melting, physical and chemical changes within the surface layer and the melted layer recrystallization in the fast cooling conditions are the basic characteristics of the interaction between the plasma from the magnetoplasma compressor and piston alloys. The major changes are conducted in the surface layer inducing hardness increment and piston resistance. Micro hardness of plasma treated surface is 750 to 883 HV_0.2, in comparison with 90 to 130 HV_0.2 in initial stage. The X-ray structure analysis of samples in cast state before and after plasma treatment shows existence of modified zones in hardened layer with phase transformations. In plasma flow treated surface at least two new phases are registered, with redistribution of already existing phases. These intermetallic phases contributed to treated surface hardness increasing. These observations show that quasi-stationary compressed plasma flow treatment of sample surface influences remarkably modification and improvement of surface properties.     

Multilayered coatings: Tuneable protection for metals

The role of oxide bi-layers in controlling the onset of corrosion has been explored. A high-throughput electrochemical approach was employed to determine the breakdown potential of aluminium metal over-coated with combinations of silicon, titanium, aluminium and magnesium oxides. Bi-layered coatings consisting of two 100 nm thick metal oxide layers provided increased protection against breakdown, and combinations with vastly different iso-electric point of solid (IEPS) were found to exhibit improved barrier properties in comparison to single-component oxides. Furthermore, the most protective oxide bi-layers were produced when a high IEPS oxide was deposited directly onto the metal surface and subsequently over-coated with a low IEPS oxide. The barrier properties of bi-layer coatings appear to be tuneable, with notable dependencies on surface charge and thickness.     

SiO2 based hybrid inorganic–organic films doped with TiO2–CeO2 nanoparticles for corrosion protection of AA2024 and Mg-AZ31B alloys

Hybrid sol–gel coatings provide an approach as protective layers on metals. In this work, corrosion protection of aluminium and magnesium alloys by SiO₂-methacrylate coatings doped with TiO₂–CeO₂ nanoparticles was studied. The films show an improvement of the barrier properties at initial immersion. The reactivity of both alloys produces a deterioration of the protection with longer immersion, although TiO₂–CeO₂ nanoparticles let to observe signals of self-healing effect. Aluminium oxide/sol–gel interface was found to be stable. In combination with excellent paint adhesion on sol–gel films, these coatings can be a promising alternative pre-treatment for high strength aluminium alloys prior to painting.     

Influences of ion migration and electric field on the layered anodic films on Al–Mg alloys

Anodizing of sputtering-deposited Al–Mg alloys containing 27 and 32 at.% magnesium in sodium hydroxide electrolyte is shown to develop two-layered anodic oxide films. The outer layer contains aluminium and magnesium species, and is enriched in the latter species relative to the alloy, particularly towards the film surface. The inner layer also contains the two alloy species but is depleted in magnesium, due to Mg²⁺ ions migrating to the outer layer faster than Al³⁺ ions. The ratio of the thickness of the outer layer to that of the film increases with increase of magnesium content of the alloy. The presence of aluminium species in the outer layer is attributed to the penetration of the outer layer by oxide of the inner layer with lower ionic resistance. This mechanism of film growth appears to be sustainable to alloy concentrations to 40 at.%Mg, when the inner layer may no longer form. Enrichment of alloying elements can accompany film growth on Al–Mg alloys, as shown by enrichment of tungsten to 2–3 × 10¹⁵ atoms cm⁻² in an Al–26 at.%Mg–1 at.%W alloy.     

Corrosion properties of plasma nitrided AISI 410 martensitic stainless steel in 3.5% NaCl and 1% HCl aqueous solutions

Samples of an AISI 410 martensitic stainless steel were plasma nitrided at a temperature of 420 °C, 460 °C or 500 °C for 20 h. The composition, microstructure and hardness of the nitrided samples were characterised using a variety of analytical techniques. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarisation tests in 3.5% NaCl solution and immersion tests in 1% HCl acidic water solution. The results showed that plasma nitriding produced a relatively thick nitrided case consisting of a compound layer and a nitrogen diffusion layer on the 410 stainless steel surface. Plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the martensitic stainless steel. In the immersion test, nitrided samples showed lower weight loss and lower corrosion rate than untreated one. In the electrochemical corrosion tests, the nitrided samples showed higher corrosion potentials, higher pitting potentials and greatly reduced current densities. The improved corrosion resistance was believed to be related to the iron nitride compound layer formed on the martensitic stainless steel surface during plasma nitriding, which protected the underlying metal from corrosive attack under the testing conditions.     

Corrosion properties of steel protected by nanometre-thick oxide coatings

A comprehensive study of the corrosion properties of low alloy steel protected by 40–50 nm aluminium and tantalum mixed oxide coatings grown by atomic layer deposition is reported. Electrochemical and surface analysis was performed to address the effect of substrate surface finish and whether an oxide mixture or nanolaminate was used. There was no dissolution or breakdown for nanolaminate alumina/tantala stacks in acidic NaCl solution. Localised corrosion (pitting) took place when defects exposing the substrate pre-existed in the coating. Substrate pre-treatment by brushing and H₂–Ar plasma was instrumental to block or slow down pit initiation by reducing the defect dimensions.     

Galvanic coupling between copper and aluminium in a thin-layer cell

The Al/Cu coupling was investigated in a thin-layer cell formed by a large Cu electrode and an Al microelectrode embedded in an insulator placed above the Cu electrode. By using a scanning electrochemical microscope (SECM) the thickness of the thin layer was perfectly controlled with a precision in the micrometer range. A copper deposit on an electrochemical quartz crystal microbalance (EQCM) was also used as SECM substrate to quantify the copper dissolution rate. It was shown that such an experimental set-up allows to mimic the galvanic corrosion of intermetallic particles embedded in the aluminium matrix of the 2XXX series aluminium alloys. The combination of the SECM and the EQCM permitted the evaluation of the corrosion rate of copper at the corrosion potential of the 2024 Al alloy, whereas cyclic voltammetry performed on the SECM tip indicated the enrichment in Cu²⁺ ions in the thin electrolyte layer.     

Microstructural characteristics of the oxide films formed on Alloy 690 TT in pure and primary water at 325 °C

Microstructural characteristics of the oxide films formed on Alloy 690 TT after the exposure in oxygenated pure and primary water at 325 °C were investigated. Change in water chemistry from pure to primary water was found to vary the film structure from duplex to triple layer, and lead to change in the morphology of NiO on the surface. The increased pH and solution conductivity of primary water also accelerate the dissolution of Cr species and enhance the electrochemical corrosion. The Ni-rich and Cr-depleted inner layer formed under both water chemistries is porous and not protective. The related oxidation mechanism is discussed.     

Influence of the surface activation and local pitting susceptibility on the AC-electrograining of aluminium alloys

AC electrograining of aluminium is strongly influenced by the surface microstructure. The mechanical and electrochemical properties of the sub-surface present in aluminium alloys affect the electrochemical reactions that prevail during electrograining. Etching pre-treatment of aluminium removes intermetallics and rolled-in oxides; as a result, the attack on the aluminium substrate starts with the initial cycles of the electrograining process. Local electrochemical investigations show differences in corrosion and passivation properties between rolled-in oxides and clean surfaces. The interface between rolled oxides and aluminium matrix acts as a weak point for pit initiation.     

The role of chelating agents on the corrosion mechanisms of aluminium in alkaline aqueous solutions

The influence of 1,2-diaminoethane (DAE) on the mechanism of aluminium corrosion in KOH solutions at pH 13 was investigated by combining time-resolved inductively coupled plasma optical emission spectrocopy, open-circuit potential measurements and X-ray photoelectron spectroscopy. In pure KOH solutions, a very slow corrosion rate is initially observed, corresponding to the dissolution of the native oxide layer. Following this incubation stage, the corrosion rate is increasing due to the formation and oxidation of Al hydride, until a steady state is reached. DAE behaves as a strong initial corrosion accelerator, due to synergistic effects with hydroxyl ions and a dissolution mechanism in three successive steps has been proposed: (i) a rapid initial dissolution induced by the formation and detachment from the surface of bidentate (chelate) Al-DAE metal bound surface complexes; (ii) a slower step ascribed to the formation and release of monodentate Al-DAE metal bound surface complexes and (iii) a final step dominated by direct oxidation of surface aluminium hydride by hydroxyl species as in pure KOH.     

Oxidation behaviour of Al-alloyed ZrSi2 at 700°C

The oxidation resistance of ZrSi₂-4 at.% Al prepared by reaction sintering was evaluated in air at 700°C. Two different stages were distinguished. In the first regime the oxidation rate remains high whereas the second one is characterised by a marked decrease on the oxidation rate. Passage from the first to the second stage, associated with a transition stage, is determined by the formation of a continuous protective alumina-rich layer at the scale/alloy interface. An oxidation mechanism is proposed to explain the different stages occurring through the oxidation. Grain boundaries contribute to the fast inward oxygen transport into the alloy, leading to internal oxidation of non-reacted silicon and aluminium during the reactive sintering stage of the processing. Oxidation resistance could be improved by increasing the grain size of the alloy and by using an appropriated processing method addressed to minimise or suppress the presence of silicon and aluminium particles at ZrSi₂ grain boundaries.     

Das Oxidationsverhalten von Cu-Zn-Al-Legierungen bei hoher Temperatur

Oxidation behaviour of CuZnAl alloys at high temperature The behaviour of CuZnAl alloys in hot oxidizing atmospheres is characterized by the formation of surface layers of good adhesion, the properties of which are determined by aluminium oxide which is partially converted to spinell. The formation of protective layers is enhanced when the alloy contains larger quantities of Zn which - after an initial aluminium depletion of the alloy surface - acts as an oxygen acceptor allowing further aluminium to diffuse to the surface from the bulk of the material, so that the surface layer is improved. The aluminium content required is reduced at increasing zinc contents; at about 22% Zn 3–4% Al are sufficient to form a dense protective layer. These alloys are also most resistant to cyclic to form a dense protective layer. These alloys are also most resistant to cyclic temperature changes during oxidation. The adhesion of the layers may be further improved by addition of rare earth elements.