Effect of atmospheric pollutants on the corrosion of high power electrical conductors - Part 2. Pure copper

The research work performed during this study was simultaneously followed with another one published in this journal as Part I. A1 and 6201 A1 alloy. Its aim was to reveal a comparative picture of the joint effect of marine and industrial atmospheric pollutants on the corrosion resistance of wire metals employed for electric transmission conductors. Weight loss after 4, 11, 16 and 24 months exposure was determined and morphology of the attack analysed through SEM-ESEM-EDX. Cu corrosion products showed higher protectiveness than those of Al in marine sites for the lowest [Cl⁻] and in marine-industrial atmospheres even for the highest SO₂ contents. Respect to marine sites where [Cl⁻] was higher than [SO₂] Cu was more susceptible than A1.     

Effect of unusually elevated SO2 atmospheric content on the corrosion of high power electrical conductors - Part 3. Pure copper

This research work belongs to a series of studies simultaneously performed with two previous publications in this journal as Part I. Al and 6201 Al alloy and Part 2. Pure copper. The aim of the project was to have a comparative picture of the joint effect of marine and industrial atmospheric pollutants on the corrosion resistance of wire metals employed for high power electric transmission. This one is also based on the pure Cu behaviour, but limited to six marine-industrial atmospheres with extremely high SO₂ contents. The interest in this study was triggered by unusual results considered appropriate to investigate. Weight loss after 4 and 11 months exposure was determined and morphology of the attack was analysed through SEM-ESEM-EDX. Polarisation of samples after exposure to all the test sites as compared to bare Cu clarified the effect of unusually high SO₂ pollutant contents in these atmospheres on the high protectiveness of the corrosion products formed.     

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

AC impedance study on the activation mechanism of aluminium by indium and zinc in 3.5% NaCl medium

Activation of aluminium by In³⁺ and Zn²⁺ ions in chloride media has been investigated by using potentiodynamic polarisation, potential vs. time measurements, electrochemical impedance spectroscopy and scanning electron microscopy studies. It is suggested that indium exhibits In⁺ and In²⁺ intermediates during the dissolution and redeposition processes. Incorporation of any of these lower valent ions changes the defect structure of the passive film, leading to an increase in anionic vacancies and a decrease in the number of electrons, promoting active dissolution of aluminium. The role of zinc is to moderate localised attack by increasing the electron concentration and cation mobility, thereby lowering the corrosion rate at activated sites.     

Corrosion properties of laser beam joints of aluminium with zinc-coated steel

This paper presents results of accelerated corrosion tests in a salt spray chamber as well as microelectrochemical measurements of thermally joint steel-aluminium mixed materials. The focus was set on analysing the corrosion behaviour of the different metallic materials (brazed seam, intermetallic phases, aluminium and steel sheet) in or within the vicinity of the brazed seam.Both corrosion tests show that the joining zone has the most negative corrosion potential and is the first to corrode. The degree of corrosive deterioration depends on the cathodic behaviour of the adjacent metal. Next to effective cathodes, such as steel or Fe-containing intermetallics, the attack is the most. However, contact to an aluminium alloy (AA6016) with its insulating oxide layer does not affect the corrosion properties of the respective filler materials.     

Anodic oxides on InAlP formed in sodium tungstate electrolyte

Amorphous anodic oxide films on InAlP have been grown at high efficiency in sodium tungstate electrolyte. The films are shown to comprise an outer layer containing indium species, an intermediate layer containing indium and aluminium species and an inner layer containing indium, aluminium and phosphorus species_. The layering correlates with the influence on cation migration rates of the energies of In³⁺-O, Al³⁺-O and P⁵⁺-O bonds, which increase in this order. The film surface becomes increasingly rough with increase of the anodizing voltage as pores develop in the film, which appear to be associated with generation of oxygen gas.     

Effect of carbon on corrosion and passivation of iron in hot concentrated NaOH solution in relation to caustic stress corrosion cracking

Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H⁺ and HFeO₂⁻, thereby favouring the formation of Fe₃O₄. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.     

Effects of pretreatment on the aluminium etch pit formation

The effect of chemical pretreatments on the electrochemical etching behavior of aluminium was investigated with the topographic studies of surface and the analysis of initial potential transients. Two-step pretreatments with H₃PO₄ and H₂SiF₆ result in a high density of pre-etch pits on aluminium surface by the incorporation of phosphate ion inside the oxide film and the removal of surface layer by aggressive fluorosilicic acid solution. It generates a high density of etch pits during electrochemical etching and results in the capacitance increase of etched Al electrode by expanding the surface area, up to 61.3 μF/cm² with the pretreatment solution of 0.5 M H₃PO₄ at 65 °C and 10 mM H₂SiF₆ at 45 °C.     

Investigation of the mechanism of plasma electrolytic oxidation of aluminium using O tracer

¹⁸O is used as a tracer to investigate the mechanism of plasma electrolytic oxidation of aluminium under AC conditions, with distributions of ¹⁸O species in the coatings determined by imaging SIMS. The transport of the oxygen species to the inner part of the coating occur through short-circuit paths, leading to formation of fresh alumina within the coating material near to the substrate. With increased times of PEO treatment the coating growth rate declines substantially, and concentration of ¹⁸O tracer is much reduced in the coating, either due to dispersal of the tracer in the coating or loss of coating material.     

In situ monitoring of the microstructural corrosion mechanisms of zinc–magnesium–aluminium alloys using time lapse microscopy

A novel technique has been developed that enables in situ monitoring of the microstructural wet corrosion mechanisms of zinc–(1–2 wt.% magnesium)–(1–2 wt.% aluminium) galvanising alloys using time lapse optical microscopy. The technique enabled the imaging of the progression of anodic attack, the development of corrosion product rings radially to the anode and pH gradients between anodes and cathodes using an indicator. It was found that corrosion initiated in the binary and ternary eutectic regions within the microstructure of the alloy with preferential de-alloying of MgZn₂ lamellae. After eutectic dissolution, anodic attack proceeded on the primary zinc rich dendrites.     

Influence of reinforcement proportion and matrix composition on pitting corrosion behaviour of cast aluminium matrix composites (A3xx.x/SiCp)

The influence of silicon carbide (SiCp) proportion and matrix composition on four aluminium metal matrix composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) immersed in 1-3.5 wt% NaCl at 22 °C was investigated by potentiodynamic polarization. The kinetics of the corrosion process was studied on the basis of gravimetric measurements. The nature of corrosion products was analysed by scanning electron microscopy (SEM) and low angle X-ray diffraction (XRD). The corrosion damage in Al/SiCp composites was caused by pitting attack and by nucleation and growth of Al₂O₃ · 3H₂O on the material surface. The main attack nucleation sites were the interface region between the matrix and the reinforcement particles. The corrosion process was influenced more by the concentration of alloy elements in the matrix than by the proportion of SiCp reinforcement and saline concentration.     

Corrosion behaviour of aluminium in copper containing environment

Corrosion behaviour of pure aluminium galvanically connected to metallic copper or in the presence of Cu²⁺ ions was investigated by electrochemical measurements in Na₂SO₄ and Na₂SO₄ + NaCl test solutions. It has been found that in aerated Cl⁻ ion containing solutions pitting corrosion of aluminium emerged immediately, while in the absence of oxygen this process was less violent. Effect of passivating pre-treatment of aluminium surface on corrosion behaviour Cu-Al bimetallic system is also demonstrated.     

Influence of chloride ion concentration and temperature on the corrosion of Mg-Al alloys in salt fog

The influence of temperature and chloride concentration on the corrosion behaviour of Mg-Al alloys exposed to salt fog was evaluated. Corrosion attack increased with decreasing aluminium content in the alloy and increasing Cl⁻ concentration and temperature. The effect of Al-Mn inclusions, which revealed several stoichiometries and were up to 300 mV more noble than the magnesium matrix, was only noticeable in the early stages of corrosion of the AZ31 alloy. Aluminium segregation and β-phase distribution were the main controlling factors for the AZ80 and AZ91D alloys, the latter being more susceptible to variations in the saline concentration.     

Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D

Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with a deposition of 70 μg/cm⁻² NaCl. The corrosion products formed and the surface electrolyte were analysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffraction and Ion Chromatography. The results show that magnesium carbonates are the main solid corrosion products formed under these conditions. The corrosion products identified were the magnesium carbonates hydromagnesite (Mg₅ (CO₃)₄ (OH)₂4H₂O) and nesquehonite (MgCO₃ 3H₂O). The corrosion attack starts with the formation of magnesite at locations with higher NaCl contents. At 95% RH, a sequence of reactions was observed with the initial formation of magnesite, which transformed into nesquehonite after 2-3 days. Long exposures result in the formation of pits containing brucite (Mg(OH₂)) covered with hydromagnesite crusts. The hydromagnesite crusts restrict the transport of CO₂ and O₂ to the magnesium surface and thereby favour the formation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on the surface at the beginning was essentially preserved during the initial corrosion process. Since the applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolyte was present on the surface during the whole exposure. Thus, Na⁺ and Cl⁻ ions can participate in the corrosion process during the whole time and the availability of these species will not restrict the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosion behaviour of AZ91D is rather controlled by factors related to the microstructure of the alloy and formation of solid carbonate containing corrosion products blocking active corrosion sites on the surface.     

Evolution of carbides and chromium depletion profiles during oxidation of Alloy 602 CA

The microstructure evolution in carbide strengthened Alloy 602 CA during exposure to a synthetic flue gas (N₂–2.5%O₂–8.6%H₂O–16.4%CO₂) at 1100 °C has been studied. The chromium and aluminium loss resulted in a chromium depleted alloy subsurface area and the dissolution of the carbides within this area. An increase of the carbide fraction in the sample core was observed and quantified. Phase equilibria calculations revealed that the depletion of aluminium as well as that of chromium triggers carbon to leave the depleted area. The overall carbon depletion in that area corresponded to the observed increase in carbide fraction in the sample core.     

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.     

The valence state of copper in anodic films formed on Al-1at.% Cu alloy

During anodising of Al-Cu alloys, copper species are incorporated into the anodic alumina film, where they migrate outward faster than Al³⁺ ions. In the present study of an Al-1at.% Cu alloy, the valence state of the incorporated copper species was investigated by X-ray photoelectron spectroscopy, revealing the presence of Cu²⁺ ions within the amorphous alumina film. However, extended X-ray irradiation led to reduction of units of CuO to Cu₂O, probably due mainly to interactions with electrons from the X-ray window of the instrument and photoelectrons from the specimen. The XPS analysis employed films formed on thin sputtering-deposited alloy/electropolished aluminium specimens. Such an approach enables sufficient concentrations of copper species to be developed in the anodic film for their ready detection.     

Pit growth behaviour of aluminium under galvanostatic control

The pit growth process on (1 0 0) aluminium under anodic pulse current in a mixed solution of 1 M HCl and 0.1 M H₂SO₄ at 30 °C has been evaluated using potential transient measurements and pit size distributions obtained by scanning electron microscopy. Sustained pit growth is observed for all pits during the initial anodic potential rise before reaching a steady-state etch potential, whereas a substantial fraction of the pits passivate at the steady-state etch potential. The pit growth rate during the initial potential rise is 3.4 μm s⁻¹, which is similar to that at the steady-state etch potential. The growth rates of active pits are potential-independent.     

Long-term monitoring of atmospheric corrosion at weathering steel bridges by an electrochemical impedance method

Weathering steel corrosion was monitored for one to two years under natural atmosphere by an electrochemical impedance technique. Two identical comb-shape weathering steel sheets embedded in epoxy resin were used as monitoring probe electrodes at two different bridges in Japan. Impedances at 10 kHz (Z_10kHz) and 10 mHz (Z_10mHz) were automatically measured every hour. Coupons (50 × 50 × 2 mm³) prepared from the same steel sheets were exposed together to measure the corrosion mass loss. The average (Z_10mHz)⁻¹ value for half to one year exposure correlated well with the average corrosion rate determined from the corrosion mass loss.     

Corrosion of N80 carbon steel in oil field formation water containing CO2 in the absence and presence of acetic acid

Corrosion behaviour of N80 carbon steel in formation water containing CO₂ was studied by polarization curve technique, electrochemical impedance spectroscopy, weight loss test, scanning electron microscope, and X-ray diffraction. Effects of temperature and acetic acid concentration on the corrosion behaviour of N80 carbon steel were discussed. The results showed that increasing temperature not only enhanced the dissolution of steel substrate, but also promoted the precipitation of FeCO₃, the addition of acetic acid enhanced localized corrosion attack on N80 carbon steel. FeCO₃ was the main corrosion product. And there was a transition region between CO₂ corrosion control and HAc corrosion control.