Effect of sulfuric acid on pit propagation behaviour of aluminium under AC etch process

Pit propagation on high purity aluminium electrode in 2 M HCl solutions with and without H₂SO₄ under an alternating current (AC) has been examined. Pit development and potential transients were dependent on the H₂SO₄ concentration. In the sulfate-free etchant, most pits developed from the pretreated surface, with little tendency to form clusters of pits. With increasing H₂SO₄ concentration the size of the pit clusters increased. There is an optimal H₂SO₄ concentration, which is 0.01 M H₂SO₄ in this study, to form a deep etched layer of uniform thickness with high surface area. At H₂SO₄ concentrations higher than 0.01 M, the pit propagation proceeded on limited foil surface sites and deep etched regions were formed locally, since sulfate ions assisted passivation and reduced the number of pit nucleation sites on foil surface. Analysis of potential transients during the anodic half-cycle supports the hypothesis that sulfate ions retarded the pit nucleation.     

Role of cathodic half-cycle on AC etch process of aluminium

The AC etch process effectively expands the surface area of aluminium foil as compared with DC etch process. This study explored the effect of cathodic half-cycle. The cathodic half-cycle enhanced passivation of pits developed during the preceding anodic half-cycle, making pit nucleation random. Anodic pulse without cathodic half-cycle produced hollows, due to preferential pit nucleation on pits produced in the preceding anodic half-cycle. The open circuit before the cathodic half-cycle does not largely influence the etch factor, but etch film formation is largely suppressed. The precipitation of aluminium hydroxide may not have a crucial role in porous layer formation.     

Corrosion behaviour of several aluminium alloys in contact with a thermal storage phase change material based on Glauber’s salt

This article presents the results of a study about the corrosion behaviour of four aluminium alloys (EN AW 2024, 3003, 6063, and 1050) in contact with a commercial thermal storage material based in the Glauber’s salt (Na₂SO₄·10 H₂O). Results indicate that the Al 2024 alloy is not compatible with this material due to the extense formation of NaAlCO₃·(OH)₂ in contact with air. The aluminium alloys 3003, 6063 and 1050 showed to be fully compatible with the material.     

Large-area self-ordered aluminium sub-micrometre dot arrays prepared by electropolishing on polycrystalline aluminium at constant current

The effects of electropolishing on the sub-micrometre structure pattern on the surface of polycrystalline aluminium were investigated. Under constant current, the electropolishing process results in large-area uniform sub-micrometre dot arrays despite the polycrystalline aluminium surface. Effects of temperature on the pattern of the surface structures were also investigated. Experimental temperature was shown to have a great influence on the structure pattern. When the electropolishing temperature increases, the structure of aluminium surface tends to transform from a dot pattern to stripe one and then to a corrosion structure.     

Effect of placement of aluminium foil on growth of etch tunnels during DC etching

The tunnel growth behaviour of aluminium foil etched in different placement has been investigated. The limiting length of tunnel of specimens in increasing order is: face-down, vertical, and face-up placement. Many sub tunnels branch on the wall surface of the main tunnel when aluminium is etched in face-down placement, while much fewer sub tunnels are formed when aluminium is etched in face-up placement. A novel etching model has been proposed to interpret the phenomenon, by considering the electrocapillary effect. It reveals that the status and transport of hydrogen bubbles inside the tunnel has a great influence on the tunnel growth.     

Effect of microcrystallization on pitting corrosion of pure aluminium

A microcrystalline aluminium film with grain size of about 400 nm was prepared by magnetron sputtering technique. Its corrosion behaviour was investigated in NaCl containing acidic solution by means of potentiodynamic polarization curves and electrochemical noise (EN). The polarization results indicated that the corrosion potential of the sample shifted towards more positive direction, while its corrosion current density decreased compared with that of pure coarse-grain Al. The EN analysis based on stochastic model demonstrated that there existed two kinds of effect of microcrystallization on the pitting behaviour of pure aluminium: (1) the rate of pit initiation is accelerated, (2) the pit growth process was impeded. This leads to the enhancement of pitting resistance for the microcrystallized aluminium.     

Analysis of the galvanostatic polarization method for determining reliable pitting potentials on stainless steels in crevice-free conditions

In this paper the potential of the galvanostatic polarization technique as accelerated method for determining the characteristic pit potentials on stainless steels in crevice-free conditions is examined. Measurement of the potential change as a function of time shows a maximum that agrees with the nucleation pit potential. Thereafter, a stationary potential is reached corresponding to the protection potential against pit. Possible limitations of this kind of measurements have been remedied by refinements in the test procedure and conditions. The state of the surface oxide film and the applied anodic current are two basic parameters that must be well defined because they govern the pitting susceptibility. It has been found that with applied anodic currents in the range 40–200 μA/cm² and with prior electrode exposures to solution between 30 and 60 min it is possible to obtain results in excellent agreement with the conventional potentiodynamic tests with the advantage of smaller data scattering and absence of crevice at electrode/holder interfaces. These effects are the result of the rapid pitting stimulated in the galvanostatic method. This implies a short duration of the experiment thus also favouring the elimination of the time-dependent crevice, which notoriously contributes to the poor reproducibility of pit potentiodynamic potentials. A detailed series of experiments have been conducted on several stainless steels and in different test conditions to validate the accuracy of the galvanostatic polarization method.     

Corrosion behaviour of sintered NdFeB deposited with an aluminium coating

A protective, pure Al coating was deposited by direct current (DC) magnetron sputtering onto sintered NdFeB magnets. Separated, single phases of sintered NdFeB (the Nd-rich phase, the B-rich phase and the matrix phase) were prepared by arc melting for open circuit potential (OCP) tests. The corrosion process of the sintered NdFeB magnets coated with Al (Al/NdFeB) was studied experimentally. It was found that the corrosion process can be divided into three different stages. The Al coating cannot provide complete sacrificial protection for the sintered NdFeB magnets.     

The corrosion of two aluminium sacrificial anode alloys in SRB-containing sea mud

Corrosion of two sacrificial anodes in marine sediment with and without sulphate-reducing bacteria (SRB) was performed. Weight loss experiments indicated that the corrosion rate of Al–Zn–In–Sn was 2–3 times higher than that of Al–Zn–In–Mg–Ti in the SRB-containing sediment. Electrochemical analysis suggested that the corrosion rates of the two anodes were enhanced substantially by SRB. Surface analysis revealed the localised corrosion of the two sacrificial anodes in abiotic and biotic sediments. The concentration of Al³⁺ in the surface of the samples immersed with SRB was lower than that of the samples without SRB.     

Effect of rapid solidification on the microstructure and corrosion behaviour of Al-Zn-Mg based material

The corrosion behaviour of Al-5Zn-3Mg-0.6Cu-0.8Zr-0.25Cr-0.15Ni-0.15Ti alloys, produced by traditional and powder technologies, with similar thermo-mechanical treatments, in 3% sodium chloride solution, has been examined by electrochemical methods, scanning electron microscopy, transmission electron microscopy and X-ray microanalysis. The alloys reveal similar precipitation but of different shape, size and distribution; further, both alloys experience localized corrosion. Copper-rich precipitates initiate the dissolution of surrounding particles, enriched in Zn and Mg. As a result, the surface is enriched with other alloying elements after a full polarisation run. Cast material has lower corrosion properties because of the higher heterogeneity of the structure. The structure heterogeneity of the cast material involves a more non-uniform distribution of the precipitates, larger Zn- and Mg-rich particles, and depletion of the matrix and areas around the precipitates by alloying elements compared with the powder material.     

Electrochemical corrosion behaviour of microcrystalline aluminium in acidic solutions

The electrochemical corrosion behaviour of microcrystalline pure aluminium coating, fabricated by a magnetron sputtering technique, has been investigated in both 0.5 mol/l NaCl and 0.5 mol/l Na₂SO₄ acidic (pH = 2) aqueous solutions. The corrosion resistance of the microcrystalline Al coating has deteriorated more compared with that of the cast pure Al in Na₂SO₄ acidic solution. However, its oxide film has a higher pitting resistance in the NaCl acidic solution. Chloride ions play a big role in the formation of the oxide film on the microcrystalline Al coating. The higher pitting resistance was attributed to the more acidic isoelectric point which the oxide film achieved.     

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.     

In-situ investigation on the pitting corrosion behavior of friction stir welded joint of AA2024-T3 aluminium alloy

The surface corrosion behavior of an AA2024-T3 aluminium alloy sheet after friction stir welding was investigated by using an “in-situ observation” method. SEM observations showed that the density and degree of the pitting corrosion in the shoulder active zone were slightly larger compared to the other regions on the top surface. The origins of the pitting corrosion were in the regions between the S phase particles and the adjacent aluminium base. The effect of Al-Cu-Fe-Mn-(Si) intermetallic compounds on the pitting corrosion was attributed to their high self-corrosion potential which induced the anodic dissolution of the surrounding aluminium matrix.     

Quantification of pitting in two tempers of 7075 aluminium alloy by non-destructive evaluation

An attempt was made to use the damage parameter, β obtained from non-linear ultrasonic assessment, to quantify pitting damage in 7075 aluminium alloy. β values were found to be different for the two tempers, T6 and T73. β reflected the pitting damage quite well in the initial stages of pitting, during which the non-linearity change in the lattice was mostly governed by the creation of free surfaces due to pitting. However, on prolonged pitting, the β values showed anomalous behaviour. The findings are explained by the operation of counteracting microstructural influences on lattice non-linearity.     

Temperature dependence of the pitting potential of high purity aluminium in chloride containing solutions

Slow scan anodic polarization experiments were conducted on high purity (99.99%) aluminium exposed to chloride containing solutions at temperatures ranging from 1 to 70 °C. The total anodic charge passed prior to stable pitting shifts towards higher values with increasing chloride concentration and there is a transition point at 30 °C above which a significantly greater amount of charge is involved. For all chloride concentrations, the pitting potential decreases with temperature according to a linear relationship, and also shows a transition at approximately 30 °C, with a much steeper decrease observed for the higher temperatures. Possible explanations for these two effects are discussed.     

The role of silicon in the corrosion of AA6061 aluminium alloy laser weldments

The galvanic corrosion temporal increase observed on examination of the weld fusion zone (WFZ) of AA6061 laser weldments in 3.5 wt.% NaCl solution cannot be attributed to electron tunnelling as the surface oxide layer is too thick, or the presence of Cl⁻ within the surface layer as this element was not found to be present. Aluminium alloy and WFZ galvanic and surface analyses indicate that the cathodic WFZ corrosion characteristics are due to increases in silicate concentrations in the surface oxide layer, leading to increased ionic and/or p-type semi-conductor conductivity, intermetallic concentrations and surface area.     

A morphological study of filiform corrosive attack on chromated and alkaline-cleaned AA2024-T351 aluminium alloy

In order to characterise filiform corrosion on a commercial AA2024-T351 aluminium alloy, a detailed microscopical study using SEM and EDS was performed. One set of AA2024-T351 aluminium alloy samples was alkaline-cleaned and deoxidised and chromate conversion coated. Another set was alkaline-cleaned only. Both samples were similarly spray coated with a 42 μm clear polyurethane topcoat. Filaments were subjected to a range of specimen preparation techniques. Sections and top views examined by SEM revealed varying degrees of attack ranging from generalised etching without local attack to severe local attack in the form of pitting, resulting in grain etchout, grain boundary attack and subsurface etchout. EDS revealed the presence of chloride deep into the pits and the subsurface etchout.     

Corrosion behaviour of a 2219 aluminium alloy treated with a chromate conversion coating exposed to a 3.5% NaCl solution

This study investigates the formation of a chromate conversion coating at Al–Cu–Fe–Mn intermetallic sites of an Al2219 alloy and the corrosion initiation at these sites in a 3.5% NaCl solution, using SEM, AES and EDX. Changes in the surface chemistry were monitored after progressive exposures to the solution up to 42 h. The coating was found to be thinner and more defective on the intermetallic. Initially, Al is dissolved and Al(OH)₃ deposited on and around the intermetallic. After 42 h of exposure, Al(OH)₃, Fe and Mn oxides and small particles of elemental Cu are deposited as corrosion products.     

Effect of temper on the distribution of pits in AA7075 alloys

A comparative study of pitting severity in T6 and T73 tempers of AA7075 in 0.1 M NaCl is reported here. Pitting was more severe for T6 tempers compared to T73 tempers. This could be attributed to higher pit nucleation sites in the former. The statistical distributions for pit areas indicated nucleation saturation in T73. The extreme value distributions showed differences for the two tempers. These differences in pit distribution are explained on the basis of the role of constituent as well as strengthening particles as well as the role of grain boundary η precipitates.     

Electrochemical aspects of exfoliation corrosion of aluminium alloys: The effects of heat treatment

Electrochemical approaches are used to investigate the exfoliation corrosion (EFC) of a 7XXX series aluminium alloy that has undergone different tempering treatments. EFC was produced under an artificial crevice at open circuit potential in neutral chloride solutions, and is found to be associated to current and potential transients. EFC was also produced under galvanostatic control conditions. Observations made through Scanning Electron Microscopy (SEM) suggest that these transients result from the progression of inter-granular cracks. Last, over-ageing heat treatments that are known to decrease both metal hardness and EFC sensitivity were found to decrease the number of transients.