Surface oxide formation during corona discharge treatment of AA 1050 aluminium surfaces

Atmospheric plasmas have traditionally been used as a non-chemical etching process for polymers, but the characteristics of these plasmas could very well be exploited for metals for purposes more than surface cleaning that is presently employed. This paper focuses on how the corona discharge process modifies aluminium AA 1050 surface, the oxide growth and resulting corrosion properties. The corona treatment is carried out in atmospheric air. Treated surfaces are characterized using XPS, SEM/EDS, and FIB-FESEM and results suggest that an oxide layer is grown, consisting of mixture of oxide and hydroxide. The thickness of the oxide layer extends to 150–300 nm after prolonged treatment. Potentiodynamic polarization experiments show that the corona treatment reduces anodic reactivity of the surface significantly and a moderate reduction of the cathodic reactivity.     

Formation and characterisation of a chromate conversion coating on AA6060 aluminium

AA6060-T6, an AlMg0.5Si0.4 model alloy and α-Al(Fe,Mn)Si phase electrodes have been subjected to chromate treatment in a commercial chromate-fluoride based solution. The coated surfaces were subsequently examined by use of field emission SEM, TEM, AES and electrochemical measurements in 0.1 M NaCl solution in order to study the effect of substrate microstructure on coating formation and properties. Non-uniform growth of the chromate conversion coating (CCC) on AA6060-T6 resulted in a porous morphology, with cracks extending down to the base metal. Poor coverage was particularly observed at the grain boundaries. The thickness of the CCC after completed treatment was about 150-200 nm, while significantly thinner films were formed on the α-Al(Fe,Mn)Si particles. AlMg0.5Si0.4 in the artificially aged (T6) condition exhibited a coating morphology similar to AA6060-T6, while CCC formation on homogenised AlMg0.5Si0.4 was characterised by growth of localised oxide particles and filaments, resulting in poor coverage. These observations indicated that precipitation of Mg₂Si particles due to heat treatment promoted nucleation of the CCC. Chromate treatment caused a significant reduction of cathodic activity on AA6060 during subsequent polarisation in chloride solution. This was attributed to rapid formation of a thin chromium oxide film on the α-Al(Fe,Mn)Si particles during the chromate treatment, resulting in significant cathodic passivation of the phase. Inhibition of the oxygen reduction reaction at cathodic intermetallic particles is suggested as an important factor contributing to the high performance of chromate pre-treatments on aluminium.     

Dielectric breakdown and healing of anodic oxide films on aluminium under single pulse anodizing

Single pulse anodizing of aluminium micro-electrode has been employed to study the behaviour of dielectric breakdown and subsequent oxide formation on aluminium in alkaline silicate and pentaborate electrolytes. Current transients during applying pulse voltage have been measured, and surface has been observed by scanning electron microscopy. Two types of current transients are observed, depending on the electrolyte and applied voltage. There is a good correlation between the current transient behaviour and the shape of discharge channels. In alkaline silicate electrolyte, circular open pores are healed by increasing the pulse width, but such healing is not obvious in pentaborate electrolyte.     

Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte

Spark anodizing of aluminium at 5 A dm⁻² in sodium metasilicate/potassium hydroxide electrolytes is studied, with particular emphasis on the mechanism of coating growth, using transmission electron microscopy and surface analytical techniques, with coatings typically 10 μm, or more, thick. Two-layered coatings develop by deposition of an outer layer based on amorphous silica, associated with low levels of alkali-metal species, at the coating surface and growth of an inner, mainly alumina-based, layer, with an amorphous region next to the metal/coating interface. Formation of crystalline phases in the inner layer, mainly γ-Al₂O₃, with some α-Al₂O₃ and occasional δ-Al₂O, is assisted by local heating, and possibly also by ionic migration processes, arising from the rapid coating growth at sites of breakdown. Due to local access of electrolyte species in channels created by breakdown events, the silicon content in the inner coating regions varies widely, ranging from negligible levels to about 10 at.%. Silica deposition at the coating surface and formation of Al₂SiO₅ and Al₆Si₂O₁₃ phases is promoted by increased time of anodizing and concentration of metasilicate in the electrolyte. However, at sufficiently high concentration of metasilicate and pH, when more extreme voltage fluctuations accompany breakdown, the two-layered nature of coatings is replaced by a mixture of aluminium-rich and silicon-rich regions throughout the coating thickness.     

Effect of heat treatment on anodic activation of aluminium by trace element indium

The presence of trace elements in Group IIIA-VA is known to activate aluminium anodically in chloride environment. The purpose of this paper is to investigate the surface segregation of trace element In by heat treatment and resulting surface activation. Model binary AlIn alloys, containing 20 and 1000 ppm by weight of In, were characterized after heat treatment at various temperatures by use of glow discharge optical emission spectroscopy, electron microscopy and electrochemical polarization. Heat treatment for 1 h at 300 °C gave significant segregation of discrete In particles (thermal segregation), which activated the surface. Indium in solid solution with aluminium, obtained by 1 h heat treatment at 600 °C, also activated by surface segregation of In on alloy containing 1000 ppm In, resulting from the selective dissolution of the aluminium component during anodic oxidation (anodic segregation). The effect of anodic segregation was reduced by decreasing indium concentration in solid solution; it had negligible effect at the 20 ppm level. The segregated particles were thought to form a liquid phase alloy with aluminium during anodic polarization, which in turn, together with the chloride in the solution destabilized the oxide.     

Corrosion inhibition by anionic surfactants of AA2198 Li-containing aluminium alloy in chloride solutions

This paper evaluates the inhibiting action of some anionic surfactants towards AA2198 corrosion in NaCl solutions; the effect of surfactant concentration in relation to chloride amount was determined.On separate electrodes, polarization curves were recorded after 1, 24 and 168 h immersion in the aggressive media, while EIS technique continuously monitored the alloy corrosion process.In general, these substances stifled both the cathodic and anodic processes and noticeably shifted the pitting potential (breakdown potential, E_BR) in the positive direction. The most efficient compounds were N-lauroylsarcosine sodium salt and sodium dodecyl-benzenesulfonate, able to withstand the effects of 0.1 M Cl⁻.     

Plasma electrolytic oxidation of pre-anodized aluminium

The influence of an anodizing pre-treatment in sulphuric acid is investigated on plasma electrolytic oxidation (PEO) of aluminium in silicate electrolyte under constant rms current. The presence of the anodic film is shown to promote the establishment of a micro-arc regime that is favourable for growth of the PEO coating. The incorporation of the pre-formed film into the coating appears to proceed by thermal transformation of the anodic alumina, accompanied by formation of oxide beneath the pre-formed layer. The final coatings contain α- and γ-Al₂O₃, with increased concentrations of silicon, sodium and potassium in an outer region of the coating.     

The behaviour of second phase particles during anodizing of aluminium alloys

Several model second phase particles and a practical alloy (AA7075-T6) have been anodized in chromic and sulphuric acid to disclose the relation between particle composition, electrolyte nature and oxidation behaviour. Generally, magnesium-containing second phases are readily oxidized, because the presence of sufficient magnesium hinders the formation of a stable oxide, while copper- and iron-containing particles oxidize at reduced rates and support a relatively stable oxide. At low potential, in chromic acid, the oxidation rate of magnesium-containing particles is reduced, due to passivation induced by chromate anions. Conversely, for particles containing only copper and/or iron, chromate anions increase the oxidation rate.     

Effect of Mn/Mo incorporated oxide layer on the corrosion behavior of AA 2024 alloy

The oxide layer formed over AA 2024 using 10 wt.% H₂SO₄ (plain oxide, PO) was modified by Mn/Mo oxyanions (permanganate/molybdate modified oxide, PMMO) as an alternative to Cr(VI) ions to enhance the corrosion resistance. The corrosion current density values obtained for PMMO was found to be 2.8% and 1.4% of hydrothermal treated oxide (HTO) and PO respectively after 168 h immersion in 3.5% NaCl solution. The electrochemical studies showed the higher barrier layer resistance for PMMO. The improved corrosion behavior of PMMO was observed based on the damage function calculated. Similar observations were confirmed by continuous salt spray test.     

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.     

Formation of dicalcium phosphate dihydrate on magnesium alloy by micro-arc oxidation coupled with hydrothermal treatment

A layer containing dicalcium phosphate dihydrate (DCPD) and β-Ca₃(PO₄)₂ was prepared on magnesium alloy by hydrothermal treatment of micro-arc oxide (MAO) layer. The biocorrosion resistance of the oxide layers before and after hydrothermal treatment was analyzed by anodic polarization and electrochemical impedance spectroscopy (EIS) in Hank’s solution. The prepared MAO layers consisted mainly of MgO and MgAl₂O₄, and Ca and P inside the oxide layers existed with amorphous phase. Hydrothermal treatments not only made the amorphous Ca and P change into DCPD and β-Ca₃(PO₄)₂ crystals, but also improved the biocorrosion resistance of magnesium alloys, especially the pitting corrosion resistance.     

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.     

Contradictory effect of chromate inhibitor on corrosive wear of aluminium alloy

The corrosive wear of D16T aluminium alloy in artificial acid rain was studied. A special tribometer with the linear reciprocating ball-on-flat geometry was used. The setup allows to measure simultaneously an open circuit potential, to carry out potentiostatic and potentiodynamic polarization studies of the alloy corrosion and to record the friction coefficient. It was established that the addition of strontium chromate inhibitor to the working environment decreases an electrochemical corrosion of the aluminium alloy under wear conditions, but in general accelerates its destruction due to insufficient wear resistance of a formed surface film.     

A morphological study of filiform corrosive attack on cerated AA2024-T351 aluminium alloy

SEM and EDS studies were carried out to characterise filiform attack on a cerated AA2024-T351 aluminium alloy with a polyurethane topcoat. The filiforms developed on AA2024-T351 were sectioned, stripped of corrosion product and etched to reveal the grain structure. Examination of sections through the filaments and the filaments themselves, revealed severe local attack in the form of pitting resulting in grain etch out, grain boundary attack and subsurface etch out. Chloride ions were detected deep within pits and the subsurface etch out. The observations were similar to those found with filiform corrosion on chromated and coated surfaces. The observations led to development of a filiform corrosion model naming the volume expansion of the corrosion product as the principal cause for delamination.     

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.     

Characterization of the plasma electrolytic oxidation of aluminium in sodium tungstate

Morphology, composition and microhardness of oxide coatings formed during DC plasma electrolytic oxidation (PEO) of aluminium in sodium tungstate were investigated. Oxide coatings were characterized by optical emission spectroscopy, AFM, SEM-EDS and XRD. Oxide coatings morphology is strongly dependent on process duration, while microhardness decreases with extended PEO time. The microdischarges characteristics were studied as well and it is shown that size of microdischarges becomes larger, while the surface density of microdischarge sites becomes lower, with increasing PEO time. Optical emission spectrum of microdischarges has several intensive band peaks caused by electronic transition in Al, W, Na, O, H atoms.     

Galvanic corrosion of laser weldments of AA6061 aluminium alloy

Galvanic corrosion of laser welded AA6061 aluminium alloy, arising from the varying rest potentials of the various weldment regions, was examined. The weld fusion zone is found to be the most cathodic region of the weldment while the base material is the most anodic region. The rate of galvanic corrosion, controlled by the cathodic process at the weld fusion zone, increases with time until a steady state maximum is reached. On galvanic corrosion the corrosion potential of the weld fusion zone shifts in the positive direction and the free corrosion current increases. It is proposed that the cathodic process at the weld fusion zone causes a local increase in pH that in turn causes dissolution of the surface film resulting in the loss of Al to solution and the increase of intermetallic phases. The increase in galvanic corrosion may result from either the build up of the intermetallic phases in the surface layer and/or significant increase in surface area of the weld fusion zone due to the porous nature of the surface layer.     

Effect of solution treatment on the corrosion behaviour of aluminium alloy AA7150: Optimisation for corrosion resistance

Studies for Al-alloys normally employ specimens subject to a fixed solution heat treatment (SHT), and hence the specific role of solutionising is overlooked. It is revealed that SHT has a major role in corrosion of AA7150 as judged by electrochemical, microscopic and profilometry studies. SHT dictates the constituent particle type and population remaining in the alloy for subsequent processing, and is thus not only a principal factor in corrosion, but a factor that can be engineered in developing more damage tolerant AA7150. Effect of an optimised SHT minimising the residual MgZn₂ and Al₂MgCu content on corrosion resistance is demonstrated.     

Corrosion properties of Nd:YAG laser-GMA hybrid welded AA6061 Al alloy and its microstructure

The paper presents the corrosion behaviour of the Nd:YAG laser-gas metal arc (GMA) welds of AA6061-T6 alloy. The laser-GMA hybrid welding enhances the corrosion susceptibility of AA6061 alloy. The surface morphology observation and composition analysis were investigated by the scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. An increasing of the precipitate phase is observed in the weld fusion zone (WFZ). The WFZ suffers more severe pitting and cracks are associated with pitting. It is proposed that the increased precipitate phase increases the galvanic corrosion couples and results in the aggravation of pitting and cracking in the WFZ.     

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.