Jan Halvor Nordlien

Morphology and Structure of Oxide Films Formed on Magnesium by Exposure to Air and Water

Oxide films formed naturally on pure Mg are investigated by the use of ultramicrotomed cross sections and transmission electron microscopy. The film formed in air immediately after scratching the metal surface is initially thin, dense, amorphous, and relatively dehydrated. Continuing exposure to humid air or exposure to water leads to the formation of a thicker hydrated film adjacent to the metal. The film formed in water contains an additional top layer characterized by a plateletlike morphology. The structure of these layers and their significance in corrosion protection are discussed. The changes occurring in these structures as a result of exposure to the electron beam are reported.

Morphology and Structure of Water‐Formed Oxides on Ternary MgAl Alloys

Oxides on ternary magnesium alloys MgAlZn and MgAlRE were investigated by transmission electron microscopy using ultramicrotomed film sections. These films have a three-layered structure, similar to pure Mg and binary MgAl alloys, characterized by a hydrated inner layer, a thin and dense intermediate region, and a platelet-like outer layer. Zinc and rare-earth elements present in the two types of ternary alloys become incorporated in the oxide film so as to increase its stability in an aqueous environment, in particular by reducing hydration and increasing resistance to magnesium egress of the inner layer, which is responsible for the passivity of the surface. The apparent presence of trace amounts of rare-earth oxides in the film is particularly effective in improving passivity of the surface and, thereby, the corrosion resistance of MgAlRE alloys. The presence of aluminum together with rare-earth elements (RE) in the alloy is an essential factor in obtaining these results.

Formation of a zirconium-titanium based conversion layer on AA 6060 aluminium

Abstract Structure and chemistry of zirconium-titanium base conversion layers were characterised as a function of immersion time in the aqueous conversion bath to understand the mechanism of film formation. Characterisation was performed by glow discharge optical emission spectroscopy, scanning electron microscopy and transmission electron microscopy. Preferential nucleation of the zirconium-titanium oxide film and its growth occurred on and around intermetallic particles, resulting in reduced cathodic activity of the particles. Passivation of the cathodes in this manner constituted a limitation in the formation of a good quality conversion layer.

Influence of heat treatment and surface conditioning on filiform corrosion of aluminium alloys AA3005 and AA5754

Abstract The effect of high-temperature heat treatment combined with different cleaning and pre-treatment practices on filiform corrosion resistance has been investigated for aluminium alloys AA3005 and AA5754. High-temperature heat treatment severely reduces the filiform corrosion resistance of alloy AA3005, while the corrosion properties of alloy 5754 are only moderately affected. The drastic loss of filiform corrosion resistance of alloy AA3005 after high-temperature heat treatment is attributed to preferential microstructural changes in a heavily deformed, micrograined surface layer caused by large surface shear strain during rolling. The enhanced deformation of the near-surface region promotes precipitation of a fine dispersion of intermetallic particles during subsequent heat treatment. The higher density of intermetallic particles combined with lower supersaturated solid solution levels of manganese in the surface layer as compared to the bulk structure makes the heat-treated AA3005 material susceptible to superficial corrosion attacks and results in poor filiform corrosion resistance. Application of a commercial acid cleaning/chromating pre-treatment did not restore the filiform corrosion resistance of heat-treated alloy AA3005. Alloy AA5754, containing lower levels of manganese and iron than AA3005, did not undergo similar preferential microstructural changes during heat treatment. A moderate increase in the amount of filiform corrosion of heat-treated AA5754 samples is attributed to poor protective properties of the thick, magnesium enriched, thermally formed surface oxide on this alloy. Any cleaning/pre-treatment practice that removes the thermally formed oxide on this alloy results in a very high filiform corrosion resistance.

Effect of heat treatment on filiform corrosion of aluminium alloy AA3005

Abstract The effect of heat treatment on the filiform corrosion (FFC) susceptibility of rolled aluminium alloy AA3005 at temperatures in the range 150–500°C is investigated. A drastic loss of corrosion resistance takes place as a result of annealing at temperatures in excess of 350°C. TEM investigations revealed a highly deformed near-surface region, approximately 1 μm thick, on the as-received material. The presence of this surface layer is caused by shear deformation of the surface region during rolling. The enhanced deformation undergone by the surface layer promotes secondary intermetallic precipitation in this region during subsequent heat treatment. The resulting microstructure, where the near-surface region contains a higher density of fine intermetallic particles and lower solid solution levels of the more noble alloying elements than the underlying bulk, results in preferential attack of this layer during corrosion processes and is identified as the main cause of the observed loss of FFC resistance.

Effect of thermo-mechanical processing on filiform corrosion of aluminium alloy AA3005

The effect of laboratory scale thermo-mechanical processing of aluminium alloy AA3005, supplied as 2.9 mm sheet at the hot roll transfer gauge, on filiform corrosion (FFC) susceptibility, electrochemical properties and near surface microstructure was investigated. Corrosion testing showed that the as hot rolled material exhibited poor FFC resistance and that heat treatment of both the as hot rolled and subsequently cold rolled samples resulted in a drastic loss of corrosion resistance. Microstructural characterisation showed that both hot and cold rolling resulted in enhanced surface shear deformation and the formation of a thin layer of metal in the surface with different properties and microstructural characteristics than the underlying bulk material. Enhanced surface deformation resulted in a redistribution of intermetallic particles in this region. Poor FFC properties were correlated with the presence of a higher density of fine intermetallic precipitates in the surface layers than in the underlying bulk. It is suggested that enhanced secondary precipitation of intermetallic particles in the deformed surface layers during high temperature exposure, e.g., during hot rolling or annealing steps, is the main contributing factor to the observed poor FFC resistance.

Effect of heat treatment on electrochemical behaviour of aluminium alloy AA3005

Abstract The effect of heat treatment on the electrochemical properties of bare metal samples of aluminium alloy 3005 in an acidified sodium chloride solution was investigated. Annealing at increasing temperatures in the range 150–500°C causes negative potential transients of increasing magnitude and longevity for samples exposed at open circuit. Electrochemical characterisation shows the potential transients to be caused by an increased anodic reaction rate of the annealed material surfaces as compared to the cold rolled, hard temper substrate. It is suggested that this increased anodic activity during short time exposure of the bare metal samples may be attributed to changes in the protective properties of the surface oxide as a result of the high temperature heat treatment. The mechanisms controlling short term electrochemical properties of bare metal samples may not be identical to those controlling the filiform corrosion susceptibility of coated products and a cautious approach is recommended when using electrochemical techniques for predicting corrosion properties.

Filiform corrosion of binary aluminium model alloys

The effect of alloy composition and microstructure on filiform corrosion (FFC) susceptibility was investigated for super-purity based binary model alloys of the systems Al–Mg, Al– Mn, Al–Fe and Al–Cu. Corrosion testing indicated that the presence of electrochemically noble second phase particles is a necessity for FFC to occur. Single phase, solid solution alloys of the Al–Mg, Al–Mn or Al–Fe systems did not support FFC. Heat treatment caused precipitation of the electrochemically noble intermetallic phase FeAl3 in the Al–Fe alloy, resulting in extensive FFC. Precipitation of the phase MnAl6, which has electrochemical properties similar to that of the aluminium rich matrix, by heat treatment did not impair the corrosion properties of the Al–Mn alloy. Significant surface oxidation and magnesium enrichment of the surface oxides by heat treatment did not affect the FFC properties of Al–Mg alloys. However, the solid solution binary Al–Cu alloys exhibited severe FFC. The detrimental effect of copper in solid solution is attributable to selective dissolution phenomena during the corrosion process, whereby copper was locally enriched on the surface as copper-rich particles providing efficient cathodic sites. � 2002 Elsevier Science Ltd. All rights reserved.

Filiform corrosion of AA3005 aluminium analogue model alloys

Abstract The effect of the metal substrate microstructure on filiform corrosion (FFC) susceptibility was investigated for super purity based model alloys with compositions based on the specifications of AA3005. Variations in alloying levels of the elements iron, silicon and copper were investigated. Alloys with high silicon content were more susceptible to FFC than alloys with low silicon content. The iron content, at the levels investigated, did not strongly affect FFC properties. The apparent detrimental effect of a high silicon content is attributed to the influence of silicon on secondary intermetallic particle precipitation. Given the same thermo-mechanical treatment, alloys with high silicon content underwent more extensive secondary precipitation of manganese containing intermetallic particles than those alloys with a low silicon content. The resulting microstructure is characterised by a higher density of finely dispersed intermetallic particles and a lower content of manganese in the adjacent supersaturated solid solution. These conditions provide a large number of potential corrosion initiation sites on the surface and also enhance microgalvanic coupling between intermetallic particles and the surrounding aluminium rich matrix, thus promoting the propagation of filamental corrosion attacks. Additions of copper had a detrimental effect on the FFC resistance. The role of copper in promoting FFC is attributed to preferential dissolution phenomena during the corrosion process, whereby copper is locally enriched on the corroding surface. This copper enrichment provides additional area for cathodic reaction, thus enhancing the corrosion process.

Effect of trace elements on electrochemical properties and corrosion of aluminium alloy AA3102

Abstract The purpose of this work is to study the effect of heat treatment and chemical processing on the electrochemical behaviour of aluminium alloy AA3102. Aluminium alloy 3102 was electrochemically activated in chloride solution as a result of heat treatment for periods exceeding 10 min at temperatures higher than 400 °C. The electrochemical activation was determined by the presence of deep negative potential transients when exposed to an acidified chloride solution. Furthermore, the anodic current densities became large at a given potential relative to the as-extruded surface as a result of high temperature heat treatment. This activation phenomenon was attributed to enrichment of the surface by lead, which was present in the material as a trace element. Enrichment of lead at the metal–oxide interface was ascertained by GD-OES depth profiling. Chemical and structural changes occurring in the oxide as a result of heat treatment did not have a direct role in the activation process. It was also shown that enrichment of the surface by lead had a sacrificial effect in protecting the surface against pitting corrosion.