Andreas Afseth

Intergranular Corrosion and Stress Corrosion Cracking of Sensitised AA5182

AA5182 (Al-4.5 wt% Mg) can become susceptible to intergranular corrosion (IGC) with time at moderately elevated service temperatures owing to precipitation of Mg-rich β-phase at grain boundaries, which can lead to stress corrosion cracking (SCC). The IGC and SCC susceptibility of AA5182 was found to depend strongly on sensitisation heat treatments. AFM and TEM studies demonstrated that the degree of precipitation and thus susceptibility to attack for a boundary can be related to its crystallographic misorientation. Low angle boundaries (<20°) are most resistant to attack as they do not show β-phase precipitation. However, higher angle boundaries show highly variable precipitation and corrosion susceptibility: critical factors are the grain boundary plane and precipitate/matrix crystallographic relationship.

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.

Filiform corrosion of aluminium sheet. Ii. Electrochemical and corrosion behaviour of bare substrates

Abstract Colled rolled aluminium substrates of the AA1000, 3000 and 8000 series in various tempers were characterized by corrosion potential (Ecorr) and potentiostatic measurements in an acidic chloride solution. The time dependence of these data indicated the presence of an active surface layer on the annealed substrates that were susceptible to filiform corrosion (FFC) after painting. This layer was electrochemically more active than the bulk of the material whose Ecorr was stable at the well known critical pitting potential (Ec) of the alloy. The Ecorr of an active substrate was more negative than the Ec of the bulk metal immediately after exposure to the test solution, rising to the bulk Ec with immersion time, as the active layer dissolved in the solution. Analogously, potentiostatic measurements at Ec indicated high anodic current transients, decaying with time to lower steady-state values after an equivalent amount of metal (of the order 1 μm in thickness) was removed from the surface. The reactivity of the layers, quantified in this manner, depended on the chemical composition of the substrates in the same way as the susceptibility of the layers to FFC.

Electrochemical Behavior of the Active Surface Layer on Rolled Aluminum Alloy Sheet

The highly deformed, micrograined layer on the outermost surface of a rolled Al-Fe-Si-Mn model alloy was electrochemically characterized. The thickness of this deformed surface layer in a 1.0 mm thick sheet was approximately 1 μm. Polarization curves in 5% NaCl solution at pH 3.0 and 11.5 were obtained at different depths from the surface using controlled sputtering in a glow discharge optical emission spectrometer for sample preparation. Both the anodic and the cathodic reactivity of the deformed surface layer were significantly higher than that of the bulk. Consistent with this, image analysis of scanning electron microscopy backscattered images revealed an increased number of fine intermetallic particles in the surface layer as compared with the bulk of the material. The corrosion morphology of the outermost surface was characterized by a high density of fine pits, while fewer and larger pits were observed in the bulk. The results highlight the importance of heavily deformed surface layers in controlling corrosion behavior of rolled aluminum products.

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.

Significance of Thermomechanical Processing in Determining Corrosion Behavior and Surface Quality of Aluminum Alloys

The paper first gives a brief review of the fundamentals of localized corrosion on multicomponent, multiphase alloys based on a knowledge of the bulk structure and the significance of such knowledge in developing materials with improved corrosion resistance. Recent advances in investigating the near surface microstructure of aluminum alloys are next discussed along with the developing understanding of the relationship between the surface structure, electrochemistry, corrosion behavior, and surface quality in general. In particular, recent electrochemical work and TEM studies of common types of architectural alloys and their model analogues revealed the presence of a surface layer with properties very different from the bulk. The surface layer is a result of the thermomechanical processing of the wrought product, especially exposure to high shear and elevated temperatures, and it is characterized by a refined grain structure with grain sizes one to two orders of magnitude smaller than the bulk grains. These are intermixed with rolled-in oxide particles and a very fine distribution of secondary intermetallics, which precipitate preferentially in the surface layer as a result of heat treatment. The significance of such layers to localized corrosion is discussed along with possible significance also to other type of surface properties, such as in relation to adhesive bonding and optical quality.

Effect of Equal Channel Angular Extrusion on Corrosion Behavior of Al-Mn ALLOY

Effect of deformation on the corrosion behaviour of Al-1Mn0.4Fe-0.3Si model alloy was studied after equal channel angular extrusion (ECAE) to different levels and subsequent annealing treatment. During polarisation, cathodic reactivity of the samples increases with the degree of ECAE deformation. Microstructure showed more number of intermetallic particles after ECAE deformation as well as after annealing. ECAE deformation also increased the anodic reactivity. Observed electrochemical behaviour is attributed to intermetallic particles as they good cathodic sites and precipitation of particles deplete manganese from matrix leading to higher anodic activity.