Local electrochemical behaviour of 7xxx aluminium alloys

Abstract

Aluminium alloys of the 7xxx series (Al-Zn-Mg-Cu) are susceptible to localized types of corrosion like pitting, intergranular corrosion and exfoliation corrosion. This represents a limitation for the application of these alloys in the aerospace components because localized corrosion might have a negative effect on safety and costs. This PhD thesis investigates the relation between electrochemical behaviour and microstructure of a number of 7xxx aluminium alloys: AA7075, AA7349 and an experimental alloy (EA1) with composition in the range of AA7449. To this aim, the experimental approach of this PhD consisted in the characterization of the microstructure of the alloys combined with the study of their electrochemical behaviour. The investigation of the microstructure focused on intermetallics and strengthening particles because these second phase particles strongly affect the corrosion behaviour of 7xxx aluminium alloys. The study of the electrochemical behaviour was approached on two different levels: macro- and micro-electrochemical characterization of the alloys. The macro-electrochemical characterization employed open circuit potential and potentiodynamic polarization measurements to study the overall corrosion behaviour of the alloys, while the micro-electrochemical characterization applied local electrochemical techniques (scanning Kelvin probe force microscope and micro-capillary cell) to study the local behaviour of the intermetallics. There are three main types of intermetallics in the 7xxx alloys investigated: - Al7Cu2Fe - (Al,Cu)6(Fe,Cu) - Mg2Si In AA7349 and EA1 the intermetallics contain small amounts of Mn and Si. The strengthening particles are Mg2Zn phase for all alloys. The intermetallics are not affected by heat treatments (solution heat treatment, aging and overaging), while the strengthening particles undergo strong changes. The strengthening particles are dissolved by solution heat treatment resulting in supersaturation of solute elements (Zn and Mg) in the matrix and in segregation of these elements at the grain boundaries. They precipitate in the matrix and at the grain boundaries during aging and overaging progressively reducing the supersaturation of the matrix and the segregation at the grain boundaries. The intermetallics are the initiation sites for localized corrosion, which takes place as local dissolution of the matrix at the location of the intermetallics and results in pitting of the alloy surface. At a later stage, the localized attack propagates as intergranular corrosion and might turn into exfoliation corrosion for microstructures particularly susceptible to the intergranular attack. The types of intermetallics in the microstructure of the alloys exhibit different micro- electrochemical behaviour. The Al7Cu2Fe and (Al,Cu)6(Fe,Cu) intermetallics have cathodic behaviour relative to the matrix (positive Volta potential difference relative to the matrix), while the Mg2Si intermetallics have anodic behaviour (negative Volta potential difference relative to the matrix). Moreover, the Al7Cu2Fe intermetallics have stronger cathodic behaviour than the (Al,Cu)6(Fe,Cu) intermetallics. The microstructural changes taking place during solution heat treatment strongly affect the electrochemical behaviour of the alloys. Solution heat treatment strongly increases the Volta potential difference between the intermetallics and the matrix (for all types of intermetallics) increasing the susceptibility to pitting. Moreover, it increases the susceptibility to intergranular and exfoliation corrosion because of the segregation of Zn and Mg at the grain boundaries. Aging and overaging decrease the Volta potential difference between the intermetallics and the matrix improving the resistance to pitting. Aging improves the susceptibility to intergranular corrosion as compared to solution heat treatment. However, the alloys aged to the peak-strength tempers are still prone to severe intergranular corrosion and exfoliation corrosion because they undergo precipitation of small strengthening particles at the grain boundaries with very small interparticle spacing. Overaging strongly improves the resistance to intergranular corrosion and eliminates the susceptibility to exfoliation corrosion because it increases the size and the interparticle spacing of the particles at the grain boundaries. The characterization using the local electrochemical techniques (scanning Kelvin probe force microscope and micro-capillary cell) showed that the micro- electrochemical behaviour of the intermetallics is different for each intermetallic. This means that some areas of the alloy surface might be very susceptible to localized attack and might be preferential sites for the initiation of localized corrosion. The local electrochemical techniques used in this PhD enable the relation of the electrochemical behaviour to the microstructure of the alloys. In particular, it was possible to establish this relationship for the intermetallics due to the lateral resolution of these techniques (in the micrometer range). This thesis has proved that the complementary use of local electrochemical techniques is very useful for the characterization of metals that are susceptible to localized types of corrosion.