Mechanism of Exfoliation (Layer Corrosion) of AI-5%Zn-1%Mg

Abstract

The mechanism of exfoliation in AlZn5Mg1 alloy has been studied by making a survey of the phasesb occurring in the alloy in the naturally aged and artificially aged conditions, preparing the phases in pure state and investigating their electrochemical properties. It has beenfound that in the naturally aged alloy, the attack is confined to lamellar zones in the structure giving rise to the exfoliation. Finely dispersed α-Al(Fe, Me)Si phase particles arranged in streaks along the extrusion or rolling direction act as cathodes and the anodic areas consist of narrow zones of the Zn- and Mg-rich matrix, next to the particles. Between the streaks of α-Al(Fe, Me)Si particles, layers of matrix are left unattacked. Since the main factor determining the amount of α-Al(Fe, Me)Si phase is the Fe content, an increase in Fe content will decrease resistance to exfoliation. In the artificially aged condition, the alloy is not prone to exfoliation, but shows a type of general attack. α-Al(Fe, Me)Si particles again constitute the cathodes of the corrosion cells, but the anodic phase is the M-phase (MgZn₂), which is evenly distributed.

Since the zone next to a weld bead is essentially in the solution treated condition, it will become resistant to exfoliation on post-weld artificial ageing. At some distance from the weld bead, however, there will be an ‘over-aged’ zone, where neither the hardness nor the resistance to exfoliation will be very much increased on artificial ageing. This is due to the formation of especially wide precipitate-free zones around α-Al(Fe, Me)Si and E-phase particles and around grain boundaries on ‘over-ageing’. Since the precipitate-free zones are due to vacancy depletion, these zones are supersaturated with respect to Zn and Mg and thus prone to corrosion. The attack will be confined to the matrix, especially along streaks of α-Al(Fe, Me)Si and E-phase particles.