The effect of heat treatment on filiform corrosion in aluminium alloy wheels

Abstract

Filiform corrosion in aluminium alloy 356 used for vehicle wheels was investigated using a combination of accelerated corrosion testing, linear potential scanning, contact angle measurement and tensile testing. Three different causes of filiform corrosion were identified. Contact angle measurement was found to be an indicator of the effect of time in the solution heat treatment on corrosion resistance. A maximum in the contact angle corresponded to poor corrosion resistance and low exchange currents. While the exchange current did not change after the end point of silicon coagulation was reached, the contact angle and filiform length both decreased after the maximum had been reached. Decreases in exchange current in the solution heat treatment indicated the coagulation of silicon and an increase in filiform growth as the grain boundaries became more active. During the ageing heat treatment in the paint oven the trend reversed and an increase in exchange current corresponding to an increase in filiform growth. This was linked to precipitation of magnesium silicide indicated by a negative shift in the equilibrium potential. There was some evidence from contact angle increases that silicon coagulated during ageing in the paint oven in the case of the low temperature solution heat treatment. The appearance of a second minimum in linear potential scans in a position negative to the equilibrium potential was a useful indicator of the precipitation of magnesium silicide. The overall conclusion was that the lowering the temperature of the solution heat treatment increased corrosion resistance but corrosion resistance could be reversed by a long high temperature aging heat treatment. The wheels needed to be heat treated for 270 minutes at 540C to allow the silicon to finish coagulation and some silicon to migrate into the silicon depleted zone in the grain boundaries to make them less anodic. Treating the wheels in the aging (paint) oven twice reduced corrosion resistance.