What causes Pitting and Corrosion to Alloy Wheels

Paintwork and surface textures now adopted in modern vehicle construction have a bright life expectancy of in excess of 10 years in normal road use conditions.

One area of exception is the road wheels where due to the direct contact with the road and other curb height objects abrasion and surface damage readily occurs, especially to the Aluminium and Magnesium Alloy variety.

Invariably this damage is slight and may only be a glancing blow from a chip of gravel but this sometimes is sufficient to breach the protective layer. These layers may be either paint or lacquer or an anodised finish.

Whichever method is employed, even the slightest pin hole in the protective layer will provide a direct path for corrosion to occur.

This process of corrosion is further exasperated due to the increasing amount of salt being spread on the roads during cold and frosty days to prevent cars and other vehicles from skidding.

Greater problems with salt accumulation occur in snow and slush conditions where the salt is suspended in large quantities and for extended periods until the surface of the wheel has dried out. While dry salt has little effect on exposed metal, as soon as it becomes moist or wet this allows a whole new generation of conditions to emerge.

Simply the wetted salt becomes an electrolyte that allows the passage of a small electrical current that combined with the salts causes a chemical change to occur resulting in corrosion and staining.

In the case of the Aluminium or Magnesium Alloy wheels these currents are amplified due to the complexity of other metals present in the structure of the vehicle and to compound the problem further the direction of corrosion is preferentially towards the ion of greatest potential. This being the Aluminium or Magnesium Alloy wheel that slowly corrodes, to all intent it effectively dissolves. Initially starting as microscopic inclusions, the process continues causing blistering to the surface of the wheel as the oxidised elements push their way out from under the now broken protective layer.

Further deterioration occurs as the blisters increase in number and deepen to form small pits in the surface, simultaneously the protective finish to the wheel is being forced off the its surface in a peeling action as the effect of the corrosion spreads to form areas of greater disfigurement. These blemishes normally take the form of darkened areas beneath the protective layer and express white powdery deposits on areas where the protection has completely failed.

Due to the safety factors built into the design of the Alloy wheels, surface corrosion and blemishes will not compromise the wheels mechanical integrity and to all intent the deterioration will remain a cosmetic issue. It however can, if allowed to go unchecked, eventually cause the wheel to fail.

This probably would not be a catastrophic failure but corrosion has been seen to aggravate microscopic cracks and inclusions to such an extent, the tyre is no longer able to retain its pressure and rapidly deflates.

A further effect of corrosion occurs around and under the bead edge of the tyre. Although the fit of the tyre on the Alloy wheel is extremely tight, the process of corrosion continues between the protective outer layer and the surface of the metal and can bridge across the pressure sealing element or bead edge of the tyre.

So there you have it. Your precious Alloys are there as a sacrificial element to protect the exhaust pipe or tow hook and; yes, the rest of the car.

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