Corrosion in the Motor Industry

During the 20th century, the chemistry of the automobile environment throughout the world has changed dramatically. As a result of these changes, the corrosivity of the automobile environment has increased.

Cars and trucks rust, some faster than others, because they are exposed to the elements whenever they are on the road, and in most cases also while they are parked. Much progress has been made in vehicle anti-corrosion treatment in the last 20 years, but the problem still remains.

The importance of the bodyshell

The bodyshell forms the load-bearing structure of almost all private vehicles and most vans and minibuses. This shell must be light, so that the vehicle can have good performance and be fuel-efficient. It must be strong to absorb both static and dynamic forces without deformation. It must provide a large and uncluttered interior space, to maximise passenger comfort and payload. It must havefront and rear crumple zones, and a strong central section for collision safety.

All of the above, together with economic considerations, dictate that the bodyshell be made of steel, and have an intricate structure with many inner structural members.


How does corrosion attack the bodyshell?

The short answer: from above, from below and from within.

From above: Vehicles which are normally parked outside may be subject to continuous exposure to rain and dew, which form a wet surface film, which eventually erodes the anti-corrosion protection layers. Also, the rain, and often the dew, work their way past imperfect seals, into inner structures. Additionally, any chrome body trim forms a parasitic galvanic cell with the steel body, with the rainwater (plus impurities) acting as an electrolyte. This causes galvanic corrosion.

From below: Stones impacting the underbody perforate the anti-corrosion protection layers. Mud, grit, slush and snow (and where used, anti-ice salt) get sprayed from below into every nook and cranny of the often-complex under-structure and accumulate there. Accumulated dirt and grit act as a sponge, keeping the underlying metal surface damp, long after the wet weather has come to an end – and will eventually clog drainage holes, aggravating the situation. Even dirt accumulated in dry weather will be transformed into a damp sponge as soon as the wet weather sets-in.

Particularly critical areas are underbody cross-members and mounting points for suspension and mechanical components.

From within:  Below its outer skin, the bodyshell is an intricate structure. As mentioned above, water and dirt have many paths into the inner structure. And, often unseen, the dirt accumulates there over the years, acting as a sponge whenever it is exposed to humidity. Of course, a humid sponge sitting on a steel panel will over time attack the protective layers and start oxidising the steel itself. Condensation also deposits on inner panels, whether from sudden drops of ambient temperature in humid weather (e.g. at nightfall), or by the use of air conditioning in the cabin, especially in hot, humid weather – where the ambient humidity condensates on the cooled steel panels.

Another mechanism by which water gains the inner structures, is where corrosion has already started under rubber door or window seals. These seals are lifted from their place by the corroded steel and allow rainwater to go past the seal. Flooded footwells and soaked baggage compartment carpets are a symptom of this.

In recent years, much progress has been made in corrosion protection techniques (since approx. the 1990s). Up to the 1980s vehicle bodies were made from cold-rolled steel, which was then primed and painted. From the 1990s onwards, steelmakers installed continuous electro and hot-dip galvanizing lines producing two-sided galvanized steel for car and light van bodies. This resulted in a significant improvement in corrosion withstand, but still does not eliminate the problem, as the protective galvanisation is eventually eaten away, exposing the steel to corrosion. Also, if a vehicle has been in an accident, unprotected steel will inevitably be exposed, leading to corrosion.


What other motor vehicle components are prone to corrosion?

Apart from the bodyshell, suspension components are exposed to mud, grit, slush, snow (and where used, anti-ice salt) sprayed from the road below.

Exhaust systems corrode because of the water vapour exhausted by the engine in cold humid weather. If the exhaust system does not reach high temperature during operation, this water vapour condenses on its inner surfaces, causing corrosion.


Safety Implications

As mentioned above, the bodyshell is a vehicle’s main structural element (for cars, vans and minibuses). As little redundant metal as possible is used in the quest for a spacious, strong and light structure – numerical design methods optimise the structure to achieve this goal. So, in the absence of much redundant metal, any serious corrosion significantly weakens the vehicle structure, and therefore compromises safety.


Cost of Corrosion

According to the NACE publication “Corrosion Costs and Preventive Strategies in the United States – publication no. FHWA-RD-01-156”, released by the U.S. Federal Highway Administration (FHWA) in 2002, the direct annual cost of corrosion for the US motor vehicles sector was estimated to be $23.4 billion, of which $14.46 billion is attributed to corrosion-related vehicle depreciation, $6.45 billion to corrosion-related repair and maintenance, and $2.56 billion to additional manufacturing costs due to corrosion protection operations and materials.