Tuesday, July 15, 2008

Engineering Essentials - Segment Three (Dissimilar Metals)

The other night I replaced the Vehicle Speed Sensor (VSS) on my car. The bracket holding the sensor in place was made from Aluminum, while the bolt holding it to the transmission housing (also Aluminum) was made from Steel.

When designing systems in which conditions like this exist, certain precautions have to be taken to avoid dissimilar metals corrosion, also known as galvanic corrosion. For galvanic corrosion to occur, two different metals must be in contact, subjected to a corrosive or conductive environment (such as salt covered roads) causing a flow of current between them. The current flow will increase the chance for the metal more likely to corrode to do so and the opposite applies for the more corrosion-resistant metal.

To prevent galvanic corrosion between materials like the Steel bolt and the Aluminum bracket and transmission housing, one technique is the use of coatings, like Zinc. The reason here is that Zinc is higher than both Steel and Aluminum on the Galvanic Series.

As seen in the table, the metals higher on the table offer protection to those lower on the table. This of course works in theory, but as anyone that's worked on cars that have been subjected to corrosive environments like here in the Northeastern United States, if those coatings are compromised, the results can be pretty nasty. For example, it can cause bolts to break when you apply torque to them in trying to remove them, one of the least cool parts about working on cars. However, like with the VSS the other night, there's always more than one way to skin a cat...if you have the right tools that is!

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