On the other hand, if stainless steel fasteners are used to fasten zinc-coated steel plates, neither will corrode particularly fast, because the anode (the zinc plates) is so large compared to the cathode.
Here are a few design guidelines for dealing with galvanic corrosion:
Where possible, use metals that are close together in the galvanic series. The closer two metals are in the galvanic series, the less the tendency for corrosion will be.
If dissimilar metals are used, try to prevent electrical contact between them. In order for there to be corrosion, there needs to be a flow of current from the anode to the cathode. Using plastic or rubber spacers between dissimilar metals can prevent the flow of current.
Keep water out. We've read about cathodes and anodes, but as I mentioned before, there is one more thing which is necessary for galvanic corrosion: an electrolyte. If no water can reach the junction between the two dissimilar metals, galvanic corrosion can't take place.
Make sure the anode area is larger than the cathode area. If you have to use dissimilar metals, and you can't keep them dry and out of electrical contact, then make sure the larger part is made out of the less noble metal, and the smaller part is made out of the more noble metal. This will keep the corrosion rate low.
Following these simple rules can help keep your designs safe from galvanic corrosion.
Thanks for a great article on a basic and important subject. I've actually been wondering about galvanic corrosion since I'm coming across the topic here at DN a lot when writing about coatings and adhesives.
I'm with Beth--I've had to learn or re-learn a lot of basic chemistry in this beat, and am astonished that engineers have so little training in it. Aside from basic principles of ME, it's the other main subject I keep running into here.
That's really hard to believe. I would think now with so much (or somewhat, however you want to look at it) of the focus on sustainability and alternative energy, chemistry and the make up of matter would be more important than ever to basic engineering work. Hopefully, we'll start to see a change in engineering curriculum to reflect that.
Thanks, Beth! You're right that this is something that should be part of a basic engineering education. Unfortunately, most mechanical engineering programs only require one semester of chemistry. Most universities which have materials engineering departments offer an upper-level undergraduate course in corrosion as an elective, but it's not usually required.
Great primer, Dave, on a topic that should be top of mind for engineers. I'm actually surprised this kind of material isn't duly covered in the standard engineering curriculum. Seems like materials/chemistry 101, especially for engineers trying to create products that have lasting legs.
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