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.
In EE, we had two semesters of chemistry in freshman year. (Never got a lot of use out of it or learned much.) We had no metallurgy at all, no materials science. I guess that was in the ME curriculum, but from reading Dave's excellent piece here, I can see that some exposure would've been beneficial. I guess everything eventually returns to its base form if exposed to the elements long enough.
Good point, Beth. When I studied engineering, we took two semesters of chemistry and one of material science (or metallurgy, which was more typical back then), but the chemistry of corrosion was never discussed, as I recall. Very nice article.
This is a simple and yet very informative write up. THANKS! It would have been useful for some of the engineers at Chrysler Corp to have been a bit more familiar with this a few years ago. The corrosive failure mode of extruded aluminum alloy bumpers is very impressive, in addition to being quite discouraging. Of course, sea water is not nearly as salty as the saturated brine solution found on our Southeast Michigan roadways every winter. So of course, just because cars might survive a dunk in the ocean idoes not mean that they would last very long here.
Probably it would be a good idea for all schools offering engineering courses to add a course in corrosion, and to put it at a senior level.
Thanks for that reference Dave Palmer. I will definitely look into it. I became interested in the subject several years ago when i installed a Ground Source Heat Pump in my home. I was interested in how to protect metallic pipe in the ground, or immersed in a well. Well water is mildly acidic in my area. But im assuming pipe in the ground in contact with various types of sand/clay might be a different, more complex problem. Metallic pipe, or an immersed metallic coil heat exchanger would be much better for heat transfer than the recommended formulation of poly pipe recommended by the Ground Source Heat Pump industry. (After looking at the problem a bit, i decided only titanium might be viable. I did not install metallic pipe because i could not obtain a cheap source of surplus titanium tubing or coils, nor reliably solve the interconnection problem due to corrsion of clamps)
PS: WOW I just looked at that document - way more info than i found before. I will digest it at home thoroughly!
@evofxdwg: Yes, it's definitely possible to prevent corrosion using impressed DC current. This is done mainly for large structures; it tends not to be practical in smaller applications. Of course, this works the other way around, too: stray currents in the water can accelerate corrosion of outboard engines, for instance. A good source of information on designing cathodic protection systems for structures is U.S. Army Corps of Engineers Technical Manual 5-811-7, which can be found online.
Thanks a lot for this great article on understandind galvanic corrosion. This is a very important topic in the ever increasing "mechatronics" design and development enviornment. These types of articles are worth their value in gold. This is a must read article.
Interesting article. I would also like to see a basic tutorial of how to prevent galvanic corrosion actively with electric current. What are the rules of thumb for current flow or voltage, magnitude/polarity, critical control parameters, reliable connection, etc? I know such devices exist but not much about them. This information might be useful for protection of metallic pipes in the ground, marine fixed and floating structures, expensive industrial and residential plumbing systems, and any metal structure with dissimilar metals and/or ground/electrolyte contact. Would this method be any better than the common "sacrificial anode" method in saltwater?
Seems like a subject that ought to covered in more depth. Easy for a bridge designer for instance to build a time bomb into a bridge without intending to. Important for field engineers to make clear to maintainers why certain hardware is used to eliminate entirely avoidable failures. Really glad he brought the subject up.
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