When zinc-coated screws are used to fasten stainless steel plates, the screws will corrode rapidly. But when stainless screws are used to fasten zinc-coated steel plates, neither the screws nor the steel will corrode particularly fast. Why?
To answer this question, we need to understand galvanic corrosion. Most engineers have heard something about it. They may have heard that certain materials shouldn't be used in combination with others, but the details of this are often only partially understood. After all, most mechanical engineering curriculums don't include classes on corrosion.
Corrosion is an electrochemical reaction, and for it to take place, you need three things: an anode, a cathode, and an electrolyte (usually water). For any combination of two metals, the less noble of the two will be the anode, and the more noble of the two will be the cathode. The anode will corrode; the cathode will not.
Which metal of a given pair is more noble? This can be determined by reference to something called the galvanic series. The most noble metals are at the top of the list, while the least noble metals are at the bottom. (Sometimes it is given in the opposite order, with the least noble metals on top.) The most noble metals include platinum, gold, silver, and titanium. The least noble metals include magnesium, zinc, cadmium, and aluminum.
The galvanic series is for seawater. The order of some metals may be slightly different in other environments. However, in the absence of detailed information about a particular environment, the galvanic series in seawater is usually a good reference point.
The farther apart two metals are in the galvanic series, the greater the tendency will be for the less noble of the two to corrode. For example, the corrosion current between titanium and aluminum will be greater than the corrosion current between aluminum and cadmium.
Corrosion rates are related to the anode-to-cathode surface area ratio. The smaller the anode is relative to the cathode, the more quickly the anode will corrode. Going back to the question at the beginning of the article, we can now see why zinc-plated bolts used to fasten stainless steel plates will corrode rapidly. Zinc is less noble than stainless steel, so the zinc-plated screws will act as anodes. The screws are small compared to the steel plates. This means that the anodes are small compared to the cathode, leading to a fast corrosion rate.
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.
Welcome, Dave, as Design News' newest guest blogger. For readers who enjoyed this -- as did I -- we'll have another blog from Dave in a few weeks. As you can tell, he's a serious materials expert.
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