I’ve been asked to give a 15-minute work-related demo to some of my junior colleagues next week. I get to choose whatever topic I like. Since I work at a warship museum, I’d like to create a tabletop demo that shows the principles behind the practice of bolting sacrificial zinc or magnesium plates (anodes) to the underside of steel ships to protect their hulls from corrosion.
It’s been a long time since I took high school chemistry, so can you science-minded Dopers help me out here?
Here are my criteria for the demo:
The demo has to show the real chemical principles involved in the use of sacrificial anodes. But since it would take forever to see salt water, zinc and steel react, please suggest a combination of materials that will work (at least to some noticeable degree) immediately, or at most, within a few minutes.
Keeping with the “real principles” criteria stated above, I do not want create some phony-baloney demo that only looks like its working like a real sacrificial reaction but isn’t really. (For example, don’t suggest dropping a silver-painted sugar cube and a nail in a glass of water and saying, “Look, everybody. See how the cube dissolves and the nail stays intact! That’s why steel ships don’t rust!”)
The demo has to include a control that visibly shows that anodes make a difference. I don’t want my colleagues to just “trust me” that the anode helped preserve the second metal. So there should be a with anode part of the demonstration, and a without anode part of the demonstration.
I don’t want to go nuts with this, so please suggest inexpensive, accessible, and reasonably harmless materials that I can find in your average hardware store or supermarket. (Think Mr. Wizard stuff like drinking glasses, aluminum foil, vinegar, nails, pennies, etc.)
Fish bowl + car battery + jumper cables + thin steel sheet + thin zinc sheet or rod + carbon or copper rod + plastic rods (cut from cheap plastic coat hanger).
Hang steel and carbon/copper from plastic rods placed across top of bowl and connect power
First, watch steel get eaten
Second, clip electrical cable from steel plate to zinc
Third place zinc in bowl with steel
See zinc get eaten.
If you can score a 5 gallon aquarium, the optics will be much better
If possible, I’d try to run the experiments in parallel - set up a rack with each configuration dangling from a piece of fishing line, then dunk them all at the same time and observe.
I’d suggest starting off with equal-sized tufts of steel wool stuffed through washers of different material - a plain steel washer, a zinc plated washer, a plastic washer etc.
Take a look at the galvanic series, which describes what happens when you put two dissimilar metals together in a conductive environment (e.g. seawater):
-the metal to the left on the chart is the sacrificial anode; the metal to the right is the one that gets protected.
All other things being equal, the voltage potential between the two is indicative of the time-rate at which the sacrificial anode will be consumed.
Want a fast demo? Pick two metals from the chart as far apart as possible; immerse them in a highly conducitve environment; heat it up; and agitate the water, either with a mechanical stirrer or just by gently boiling the water on a stove. Zinc-to-steel has a galvanic potential of just 0.4 volts. Attach a small magnesium block on a sheet of graphite, and the potential rises to 1.8 volts. You are now demonstrating galvanic corrosion, albeit at an accelerated rate. The reaction between zinc anodes, steel ship hulls and seawater is the exact same thing; it just happens more slowly. In fact, magnesium sacrificial anodes are commonly used to protect residental water heaters. Show your audience the galvanic series chart to help them understand what’s happening.
I don’t know that unprotected graphite will appreciably corrode in the absence of the galvanic cell formed with magnesium, so you might not be able to demonstrate “control” behavior. If that’s important, then try magnesium with steel; unprotected mild steel should corrode quite nicely in salt water. You would want to have the protected steel and the unprotected steel in two separate pots of water, otherwise the anode could possibly protect both pieces of steel.
Here’s your magnesium anode. Just drill a hole through it (slowly, and use plenty of cutting fluid; also have a type D fire extinguisher on hand in case the chips catch fire), and bolt it to the steel.
I suggest a private demo for yourself beforehand, so you can verify whether this setup will exhibit the behavior you’re looking for in a timely manner.