Magnesium anode nickel contamination failure

Hot-water-tank sacrificial anode. Nickel solubility in magnesium is ~7 ppm; excess nickel forms precipitates that cause local galvanic self-consumption — anode disappears in hours without protecting the steel vessel.

I mentioned to someone after class yesterday — I think I mentioned it in class — what does a magnesium anode look like for a hot water tank at home? They basically extrude magnesium over a steel wire and pull the steel wire out, and that becomes your sacrificial anode. This magnesium has to be extremely pure — less than seven parts per million of nickel. Nickel and magnesium do not mix in equilibrium proportions. The solubility of nickel in magnesium is only about seven parts per million. If you have more nickel than that, you'll get little nearly-pure nickel precipitates, and your magnesium anode will consume itself by galvanic action. You'll have local galvanic cells from here to here, and two hours later it's completely gone, and it didn't do anything to protect your steel vessel. It just protected itself in some areas where there was nickel and corroded itself everywhere else. It looks like Swiss cheese. So some of these things get fairly tight on composition control for corrosion.

Spine of the corrosion thread. The anode's high-purity magnesium spec (<7 ppm nickel) used to teach concentration-cell corrosion; the Juarez fabrication line used to teach how design accommodates real-world coating defects.

One of the students came up afterwards last time — I passed this magnesium around — and asked, what's that little dot in the middle of it? [Tom holds up the magnesium bar.] If you pass this around, it turns out it's a piece of steel. I bought these last spring when I was teaching this course on the side — wanted something more touchy-feely, so people can feel the density of something light like magnesium. I couldn't afford to buy a one-inch diameter, twelve-inch-long bar of beryllium, which is actually about the same density.