I stared at a strange brightness on the point of the lathe tool as I machined a brake-disc carrier. Then, I understood. Cutting friction had ignited magnesium chips. Brilliant, white light flared. White smoke billowed to the ceiling and then died away as the fuel was exhausted. I hardly had time to think, “Pooh-bah, I’ve burned my shop down!”
Magnesium is that compromised metal whose lightness is so attractive (much cheaper than similarly light beryllium!) but whose softness, ease of catching fire and rapid corrosion have made engineers think twice. In 1972, when the late mag-wheel pioneer, Elliott Morris, asked his friend at Lockheed why magnesium wheels were no longer used on aircraft, the reply was, “We don’t use that damn metal anymore. Now, we have engines that’ll lift anything!” In the 1930s, Velocette’s racing manager, Harold Willis, called magnesium “trouble metal.”
A good many racing motorcycles have had lightweight magnesium case covers (today, on factory prototypes, they are carbon fiber). The cam-chain covers on AJS 7Rs and Matchless G-50s were painted a distinctive gold color, and factory Kawasaki H2-R 750s had primary-gear covers protected from corrosion by the Dow-19 chemical process, which produced a gold color. That color, and a competing matte brown found on the mag carbs of Yamaha factory 0W-31 750s, shouted, “Secret factory stuff—not for you!”
In the back lot of the New England Air Museum was an 18-cylinder radial engine whose beautifully cast mag nosecase and accessory sections had corroded to masses of white powder, exposing the gears and other parts within. Metallurgists say that all metals seek to return to being ore—metal oxides, sulfides and the like. That happens because metals are chemically reactive, especially vulnerable to combining with oxygen from the atmosphere. Schemes to stop corrosion operate mainly by excluding oxygen and water from the surface or by chemically altering the metal’s surface to reduce its reactivity.
An oil-pump rotor from an R-3350 engine, surrounded by white magnesium corrosion product, with the pieces held together by stainless safety wire.
Stainless steel contains a substantial percentage of chromium, which, by combining with atmospheric oxygen to produce chromium oxide, forms an oxygen barrier that protects the majority constituent, iron, from doing what it otherwise does so quickly: rust.
We’ve all seen brand-new steel bridges that are rust brown; this is a Cor-Ten steel alloy developed in the 1930s. Normally, as steel rusts, the oxide layer expands, breaking flakes loose from the metal beneath in a process called “exfoliation.” But “weathering steels,” such as Cor-Ten, are altered to stop this expansion, allowing the oxide layer to remain strongly attached, preventing further corrosion.
In the case of magnesium, the idea of luring the oxygen to a more reactive constituent led to adding two percent of even more reactive calcium to the mix. The resulting surface layer of calcium oxide then protects the magnesium. Combustion is just runaway oxidation, so this addition of calcium also raises magnesium’s normal ignition temperature of about 1170 Fahrenheit by 350 to 550 degrees, making it generally unnecessary to machine the resulting alloy in an oxygen-free atmosphere to prevent ignition of hot machining chips.
Another approach seeks to stop the action of electrolytic cells on the surface of the magnesium. Trace metals in the magnesium—iron, nickel, copper and cobalt—form intermetallic clusters. Because their electrons are bound at a different energy from those of magnesium, if you add water, you can form an electric cell. This is just like poking strips of copper and zinc into a lemon and then finding there is a voltage difference from one strip to the other.
The operation of these cells is similar to the electrolysis of water experiment so many of us performed in high school chem lab: Oxygen is produced at one electrode and hydrogen at the other. In the case of magnesium, the oxygen readily combines with it to form corrosion. Indeed, magnesium is commonly used as a sacrificial electrode to prevent the corrosion of underwater steel structures.
F-102 cockpit panel, showing many holes from the formation of corrosion cells.
In the accompanying photo of a corroded Convair F-102 cockpit panel, you can see that such corrosion cells, operating at every point where a crack in the protective paint allowed, have eaten their way right through the material. One way to prevent this is to eliminate the trace metals, but this level of refinement is too expensive to be widely used.
A group at Monash University in Australia, directed by Associate Professor Nick Birbilis, has adopted the approach of using a “cathodic poison” to greatly slow magnesium corrosion. In other alloy systems, materials such as antimony, selenium or tellurium effectively “unplug” corrosion cells. The group found that adding one third of one percent arsenic to magnesium reduced corrosion 90 percent. Such “poisons” stop the reaction by preventing single hydrogen atoms, produced by the electrolytic reaction, from giving up electrons as they join in pairs to make hydrogen molecules. The arsenic combines with the bare single hydrogen atoms, stopping the reaction. With electron flow halted, magnesium cannot “become ore.”
Even though the need for lighter-weight vehicles has increased the use of magnesium for such applications as seat frames, it is pointed out that only 1/50th as much magnesium is used as aluminum. With improved corrosion resistance available at low cost, thanks to the Monash University research, magnesium may find many more applications.