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Textile conservator, Gwen Spicer of Spicer Art Conservation at work

Thursday, February 15, 2018

What is Magnetized Stainless Steel?

If you don't know it by now, we at Spicer Art Conservation think about magnets a lot. And with Gwen's book, Magnetic Mounting for Art Conservation and Museums (to be published by Archetype later this year), we like to share what we're discovering about their properties and applications.

The other day a client called about how to mount an artifact in their institution using magnets. We worked out a system where counter-sunk disc magnets would be secured to the wall allowing the artifact to be held in place with thin stainless steel discs. The registrar proceeded to order the supplies. She called back a few minutes later asking, "what is magnetized stainless steel?" and then stated, "but stainless steel is not magnetic!"

As it turns out stainless steel is not just one metal, but instead is composed of a group of metals or alloys. All of the metals in this group are magnetic, except one. The confusion lays in the fact that the non-magnetic type of stainless steel called "austenitic" is the most commonly used stainless steel for producing domestic products, and thus it is the type of stainless steel that we are most familiar with. (An example is stainless steel utensils/flatware that have 18-20% of chromium and 8-10% nickel, which is not magnetic.) 

When nickel is added, as with the utensils/flatware example above, stainless steel becomes non-magnetic and its anti-rust properties are enhanced. The more nickel the greater the corrosion resistance. But, its presence also causes the stainless steel to be non-magnetic. This stainless steel is the austenitic type.

The stainless steel alloy has at least 10.5% chromium. It is the added chromium that creates the protective layer of chromium oxides on the surface that prevents the development of iron oxide rust. It is the added chromium that makes the metal both rust and scratch resistant, and with the increase of chromium, resistance is also increased. Chromium can make up as much as a quarter of the weight.

Magnetic stainless steel is based on the amounts of alloying elements as described above as well as on the grain structure and the amount of cold working. Another interesting fact is that austenitic type stainless steel with a low amount of nickel can be reverted to a magnetic type when cold hardened. However, it is true that the metal has a crystalline structure that has a lower magnetic permeability than just steel alone.

The odd thing you might now being asking is, "But nickel is ferromagnetic! How can it NOT be magnetic?" Therefore, you would think that when nickel is added to iron and chromium it would be even more magnetic. But this is not the case! Why this happens is based on the different atomic arrangements between face-centered cubic (FCC) and body-centered cubic (BCC) -- austenitic with nickel and ferritic without nickel, respectively.


 
Face-centered cubic (FCC)                        Body-centered cubic (BCC)

Therefore, if you want a ferromagnetic material that will not easily corrode and has a thin profile, stainless steel is a great option for that magnetic system.