Do all grades of Nd-Fe-B material still need to be coated/plated?
Nd-Fe-B magnets have improved dramatically over the last 10 years. But they still contain iron and other reactive phases, and will rust. Nd-Fe-B is a blended powder before it is pressed into a magnet. The better the powder is mixed, the less “free” iron is available [small clumps of iron that did not mix with the other chemicals].
Dexter recommends that Nd-Fe-B magnets be coated. This greatly extends the lifetime of the magnet. The benefit over time is great. Newer, uncoated Nd-Fe-B materials can be used for prototypes and experiments as long as there is not an expectation that the material will remain rust free, particularly in harsh environments.
To what specifications are Dexter’s coatings rated, with respect to corrosion resistance?
There are no specifications for rating coatings. We have done extensive testing in-house and can use this as a guideline for choosing the best coating. It is best to consult a Dexter Applications Engineer to determine the best coating for a specific application.
What kind of performance can we expect from the various coating types used for Nd-Fe-B and Neoform® materials?
There are a variety of coatings/platings that can be applied to Nd-Fe-B and Neoform® material. Dexter has tested and can recommend three coatings / platings that were exposed to a 5-day salt fog test per ASTM Standard B117-97. Dexter has the ability to supply other coatings, but cannot predict their performance with our product. Three application methods are possible: barrel coating (Parylene), spray (Ever-Slik 1221), and electrolytic plating (nickel). in order of performance, the coatings ranked as follows: None [Poor], Nickel [Good], Ever-Slik 1221 [Very Good] and Parylene [Excellent]
What protection can be placed around a magnet when being used in an autoclave environment?
This depends – if the autoclave is only going to pull to the milliTorr level, then regular magnet materials will hold up. Bonded magnets will lose 30 to 40% of their output and will outgas, but after initial exposure it won’t get any worse. If the autoclave will pull a stronger vacuum than that, then bonded materials should be avoided.
What’s the difference between aluminum and nickel coating other than the physical appearance?
The differences are many. A sputtered aluminum coating is much thinner than a plated nickel finish. Although aluminum is softer than nickel, the coating is not generally intended for anything other than surface protection. A thinner coat creates less air gap, so that is an advantage. A sputtered coating is also “pounded” into the surface of the substrate, so it bonds strongly with the substrate. This is great for adhesive bonding of assemblies. Plated nickel has more “lubricity” than sputtered aluminum, so it does not bond as well. Plating is a surface treatment that can alter surface chemistry; this is a “skin depth” effect that is usually not a problem unless the magnets are very small.
When does Sm-Co require coating?
Sm-Co is typically plated for medical applications to give a “sealing” coat of nickel. This is extra insurance against the rare flake of iron. Nickel can be good insurance against such flaking, by making the surface tougher. It also protects the magnet from sterilizing chemicals and other contaminates.