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What are files coated with?

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A file, a tool that can be used for smoothing, deburring, shaping and sharpening Usually, files do not have protective coatings or finishes added when they are made.
A woodworker demonstrating abrasion by wearing down a piece of wood with a rasp This is because files are abrasive tools, i.e. their main function is to rub against a surface and wear it down.
Image to show that coating a file that has been designed to be resistant to abrasion with a coating to protect it from abrasion is a waste of money Most coatings are unsuitable for this, as they would wear off very quickly, therefore making it a waste of money and resources to put them on!

File coating: the only option…

A file coated with black oxide One type of protective coating that can be used is black oxide coating.
A spanner with a black oxide coating, which is also used on files This is the same kind of coating that is used for guns and knives, as well as some other tools such as spanners and drill bits.
A file suspended in the chemical bath used to coat steel in black oxide The process of coating steel in black oxide involves soaking it in a bath of caustic chemicals that react with the surface of the metal.
Diagram to illustrate that black oxide coats the surface of the steel This reaction causes the metal to oxidise in a specific way, coating the entire tool in a thin layer of magnetite.
Magnetite in its naturally occuring form. This is the same material that forms on a file during the black oxide coating process. Magnetite (Fe2O3) is a specific type of iron oxide that occurs in nature in rocky mineral deposits. Using this coating is like wrapping your file in a thin layer of rock!
A protective coating on an abrasive tool such as a file will wear through The coating provides limited protection against corrosion, but does help to keep the tool sharp; a DIYer would have to wear through the black oxide coating before they could start to blunt their file.
A happy piggy bank thanks to a cost effective way of heat treating the steel used to make files Even though it’s quite cheap to coat tools in black oxide, you are still more likely to find files that are uncoated.

This is because the elements used in the tool steel that the files are made from increase the file’s hardness as well, and also provide some resistance to corrosion.

Coating a file that is made from chromium alloy steel with black oxide is very much a belt and braces approach Still, it can be argued that a black oxide coating can be beneficial, especially if you’re a fan of the belt and braces approach!

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