The coal will absorb a sufficient amount of oxygen to prevent the rusting process.
You can prevent tools from rusting with coal by simply inserting coal pieces in your toolbox.
Ladies and gentlemen, I stumbled onto this one somewhere and thought I'd give it a try. Prevent tools from rusting with coal, put a couple of chunks of coal in the toolbox. The coal will absorb a sufficient amount of oxygen to prevent the rusting process. I put about three fist-sized chunks in a toolbox that was mounted behind the cab of a pickup truck, and I had no problem with tools rusting or seizing up.
Henry R. Bragg, Jr.