What is the wood? Oak will get darker when exposed to any sort of finishing chemicals, moisture or for any of several other reasons. Whoops - I see you wrote Walnut! Walnut will suck up any microscopic materials on the steel, so the rolling lubricant is now the most likely suspect.
Unless you need the steel panel to be unfinished for grounding and connection reasons, a half-dozen light coats of Krylon Workable Fixatif (accept no substitutes), will do a good job of sealing the steel from the wood, without changing the appearance. If you do need to connect to the steel and have good contact, install these little rings at each fastening point, (
http://www.target.com/p/binder-hole-rei ... lsrc=aw.ds ) prior to spraying the Krylon. It works.
Keep in mind that steel sold in the manner you describe is pretty much straight off the rolling mill, and is covered with a very fine layer of rolling lubricant. This stuff is an interesting mixture of rendered fat, wetting agents and other materials and is probably the actual cause of your darkened wood. Removing this stuff takes either a very strong solvent or extremely hot water and a very strong cleaning agent such as TSP - altogether environmentally nasty. But in small amounts, not so bad.
Oxalic Acid will remove the darkening from Walnut - use it with great care, however. This may be had at about any hardware store or paint store.
Good luck with it!
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