The black finish probably is basically another form of rust. It's similar to what is called "mill scale" that occurs on hot rolled steel. It's ferric oxide but with the iron and oxygen in different proportions than in the usual red rust.

The black oxide finish is common on tool steels such as drill bits. It's cheaper than other coatings, although it needs to be after-finished with oil, wax, etc to provide corrosion resistance.

There are a number of ways to produce such a coating.

http://www.heatbath.com/black-oxide-pentrate.htm

http://www.findarticles.com/p/articles/mi_m0PIL/is_2004_Nov_2/ai_n6275141

http://www.finishing.com/309/88.html

http://www.caswellplating.com/kits/black.htm

http://www.swdinc.com/black_oxide.html

Jane,

Gun blueing!

I have a few drywall tools that often need touchup because of surface rusting after use and cleaning. This seems to be the answer to restore the finish.

The link below also mentions black oxide finishing, which you may want to look at.

GG

Here is a link that might be useful: Ampere metal finishing

spamb - do you have experience w/ these products? I noticed some mfgs offer gel applications (instead of 200 deg F baths). Sounds DIY and I'm surpirsed none of the fabricators are aware of them. Perhaps I've been barking up the wrong tree?

hmmm. looks like i need to have the piece made first, then find another company that will dip the hood for me.

thanks.

Spam,

I'm not sure if the mfg. website comments about cold process oxide would include my suggestion about gun blueing, but once a steel item is treated with this, it takes a whole lot to remove it. I don't know if this product would be considered as cold process oxide.

GG

GG,

I see. Well, I was refering to cold black oxide finish, not gun blue finish, which is different.

In any case, the black oxide finish sounds like too much of a maintenance issue. A flat black enamel paint would give much the same look on the hood, plus it would require far less upkeep. On a countertop, I don't think the black oxide would last very long, and then one would be stuck with scratches that would initially stand out as bright steel streaks and would wind up as red rust stains.

The main value of a black oxide finish is that it is slightly corrosion resistant and imparts a thin hard surface to a part that cannot tolerate a change in dimensions due to the finish. This requirement obviously does not apply to kitchen hoods or countertops, where a few thousands of an inch is not going to matter.

More thoughts on these finishes:

It seems to me, a black oxide finish would be the same as what you would find on impact sockets and many automotive fasteners, and which is usually a heavy, flat finish. These finishes are more of a coating than a plain oxidizing steel treatment, which is what the gun blueing would be. I tried a little gun bluing on a scrap piece of steel sheet tonight, and after about 5 minutes, washed it off and used a very fine Scotchbrite pad to buff it out and even up the shading. Using this, it darkened the metal and still allowed the sheen of the metal to stand out. Once steel is oxidized to black with this, it will be fairly resistant to red rust, which is why it is used on guns.

GG

Well, actually, black oxide is not a coating. That is why it doesn't change dimensions. It is a chemical change of the iron atoms in the very upper layer steel. It is close kin to red rust. It wants to become red rust, which is why it isn't all that corrosion resistant and must be oiled or waxed to maintain its appearance. It is quite hard, however, as long as it doesn't turn to red rust. However, if it does get scratched, it may become cathodic to the scratched surface, which means that it will accelerate the rusting of the exposed metal.

i definitely will not be using this setup for countertops, just the hood. I've been hesitant of using paint b/c of it would look too dull/monochrome. The "oxided" hood looked mottled or blotchy and had depth...an antique feel of sorts.

Spam,

Black phosphate is the coating I was thinking of that is used on fasteners and impact sockets. This is finished off in a very high temperature environment, so a thin sheet metal range hood would likely be deformed if this coating process was used to color it.

Black oxide is the results of the the gun blueing process. The last site you had listed, had a good explanation of what their process was, which used hot chemicals. The liquid gun blueing I was using to recoat my drywall tools, is a cold process and is more sturdy than that website says.

Jane,

I would suggest you get the bluing and try it on a section of steel sheet that has been totally cleaned of all grease and dirt, and see if that is what you want for a finish. The gel you mentioned above, may be an easier product to use and may leave a more even color to the steel. It's just something you should try. Having this work done in a job shop may be a waste of money if you can get the same results from your own efforts. Good luck!

GG

Here is a link that might be useful: Difference of black phosphate and black oxide coatings

Jane,

There are lots of paint processes that could add interest. Such as a black crinkle finish, which seems to repel dust, scratches, and smudges quite well. Or a metal flake paint job. Airbrushing could be used to add interest as well. You could take the entire hood to an automotive paint shop and they could bake on an extremely durable finish. Or powder coating, which is even more durable, almost like porcelain.

I've got the opposite problem than you. The existing cooking area hood is done in classic 70's Harvest Gold. You know, that dull yellow with brown shading. Then it's got these fake metal straps with cheesy antique brass buttons. Way too much interest.

Since I've swapped out all the appliances for black or stainless, it kind of clashes. So I'm thinking of taking it down, cleaning it real well, removing the fake straps and filling the holes where the buttons go (or replace them with stainless cap nuts), and then shooting a nice glossy black enamel finish on it. The big hassle is taking it down... It's big and ungainly.

you wait another 20 yrs and that harvest gold will be the rage of the neighborhood.

Try Brownells for a decent range of gun bluing chemicals. Dicropan and T44 are both very good. Cold bluing does not produce as tough a finish as hot bluing, but hot bluing is not practical for anything as large as a range hood.
Using a heat gun and Dicropan will produce a very good blue/black finish. It may look blotchy and uneven after the first coat, but additional coats will even up the color and the finish. Brownells has good directions, they leave out the heating step.