Blackening formulas for steel and cast iron

A. Try any old-school blackening formula for blackening steel (50 gms ferric chloride [on eBay or Amazon]/ 1 lit water, brown finish boil it in water for 30 minutes, repeat it at least 3 times). tannic acid [on eBay or Amazon] / phosphoric acid mix can be used too (20 gms tannic acid [on eBay or Amazon]/15 mll phosphoric acid/ 1 lit water). Hope it helps and good luck!

A. According to old D.Fishlocks book on metal coloring you can use sodium hydroxide and Sodium Nitrate [affil links] formula. Sodium hydroxide ... 1200-1500 gms sodium nitrate ... 30 gms water 1 lit.,5-30 minutes immersion, boiling solution (140-150 °C). Hope it helps and good luck!

A. Expired US patent US2817610A is good source of information on that subject.Hope it helps and good luck!

A. Dear Mr. Ricardo B.,

Blackening on steel can be achieved by oxidation process using sodium hydroxide along with oxidising agents such as sodium nitrite and sodium nitrate. A typical formula for blackening on steel is given below. 1. sodium hydroxide (caustic soda [affil links] ) - 625 gms/litre 2. Sodium nitrite - 100 gms/lit 3. Sodium Nitrate - 25 gms/litre Here the temperature should be around 140 °C to get uniform blackening. with best regards, ramesh.

A. Answer to letter of Ricardo Bastos: There is really not much information about blacking of steels and cast iron available. All I can add is that it is simple and at everyone's site possible by means of a new blacking at room temperature. You need no costly apparatus and have no harmful chemicals-no heating-up-no exhaust-no environmental problems. It is as easy as dipping parts into paint. Klaus S [last name deleted for privacy by Editor] - WIESBADEN - Germany Hi Klaus. I claim below (as others have in published articles) that cold blackening is inferior to hot blackening, but hot blackening is extremely dangerous so the world is very interested in alternatives! If you can provide technical data on these issues, and results from independent testing, we'd love to hear more! You're also welcome to order paid advertising here or anywhere. But we don't include commercial promotion in postings in this public forum ( huh? why?), which is made available to us all for camaraderie & technical info by supporting advertisers. Thanks for understanding that we can't possibly spend their support money maintaining ads for their competitors. Regards, Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey

A. Hi, Janarthana. There are proprietary blackening solutions that operate at room temperature available from suppliers like EPI / Electrochemical Products Inc. [a finishing.com supporting advertiser], Heatbath, and several others. Some if not all are based on deposition of selenium compounds. We have an FAQ about black oxide and cold blackening.

However, the best of the proprietaries, after decades of trial-and-error and gradual improvement, are still inferior to hot blackening in my personal opinion. So I would not propose a non-proprietary cold blackening process because the likely quality of the finish would likely be very low. I'd say stick to hot blackening or purchase a proprietary room temperature bath. Good luck!

A. Phosphoric acid

A. Hi. I appreciate Sean's answer and help, but I don't think phosphoric acid is up to this job :-)

I think you'll need hot alkaline cleaning followed by rinsing and a dip in HCl.

Regards,

A. Hi, Rajendar. In most cases black oxiding will have no meaningful effect on hardness, dimensions, or smoothness. Some hardened components like rifle barrels are blackened.

But we can give you a more accurate and useful answer if you are able to describe the details of your own particular situation; there are a dozen "ifs, ands, and buts" which apply to the general case, but probably not to your case. Good luck.

Regards,

A. Hi Vikas. Ramesh Varadhan advised us "caustic 625 gm/ltr", but also to add sodium nitrite 100 gms and sodium nitrate 25 gms per liter of water.

In the 1998 Metal Finishing Guidebook, Nat Hall suggests caustic 80 oz/gal, potassium nitrite 30 oz/gal, and potassium nitrate 20 oz/gal. Biestek & Weber's "Conversion Coatings" .
offers five more variations, and a very good treatment about how the background chemistry works. One of their formulas is 650-700 gm/ltr caustic, 30-35 gms/ltr NaNO3, 16-18 gms/liter NaNO2, 18-20 gms/ltr NaCl.

I'm confident that any of these formulas will work, and that small variations are highly unlikely to be the main cause of any difficulties which you are encountering. Reprinting variations in formulas from textbooks is probably less useful than people describing their own situation and letting us try to help them with it :-)

Good luck, regards,

A. Probably oldest electroblackening process for iron and steel is Becquerels process (1861). He used a solution of iron-II-oxide in ammonia. According to Fishlock 400 gm NaOH/ 1 lit water solution can be used (50-100A/sq.ft.,120 °C temp, 10-30 minutes. Object= anode. Hope it helps and good luck!

A. According to Angiers' book Firearm Bluing and Browning there are very many formulas and many of them are based on use of more or less toxic components (mercury compounds, antimony compounds, potassium bichromate, nitric acid). If compared with that type of compounds, sodium chloride is harmless but still effective. Hope it helps and good luck!

A. The best answer to my secondary question regarding the reason for the NaCl dissolved in hydrogen peroxide seems to have been found on Reddit":

My commentary which relates to firearms coatings:

The blacking conversion (from red to black) is accomplished in a boiling water tank using the same process that has been used to form the classical 'cold rust' seen on many old firearms that post date the 'browning' era but pre-date the 'hot blue' using a high temperature saturated NaO/NaNO3 solution.

Thanks for the input.

A. My company has used the supersaturated aqueous solution of sodium hydroxide to blacken antique steel and cast iron hardware, such as sash pulleys for many years. Without any other additives, it works very well. Quite a durable finish.

Boil for 20 minutes, approx. 280 °F. We use a gas burner on a stand OUTSIDE. And handle very carefully, with protective clothing and face protection. Water being added can erupt as it superheats beneath the surface.

A. Any steel pot will do. Deeper is better. Reasonably thick. Don't use a thin steel can. Use tongs with thick rubber gloves and goggles to put steel objects into the pot