Browning/Blackening stainless steel: black oxide, chemical dip, PVD, electroplating?

A. Hi Chris. Isopropyl alcohol and acids are "sort of" cleaners, but not quite. Is your tube "water break free" when you pull it out of the deionized water in step 8? If not, before searching further for causes I think you should make sure it's clean by inserting step 5A, scrubbing with a scrub brush ⇦this on eBay or Amazon [affil links] and wet powdered pumice ⇦this on eBay or Amazon [affil links] .
Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Dear Mukesh,  

My company is a manufacturer's agent that specializes in finishes for stainless steel (type 304/316). We provide Titanium sputtered (vacuum deposition) of titanium oxide onto stainless steel. Colors available are: Gold, Black, Bronze & Blue. Titanium oxide plated sheets are durable, abrasive resistant, will not fade and do not require lacquer coatings replaced every 2-3 years. Because the base material is type 304/316 stainless, the sheets will not corrode or rust, not even near salt water environment (where, by the way, we recommend Type 316).

A. Hi, Mukesh. In addition to sputtering with titanium, stainless steel can also be electroplated with black materials like black chromium; it can be black oxided, although this may not offer enough corrosion resistance; it can be anodized into black and other colors. And, of course, it can be painted or powder coated.

Salt water is very corrosive and I think it would need to be a minimum of grade 316. Good luck.

Regards,

A. Black Oxide coatings on stainless steel are common. Type 303 has a high sulfur content that seems to interfere with the black iron oxide (Fe3O4) formation, but the rest of the 300 series can be coated to MIL-DTL-13924 [on DLA], Class 4. (I like 316 or 316L over the 304 alloy.) Remember that black oxide coating is not prescribed for corrosion protection. If you're not going to paint it, use a water displacing preservative coating such as MIL-PRF-16173 [on DLA, grade 3 or 4. The process details are found in MIL-HDBK-205A.

Get the military specs - they are free (you've already paid for them in taxes :). Google "Assist Quicksearch" and put in the numeric part of the spec in the "Document Number" box.

A. As an alternative to trying to formulate your own black oxide process for stainless steel, proprietary processes are available from companies like EPI / Electrochemical Products Inc. [a finishing.com supporting advertiser] and others.

A. Have your tried titanium nitride colored stainless? Physical vapor deposited onto type 300 or 400 series?

Any MSDS data from your examinations/evaluations would be appreciated.

Regards,

A. Hi Jim,

I had experience with a black finish on stainless steels. I believe that sulfuric acid/dichromate solutions can help you achieve the goal if you use proper ratio of sulfuric acid to dichromate, high concentration and high temperature, and enough immersion time. But be very careful, this operation is quite dangerous.

A. We frequently put what we call "black stain marks" on type 300 & 400 stainless steels by local spot annealing with laser. The result is an indelible jet black mark (it looks just like 100% black ink) 0.001" - 0.002" into the metal without any visible erosion of the surface (visible under 30X magnification). I recognize that rastering a surface with a 0.004" dia. spot isn't an economical process for areas of any size (although Penn State's Advanced Research Center (ARC) has pioneered work with laser cladding of large areas using just such a rastering technique) but it suggests that there may be a non-chemical heating method for obtaining this finish.

A. There is a product available from Heatbath called PX5; it is a blackening salt designed specifically for stainless steel. Good luck.

A. Hi Chris and Victor,

I'm not familiar with Heathbath's PX 5 product (though I'm sure it's a good one). But I know Hubbard Hall has a room temperature Black that works on most stainless steels.

Good Luck,

A. John

adv.
Please feel free to contact me for more information.

A. Maybe this solution can help: 50 grams H2SO4 200 grams sodium thiosulphate ⇦this on eBay or Amazon [affil links] 1 liter water, 30 °C temp.

Oxynate No. 7



(affil links)

A. Hi Walt. If the volume is too low to interest a local distributor of plating chemicals, it's also possible to buy in small quantities from consumer-oriented companies like Brownells.

Regards,

A. Black oxide is just that - chemical conversion of IRON to the black oxide form of rust (as opposed to the all too familiar red form). It has an appealing black color, is fairly well adherent to the basis metal, and adsorbs post treat preservatives such as oil quite well.

This should be compared versus a black etch smut that forms upon immersion of certain stainless steels in various acid media such as hot sulfuric or hot muriatic (hydrochloric). This black smut is non-adherent and non-uniform in appearance. They are obviously not the same!

A. Black oxide conversion coatings for steel are formed in alkaline solutions containing strong oxidizers (nitrite/nitrate).
The chemical reaction for the formation of magnetite, black oxide, is as follows: The iron for the blackening process is contributed by the base metal. Byproducts of the blackening reaction are colloidal iron and carbonates, which are removed from the bath by de-sludging and skimming.
Black oxide coatings do not change the dimensions of the parts blackened. The coating will not chip or peel and acts as an absorbent to hold oils or waxes.
The black oxide bath is simple to operate. A rolling boil should be maintained at 150 °C with additions of water or blackening salts. Cleaned parts are immersed from 5-20 minutes in the bath and then rinsed thoroughly. Finally, a rust preventive coating is applied.
The black oxide coating itself does not afford much corrosion protection (only 1-2 hours salt spray exposure); although the rust preventive top coat is absorbed by the black oxide and can achieve up to 100 hours of salt spray exposure depending on the product used.

A. Blackening on SS can be done on various methods. But all depends on the usage and its utility and the purpose and the environmental condition. If the answer is submitted, we can suggest many blackening processes.

A. Hi. Last I heard Ebonol was for copper and zinc and alloys of the same, such as brass, and nickel. But I think you folks need to contact the manufacturer (Enthone) since Ebonol is a trade name for a proprietary blackening process and there may be a number of different formulations.

But Kaka, don't increase the strength of your HCl -- 50% is already probably too much, and not great for stainless steel. I suspect that the metal is simply dirty. You need to do hot alkaline cleaning until you have a waterbreak-free surface after rinsing before you do the acid dip. Good luck.

Regards,

A. Hi Guy. You say that you have a "precise temperature controlled process", but does that mean you turn heaters on and off to maintain temperature? Not good enough!

As Rod Henrickson has noted several times on this site, you must never attempt to modulate the temperature by controlling the heat input; rather you must maintain a rolling boil and add water if the temperature is too high and evaporate it if too low ... because the solution must always be at a rolling boil or you will get brown parts. You must control the temperature by controlling the concentration and thus the boiling temperature, not by modulating the heat input.

Regards,

A. Hi. Sorry, I don't know of an instrument that can detect the boiling point of a liquid. That is a good question, though -- obviously it would be nice to not madly pour heat into a solution beyond the amount necessary to maintain a rolling boil.

But in the meanwhile, remember to not use those thermostats to modulate the heaters -- instead, the solution must be kept at a rolling boil, and you must add water if that rolling boil temperature is higher than your spec, and stop adding water if that rolling boil temperature is lower than your spec. Good luck.

Regards,

A. Chemically Blackening Stainless is not easy. It is usually done in an Alkaline salt bath at elevated temperatures (230 °F? if I recall) as well as some cleaning before hand. Heatbath used to sell a line of product for this purpose.

There are room temperature systems out there to blacken steel, but these are for carbon steel, not stainless.

A. Try next download free booklet, there you can find some black finishes for stainless steel :
attachments.wetpaintserv.us/ZLSnLKosv0l3CnE9hGDi7w488720

Hope it helps and good luck!

Ed. note Jan 2017: Although wetpaintserv.us was a legitimate site when Goran posted this, it is currently a malware site. DON'T GO THERE!! Goran has been kind enough to upload his booklet to https://www.finishing.com/library/budija/budija.pdf.

A. Hi Don. We attached your inquiry to a thread which introduces many different ways of blackening stainless steel. But of these, I think only sputtering or black oxiding would meet your tolerance requirements, and sputtering will not get into those holes and threads.

Allan Lang notes that black oxiding of 303 stainless can be problematic, so I think you should contact a supplier of proprietary black oxide processes and see what they suggest. Best of luck.

Regards,

? Hi Adeel. When you say you need to 'oxidize' stainless steel, I'm thinking you want to blacken it through that oxidation process (black oxide). We appended your inquiry to a thread about blackening stainless steel, and several other applicable threads can be found with that search term. If you are not intending to blacken the stainless, please help me understand what you mean by oxidize.

Regards,

A. Hi Thomas. I would be surprised if 13% sulfuric acid has the required activating power to remove heavy oxides and expose the stainless steel. I'd probably focus on trying a more powerful acid. You could try activating a scrap part in hydrochloric acid or a tri-acid salt and see what happens.

Regards,

A. Hi again. Although black oxide lines do not usually have provision for electrocleaning, I wonder if, as an experiment, anodic electrocleaning would be useful before the HCl?

Regards,

A. Thomas,
Consider sending some sample parts to the (PX-5) manufacturer for processing. They should be able to help you dial in the process.