Black Oxide coating

I had a tour of the Axminster Tools facility yesterday which was very interesting. In the manufacturing dept the guide explained that they have changed to using stainless steel for some of their products due to the fact that the Black Oxide coating applied to ordinary steels is no longer environmentally viable. At least I think that’s the term he used. Interestingly though I noticed they’re still using black Unbrako socket screws. I wonder how they’re blacked and if the process used will change soon? Well worth a trip there if you get the chance, they have some amazing Japanese CNC machines there and they’re installing an even bigger one soon requiring an extension to the building.

If you plate or other coat bolts and cap screws etc you can cause embritalnent or lower the tensile strength of items but there is no change to steel strength or structure with blackodising which is done by diping in to a caustic solution at 135 deg which causes a black coating Fe 3O4 which us rust preventative no change to steel structure. That is why Unbrako blackodise their cap screws.

David

Edited By David George 1 on 20/07/2019 10:03:50

If you want to add a black finish to your homemade steel masterpiece, heat it to dull red & dunk it in old sump oil.

Do this outside with the part on a longish wire, as it will flare & smoke.

Obviously won't work with heat sensitive parts, but you will be amazed how professional it looks.

Yes I’ve done that in the past. I used to keep a small quantity of old oil for the purpose.

I've been told used diesel oil is the best for this, haven't got around to trying it

cheers

A little of topic but today I blued several small parts but two M1.7 mm screws which came from package which I bought did not colour. I replaced them with new ones I made from silversteel and that of course coloured without problems. Any idea what steel the others are made from, perhaps stainless? Niko.

For blackening steel my choice is used cooking oil as there are fewer nasty chemicals in it compared to old engine oil.

Speelwork, it's possible your screws were either stainless steel or bright zinc plated.

Do you know what is in the caustic solution?

I was given a 'home brew' recipe for blackening steel many years ago, it involves three different chemicals dissolved in water and boiling at some elevated temperature, sounds iffy, but if you want to try it I'll dig it out.

The classic industrial blacking process is phosphating, which uses phosphoric acid to create the black surface, which is then sealed with oil. This maybe what the OP is referring to.

It does not need to be used engine oil (which I understand is rather toxic ) but unused oil works fine. I had some unused 5W-30 engine oil which I no longer needed and tried it with good results. Rapeseed oil from the supermarket also works a treat. Best done outside if possible, where the fumes don't stink the house and set off the smoke alarm, as I found to my cost.

Doug

Lots of different processes for blacking/blueing or browning steel. Some - for example steam heating, or boiling in strong caustic soda solution - are mostly harmless. Others use chemicals covered by the Poisons Act or are licensed explosive precursors. Take your pick: A molten mix of Sodium Nitrite and Sodium Nitrate which fumes toxic Nitrogen Oxides, or wet mixes containing various combinations of Mercuric Chloride, Antimony Chloride, Bismuth Chloride, Selenium compounds, with Phosphoric, Oxalic, Hydrochloric and/or Nitric Acids. Now spoilsports have banned simply tipping toxic waste into the nearest river at zero cost, disposal of effluent is expensive.

Bloody tree-huggers, there's no proof Mercury in drinking water ever harmed anybody...

As always in engineering, production is a series of balanced compromises with cost playing the major part. Methods using heat alone are cheap and safe but the protective film is thin and the metal can be damaged internally. Chemical methods produce thicker protection and lower temperatures but are expensive and likely to involve unpleasant chemicals requiring special handling. The good safe cold processes don't always fit the bill. When all the costs are added up a particular blackening process could be more trouble than it's worth, and dodging the bullet by switching to stainless steel could well be advantageous. But blackening is still available and appropriate in the right circumstances.

Dave

Hi All

One of my treasured books.

Howe does an outstanding job describing formulas on metal and wood finishing.
Also high precision toolmaking in general from an old school manual perspective.

I bought mine many years ago. for the tech info, The gunsmithing part is secondary.

**LINK**

Regards
John

Slightly different question but on the same theme..... I have flame blued steel successfully (dunked in clean engine oil by the way) but I have never tried to 'blue' a mixed steel and brass assembly. Has anybody tried it? Does the brass just go dull?

Phil H

Beg to differ.

New motor oil leaves a browned finish. The best for a dark, deep black is used diesel oil. I've tried them all inc cooking oil, used diesel oil leaves the best consistent deep black.

Edited By thaiguzzi on 12/08/2019 10:44:18

I agree about used engine oil but it’s nasty stuff, especially from a diesel. I used to do it outside being careful not to breath in any fumes. It may be worth wearing gloves as well.

Does the colour make any odds to the corrosion resistance?

Regards Martin

I am sure there is a correct oil to use.

About a year ago I blacked a batch of small steel fixings using fresh clean Morris 32 hydraulic oil. I am more than happy with the results. The parts have been kept with a little bit of oil in a jar which is why there is a slight glint on some items in the photograph.

Blacking produces an porous coating which holds oil. As long as the item is kept damp with oil it should not rust. I don't think the colour of the coating is of any importance as far as holding oil.

JA

Off topic story. Some time ago we were making a part for a military aircraft from white plastic (type of ptfe I think) and someone accidently set the drying oven too hot so they turned black throughout. We were allowed to deliver them on concession but then got a request to make them all black as they had never liked the white finish (too visible).

I usually just heat and dunk in some old engine oil.

This weekend I tried a 3 stage chemical blacking kit I got off Ebay. I must say it is far easier and quicker (and probably a lot safer) than heating things up.

Jim