Black Oxide on SS 304 looking for tips on improving appearance and coverage

Hi Folks,
We have recently started running Black oxide finish on Stainless Steel 304 with a local finishing house.

A prototype lot of black oxide on about 10 part types came back with overall nice appearance and coverage.

A 1st production lot of black oxide on the same 304 SS part types (higher volume) came back with quite a few missing spots/regions. We see patches on some parts were the parts are not taking the black finish (sometimes 1/2" voids) but other regions on the same piece look fine.

The worst parts were 304SS precision ground rod

We are going to discuss this with the finisher, but does anyone have pointers, tips or guidance on common problems that we can look for?

Some questions we have are if the material could have composition or impurities, variations that cause problems.

Is there anything we can do prior to sending parts out that will increase the uniformity of black oxide (i.e. bead blast, chemical etch or cleaning process?)

Can black oxide be redone/reworked and if so what is the process (any stripping required?) If we don't yet know where the variations are coming from, I am not sure if rework might help, or what to change to prevent it.

Perhaps the finisher did something wrong, the bath was degraded, bath concentrations were wrong, dirty/contaminated, temp was low. We will ask them to comment on this.

Perhaps the metal quality or cleaning/prep on our side might the root cause. We are not sure yet why this is happening.

We are going to talk this through with the finisher, but would like to know if anyone with experience in black oxide of stainless steel 304 has ideas as to what to look out for to improve the uniformity of finish.

Thanks!
Steve

I am surprised it worked, black oxide is for carbon based steels.
It is matter of preparation, parts have to be super clean. but that is for the supplier to do.
parts have to be clean in degrease, then maybe a light blast, care full with aluminum oxide it will remove material
and is very aggressive, I would suggest to go there and follow the process.
I would think the SS oxide build up on the parent material would hinder black oxide.
P.S. I never had luck with stainless steel and actually never tried it.
so something new for me.

Excuse me for asking, but what's the point of black-oxide on stainless steel...??

ChipSplitter said:

Excuse me for asking, but what's the point of black-oxide on stainless steel...??

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Fair question, simple answer: Customer request

They wanted a specific look so we are providing what they ask for

ChipSplitter said:

Excuse me for asking, but what's the point of black-oxide on stainless steel...??

Click to expand...

We do it on 303 SS gun parts all the time. You need corrosion resistance but everything on the gun has to be black.

Not specific to SST and black oxide necessarily, but you should get a clear understanding of how the parts are being held/fixtured for immersion. If you have parts which can trap air, this will give obvious non-uniformity where air bubbles are preventing solution contact. There could also be localized material variation causing some appearance differences.

In my experience, getting SST parts black-oxide processed is a pain, and subject to severe variation in appearance, probably because it doesn't work THAT well to begin with. Some bead blasting may help to get more uniformity. I have run batches of McMaster 18-8 stainless washers through black oxide and never was very happy with what I got back.

MIL-DTL–13924D (except class 1, for carbon steel) should work.

specfab said:

Not specific to SST and black oxide necessarily, but you should get a clear understanding of how the parts are being held/fixtured for immersion. If you have parts which can trap air, this will give obvious non-uniformity where air bubbles are preventing solution contact. There could also be localized material variation causing some appearance differences.

In my experience, getting SST parts black-oxide processed is a pain, and subject to severe variation in appearance, probably because it doesn't work THAT well to begin with. Some bead blasting may help to get more uniformity. I have run batches of McMaster 18-8 stainless washers through black oxide and never was very happy with what I got back.

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Our local shop only does it one day a week. I don’t know the process, but it takes different chemical baths from normal black oxide. The SS parts actually look better than the low carbon steel parts.

i didnt even know you could blacken ss. found "ultra blak" even cold. i wonder whats in it to make ss rust.

dian said:

i didnt even know you could blacken ss. found "ultra blak" even cold. i wonder whats in it to make ss rust.

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It is becoming common in the appliance industry too. Black SS ovens , refrigerators, etc.

Kyle Smith said:

Our local shop only does it one day a week. I don’t know the process, but it takes different chemical baths from normal black oxide. The SS parts actually look better than the low carbon steel parts.

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It may very well be that the shop I usually use doesn't really know how to work the chemistry correctly. It was never a terrifically important aspect of what I had them do for us. 99% of our plating work is anodizing aluminum.

1953chevB said:

I am surprised it worked, black oxide is for carbon based steels.
It is matter of preparation, parts have to be super clean. but that is for the supplier to do.
parts have to be clean in degrease, then maybe a light blast, care full with aluminum oxide it will remove material
and is very aggressive, I would suggest to go there and follow the process.
I would think the SS oxide build up on the parent material would hinder black oxide.
P.S. I never had luck with stainless steel and actually never tried it.
so something new for me.

Click to expand...

It is done all the time.

There are countless situations where you need the corrosion resistance of stainless, but you need low reflectivity or a certain look.

Black oxider I use always has many bins full of stainless parts when I'm there. Seems they do more stainless than steel.

The black oxide process is pretty archaic. Probably parts getting stuck together or just laziness.

Black oxide looks pretty, but really offer no corrosion prevention.

1953chevB said:

Black oxide looks pretty, but really offer no corrosion prevention.

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It's stainless steel underneath.....

digger doug said:

It's stainless steel underneath.....

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Is it from sewer-grade billet...??

Ive done the cold "black oxide" kits from caswell on stainless gun parts, both old and new, and like you've found, its hit or miss with the color. I tried different cleaning solutions, blasting, suspension, etc. No change. Even the ones that were nice were not durable.
Sent some parts out ( cast stainless m1 garand cylinders) for stainless hot blackening with a gun shop that uses Brownells Oxinate 84, they all came back beautiful and the finish was durable.
Also have you thought about having them Melonited? then you'd have the added bonus of better surface hardness and durability. All my Melonited parts look great.

Garwood said:

It is done all the time.

There are countless situations where you need the corrosion resistance of stainless, but you need low reflectivity or a certain look.

Black oxider I use always has many bins full of stainless parts when I'm there. Seems they do more stainless than steel.

The black oxide process is pretty archaic. Probably parts getting stuck together or just laziness.

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right on, in my line of work, never seen it, go fiquire

digger doug said:

It's stainless steel underneath.....

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right so what's point to black oxide stainless, just to make them pretty, as a matter fact being its a solution
it could cause detrimental effects. stainless does and can get corrosion. salt spray is one.

carbon steel even with black oxide will corrode if not oiled properly right.
we have oil parts to prevent corrosion, hey but they look pretty.

1953chevB said:

right so what's point to black oxide stainless, just to make them pretty, as a matter fact being its a solution
it could cause detrimental effects. stainless does and can get corrosion. salt spray is one.

carbon steel even with black oxide will corrode if not oiled properly right.
we have oil parts to prevent corrosion, hey but they look pretty.

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I don't know... When I worked at a shop that did our own black oxide back in 2013, I had some of personal tools done. No rust to this day, and I don't oil them. I also don't abuse them and get them wet, and I work in AC....

Garwood said:

It is done all the time.

There are countless situations where you need the corrosion resistance of stainless, but you need low reflectivity or a certain look.

Black oxider I use always has many bins full of stainless parts when I'm there. Seems they do more stainless than steel.

The black oxide process is pretty archaic. Probably parts getting stuck together or just laziness.

Click to expand...

Thanks for the feedback, you may have hit the nail on the head. I can see what looks like fingerprints in some of the problem areas, and it looks like parts may have been in contact with each other.
We let the plater know about the problem, and they are going to rerun the problem parts.
They did not comment on what may have happened as to a specific cause.
However I know they got backed up on production, and lost power the day before from a storm, so maybe they just rushed things too much trying to catch up.