Black oxide - Dimension change?

[lakeside53]

I need to make a bunch of fitted parts from 1018 steel and would like to give them a finish and mild rust protection with back oxide.

Does the process materially change the dimension?

[macona]

From what I understand there is, but unless you have a comparator mic that can read millionths I dont think you will be able to measure it. I believe the black is just a layer of magnetite (good rust)

[lazlo]

Are you talking about hot blue? Hot blue is 1 or 2 microns thick.

Cold blue is even thinner, but cold blue is purely decorative -- it doesn't prevent rust.

[KiddZimaHater]

Black Oxide won't change the dimensions of your part.
Only the color of the part changes.
It may be only a few microns thick. Nothing to worry about.

[Bill Pace]

I agree on the dimensional change, its not anything we'd likely be able to measure in the home shop.

I disagree - somewhat - with Laz that it (cold blue) is ineffective in preventing rust --- if applied correctly.

The key to it is having the piece clean, clean, clean! Some folks have good luck with mineral spirits - lacquer thinner, MEK, etc, but in my experience unless I bead blast the piece and immediately apply the solution, I can have spotty rust later. Of course this isn't to say that my stuff doesn't need an occasional wipe down with LPS, or similar.



This pic is one of the 4 guns that I've built and as you can see there is plenty of blu/blacking on it, and they sit on a shelf out in the open in my shop all the time, the oldest one is now about 4 yrs old and still looks good. As I said I will occasionally take them down and wipe/clean them with a light oil cloth - and occasionally will have to ues a 3M pad to take a small rust spot off, but they, and many other parts I cold blue hold up very well.

Of course, it can depend a lot where you live and the weather conditions, here I really dont have much rust problems on anything...

[MTNGUN]

Never been able to measure a dimension change in the parts that I hot blue.

[kf2qd]

Hot blue does change sizes

I have had to polish shafting with bearing fits because the bearing would no linger slide on the shaft because of the effect of the blacking. What are you trying blacken and what fits on it?

[MTNGUN]

here I really dont have much rust problems on anything...

? ? ? N. Louisiana is a desert now ? ? ? When did this happen ?

I grew up in Arkansas/Louisiana. Had to wipe all fingerprints off firearms with an oily rag or else the fingerprint would rust.

Now, I clean my firearms once a year.

[Bill Pace]

Had to wipe all fingerprints off firearms with an oily rag or else the fingerprint would rust

.

Hah! I admit it does sound weird, but I'm not alone nor have any kinda special set-up shop -- Lane lives bout 10 miles east away and another friend lives about 10 mi west, and none have any real problem with rust, -- though it does pop up on occasion.

[lakeside53]

I have had to polish shafting with bearing fits because the bearing would no linger slide on the shaft because of the effect of the blacking. What are you trying blacken and what fits on it?

Several parts, but those more critical are shafts into reamed bushings... both steel. It would be "hot black". I'l like a "light press fit".

[lazlo]

I disagree - somewhat - with Laz that it (cold blue) is ineffective in preventing rust --- if applied correctly.

The key to it is having the piece clean, clean, clean! Some folks have good luck with mineral spirits - lacquer thinner, MEK, etc, but in my experience unless I bead blast the piece and immediately apply the solution, I can have spotty rust later. Of course this isn't to say that my stuff doesn't need an occasional wipe down with LPS, or similar.

Bill, I think perhaps we're disagreeing on duty cycle. I'm talking about cold blue being ineffective at preventing rust on daily-use tools. I do everything you suggest except bead blasting: thorough cleaning, wipe with acetone, heat the part to ~ 200°, apply with nitrile gloves. I can get a dark bluing with cold blue, but it's exceptionally fragile, and it rusts quickly in Austin, which is admittedly hotter than Hell during the Summer...