intentionally oxidizing zinc bolts to retain grease?

[coppercone2]

So I had some rusty bolts that were used for braze fixturing and I thought they were stainless, so I cleaned them up with a brush and left it in a citric acid bath. I realized later they are actually zinc plated bolts. I was a little suspicious but I swore I thought I used stainless for this.

Anyway, they developed a kind of dark grey patina that seems quite porous. It looks like something that could absorb oil pretty good.

I don't think the zinc plating is completely compromised. But is this some kind of plausible finish for a bolt? Oxide layer that is saturated in grease? I would do it under more controlled conditions, but I find it interesting since its quite rough. In this case I will likely throw the bolts out because I see rust on the threads because they got a bit too beat up, but it seems interesting. Its not a very durable oxide layer though, scratches off easily if I rub them together.

[jpanhalt]

Why would you want to grease bolts used for a welding fixture?

Why would you want to grease any bolt?  Aren't they for holding stuff together tightly rather than being slippery?

[coppercone2]

I mean you usually grease bolts with something or another. I just meant in general as a coating. I wonder if it would be more resistant to further corrosion if you say dipped it in a pot of molten grease. It seems like it would really get in there and bond well to the surface. Kind of like black oxide coating. But black oxide coating is on the steel, this would be a tertiary layer on the zinc of a galvinzied bolt to allow for oil impregnation.

I think you could check by preparing two bolts, that is a greased galvinized bolt and a greased whatever this is bolt and weigh them to see the oil retention.

[SeanB]

Zinc will slowly oxidise in air outside, which is why it normally is either left bare for 6 months before painting, or you use etch primer on it, or wipe down with phosphoric or sulphuric acid and wash before painting. Yes the oxide layer will hold a grease or oil, just like a black oxide coating does, but will also provide extra cathodic protection. Your threads likely rusted because the zinc coat was completely worn off from multiple cycles, which will strip the soft coating off.

[coppercone2]

I am curious if you can delicately partially rust the zinc coating with a acid so that grease adheres to it better but its not totally compromised to make a improved zinced bolt

And is the chemical zinc oxide or zinc citrate? Because I thought zinc oxide would be white.

[Kleinstein]

Zink oxide is transparent, it only gets white when as a fine power. This is common to many white powders to get the reflection from the surfaces of an otherwise transperent material.
It is well possible to have some other zinc salt at the surface depending on the acid and an organic acid may indeed provide some anti sieze / lubrication without any grease. A know good anti sticking material is zink stearate. This is kind of similar to soap.

[coppercone2]

Hmm I was not thinking of the lubrication provided by the dry oxide, just the fact that it seems to wick/absorb/hold grease due to its surface finish/porous feeling.

They feel rougher for sure. Usually thats bad because it messes with tightening torque causing it to increase. Then you need to measure bolt elongation to be sure of proper engagement.

Not sure about the dry anti sieze component. On one hand you would imagine rougher is worse but I have a feeling that one might not be so obvious.

But its interesting also as a dry film for the roughness because lets say you dust it with graphite powder, it seems that the graphite would get stuck in the porous layer and be rather difficult to get off the bolt without ultrasonic cleaning or whatever. Kind of like turning a bolt into a powder pillow (used for makeup, chalk disposer too for gynmnasts grip)

I wonder if the graphite would grow into the oxide if you had a agitated solution of acid and very fine graphite to slightly corrode the bolt. Maybe electrically charge them so particles attract during the mild corrosion process to get embedded.

[Kleinstein]

I see no chance for the graphite to grow into the oxide, as the oxide is growing from the metal surface out, not from the liquid side.

[mfro]

I don't think the zinc plating is completely compromised.

Rusted bolts should be replaced. The zinc plating is meant as a sacrificial anode (besides as a means of lubrication) to ensure even if plating is bruised, the zinc will corrode before the steel.

If most of the zinc is oxidized already, this won't work reliably anymore. New bolts with new zinc plating will provide a lot better rust prevention than any amount of grease or oil can do (besides that screw joints are usually dimensioned for dry assembly).

[coppercone2]

well yeah I threw the bolts out the idea is to try to make one in controlled conditions to act as a grease retainer maybe but I am starting to lose interest now

[Stray Electron]

  If you don't need lubrication on the bolts and are thinking of greasing them only to prevent corrosion then I would say don't spend your time over thinking this and just buy some stainless steel bolts and be done with it.  Unless your bolts are some odd size, or there is something special about them, new SS bolts can be had cheaply from Amazon and/or E-bay. Especially if you buy a package of 50 to 100 at a time. 

   FWIW, I have heat blued plain steel bolts in the past and that adds some corrosion resistance to them but hot-dipped galvanized (not electroplated) are much more corrosion resistant. But HD bolts have a much rougher finish so they can't be used for some applications.

   PS Thank GOD for Amazon and Ebay!  The price of SS hardware is finally coming down to something that we mere mortals can afford!  I now buy SS for all of my outdoor projects.  I just bought a package of about 300 SS self-tapping screws to put up a trellis with. I think 300 SS ST screws from China cost me about $10 from E-bay.  I can't buy plain steel fasteners locally for that price.

[coppercone2]

I noticed they are cheap but I never saw strength tests done on them. Just putting it out there incase of a guard rail on a deck or something

[mfro]

I noticed they are cheap but I never saw strength tests done on them. Just putting it out there incase of a guard rail on a deck or something

Nuts and bolts have to have their strength class imprinted. If they haven't, stay away. Probably soft cheese instead of steel.

[T3sl4co1l]

The wrinkly finish, I think, is a combination of surface oxidation plus intermetallic growth.  I wouldn't recommend it for anything requiring strength or flexibility.  Fe-Zn intermetallics, like most, are brittle.  (Curiously, Fe has a large solid solution range, i.e. capacity to dissolve zinc in it, analogous to brass being a solid solution of Zn in Cu; I'm not aware of any application of such alloys though, and I'm sure they're a bit of a pain to make, for obvious* reasons.)

*Obvious to anyone familiar with zinc's boiling point, that is!

Also, I don't think there are any Zn-ZnO mixtures, nor much if any solubility at standard pressures, and below ludicrous temperatures.  So, no suboxide thing, just how the surface tension and bulk diffusion happens to crinkle up.

Zinc citrate is water soluble, or at least modestly so, so can be washed away.  Anything that doesn't remain is either metal or more oxide (or other insoluble contaminants).

An anisotropic etch + electropolish process would probably give a more consistent and durable roughness for purposes of lubrication and wearing.

I suppose that more or less describes a black oxide or phosphate finish on steel, give or take how much the roughness can be controlled.

There's a zinc phosphate process as well, which perhaps has similar surface microstructure.

Mind that zinc is a soft metal so the asperities of such a surface will wear down particularly quickly; I wouldn't think such a surface is suitable for, like, crankshaft bearing journals, but maybe where less pressure and precision are required, it would be fine.

A rough surface of a tougher metal, like bronze, seems feasible.  Oilite(R) is an oil-impregnated sintered bronze product; the surface of which will be largely smooth (the wide faces of grains cut smooth by finish machining, or wear), but which is marked by a network of pores between grains (supplying oil by capillary action).

Tim