An oak framer told me a while ago to always use stainless fixings in green oak because the tanins ruin the fixing over time, with that in mind, if you re-forge a hex head stainless coach screw into a square head, do you need to passivate the threads?

Depends on the alloy. But for the most part its an appearance as apposed to performance issue. Surface contamination can lead to surface corrosion and staining.

Is the reforged screw going to go into green oak after forging?  Your question is not entirely clear.

Passivation is designed to neutralize free iron on stainless steels.  Not sure what alloy you are considering but as an example, you could forge 304 then passivate in citric acid in the shop.  Alternatively, you could send out the work to someone who does that type of process using citric or nitric acid.

Is the reforged screw going to go into green oak after forging?

Yes.
Most likely to use 304.

If you hold it in stainless steel tongs and hit it with a stainless hammer on a stainless block/anvil you will reduce the contamination, but you can get a stainless pickling paste from your welding supplier which would do the job having worked it with ordinary tools. Of course if you are going to be doing hundreds it would be well worth getting them passivated by someone like Anapol. Do you want the finish to be silver or black?

 

Of course you will be putting them in with a stainless spanner to save contaminating the facets when screwing them in?  It just goes on! I have just bought some Stainless Allen keys having used steel ones on stainless bolts for forty years...

 

Years ago I used some air dried oak for bench seats and backs for High Wycombe High Street and I finished them with a micropourous sealer. Having installed them I received a call the day before the grand opening that the new York Stone paviours had been stained by rust under my benches and what was I going to do about it? I ended up taking a gang down with cleaning agents and wire brushes and paint wondering how on earth the steelwork could have leached rust out through the zinc spray and couple of coats of paint in less than a fortnight. It was just the oak Tannin being washed out, and I was able to explain that the benches were not corroding away.  It taught me to explain the phenomenum to a client before it happens whenever using oak.

 

 

Alan

Cheers Alan. I'm aware of the pickling paste, my local welding supplier does it for £40 a tub.

You could have gone to High Wycombe with 2 guys and a multi pack of chewing gum, chewed some up then stuck it down over the tannin patches & said the stone was now aged to match the rest of the street.

In th us, some of the electronic firms in silicon valley use lemon juice insted of citric acid, as ofcorse lemon juice containes Citric acid, If staining isnt an isue you can usualy skip that.

I made some stainless steel hooks for my wife to hang some of the back yard pool implements.   They have all held up but there is surface rust all over them.   I forged then with a carbon steel hammer, anvil and tongs.   So my experience says if you do that.  Expect rust.   And of course they are outside all the time, near a chlorinated pool.

Stainless MUST be passivated after forging or welding, simple acids like citric acid will work. There are commercial methods as well, some of which were noted above.

 

In weld shops that work with various alloys, there are color coded grinding wheels, hammers, brushes, etc. for each alloy. Having to weld up your own chipping hammer out of stainless scrap is just another day at the job........

You could also quench it in cold water from an orange heat to passivate it.